Methods for on-line radioisotope measurements for the characterization of failed fuel in primary sodium systems.

Abstract

A failed fuel pin releases cesium into the primary sodium coolant and xenon into the cover gas in the reactor vessel. A tube containing radioactive liquid sodium receives the flowing primary sodium from the reactor vessel. A radiation detector is positioned near the tube to measure gamma rays emitted from the tube. The tube may be isolated to increase the detection limit by allowing short-lived isotopes to decay. Isotope ratio 137 Cs / 134 Cs can be measured and used to determine the burnup of the fuel assemblies from within the reactor core. As a result, failed fuel assemblies can be identified based, at least in part, on their burnup. Additionally, mass spectrometry may be used to measure the ratio of stable to unstable xenon isotopes. These identification techniques may be used in combination to rapidly identify failed fuel assemblies in situ during reactor operation.

Description

Detailed Description of the Invention

[0001] [Government License Rights] This invention was made with government support under DOE Cooperative Agreement No. DE-NE0009054 awarded by the U.S. Department of Energy. The government has certain rights in this invention.

[0002] 〔background〕 In a sodium-cooled fast reactor (SFR), reactor components include a reactor vessel filled with liquid sodium coolant and a reactor core. In some cases, SFRs are once-through fast reactors that use subcritical recharge fuel with in situ breeding and burning. The reactor core is immersed in a sodium pool within the reactor vessel. In some designs, the core may include multiple fuel pins bundled into multiple fuel assemblies. These include fissile fuel and fertile fuel that can be bred into fissile fuel. Within the reactor core, the fuel pins are subject to significant stresses and strains due to mechanical stresses and strains, as well as thermal stresses, neutron damage, and internal forces within the fuel pins resulting from nuclear fission. In some cases, one or more of these fuel pins may fail, for example, leaking parent fuel, fissionable fuel, and / or fission products into the primary sodium.

[0003] Typically, a fuel pin failure is a breach in the fuel pin cladding. The breach allows direct contact between the primary coolant and the nuclear fuel. This contact can result in a reaction between the oxide fuel and sodium. This reaction can lead to the formation of a uranoplutonate phase. This formation can further lead to fuel expansion and the potential for further degradation of the fuel pins. Furthermore, in metal-fuel-based reactors, fission products can escape from the fuel. In particular, cesium accumulates in the sodium bonds of the metal fuel, and cesium vapor accumulates in the gas plenum. This cesium can be released upon pin failure.

[0004] Delayed neutron and cover gas analysis are typically used to detect the presence of a failure. However, locating a failed fuel element can be challenging. For example, triangulation with delayed neutron detectors, flux tilting, sodium sampling, wet shipping, and dry shipping are all techniques that have been attempted with varying degrees of success.

[0005] For example, dry shipping requires the fuel assembly to be completely lifted from the sodium pool and cooling discontinued, allowing it to heat up within the sealed vessel. This can be supplemented by pulling a vacuum on the fuel assembly, creating a pressure differential and "burping" the assembly. Wet shipping requires pressurizing the fuel pins to vent the fission gases. Wet shipping is typically accomplished by blocking the assembly's outlet to reduce cooling, resulting in heating and pressure buildup within the fuel pins. A failed fuel pin releases additional fission product inventory, which can then be sampled. Sodium sampling involves leaving the fuel pin in place and taking a sodium sample from the top of the fuel assembly and analyzing it with a delayed neutron detector. However, each of these methods has drawbacks. It would be advantageous if the presence and location of a failed fuel pin could be determined efficiently without affecting reactor operation.

[0006] 〔summary〕 According to some embodiments, a method for characterizing a failed fuel assembly in a nuclear reactor includes the steps of: flowing primary sodium coolant through a bypass pipe; 137 Cs / 134The method includes determining an isotope ratio of Cs; determining a burnup of the failed fuel assembly based at least in part on the isotope ratio; and identifying the failed fuel assembly based at least in part on the burnup. The isotope ratio determining step may be performed by gamma ray spectroscopy. Furthermore, the method may be performed without removing primary sodium coolant from a closed system including the reactor vessel and the bypass piping. In other words, the method may be performed in situ. In some cases, a cover gas in the reactor vessel is analyzed to detect fission products in the cover gas, thereby determining that a fuel assembly has failed. The isotope ratio determining step may be performed by mass spectrometry. In some cases, the method further includes providing tag gas to one or more fuel elements in the fuel assembly. The providing tag gas step may include providing a plurality of unique tag gases, the number of unique tag gases being less than the number of fuel assemblies located in the reactor core. In other words, a first unique tag gas may be applied to a first group of fuel assemblies, a second unique tag gas may be applied to a second group of fuel assemblies, etc. In some embodiments, the method is performed during reactor operation. Identifying the failed fuel assemblies may include determining a subset of the fuel assemblies, where the subset of the fuel assemblies may include one or more failed fuel assemblies of the failed fuel assemblies. The method may further include analyzing fuel assemblies of the subset of the fuel assemblies to identify failed fuel assemblies, which may include a lift-and-burp technique. Lift-and-burp occurs due to potential temperature increases and hydrostatic pressures caused by removing the assemblies from a forced flow path within the reactor core. In some cases, the method may include isolating the sodium coolant within the bypass pipe.

[0007] In some embodiments, a system includes a nuclear reactor core, a plurality of fuel elements disposed within the nuclear reactor core, a volume of primary sodium coolant in contact with the plurality of fuel elements, a sodium processing cell external to the nuclear reactor core, the sodium processing cell in fluid communication with the nuclear reactor core by sodium processing piping, a detector near the sodium processing piping, the detector configured to detect radioactive emissions of an isotope that has escaped from a failed fuel assembly, and one or more processors configured with instructions that, when executed by the one or more processors, cause the one or more processors to perform the following operations: determine an isotope ratio of the isotope; determine a burnup of the failed fuel assembly based at least in part on the isotope ratio; and determine a location of the failed fuel assembly within the nuclear reactor core based at least in part on the burnup of the failed fuel assembly.

[0008] The system may include a plurality of unique tag gases located in selected fuel elements of the plurality of fuel elements disposed in the reactor core. For example, a first tag gas may be provided for a first group of fuel assemblies, and a second tag gas may be provided for a second group of fuel assemblies. In other words, the number of unique tag gases may be less than the number of fuel assemblies.

[0009] In some cases, the detector is configured to detect gamma emissions from the isotopes that have escaped from the failed fuel assembly through gamma ray spectroscopy. In some embodiments, the isotopes that have escaped from the failed fuel assembly are one or more of a plurality of xenon isotopes or a plurality of cesium isotopes. In some cases, the isotope ratio is 137 Cs / 134 It is Cs.

[0010] In some embodiments, a method for locating a failed fuel assembly in a nuclear reactor includes determining that a fuel assembly is failed, flowing primary coolant near a detector, determining an isotopic ratio of fission product isotopes in the primary coolant using the detector, determining a burnup of the failed fuel assembly based on the isotopic ratio, and determining a location of the failed fuel assembly within the reactor based on the burnup of the failed fuel assembly and through core modeling and fuel assembly tracking. 137 Cs / 134 Cs. The isotope ratio of fission product isotopes in the primary coolant may be determined by measuring gamma emissions. In some cases, the method is performed while the reactor is operating. Further, the method may be performed without removing the primary coolant from the closed cooling loop.

[0011] In some cases, using the detector to determine the isotopic ratio of fission product isotopes in the primary coolant is performed on a volume of sodium in a flowing state, thereby enabling real-time online measurement of isotopic ratios. In some cases, the method further includes detecting tag gas from a failed fuel assembly.

[0012] In some embodiments, a method for identifying failed fuel assemblies in a nuclear reactor core includes, in no particular order, determining that a fuel assembly is failed; narrowing down potentially failed fuel assemblies to a first subset of a plurality of fuel assemblies by using isotope ratios to determine burnup; narrowing down the first subset to a second subset of a plurality of fuel assemblies by using tag gas; and confirming the failed fuel assemblies based at least in part on the burnup and the tag gas.

[0013] In some cases, burnup is determined by detecting isotope ratios using gamma ray spectroscopy. The method may be performed in situ without removing a volume of sodium coolant from the reactor core. In some cases, determining that a fuel assembly has failed is performed by analyzing cover gas within the reactor vessel and detecting fission products in the cover gas.

[0014] In some examples, using the tag gas includes providing a first tag gas to a first group of the plurality of fuel assemblies, providing a second tag gas to a second group of the plurality of fuel assemblies, and providing a third tag gas to a third group of the plurality of fuel assemblies.

[0015] The method may be performed while the reactor is in operation. In some cases, the method further includes analyzing the second subset of the plurality of fuel assemblies using a lift-and-burp technique.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are part of this disclosure and are incorporated herein. The drawings illustrate example embodiments of the present disclosure and, in combination with the specification and claims, serve to explain, at least in part, various principles, features, or aspects of the present disclosure. Specific embodiments of the present disclosure are described more fully below with reference to the accompanying drawings. However, various aspects of the present disclosure may be implemented in many different forms and should not be construed as limited to the implementations disclosed herein. Like numbers refer to like (but not necessarily the same or identical) elements throughout.

[0017] The following drawings that form part of this application are intended to illustrate the described technology and are not intended in any way to limit the scope of the claimed technology, which scope is based on the claims appended hereto.

[0018] FIG. 1 illustrates a partial cutaway perspective view of a nuclear fission reactor, according to some embodiments.

[0019] FIG. 2 is a cross-sectional top view of a nuclear fission reactor core, according to some embodiments.

[0020] FIG. 3A is a partial elevation view of a nuclear fission reactor core, according to some embodiments.

[0021] FIG. 3B shows a fuel element with a fuel and tag gas capsule disposed therein, according to some embodiments.

[0022] FIG. 4 illustrates, in block diagram form, a sodium-cooled fast reactor having a sampling subcell as part of a sodium handling system, according to some embodiments.

[0023] FIG. 5 is a graph illustrating activity of cesium-134 and cesium-137 versus burnup, according to some embodiments.

[0024] FIG. 6 illustrates a process for online radioisotope measurements for in-cell failed fuel characterization, according to some embodiments.

[0025] FIG. 7 is a graph illustrating mass ratio of xenon isotopes versus burnup, according to some embodiments.

[0026] FIG. 8 illustrates a process for identifying and locating a failed fuel assembly within a nuclear reactor core, according to some embodiments.

[0027] FIG. 9 illustrates a process for identifying and locating a failed fuel assembly within a nuclear reactor core, according to some embodiments.

[0028] Detailed Description This disclosure describes exemplary embodiments, which are not intended to limit the scope of the disclosed embodiments and the appended claims in any way. The embodiments have been described using functional building blocks that illustrate implementations of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately implemented.

[0029] 1 and 2 illustrate a nuclear fission reactor and core as a non-limiting overview. As shown, the nuclear fission reactor 100 includes a nuclear fission reactor core 102 disposed within a reactor vessel 104. In some embodiments, the nuclear fission reactor core 102 includes nuclear fuel in a central core region 106. The nuclear fission reactor core 102 may include fissile nuclear fuel assemblies 202, parent nuclear fuel assemblies 204, and movable reactivity control assemblies 206. In some embodiments, the nuclear fission reactor core 102 may include only fissile nuclear fuel assemblies 202 and parent nuclear fuel assemblies 204. According to some embodiments, an in-vessel handling system (not shown) is configured to shuffle fissile nuclear fuel assemblies of the plurality of fissile nuclear fuel assemblies 202 and parent nuclear fuel assemblies of the plurality of parent nuclear fuel assemblies 204 within the reactor core 102. The nuclear fission reactor 100 may further include a reactor coolant system 108 .

[0030] In some implementations, the nuclear fission reactor 100 is based on elements of liquid metal-cooled fast reactor technology. For example, in various embodiments, the reactor coolant system 108 includes a pool of liquid sodium disposed within the reactor vessel 104. In such cases, the nuclear fission reactor core 102 is immersed in the pool of sodium coolant within the reactor vessel 104. The reactor vessel 104 may be surrounded by a containment vessel 110. The containment vessel 110 helps prevent loss of the sodium coolant in the unlikely event of a leak from the reactor vessel 104.

[0031] In various embodiments, the reactor coolant system 108 includes a reactor coolant pump 112. The reactor coolant system 108 may include one pump, two pumps, or any suitable number of pumps. Furthermore, the pump may be any suitable pump (e.g., electromechanical, electromagnetic, etc.) as desired.

[0032] The reactor coolant system 108 may include one or more heat exchangers 114. The heat exchangers 114 may be disposed within a pool of liquid sodium. In some embodiments, the heat exchangers 114 have a non-radioactive intermediate sodium coolant on the other side of the heat exchanger 114. For this purpose, the heat exchangers 114 may be considered intermediate heat exchangers.

[0033] Pump 112 may be configured to circulate the primary sodium coolant through the nuclear fission reactor core 102. In some embodiments, the pumped primary sodium coolant exits the nuclear fission reactor core 102 at an upper portion of the nuclear fission reactor core 102 and passes through one side of a heat exchanger 114. In some embodiments, the heated intermediate sodium coolant may be circulated via an intermediate sodium loop 116 on the side of the containment vessel 110 to, for example, a steam generator, a heat storage system, or the heated intermediate sodium coolant may be circulated to a heat exchanger for further use. The primary sodium coolant may be circulated through the reactor core and through the fuel assemblies within the reactor vessel, and a volume of the primary sodium may be sent beyond the reactor vessel to a sodium processing cell. Further details on this point are provided below.

[0034] 3A shows a nuclear fission reactor core 100 including a plurality of nuclear fuel assemblies (e.g., fissile nuclear fuel assemblies 202, parent nuclear fuel assemblies 204, movable reactivity control assemblies 206, etc.), illustrated as fuel assemblies 302. In some embodiments, the fuel assemblies 302 may be supported in part by a core support grid plate 304. According to some embodiments, primary sodium coolant flows through the fuel assemblies 302 to absorb heat generated by the fuel within the fuel assemblies undergoing nuclear fission reactions.

[0035] In some embodiments, the fuel assembly 302 includes a plurality of nuclear fuel pins (e.g., fuel rods, fuel elements, etc.) disposed within a duct that includes a tubular body. In some cases, the tubular body has a hexagonal cross-sectional shape, as shown in Figures 2 and 3. In use, primary sodium coolant flows upward into the fuel assembly and around the fuel elements within the fuel assembly, extracting heat from the fuel assembly and delivering it to a heat exchanger.

[0036] FIG. 3B illustrates a fuel element. A fuel element is typically an elongated body including a thin-walled outer jacket (i.e., cladding 310). Note that the fuel element 302 need not be neutronically active. In some cases, the fuel element 302 need not contain fissile material but may instead include a neutron-reflective or parent material, or a combination thereof. In some cases, the fuel element 302 may include multiple fuel slugs 312, or multiple fuel slugs 312 may be stacked. Of course, the fuel element may include any suitable fuel material and form, including extruded, annular, sponge, oxide, metallic, and other types and shapes of nuclear fuel. In some cases, the tag gas capsule 314 may include a tag gas (i.e., an identifying gas or gas mixture). According to some embodiments, the tag gas capsule 314 may be filled with a specially blended gas (e.g., one or more isotopes of krypton or xenon). In some examples, once the tag gas capsule 314 is loaded and sealed within the fuel element 302, the tag gas capsule 314 may be punctured to allow the tag gas within the capsule to flow into the fuel element. Then, if a leak occurs in the fuel element 302, the tag gas can be released and a defective core assembly can be located according to embodiments described herein. In some cases, the tag gas capsule 314 may be molded into an end cap or placed inside the fuel assembly. According to some embodiments, the tag gas capsule 314 releases its gas contents when the fuel assembly reaches a desired temperature, so the tag gas capsule 314 may remain intact until the fuel assembly is placed in an operating reactor core. At some point after installation within the fuel assembly, the tag gas capsule 314 may be ruptured, punctured, broken, or otherwise unsealed to allow the tag gas to flow into the fuel assembly.

[0037] Figure 4 shows a system 400 including a sodium-cooled nuclear reactor 402. The reactor 402 has a reactor core with fuel elements disposed therein. As shown with respect to Figure 3, a fuel element is typically an elongated body including a thin-walled outer jacket (also referred to as cladding) and a fissionable and / or parent composition (including fissionable nuclear fuel) within the cladding. Depending on the reactor design, multiple fuel elements are typically arranged together in fuel bundles or fuel assemblies, and multiple fuel assemblies are contained within the reactor. The geometry of the fuel element can be any suitable shape designed to fit the physical and design constraints of the fuel assembly and the reactor.

[0038] In conventional nuclear reactors, during irradiation in the core, fuel expands, for example, due to the production of fission products, particularly gaseous fission products. The fuel expands within the available space in the inner diameter of the cladding of each individual fuel element. However, over time and with increasing burnup, fuel expansion can strain the cladding, particularly in areas where gas retention occurs and when fission products (gas or solid) begin to fill voids within the fuel. At this point, cladding strain can become proportional to burnup and begin to increase rapidly. Filling of the available space within the cladding creates an increase in pressure, resulting in hoop stresses, longitudinal stresses and strains, and fuel element deformation. This strain ultimately limits the life of fuel elements in the reactor core because fuel cladding expansion reduces the (sometimes non-uniform) coolant flow area outside the cladding. The rate of strain increase is exacerbated by the constant effects of radiation on structural materials (e.g., cladding material and fuel assembly ducts). Fuel elements can expand sufficiently to impart additional strain to the duct walls of the associated fuel assembly. This can cause them to become "jammed" and / or bow the fuel assembly due to the expansion. Fuel element expansion can sometimes cause cracks in the cladding. These cracks can result in uncontrolled release of fission products and / or coolant interaction with the fuel. In sodium-cooled fast reactors, for example, liquid sodium flows around the fuel elements, and if the fuel element fails (e.g., cracks or otherwise ruptures), the sodium coolant comes into contact with the fuel and interacts with the fuel and fission products.

[0039] In some embodiments, sodium travels from the reactor 402 through a sodium outlet line 408 to the sodium processing cell 404. A sampling subcell 406 may be placed within the processing cell 404 to measure the release from the sodium sample. After the measurement is made, the sodium may return to the reactor 402 through a sodium inlet line 410. In some cases, the sodium flow loop from the reactor 402, through the outlet line 408, through the sodium processing cell 404, and back to the reactor 402 through the sodium inlet line 410 is a closed fluid system. As used herein, a closed fluid loop or closed fluid system refers to pipes, valves, pumps, and other fluid transport devices that are closed to the surrounding environment. A closed fluid loop is one in which a fluid does not leave the loop once it is introduced.

[0040] In some cases, the sampling subcell 406 may be configured to isolate the sodium sample, for example, by valves 412, 414. The valves 412, 414 allow sodium to flow into the subcell 406 and then be isolated within the subcell 406 by closing the valves 412, 414. The sampling subcell 406 may include a radiation detector / spectrometer 416 located near and / or adjacent to the tube within the sampling subcell 406 to measure radiation emitted from the sodium sample within the tube. The valves 412, 414 allow the tube to be isolated, in part, because short-lived isotopes (e.g., 24 The goal is to cause the sodium (Na) to decay, thereby reducing the background signal. In some cases, the radiation detector 416 is configured to measure gamma emissions from the sodium in the sampling subcell 406.

[0041] According to some embodiments, a method for characterizing failed fuel in a sodium fast reactor (SFR) uses measurements of gamma emissions to determine the isotopic abundances and isotopic ratios of failed fuel products in the primary sodium coolant. By allowing the sodium coolant to flow to the sodium processing cell 404 and into the sampling subcell 406, the failed fuel assemblies can be characterized without the need to extract, process, and analyze a primary sodium sample physically taken from the reactor coolant for characterization of the failed fuel products.

[0042] Conventional approaches to characterizing failed fuel have included removing primary sodium samples for radioisotope analysis. These methods require handling of radioactive primary sodium, necessary sodium sample preparation equipment, the generation of hazardous radioactive waste during sample processing, and the time required to analyze the primary sodium samples. Other conventional approaches have required removing fuel assemblies from the reactor core to detect leaks.

[0043] According to some embodiments, characterization of the failed fuel can be performed in-situ by characterizing the radioisotopes in the primary sodium that passes to the sampling subcell 406. This can be used to determine the isotopic ratios of the failed fuel components present in the primary coolant, which allows the burnup of the failed fuel assembly to be determined and correlated to the location of the failed fuel assembly within the reactor core.

[0044] As used herein, the term “burnup,” also referred to as “%FIMA” (fissions per initial heavy metal atom), refers to a measure (e.g., a percentage) of fissions occurring in fissile fuel. For example, a burnup of 5% may indicate that 5% of the fissionable fuel has undergone fission reactions. Due to several factors, burnup may not occur uniformly along the length of each fuel element within a fuel assembly. Similarly, various fuel elements and fuel assemblies will each have different burnups based on several factors (e.g., location within the core, length of time within the core, length of time at various locations within the core, volume of fuel within the assembly, fuel enrichment, etc.). A fuel element is considered consumed when a region of the fuel element has experienced sufficient burnup to reach its burnup limit (sometimes referred to as peak burnup or maximum burnup). When any one location reaches its burnup limit, the entire fuel element is considered discharged, even if only a portion of the fuel within that element actually reached its discharge limit. The term "actual burnup," as opposed to peak or maximum burnup, may be used herein to refer to the amount of burnup that occurred within a given region of a fuel assembly at the time the entire fuel element is considered discharged because at least a portion of the fuel within the fuel element has reached its burnup limit. According to embodiments described herein, the isotope ratios of failed fuel components may be used to determine isotope ratios corresponding to either the peak burnup, actual burnup, and / or average burnup of the fuel element.

[0045] Gas tagging is a method traditionally used to identify failed fuel elements and involves adding a small amount of gas to the fuel element with a unique isotopic composition for each fuel assembly. When a fuel assembly leak occurs and fission products are released from the pressurized fuel element into the primary coolant, the tag gas can be detected, for example, by mass spectrometry of the reactor vessel cover gas. For example, gas tagging may utilize inert gases such as krypton and xenon. The unique tag gas composition can be determined by adding multiple isotopes (to name a few). 78 Kr, 80 Kr, 82 Kr, 126 Xe, and 129 This can be achieved by selective enrichment of any isotope of Xe, etc. 78 Kr / 80 Kr, 82 Kr / 80 Kr, or 126 Xe / 129 Xe) could be used to determine which fuel assembly among multiple fuel assemblies has failed. This technique can be effective in identifying the failed fuel element and location. However, producing tag gases is expensive, especially considering that each fuel assembly requires a unique tag gas, and therefore a unique manufacturing process for each fuel assembly. In some cases, this requires up to 168 or more unique tag gases, so that there is a unique tag gas for each fuel assembly.

[0046] In some embodiments, high burnup fuel is utilized in the fuel assemblies, and commonly used tagging systems are impractical for high burnup fuel due to the reduced bandgap between the many different tag gases. For example, in past reactors, tag gases were indistinguishable from one another due to direct depletion of the tag gas and alteration of its composition by fission products (e.g., Xe and Kr). The embodiments described herein (e.g., cesium isotope ratios) allow for the narrowing of potentially damaged fuel assemblies to a much smaller subset of possible fuel assemblies. Furthermore, using fewer tag gases than the number of fuel assemblies present provides significant cost savings, but perhaps more importantly, provides a tag gas system that can be used with high burnup fuel, as conventional tag gas systems do not function with high burnup fuel. As used herein, the term "high burnup fuel" is a broad term and, in some cases, refers to fuel having a FIMA greater than about 6%.

[0047] However, according to embodiments described herein, fission gas can identify a failed fuel assembly without the need to manufacture and incorporate unique tag gas into each fuel assembly. This greatly simplifies fuel element manufacturing, and all of the fuel elements can be manufactured to be nearly identical using the same materials and processes. For example, by using fewer tag gases than the number of fuel assemblies present, a larger band (i.e., initial mass difference of the tag gases) can be used, resulting in a continued determination of a failed fuel assembly with fewer unique tag gases even as higher levels of depletion and additional fission gas are added. According to some embodiments, having additional indicators (e.g., direct fission product sampling) can further narrow down the failed fuel assembly to fewer unique tag gases.

[0048] Figure 5 shows the 137 Cs / 134 The graph of Cs is shown below. As shown, 137Cs exhibits linear radioactivity as a function of burnup, while 134 Cs is nonlinear, which means that 137 Cs / 134 The measured ratio of Cs corresponds to the specific fuel burnup in the fuel assembly. 137 Cs and 134 The mass of Cs can be measured, the isotope ratio determined, and based on that, the burnup of the fuel can be determined. Through core tracking and modeling, the burnup can be associated with individual fuel assemblies.

[0049] Although the isotope ratio can be determined by mass spectrometry, the gamma ratio can also be used to determine radioactivity, which is related to burnup and ultimately to the identification of failed fuel assemblies. Radioactive cesium is a metastable nuclear isotope of barium, which is released by beta emission. 137m Ba. Metastable barium has a half-life of about 153 seconds and decays to the ground state ( 137 When it decays to Ba, 137 It causes all the gamma radiation associated with Cs.

[0050] In some embodiments, the background radiation of sodium is 137 The gamma emissions from the Cs are reduced so that they can be measured more accurately. This can be done by isolating a sodium coolant sample in the sampling subcell 406 for a predetermined period of time. The gamma emissions travel through piping in the sampling subcell 406 and reach the detector 416. The detector then measures the 137 Cs and 134 The radioactivity of both Cs and Cs can be identified to determine the burnup of the failed fuel assembly. However, in some cases, background radiation may be characterized prior to gamma testing, for example, and the background radiation may be subtracted from the gamma spectroscopy results to increase the resolution of the isotope ratios.

[0051] In some embodiments, determining the burnup of fuel assemblies by isotope ratio allows for identification of failed fuel assemblies without the need for a unique tag gas within each fuel assembly. In some reactor embodiments, 168 or more fuel assemblies may be present in the core, requiring a unique tag gas. In some embodiments, fuel batches may be separated by burnup, which allows for efficiencies when combined with a tag gas system that reuses tag gas for groups of fuel assemblies. For example, by separating fuel assemblies by burnup, fewer tag gases (e.g., 28 tag gases instead of 168) may be used to identify failed fuel assemblies by combining the tag gas identification of the failed fuel assembly with the isotope ratio and burnup. When determining the location of a failed fuel assembly, the batch of the failed fuel assembly can be determined by the tag gas, and the cesium isotope ratio provides additional information for identifying the failed fuel assembly.

[0052] According to some embodiments, the tag gas system may incorporate fewer unique tag gases than the number of fuel assemblies. For example, the ratio of unique tag gases to the number of fuel assemblies may be less than 50%, or 40%, or 30%, or 20%, or 17%, or less. As an example, in a reactor core containing 168 fuel assemblies, 28 unique tag gases may be introduced into the fuel elements during fabrication. If a fuel element fails, analysis of the cover gas will identify the tag gas. This will identify the group to which the failed fuel assembly belongs, thereby narrowing the number of possible fuel assemblies. This will narrow the identification of the fuel assembly to one of six possible fuel assemblies. Similarly, if only six unique tag gases are used during fuel fabrication, the presence of tag gas in the cover gas analysis will narrow the identification of the failed fuel assembly to at least one of the 28 fuel assemblies. Primary coolant analysis can be used to determine isotope ratios. This then provides the burnup of the failed fuel assembly. By narrowing down the number of potentially failed fuel assemblies, and by further narrowing down the number of potential fuel assemblies by burnup, reactor modeling and core tracking can be used to identify specific failed fuel assemblies with known locations within the reactor core. Additionally, once the narrowed down fuel assemblies are determined, further analysis can be performed on the likely failed fuel assemblies. For example, once the number of failed fuel assemblies is narrowed down, additional inspection techniques, such as lift-and-burp techniques, can be employed to determine which of the likely failed fuel assemblies are actually failed. This provides a significant improvement in efficiency over conventional processes that typically require inspecting each fuel assembly throughout the core to determine which assembly has failed.

[0053] 6 shows a process flow for identifying and locating a failed fuel assembly 600. In step 602, primary sodium flows through a bypass tube. The bypass tube may optionally include a cesium trap that can be used to increase the concentration of cesium, allowing for a larger signal. The bypass tube may be located within a sodium processing cell or within a sampling subcell within the sodium processing cell.

[0054] In step 604, the sodium sample may be isolated from the sodium flow loop, for example, by closing one or more valves. This allows the sodium sample to remain stationary within the bypass tube, allowing time for short-lived decay products to diffuse. A detector may be located near the tube to detect emissions from the sodium sample. In some exemplary embodiments, a detector is used to measure the isotope ratio once the sodium enters the bypass tube, without allowing time for short-lived decay products to diffuse. In some exemplary embodiments, the sodium is allowed to flow continuously, and a detector is used to measure the isotope ratio of the flowing sodium stream. In some cases, measuring the isotope ratio in the flowing sodium stream alleviates the need for a bypass tube, and a detector may be located near the sodium loop without the need for a bypass tube.

[0055] At block 606, the method may include determining that the high-energy sodium activity is below a threshold noise level, thereby detecting short-lived isotopes (e.g., 24The cesium isotope ratio of the sodium in the flowing state can be determined by measuring the isotope ratio of the sodium in the flowing state. This can be achieved by isolating the sodium sample for a predetermined period of time. Of course, as explained above, the above steps are optional, and in some cases, the isotope ratio of the sodium in the flowing state can be determined by measuring the isotope ratio of the sodium in the flowing state. This can completely eliminate the need for a bypass tube and a valve to isolate the bypass tube, and a detector can be placed near the sodium tube that can continuously measure the isotope ratio of the sodium in the flowing state.

[0056] In block 608, a detector may be used to measure the cesium isotope ratio. In some cases, this may involve measuring the ratio 137 Cs / 134 In some embodiments, the isotope ratio is determined using a gamma ray detector. The detector may be any suitable detector, such as, but not limited to, a radiation dosimeter, a radiographic film (e.g., a NaI scintillation detector), a thermoluminescence detector (TLD detector), a diode detector, a high-purity germanium detector (HPGe detector), or any other suitable gamma ray detector.

[0057] In block 610, the isotope ratios may be associated with the burnup of the fuel elements. This may be done, for example, by a dedicated computer program executed by one or more processors to determine the burnup (which may be the average burnup, peak burnup, or actual burnup of the fuel assembly) based on the isotope ratios.

[0058] In block 612, the burnup can be used to determine the failed fuel assembly. Through modeling and core tracing, the burnup of each fuel assembly can be ascertained and compared to the burnup of the failed fuel assembly to identify both the failed fuel assembly and its location within the reactor core. In some cases, tag gases can be used to narrow down the list of potentially failed fuel assemblies, and burnup can provide further information to narrow down the identification of the failed fuel assembly. The number of unique tag gas species can be fewer than the number of fuel assemblies.

[0059] In block 614, the bypass piping may be flushed, for example, by opening one or more valves, to allow the sodium sample to return to the sodium loop and back to the reactor. As noted, one particular advantage of fuel characterization through gamma spectroscopy is that it alleviates the traditional steps of shutting down the reactor to take radioactive sodium samples from the reactor vessel or extract fuel assemblies from the core. As described herein, these systems and methods allow for in-situ characterization of failed fuel while the reactor is operating, without removing primary coolant from the system.

[0060] Once a damaged fuel assembly is identified, it can be scheduled for replacement during a scheduled reactor outage, rather than having to inspect each fuel assembly to find the damaged one, find a suitable replacement, recalculate the core loading, approve the new core loading, and then replace the fuel assembly, a time-consuming process during an outage that would be required in conventional systems.

[0061] Identifying that a failure has occurred is routine and may be accomplished by sampling the cover gas for the presence of fission products. During cover gas sampling (periodic or continuous), a reactor operator may recognize that a fuel assembly has failed, for example, by detecting fission products in the cover gas. The disclosed methods may then be implemented to determine which fuel assembly has failed. The disclosed systems and methods enable identification of the specific fuel assembly that has failed through isotope ratio detection, burnup determination, and computational modeling and core tracing, and the fuel assembly having the determined burnup is identified. In some cases, tag gas may optionally help narrow the number of likely candidates.

[0062] In some embodiments, the identity of the failed fuel assembly may not be determined; instead, information obtained from the burnup determination may narrow the number of likely candidates to fewer than all fuel assemblies in the core. In some cases, the techniques described herein may narrow the number of likely candidates to less than 50%, or less than 25%, or less than 10%, or less than 5% of all fuel assemblies in the core. In these cases, additional steps may be performed to identify the specific failed fuel assembly. For example, once the fuel assembly is narrowed down, the candidate fuel assembly may be lifted from the core, which reduces hydrostatic pressure and effectively increases relative pin pressure. This may also reduce forced coolant flow through the assembly, resulting in increased temperatures and relative pressures within the fuel elements within the fuel assembly. The increased hydrostatic pressure within the fuel assembly allows fission gases to escape through breaches in the cladding, thereby enabling identification of the failed fuel assembly. This process may be repeated for each candidate fuel assembly until a failed fuel assembly is identified. In some cases, this approach is known as the "lift-and-burp" technique. In some cases, the lift-and-burp technique may be applied during a refueling operation.

[0063] Although the above description refers to a fast sodium cooled reactor, this is for illustrative purposes only and any suitable solid fuel nuclear fission reactor may be used.

[0064] Figure 7 is a graph of the mass ratio of xenon isotopes as a function of burnup. It shows the ratio of radioactive to non-radioactive isotopes of the major xenon isotopes. During irradiation in the reactor core, fission gas is generated, and various isotopes are present in the fuel elements. When a fuel element fails, the fission gas is released into the cover gas, which can be detected according to embodiments described herein. Radioactive and stable isotopes of Xe can be released from the failed fuel element, and their ratios can be compared. Stable isotopes do not decay; therefore, their inventory increases linearly with burnup. In contrast, radioactive isotopes reach an equilibrium concentration, and each decaying atom is replaced by a new atom resulting from fission. Not all of the inventory is released from the fuel pin; it must travel out of the fuel, up the fuel column, and out of the pin. The ratio of gas reaching the cover gas space to the amount actually produced by fission is also called the "release to birth ratio" (R / B). The shorter the half-life, the smaller the R / B. Therefore, the R / B ratio is taken into account when determining the ratio of stable and radioactive isotopes produced to the measured amount. Reliable sensitivity is achieved by selecting multiple radioactive isotopes that will have the same concentration at some point during their lifetime. At some point, the ratio of radioactive to non-radioactive isotopes will reach equilibrium, equal to 1, and identifying these ratios can be directly related to the burnup of the failed fuel element.

[0065] 8 illustrates a process for identifying and locating a failed fuel assembly 800 within a nuclear reactor core. According to some embodiments, the method includes determining that the fuel assembly is failed at block 802. This may be performed through any suitable process, and in some cases may be determined by analyzing the cover gas for fission products.

[0066] At block 804, candidate failed fuel assemblies are narrowed down by determining a burnup using the isotope ratio. The burnup may be associated with a first subset of the plurality of fuel assemblies having an approximate burnup associated with the failed fuel assembly. As a non-limiting example, a failed fuel assembly releases cesium into the primary coolant. The primary coolant may be analyzed, for example, through gamma ray spectroscopy, to determine the cesium isotope ratio. The cesium isotope ratio may be associated with the fuel burnup in the failed fuel assembly.

[0067] In block 806, the first subset of fuel assemblies may be further narrowed down to a second subset of fuel assemblies based on the tag gas. As previously described, tag gas may be used within the fuel elements, and once detected, the tag gas may be used to further narrow down the likely failed fuel assemblies. As described elsewhere herein, a unique tag gas may be used within a group of fuel assemblies. For example, a first tag gas may be used for a first group of fuel assemblies, a second tag gas may be used for a second group of fuel assemblies, and a third tag gas may be used for a third group of fuel assemblies. By narrowing down the likely failed fuel assemblies to a first subset through isotope ratios, the first subset may be further narrowed down to a second subset by detecting the tag gas of a failed fuel assembly. In some cases, the second subset includes a single fuel assembly, which is the failed fuel assembly. In some cases, the second subset identifies multiple fuel assemblies that are potentially failed. In this case, the fuel assemblies in the second subset may be investigated to identify the failed fuel assemblies, as shown in block 808. For example, the fuel assemblies in the second subset may be lifted and burped, or some other investigation technique may be used, to determine which fuel assemblies in the second subset have failed.

[0068] 9 illustrates a process for identifying and locating a failed fuel assembly 900 within a nuclear reactor core. According to some embodiments, the method includes determining that the fuel assembly is failed at block 902. This may be performed through any suitable process, and in some cases may be determined by analyzing cover gas for fission products, by wet shipping techniques, by dry shipping techniques, or by some other technique.

[0069] The primary coolant is passed near the detector at block 904. The primary coolant may be routed through a bypass pipe that directs a portion of the primary coolant near the detector.

[0070] At block 906, the method includes determining the isotopic ratio of the fission product isotopes in the primary coolant, which may be performed by any suitable technique (e.g., any of the techniques described herein in connection with the various disclosed embodiments).

[0071] At block 908, the method includes determining a burnup of the failed fuel assembly based on the isotope ratio. In some cases, the burnup may be associated with a first subset of the plurality of fuel assemblies having an approximate burnup associated with the failed fuel assembly. As a non-limiting example, the failed fuel assembly releases cesium into the primary coolant. The primary coolant may be analyzed, for example, through gamma ray spectroscopy, to determine the cesium isotope ratio. The cesium isotope ratio may be associated with the fuel burnup in the failed fuel assembly.

[0072] At block 910, the method may determine the location of the failed fuel assembly. This may be done using core modeling and / or fuel assembly tracking during the fuel cycle of the reactor core. Further steps may include narrowing down or specifically identifying the failed fuel assembly, for example, using tag gas. In some cases, the suspected failed fuel assembly may be investigated to determine which one or more of the suspected fuel assemblies have failed.

[0073] Of course, the narrowing techniques need not be performed in the order shown. For example, an initial narrowing can be performed by tag gas analysis to determine a first subset, which can then be further narrowed by analyzing for burnup associated with the measured isotope ratios. In this manner, damaged fuel assemblies can be quickly identified, and in many cases, this can be accomplished in situ (e.g., without having to remove the fuel assemblies from the reactor core for testing) while the reactor is in operation.

[0074] The general nature of the embodiments of the present disclosure will be sufficiently apparent from the foregoing description of such specific embodiments that others may readily modify and / or adapt the specific embodiments for various applications without undue experimentation by applying the knowledge of those skilled in the art without departing from the general concepts of the embodiments of the present disclosure. Such adaptations and modifications are therefore intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. The phraseology or terminology used herein is intended to be illustrative, not limiting, as the term or terminology would be interpreted by one of ordinary skill in the art in light of the teaching and guidance presented herein.

[0075] The breadth and scope of embodiments of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0076] Unless otherwise specified or understood differently within the context in which they are used, conditional words such as "can," "could," "might," or "may," among others, are generally intended to convey that particular implementations may include particular features, elements, and / or operations, while other implementations do not include particular features, elements, and / or operations. As such, such conditional words generally do not intend that features, elements, and / or operations are in any way required in one or more implementations, or that logic for determining whether these features, elements, and / or operations are included in any particular implementation, or whether these features, elements, and / or operations should be performed in any particular implementation, with or without user input or prompting, is necessarily included in one or more implementations.

[0077] Unless otherwise specified, the terms "connected to" and "coupled to" (and derivatives thereof) as used herein should be interpreted to allow both direct and indirect connections (i.e., connections via other elements or components). Furthermore, the terms "a" or "an" (singular words) as used herein should be interpreted to mean "at least one of." Finally, for ease of use, the terms "including" and "having" (and derivatives thereof) as used herein should be interpreted to mean "at least one of."

[0078] Disclosed herein and in the accompanying drawings are examples of systems, apparatus, devices, and techniques that can provide for control and optimization of coolant flow through a reactor core assembly. It is, of course, not possible to describe every conceivable combination of elements and / or methodologies for purposes of describing the various features of the present disclosure. However, those skilled in the art will recognize that many additional combinations and permutations of the disclosed features are possible. Accordingly, various modifications can be made to the present disclosure without departing from the scope or spirit of the disclosure. Moreover, other embodiments of the present disclosure will become apparent from consideration of the specification and accompanying drawings, as well as from the practice of the disclosed embodiments presented herein. The examples presented in this specification and in the accompanying drawings are to be considered in all respects as illustrative and not restrictive. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0079] Those skilled in the art will recognize that any process or method disclosed herein can be modified in many respects. The process parameters and order of steps described and / or illustrated herein are provided for illustrative purposes only and can be modified as desired. For example, although the steps illustrated and / or described herein may be illustrated or discussed in a particular order, these steps do not necessarily have to be performed in the order illustrated or discussed.

[0080] The various exemplary methods described and / or illustrated herein may omit one or more of the steps described or illustrated herein or may include additional steps in addition to those disclosed. Furthermore, the steps of any method disclosed herein may be combined with any one or more steps of any other method disclosed herein.

[0081] The methods described in connection with the embodiments herein may be implemented by one or more processors executing instructions that cause the processors to perform the disclosed methods.

[0082] Throughout this specification, the term "substantially" in reference to a given parameter, property, or condition may mean or include the extent to which one of ordinary skill in the art would understand that the given parameter, property, or condition is met with minor variance, e.g., within acceptable manufacturing tolerances. For example, depending on the particular parameter, property, or condition that is substantially met, the particular parameter, property, or condition may be at least about 90% met, at least about 95% met, or at least about 99% met.

[0083] From the foregoing description, it will be understood that, although specific implementations have been described herein for illustrative purposes, various modifications may be made without departing from the spirit and scope of the appended claims and the elements recited therein. In addition, while certain aspects are presented below in certain claim forms, the inventors contemplate various aspects in any available claim form. For example, while only some aspects may currently be described as embodied in a particular configuration, other aspects may similarly be embodied in that manner. Various modifications and changes may be made as would be apparent to those skilled in the art having the benefit of this disclosure. All such modifications and changes are intended to be encompassed, and therefore the foregoing description should be considered in an illustrative, rather than a limiting, sense.

[0084] The following numbered clauses also form part of this disclosure:

[0085] Article 1. 1. A method for characterizing a failed fuel assembly in a nuclear reactor, comprising: flowing the primary sodium coolant through a bypass pipe; The sodium in the bypass pipe 137 Cs / 134 To determine the isotope ratio of Cs, determining a burnup of the failed fuel assembly based at least in part on the isotope ratio; and identifying the failed fuel assembly based at least in part on the burnup; A method comprising:

[0086] Article 2. 10. The method of claim 1, wherein determining the isotope ratio is performed by gamma ray spectroscopy.

[0087] Article 3. 3. The method of clause 1 or 2, wherein the method is carried out without removing primary sodium coolant from a closed system including the reactor vessel and the bypass pipe.

[0088] Article 4. 4. The method of any one of clauses 1-3, further comprising determining that a fuel assembly has failed by analyzing a cover gas in a reactor vessel and detecting fission products in the cover gas.

[0089] Article 5. 5. The method of any one of clauses 1 to 4, further comprising determining the xenon isotope ratio by mass spectrometry.

[0090] Article 6. 6. The method of any one of clauses 1-5, further comprising providing tag gas to one or more fuel elements in the fuel assembly.

[0091] Article 7. providing a tag gas includes providing a plurality of unique tag gases; 7. The method of clause 6, wherein the number of species of the plurality of unique tag gases is less than the number of fuel assemblies located within the nuclear reactor core.

[0092] Article 8. 8. The method of any one of clauses 1 to 7, wherein the method is carried out while the reactor is in operation.

[0093] Article 9. identifying the failed fuel assemblies includes determining a subset of the plurality of fuel assemblies; 9. The method of any one of clauses 1-8, wherein the subset of the plurality of fuel assemblies includes one or more failed fuel assemblies of the failed fuel assemblies.

[0094] Article 10. 10. The method of claim 9, further comprising analyzing a plurality of fuel assemblies of the subset of fuel assemblies to determine failed fuel assemblies.

[0095] Article 11. 11. The method of clause 10, wherein analyzing a plurality of fuel assemblies of the subset of the plurality of fuel assemblies includes a lift-and-burp technique.

[0096] Article 12. 12. The method of any one of clauses 1-11, further comprising isolating the sodium coolant within the bypass pipe.

[0097] Article 13. 1. A system comprising: a reactor core; a plurality of fuel elements disposed within the nuclear reactor core; a volume of primary sodium coolant in contact with a plurality of said fuel elements; a sodium processing cell external to the reactor core, the sodium processing cell being in fluid communication with the reactor core by sodium processing piping; a detector located near the sodium processing piping and configured to detect isotopic radioactive emissions leaked from a failed fuel assembly; When executed by one or more processors, causes the one or more processors to: determining an isotopic ratio of said isotopes; determining the burnup of the failed fuel assembly based at least in part on the isotope ratios; and determining a location of the failed fuel assembly within the reactor core based on the burnup of the at least partially failed fuel assembly; the one or more processors configured with instructions to execute Including, the system.

[0098] Article 14. 14. The system of claim 13, further comprising a plurality of distinctive tag gases located within selected fuel elements of the plurality of fuel elements disposed within the nuclear reactor core.

[0099] Article 15. 15. The system of clause 14, wherein the number of species of the plurality of unique tag gases is less than the number of fuel assemblies.

[0100] Article 16. 14. The system of clause 13, wherein the detector is configured to detect gamma emissions from the isotopes that have escaped from a failed fuel assembly through gamma ray spectroscopy.

[0101] Article 17. 14. The system of clause 13, further comprising a cover gas processing system configured to measure the isotopes leaked into the cover gas from the failed fuel assembly.

[0102] Article 18. 18. The system of clause 17, wherein the isotope leaked from the failed fuel assembly is a xenon isotope.

[0103] Article 19. 14. The system of clause 13, wherein the isotope leaked from a failed fuel assembly is a cesium isotope.

[0104] Article 20. The isotope ratio is137 Cs / 134 14. The system of claim 13, wherein the system is Cs.

[0105] Article 21. 1. A method for locating a failed fuel assembly in a nuclear reactor, comprising: determining that a fuel assembly is damaged; flowing a primary coolant near the detector; determining isotope ratios of fission product isotopes in the primary coolant using the detector; and determining a burnup of the failed fuel assembly based on the isotope ratio; determining a location of the failed fuel assembly within the reactor based on the burnup of the failed fuel assembly and through core modeling and fuel assembly tracking; A method comprising:

[0106] Article 22. The isotope ratio is 137 Cs / 134 22. The method of claim 21, wherein Cs.

[0107] Article 23. 23. The method of claim 21 or 22, wherein the isotopic ratio of fission product isotopes in the primary coolant is performed by measuring gamma emissions.

[0108] Article 24. 24. The method of any one of clauses 21 to 23, wherein the method is carried out while the nuclear reactor is in operation.

[0109] Article 25. 25. The method of any one of clauses 21 to 24, wherein the method is carried out without removing the primary coolant from the closed cooling loop.

[0110] Article 26. 26. The method of any one of clauses 21 to 25, wherein using the detector to determine the isotopic ratios of fission product isotopes in the primary coolant is performed on a volume of sodium in a flowing state.

[0111] Article 27. 27. The method of any one of clauses 21-26, further comprising detecting tag gas from the failed fuel assembly.

[0112] Article 28. 1. A method for identifying a failed fuel assembly in a nuclear reactor core, comprising: determining that a fuel assembly is damaged; Narrowing the potentially damaged fuel assemblies to a first subset of the plurality of fuel assemblies by determining burnup using isotope ratios; narrowing the first subset down to a second subset of a plurality of fuel assemblies using a tag gas; determining the fuel assembly as failed based at least in part on the burnup and the tag gas; A method comprising:

[0113] Article 29. 29. The method of clause 28, wherein burnup is determined by detecting isotope ratios using gamma ray spectroscopy.

[0114] Article 30. 30. The method of claim 28 or 29, wherein the method is performed in situ without removing a volume of sodium coolant from the reactor core.

[0115] Article 31. 31. The method of any one of clauses 28-30, wherein determining that the fuel assembly is damaged is performed by analyzing cover gas within the reactor vessel and detecting fission products or tag gas in the cover gas.

[0116] Article 32. 32. The method of claim 31, wherein detecting fission products or tag gases is performed by mass spectrometry.

[0117] Article 33. Using tag gas means: providing a first tag gas to a first group of the plurality of fuel assemblies; providing a second tag gas for a second group of the plurality of fuel assemblies; providing a third tag gas for a third group of the plurality of fuel assemblies; 33. The method of any one of clauses 28 to 32, comprising:

[0118] Article 34. 34. The method of any one of clauses 28 to 33, wherein the method is carried out while the reactor is in operation.

[0119] Article 35. 35. The method of any one of clauses 28-34, further comprising analyzing the second subset of the plurality of fuel assemblies using a lift-and-burp technique.

[0120] Article 36. 36. The method of any one of clauses 28 to 35, wherein determining the failed fuel assemblies is performed through core modeling and / or fuel assembly tracking. [Brief explanation of the drawings]

[0121] [Figure 1] 1 illustrates a partial cutaway perspective view of a nuclear fission reactor according to some embodiments. [Figure 2] 1 is a cross-sectional top view of a nuclear fission reactor core according to some embodiments. [Figure 3A] 1 is a partial elevation view of a nuclear fission reactor core according to some embodiments. [Figure 3B] 1 illustrates a fuel element with a fuel and tag gas capsule disposed therein, according to some embodiments. [Figure 4]1 illustrates, in block diagram form, a sodium-cooled fast reactor having a sampling subcell as part of a sodium handling system, according to some embodiments. [Figure 5] 1 is a graph illustrating radioactivity of cesium-134 and cesium-137 versus burnup, according to some embodiments. [Figure 6] 1 illustrates a process for online radioisotope measurements for in-cell failed fuel characterization, according to some embodiments. [Figure 7] 1 is a graph illustrating the mass ratio of xenon isotopes versus burnup, according to some embodiments. [Figure 8] 1 illustrates a process for identifying and locating a failed fuel assembly within a nuclear reactor core, according to some embodiments. [Figure 9] 1 illustrates a process for identifying and locating a failed fuel assembly within a nuclear reactor core, according to some embodiments.

Claims

1. 1. A method for characterizing a failed fuel assembly in a nuclear reactor, comprising: flowing the primary sodium coolant through a bypass pipe; The sodium in the bypass pipe １３７ Cs / １３４ Determining the isotope ratio of Cs; determining a burnup of the failed fuel assembly based at least in part on the isotope ratio; and identifying the failed fuel assembly based at least in part on the burnup; A method comprising:

2. The method of claim 1 , wherein determining the isotope ratio is performed by gamma ray spectroscopy.

3. 10. The method of claim 1, wherein the method is performed without removing primary sodium coolant from a closed system including the reactor vessel and the bypass pipe.

4. 10. The method of claim 1, further comprising determining that a fuel assembly has failed by analyzing a cover gas within the reactor vessel and detecting fission products in the cover gas.

5. The method of claim 1 further comprising determining the xenon isotope ratio by mass spectrometry.

6. 10. The method of claim 1, further comprising providing tag gas to one or more fuel elements within the fuel assembly.

7. providing a tag gas includes providing a plurality of unique tag gases; 7. The method of claim 6, wherein the number of species of the plurality of unique tag gases is less than the number of fuel assemblies located within the nuclear reactor core.

8. The method of claim 1 , wherein the method is performed while the nuclear reactor is operating.

9. identifying the failed fuel assemblies includes determining a subset of the plurality of fuel assemblies; 10. The method of claim 1, wherein the subset of the fuel assemblies includes one or more failed fuel assemblies of the failed fuel assemblies.

10. 10. The method of claim 9, further comprising analyzing a plurality of fuel assemblies of the subset of fuel assemblies to determine failed fuel assemblies.

11. 11. The method of claim 10, wherein analyzing a plurality of fuel assemblies of the subset of fuel assemblies includes a lift-and-burp technique.

12. The method of claim 1 further comprising isolating the sodium coolant within the bypass pipe.

13. 1. A system comprising: a reactor core; a plurality of fuel elements disposed within the nuclear reactor core; a volume of primary sodium coolant in contact with a plurality of said fuel elements; a sodium processing cell external to the reactor core, the sodium processing cell being in fluid communication with the reactor core by sodium processing piping; a detector located near the sodium processing piping and configured to detect isotopic radioactive emissions leaked from a failed fuel assembly; When executed by one or more processors, the one or more processors are caused to: determining the isotopic ratio of said isotopes; determining the burnup of the failed fuel assembly based at least in part on the isotope ratio; and determining a location of the failed fuel assembly within the reactor core based on the burnup of the at least partially failed fuel assembly; the one or more processors configured with instructions to execute Including, the system.

14. 14. The system of claim 13, further comprising a plurality of distinctive tag gases located within selected fuel elements of the plurality of fuel elements disposed within the nuclear reactor core.

15. The system of claim 14 , wherein the number of species of the plurality of unique tag gases is less than the number of fuel assemblies.

16. 14. The system of claim 13, wherein the detector is configured to detect gamma emissions from the isotopes that have escaped from a failed fuel assembly through gamma ray spectroscopy.

17. The system of claim 13 , further comprising a cover gas processing system configured to measure the isotopes leaked into the cover gas from a failed fuel assembly.

18. 20. The system of claim 17, wherein the isotope leaked from the failed fuel assembly is a xenon isotope.

19. 14. The system of claim 13, wherein the isotope leaked from a failed fuel assembly is a cesium isotope.

20. The isotope ratio is １３７ Cs / １３４ The system of claim 13, wherein the ion exchange coefficient is 0.

05.

21. 1. A method for locating a failed fuel assembly in a nuclear reactor, comprising: determining that a fuel assembly is damaged; flowing a primary coolant near the detector; determining isotope ratios of fission product isotopes in the primary coolant using the detector; and determining a burnup of the failed fuel assembly based on the isotope ratio; determining a location of the failed fuel assembly within the reactor based on the burnup of the failed fuel assembly and through core modeling and fuel assembly tracking; A method comprising:

22. The isotope ratio is １３７ Cs / １３４ 22. The method of claim 21, wherein the compound is Cs.

23. 22. The method of claim 21, wherein the isotope ratios of fission product isotopes in the primary coolant are performed by measuring gamma emissions.

24. 22. The method of claim 21, wherein the method is performed while the nuclear reactor is operating.

25. 22. The method of claim 21, wherein the method is performed without removing the primary coolant from the closed cooling loop.

26. 22. The method of claim 21, wherein the determining with the detector is performed on a volume of sodium in a flowing state.

27. 22. The method of claim 21, further comprising detecting tag gas from the failed fuel assembly.

28. 1. A method for identifying a failed fuel assembly in a nuclear reactor core, comprising: determining that a fuel assembly is damaged; Narrowing the potentially damaged fuel assemblies to a first subset of the plurality of fuel assemblies by determining burnup using isotope ratios; narrowing the first subset down to a second subset of a plurality of fuel assemblies using a tag gas; determining the fuel assembly as failed based at least in part on the burnup and the tag gas; A method comprising:

29. 30. The method of claim 28, wherein burnup is determined by detecting isotope ratios using gamma ray spectroscopy.

30. 30. The method of claim 28, wherein the method is performed in situ without removing a volume of sodium coolant from the nuclear reactor core.

31. 30. The method of claim 28, wherein determining that a fuel assembly is damaged is performed by analyzing a cover gas in the reactor vessel and detecting fission products or tag gas in the cover gas.

32. 32. The method of claim 31, wherein detecting fission products or tag gases is performed by mass spectrometry.

33. Using tag gas means: providing a first tag gas to a first group of the plurality of fuel assemblies; providing a second tag gas for a second group of the plurality of fuel assemblies; providing a third tag gas for a third group of the plurality of fuel assemblies; 29. The method of claim 28, comprising:

34. 30. The method of claim 28, wherein the method is performed while the nuclear reactor is operating.

35. 30. The method of claim 28, further comprising analyzing the second subset of a plurality of fuel assemblies using a lift-and-burp technique.

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