Nuclear reactor system improvements

Abstract

A nuclear fission reactor system (10) comprising a reactor unit (12) having a fuel block (400) having a central axis (402) extending along a length of the fuel block (400) between a first surface (412) and a second surface (422). The fuel block (400) defines a plurality of fuel assembly chambers (404) extending from an inlet opening (410) on the first surface (412) of the fuel block (400) to an outlet opening (420) on the second surface (422) of the fuel block (400). A plurality of fuel assemblies (100) are also provided. Each fuel assembly (100) includes a fuel compact (200) defining a passage (202) extending from an inlet (204) at a first end (206) of the fuel compact (200) to an outlet (208) at a second end (210) of the fuel compact (200), the passage (202) defining an inner surface (222) of the fuel compact (200). A displacer element (300) is disposed within the cooling passage (202) and is mounted such that a clearance (212) is maintained between the inner surface (222) of the fuel compact (200) and the displacer element (300) to define an annular cooling passage (230). Each of the plurality of fuel assemblies (100) is positioned in a respective one of the plurality of fuel assembly chambers (404). At least one of the fuel assemblies (100) of the plurality of fuel assemblies (100) is configured to transfer heat to the coolant in its annular cooling channel (230) at a first rate, and at least one other fuel assembly of the plurality of fuel assemblies (100) is operable / configured to transfer heat to the coolant in its annular cooling channel (230) at a second rate.

Description

[Technical Field]

[0001] The present disclosure relates to nuclear fission reactor systems. [Background technology]

[0002] Controlling the operating temperature and temperature distribution within the reactor core is essential to the life of the system and the overall safe operation of the equipment.

[0003] Conventionally, a reactor core has multiple passages, some configured to receive nuclear fuel rods, some for neutron moderators, and some for cooling flow. Neutron moderators are media that slow the velocity of fast neutrons from the fuel rods, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. To illustrate, an example of a graphite-moderated nuclear fission reactor 1 (General Atomics GT-MHR) is shown in Figure 1. Common to many designs, this arrangement has conventional linear control rods 2 that must be positioned above the reactor vessel 3. As is well known, the control rods must extend into and out of the reactor.

[0004] The coolant and fuel passages are adjacent to each other, but there is no direct contact between the coolant and the fuel rods, and heat is conducted through the core. In another example, the fuel rods are surrounded by a graphite moderator sleeve, and coolant is provided around the outside of the sleeve. In this example, there is no direct contact between the coolant and the fuel rods, and heat is conducted through the graphite (moderator) sleeve. Both examples can also provide high thermal resistance for cooling.

[0005] Cooling alone may not be sufficient to achieve optimal temperature distribution within the core. The power density within the core can be affected by the relative performance of the fuel rods and their proximity to other features of the core. A conventional solution to balancing the power density profile of this problem is to monitor the performance of the fuel rods and either swap them between locations during use or replace fuel rods within the core with replacement fuel rods. For example, during a maintenance cycle, it may be common to replace approximately 30% of the fuel rods and move other fuel rods around the core to balance the power. However, such an operation requires a complete dismantling of the reactor, which is time-consuming, dangerous, and expensive.

[0006] Therefore, core elements and / or resulting core assemblies that provide improved cooling control and predictable power density throughout the life of the reactor are highly desirable. Summary of the Invention

[0007] According to the present disclosure, there is provided an apparatus and system as set out in the accompanying claims. Other features of the invention will become apparent from the dependent claims and the following description.

[0008] Accordingly, a fuel assembly (100) for a nuclear fission reactor (10) may be provided, the fuel assembly (100) being centered about a longitudinal axis (102). The fuel assembly (100) may include a fuel compact (200) defining a flow passage (202) extending from an inlet (204) at a first end (206) of the fuel compact (200) to an outlet (208) at a second end (210) of the fuel compact (200). The flow passage (202) may define an inner surface (222) of the fuel compact (200). A displacer element (300) may be disposed within the passage (202) and mounted such that a clearance (212) is maintained between the inner surface (222) of the fuel compact (200) and the displacer element (300) to define an annular cooling flow passage (230).

[0009] A spacer element (302) may extend between the fuel compact (200) and the displacer element (300) to position the displacer element (300) relative to the fuel compact (200) and maintain a clearance (212) therebetween.

[0010] The spacer element (302) may be airfoil-shaped with a leading edge (304) oriented towards the inlet (204) and a trailing edge (306) oriented towards the outlet (208).

[0011] A first spacer element array (310) may be provided at a first position (P1) along the length of the displacer element (300), and the first spacer element array (310) may comprise at least two spacer elements (302) spaced apart around the circumference of the displacer element (300).

[0012] A second spacer element array (312) may be provided at a second location (P2) spaced along the length of the displacer element (300) from the first spacer element array (310).

[0013] A first spacer element array (310) may be provided at the first end (206) of the fuel compact (200), and each spacer element (302) of the first spacer element array (310) may be located within a respective recess (232) provided and defined by the first end (206) of the fuel compact (200).

[0014] A second spacer element array (312) may be provided at the second end (210) of the fuel compact (200), and each spacer element (302) of the second spacer element array (312) may be positioned within a respective recess (234) defined by the second end (210) of the fuel compact (200).

[0015] At least one of the spacer elements (302) in each spacer element array (310, 312) may extend beyond a respective end (206, 210) of the fuel compact (200).

[0016] The displacer element (300) may extend from a first end (316) to a second end (318). A first region (340) may extend from the first end (316) of the displacer element to a second region (342), where the first region (340) may have a constant first diameter. A second region (342) may extend from the end of the first region (340), where the second region (342) may increase in diameter toward a third region (344). The third region (344) may extend from the end of the second region (342) to a fourth region (346), where the third region (344) may have a constant third diameter. A fourth region (346) extends from the end of the third region (344), and the fourth region (346) may decrease in diameter toward the fifth region (348). The fifth region (348) extends from the end of the fourth region (346) to the second end (318) of the displacer element, and the fifth region (348) may have a constant fifth diameter.

[0017] The spacer elements (302) of the first spacer element array (310) may extend along the second region (342). The spacer elements (302) of the second spacer element array (312) may extend along the fourth region (346).

[0018] Also provided may be a nuclear fission reactor system (10) including a reactor unit (12) having a fuel block (400) with a central axis (402) extending along the length of the fuel block (400). The fuel block (400) may define a plurality of fuel assembly chambers (404) extending from an inlet opening (410) on a first surface (412) of the fuel block (400) to an outlet opening (420) on a second surface (422) of the fuel block (400). The fuel assembly chamber inlet (410) may be operable to be in fluid communication with a coolant source (500), and the fuel assembly chamber outlet (208) may be operable to be in fluid communication with the coolant outlet (502). A fuel assembly (100) according to the present disclosure may be disposed within the fuel element chamber (404).

[0019] The fuel assembly chamber (404) may extend from the fuel assembly chamber inlet (410) with a diameter that provides a snug fit between the fuel assembly (100) and the fuel assembly chamber (404). The fuel assembly chamber (404) may decrease in diameter toward the fuel assembly chamber outlet (420) such that the diameter of the fuel assembly chamber outlet (420) is smaller than the diameter of the fuel assembly (100), thereby preventing the fuel assembly (100) from passing through the fuel assembly chamber outlet (420).

[0020] A shoulder (460) may define a reduced diameter of the fuel assembly chamber (404), and the spacer element (302) may engage the shoulder (460).

[0021] A retaining plug (462) may be provided in the fuel assembly chamber inlet (410), the retaining plug (462) defining a portion of a plug passage (464) having a diameter smaller than the diameter of the fuel assembly (100), thereby preventing the fuel assembly (100) from passing through the retaining plug (462).

[0022] The spacer element (302) may engage with the retaining plug (462).

[0023] The fuel block (400) defines a plurality of fuel assembly chambers (404), each of which is provided with a fuel assembly (100), and the annular cooling channel (230) of one of the fuel assemblies (212) may have a different flow area than the annular cooling channel (230) of at least one of the other fuel assemblies (212).

[0024] A land-based, air-based, or sea-based power plant may be provided that includes a nuclear fission reactor system (10) according to the present disclosure.

[0025] A nuclear fission reactor system (10) may be provided that includes a nuclear reactor unit (12) having a fuel block (400) with a central axis (402) extending along the length of the fuel block (400) between a first surface (412) and a second surface (422). The fuel block (400) may define a plurality of fuel assembly chambers (404) extending from an inlet opening (410) on the first surface (412) of the fuel block (400) to an outlet opening (420) on the second surface (422) of the fuel block (400). A plurality of fuel assemblies (100) may be provided. Each fuel assembly (100) may include a fuel compact (200) that defines a flow path (202) extending from an inlet (204) at a first end (206) of the fuel compact (200) to an outlet (208) at a second end (210) of the fuel compact (200). The passage (202) may define an inner surface (222) of the fuel compact (200). A displacer element (300) may be disposed within the cooling channel (202) and mounted such that a clearance (212) is maintained between the inner surface (222) of the fuel compact (200) and the displacer element (300) to define an annular cooling channel (230). Each of the plurality of fuel assemblies (100) may be positioned in a respective one of the plurality of fuel assembly chambers (404). At least one of the fuel assemblies (100) of the plurality of fuel assemblies (100) may be configured to transfer heat to the coolant in its annular cooling channel (230) at a first rate. At least one other fuel assembly of the plurality of fuel assemblies (100) may be operable / configured to transfer heat to the coolant in the annular cooling channel (230) at a second rate.

[0026] The fuel assembly chambers (404) may be evenly distributed within the fuel block (400).

[0027] In some regions of the fuel block (400), there may be more fuel assembly chambers (404) per unit volume than in other regions of the fuel block (400).

[0028] In a central region of the fuel block (400), the fuel assembly chambers (404) may be more numerous per unit volume than in the remainder of the fuel block (400).

[0029] The annular cooling channel (230) of one of the fuel assemblies (212) may have a different flow area than the annular cooling channel (230) of at least one of the other fuel assemblies (212).

[0030] At least one annular cooling channel (230) of the fuel assembly (212) may have a constant flow area along its length.

[0031] At least one annular cooling channel (230) of the fuel assembly (212) may have a decreasing flow area along its length.

[0032] The heat transfer mechanism (320) may be provided on the displacer element (300). The heat transfer mechanism (320) of one of the fuel assemblies (212) may be configured to transfer heat to at least one heat transfer mechanism of the other fuel assemblies (212) at a different rate.

[0033] The heat transfer mechanism (320) may be provided as a roughened surface on the displacer element (300).

[0034] The heat transfer features (320) may be provided as fins, pedestals, axially extending ribs, and / or circumferentially extending ribs extending from the displacer element (300).

[0035] At least one of the displacer elements (300) of the plurality of fuel assemblies (100) may comprise a first material, and each of the remaining displacer elements (300) of the plurality of fuel assemblies (100) may comprise one of a second material, a third material, or a fourth material, each of which is different from the first material.

[0036] The first material may be a neutron moderator and multiplier.

[0037] The first material may be beryllium oxide.

[0038] The second material may be a neutron moderator and absorber.

[0039] The second material may comprise graphite, boron carbide, and / or silicon carbide.

[0040] The third material may be transparent to neutrons.

[0041] The third material may comprise zirconium, zirconium hydride, and / or a low neutron capture metal.

[0042] The fourth substance may be a neutron poison.

[0043] The fourth material may comprise boron carbide particles.

[0044] The fourth material may comprise boron carbide particles in a graphite matrix.

[0045] The fourth material may comprise gadolinium oxide particles.

[0046] The fourth material may comprise gadolinium oxide particles in a graphite matrix.

[0047] A land-based, air-based, or sea-based power plant may also be provided that includes a nuclear fission reactor system (10) according to the present disclosure.

[0048] Thus, core and core assembly elements are provided that provide improved cooling control and predictable power density throughout the life of the reactor.

[0049] The solution of the present disclosure solves the problem of managing peaking power through the core without the cost of variable fuel loading or complex geometric fuel zone arrangements.

[0050] The solution of the present disclosure allows the ability to control the distribution of coolant flow while improving heat transfer performance.

[0051] The solution of the present disclosure may also allow the peak fuel temperature to remain constant throughout the life of the core without the need to perform fuel assembly shuffling. [Brief explanation of the drawings]

[0052] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which: [Figure 1] FIG. 1 shows an example of a nuclear fission reactor, as described in the Background section. [Figure 2] FIG. 2 illustrates features of a nuclear fission reactor unit according to the present disclosure. [Figure 3] FIG. 3 illustrates an example fuel block of a nuclear fission reactor system according to the present disclosure. [Figure 4] FIG. 4 shows the inlet end of a fuel block and fuel assembly according to the present disclosure. [Figure 5] FIG. 5 shows the outlet end of a fuel block and fuel assembly according to the present disclosure. [Figure 6] FIG. 6 illustrates an end view of a first example fuel block according to the present disclosure. [Figure 7] FIG. 7 illustrates an end view of a second example fuel block according to the present disclosure. [Figure 8] FIG. 8 shows an example of a displacer element according to the present disclosure. [Figure 9] FIG. 9 shows a further example of a displacer element according to the present disclosure. [Figure 10] FIG. 10 shows another example of a displacer element according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0053] The present disclosure relates to a fuel assembly 100 for a nuclear fission reactor 10. The present disclosure also relates to a nuclear fission reactor system 10 including a reactor unit 12 having a fuel block 400. The present disclosure also relates to a land-based, air-based, or sea-based power plant including a nuclear fission reactor system 10 according to the present disclosure.

[0054] For example, the nuclear fission reactor 10 may be a high-temperature gas-cooled reactor (HTGR). The system may be a direct cycle design in which the reactor core 600 directly heats a working fluid that drives a turbine to generate electricity. The coolant may be, for example, nitrogen. Features of such systems are well known in the art and therefore will not be described in detail herein.

[0055] As shown in Figure 2, the nuclear fission reactor system 10 of the present disclosure may include a reactor unit 12 having a casing 14 surrounding a fuel block 400. The fuel block 400 has a central axis 402 extending along the length of the fuel block 400. A perspective view of the fuel block 400 is shown in Figure 3, and end views of different examples of the fuel block 400 are shown in Figures 6 and 7.

[0056] The reactor unit 12 may include a core 600 made up of one or more fuel blocks 400. The fuel blocks are shown as hexagonal prisms, however, each fuel block 400 may be any suitable shape.

[0057] The fuel block 400 defines a fuel assembly chamber 404 extending from an inlet opening 410 on a first surface 412 of the fuel block 400 to an outlet opening 420 on a second surface 422 of the fuel block 400. Multiple fuel assembly chambers 400 may be provided (as shown in FIGS. 3, 6, and 7). For clarity, a single fuel assembly chamber 400 is shown in FIG. 2, but the fuel block may define multiple fuel assembly chambers 404 (as shown in FIGS. 3, 6, and 7).

[0058] The fuel block 400 may be made of any suitable conventional material appropriate for the reactor design. For a thermal spectrum reactor, the fuel block 400 must be made of a moderating material with a low neutron cross section. Examples include graphite, beryllium, beryllium oxide, and metal hydrides, zirconium hydride, yttrium hydride, etc. If a fast spectrum core 600 is utilized, the material may be selected to have a low moderating potential while being able to withstand the high temperatures and radiation flux.

[0059] Each fuel assembly chamber inlet 410 is operable to be in fluid communication with a coolant source 500 and the fuel assembly chamber outlet 208 is operable to be in fluid communication with a coolant exhaust 502 .

[0060] The coolant source 500 may provide a flow of coolant into a coolant cavity 504 defined by the casing 14 and the first surface 412 of the fuel block 400. The coolant outlet 502 may be in fluid communication with a discharge cavity 506 defined by the casing 14 and the second surface 422 of the fuel block 400. Thus, the coolant may flow into the coolant cavity 504 before entering the fuel block 400 and exit the fuel block 400 into the discharge cavity 506.

[0061] A fuel assembly 100 is disposed within each fuel assembly chamber 404.

[0062] For the avoidance of doubt, other features of the nuclear fission reactor system (e.g., heat exchangers, turbines, etc.) are not shown in the figures.

[0063] 4 and 5, each fuel assembly 100 is centered about a longitudinal axis 102. Each fuel assembly 100 includes a fuel compact 200 defining a flow passage 202 extending from an inlet 204 at a first end 206 of the fuel compact 200 to an outlet 208 at a second end 210 of the fuel compact 200. The fuel compact 200 has an outer surface 220. The flow passage 202 defines an inner surface 222 of the fuel compact 200.

[0064] A displacer element 300 is disposed in the passage 202. The displacer element 300 may be centered about the longitudinal axis 102. The displacer element 300 may be mounted such that a clearance 212 is maintained between the inner surface 222 of the fuel compact 200 and the displacer element 300 to define an annular cooling passage 230.

[0065] As shown in FIGS. 4 and 5 , the displacer element 300 may include a central cylinder 330 extending from a first end 316 to a second end 318. A first region 340 of the displacer element 300 may extend from the first end 316 of the displacer element to a second region 342, where the first region 340 has a constant first diameter. A second region 342 may extend from the end of the first region 340, where the second region 342 increases in diameter toward a third region 344. That is, the second region 342 may be frustoconical. The third region 344 may extend from the end of the second region 342 to a fourth region 346, where the third region 344 has a constant third diameter. Fourth region 346 may extend from the end of third region 344, with fourth region 346 decreasing in diameter toward fifth region 348. That is, fourth region 346 may be frusto-conical. Fifth region 348 may extend from the end of fourth region 346 to displacer element second end 318, with fifth region 348 having a constant fifth diameter.

[0066] The fuel compact may comprise conventional fissile material. In one example, the fuel compact may comprise TRISO fuel particles embedded in a graphite matrix.

[0067] A spacer element 302 may extend between the fuel compact 200 and the displacer element 300 to position the displacer element 300 relative to the fuel compact 200 and maintain a clearance 212 therebetween. The spacer element 302 may be integral with the displacer element 300. Alternatively, the spacer element may be fabricated separately and then bonded to the body 330 of the displacer element 300.

[0068] The spacer element 302 may be airfoil-shaped with a leading edge 304 oriented toward the inlet 204 and a trailing edge 306 oriented toward the outlet 208 .

[0069] 4 and 5, a first spacer element array 310 may be provided at a first position P1 along the length of the displacer element 300. The first spacer element array 310 may include at least two spacer elements 302 spaced apart around the periphery of the displacer element 300.

[0070] The second spacer element array 312 may be provided at a second position P2 spaced along the length of the displacer element 300 from the first spacer element array 310. That is, the second position P2 may be spaced along the length of the displacer element 300 from the first position P1.

[0071] The first spacer element array 310 may be disposed at the first end 206 of the fuel compact 200. Each spacer element 302 of the first spacer element array 310 may be positioned within a respective recess 232 defined by and disposed within the first end 206 of the fuel compact 200.

[0072] A second spacer element array 312 may be provided at the second end 210 of the fuel compact 200, and each spacer element 302 of the second spacer element array 312 may be located within a respective recess 234 defined by the second end 210 of the fuel compact 200.

[0073] At least one of the spacer elements 302 in each spacer element array 310 , 312 extends beyond the respective end 206 , 210 of the fuel compact 200 .

[0074] The spacer elements 302 of the first spacer element array 310 may extend along from the second region 342 of the displacer element 300. The spacer elements 302 of the second spacer element array 312 may extend along from the fourth region 346 of the displacer element 300.

[0075] The fuel assembly chamber 404 may extend in diameter from the fuel assembly chamber inlet 410 such that a snug fit is provided between the fuel assembly 100 and the fuel assembly chamber 404. The fuel assembly chamber 404 may decrease in diameter toward the fuel assembly chamber outlet 420 such that the diameter of the fuel assembly chamber outlet 420 is smaller than the diameter of the fuel assembly 100. Thus, the restriction provided by the fuel assembly chamber outlet 420 prevents the fuel assembly 100 from passing through the fuel assembly chamber outlet 420. A shoulder 460 defines the decrease in diameter of the fuel assembly chamber 404, and the spacer element 302 may engage the shoulder 460.

[0076] A retaining plug 462 may be provided at the fuel assembly chamber inlet 410. The retaining plug 462 may partially define a plug passage 464 having a diameter smaller than the diameter of the fuel assembly 100, thereby preventing the fuel assembly 100 from passing through the passage 464 of the retaining plug 462. The spacer element 302 may engage the retaining plug 462.

[0077] In an example where multiple fuel assembly chambers 404 are provided in the core 600, one of the multiple fuel assemblies 100 may be positioned in each one of the multiple fuel assembly chambers 404. In one example, the multiple fuel assemblies 100 may all be essentially identical.

[0078] In other examples where a plurality of fuel assemblies 100 are provided, at least one of the fuel assemblies 100 of the plurality of fuel assemblies 100 is configured (i.e., operable) to transfer heat to (i.e., pass through) the coolant in its respective annular cooling channel 230 at a first rate, and at least one other fuel assembly of the plurality of fuel assemblies 100 is configured (i.e., operable) to transfer heat to the coolant in its respective annular cooling channel 230 at a second rate.

[0079] 6 shows a cross-sectional (or end) view of an example fuel block 400. This illustrates an example in which fuel assembly chambers 404 are evenly distributed within the fuel block 400.

[0080] In other examples, some regions of the fuel block 400 may have more fuel assembly chambers 404 per unit volume than other regions of the fuel block 400.

[0081] 7 illustrates a cross-sectional (or end) view of a different example of a fuel block 400. In this example, in a central region of the fuel block 400 (i.e., extending radially from the central axis 402 along the length of the fuel block 400), more fuel assembly chambers 404 are provided per unit volume than in the remainder of the fuel block 400.

[0082] The annular cooling channel 230 of one of the fuel assemblies 100 may be provided with a different flow area than at least one annular cooling channel 230 of another of the fuel assemblies 100, thereby providing a set of fuel assemblies 100 with different annular coolant flow rates. For example, the diameter of the third region 344 of the first displacer element 300 in a first fuel assembly 100 may be different from the diameter of the third region 344 of the second displacer element 300 in a second fuel assembly 100.

[0083] As shown in the examples of FIGS. 4 and 5, the annular cooling channel 230 of at least one of the fuel assemblies 100 of the nuclear fission reactor system 10 may have a constant flow area along its length.

[0084] In another example, at least one annular cooling channel 230 of a fuel assembly 100 of a nuclear fission reactor system 10 may have a decreasing flow area along its length. For example, a channel 202 of a fuel compact 200 may decrease in diameter along its length from an inlet 204 to an outlet 208, and / or a displacer element 300 may increase in diameter along its length from an inlet 204 to an outlet 208.

[0085] The displacer elements 300 of the fuel assemblies 100 of the nuclear fission reactor system 10 may be provided with different diameters to provide fuel assemblies 100 each operable to limit mass flow to a different value.

[0086] The displacer elements 300 of the fuel assemblies 100 of the nuclear fission reactor system 10 may be provided in different lengths to provide fuel assemblies 100 each operable to limit mass flow to a different value.

[0087] Both allow for creating an optimized coolant mass flow distribution whereby the coolant mass flow through each fuel assembly 100 is configured using displacer elements until the exit coolant temperature is uniform across the fuel blocks 400 and / or core 600 despite the varying thermal output of each fuel compact 200. The displacer elements 300 increase the heat transfer surface area as heat radiates from the fuel compacts 200 and heats the displacer elements 300, which can then also transfer heat to the coolant.

[0088] The heat transfer mechanism 320 may be provided as part of the displacer element 300. That is, the heat transfer mechanism 320 may extend from a central body 330 of the displacer element 300. The spacer element 302 may be provided as the heat transfer mechanism 320. The heat transfer mechanism 320 of one of the fuel assemblies 100 is configured to transfer heat to at least one heat transfer mechanism of another of the fuel assemblies 100 at a different rate.

[0089] The heat transfer mechanism 320 may be provided as a turbulence enhancer.

[0090] For example, the heat transfer features 320 may be provided as a roughened surface 350 of the displacer element 300. The roughened surface may comprise raised features (i.e., features that extend from the displacer element 300 into the annular cooling channel 230). For example, the raised features may be provided as circumferential ribs 352 (as shown in FIG. 8). In other examples, the roughened surface may comprise helical / screw ribs that extend along the displacer element.

[0091] In further examples, the transmission mechanism 320 may be provided as fins, pedestals, axially extending ribs, and / or circumferentially extending ribs extending from the body 330 of the displacer element 300. Figure 9 illustrates an example in which the transmission mechanism 320 is provided as axially extending fins 354.

[0092] 10, the heat transfer features may be provided as continuous spiral fins 356 along at least a portion of the length of the displacer element 300. Such a configuration may increase flow turbulence, thereby improving heat transfer to the coolant.

[0093] In another example, the displacer element 300 may be shaped along its length to enhance turbulence. For example, the displacer element 300 may be provided as a helical element extending along the flow path 202.

[0094] At least one of the displacer elements 300 of the plurality of fuel assemblies 100 may comprise a first material having first material properties, and each of the remaining displacer elements 300 of the plurality of fuel assemblies 100 comprises one of a second material, a third material, or a fourth material, each having different material properties, each of which has material properties different from the first material and different from each other.

[0095] The first material may be a neutron moderator and multiplier, for example beryllium oxide.

[0096] The second material may be a neutron moderator and absorber. For example, the second material may comprise graphite, boron carbide, and / or silicon carbide.

[0097] The third material may be transparent to neutrons. For example, the third material may comprise zirconium, zirconium hydride, and / or a low neutron capture metal.

[0098] The fourth substance may be a neutron poison. For example, the fourth material may comprise boron carbide particles and / or gadolinium oxide particles in a graphite matrix.

[0099] A method for optimizing the performance of a reactor core 600 using the fuel blocks 400 and fuel assemblies 100 of the present disclosure may include optimizing fuel element placement, fuel loading, or moderation levels to ensure a radially flat power profile of the fuel block (and thus the assembled reactor). Because the coolant channels are positioned within the fuel compacts (i.e., fuel assemblies), cooling does not rely on the fuel blocks as a heat transfer path. This allows the fuel assemblies to be positioned at any desired location within the fuel block, optimizing the neutron moderation performance of the core 600 and offsetting the effects of neutron leakage.

[0100] Additionally or alternatively, the method can include accommodating some variation in radial power in the fuel block and controlling coolant flow through each fuel assembly to match the local power level. This can increase pressure drop and degrade heat transfer performance in low-power areas (i.e., the edges of the fuel block 400) due to localized reductions in mass flow in these areas. In one example, flow restriction is added progressively along the coolant channels, as additional turbulence created by any pressure drop also improves heat transfer. In such an example, balancing the heat load is achieved by varying mass flow and heat transfer without having to change the surface properties / profile of the displacer element.

[0101] Thus, the fuel assembly 100 and / or nuclear fission reactor system 10 according to the present disclosure can be configured to ensure that the power / heat transfer from any portion of the core 600 (e.g., a fuel block) is the same across the outlet of each fuel block (and thus the core 600 made from an assembly of fuel blocks).

[0102] Based on the teachings of the present disclosure, the fuel assembly 100 is optimized to control local coolant flow, thereby matching it to the local power level so that all gases exiting the core 600 are at approximately the same temperature. This is the crux of the idea and how it is novel in the field of core 600 design.

[0103] Thus, the coolant mass flow rate can be controlled by adding and modifying displacer elements 300. Thus, the thermal power (and therefore the temperature profile of the core 600) may be controlled using coolant flow rate control in addition to, or instead of, fuel loading.

[0104] The heat output from the fuel assemblies can be adjusted by varying the coolant mass flow rate through the fuel assemblies, and thus some fuel assemblies (i.e., those with higher temperatures, for example, at the center of the fuel block 400) can have a higher coolant mass flow rate than other fuel assemblies (i.e., those with lower temperatures, for example, at the edge of the fuel block 400).

[0105] Additionally or alternatively, heat transfer to the coolant can be varied by controlling heat conduction to the coolant (e.g., to increase / decrease heat transfer to the coolant), so that some fuel assemblies (i.e., those at higher temperatures, e.g., in the center of the fuel block 400) have higher heat transfer to the coolant than others (i.e., those at lower temperatures, e.g., at the edges of the fuel block 400).

[0106] Additionally or alternatively, the material of the displacer element may be changed to change the moderator function, so that some fuel assemblies (i.e., higher temperature fuel assemblies, for example, in the center of the fuel block 400) generate less heat than other fuel assemblies (i.e., lower temperature fuel assemblies, for example, at the edge of the fuel block 400).

[0107] As described above, the disclosed configuration employs an annular fuel compact 200 having a central coolant hole (flow passage 202) in which a displacer element 300 is disposed. The addition of the displacer element 300 allows the coolant flow through the flow passage 202 to be optimized within each fuel element chamber 404, while improving the overall heat transfer performance of the fuel block 400 (and thus the core 600 formed by the fuel blocks 400). Thus, the disclosed configuration can enable the ability to control the distribution of coolant flow while improving heat transfer performance.

[0108] By routing the coolant through the center of the fuel compact 200 without a support sleeve, thermal resistance to heat flow to the convection heat transfer surface is minimized. Thus, the disclosed configuration transfers heat directly from the fuel compact to the coolant.

[0109] Additionally, receiving the fuel compacts 200 in the counterbore holes in the fuel block 400 eliminates the need for additional support methods for the fuel compacts 200 and ensures good thermal contact between the fuel compacts 200 and the fuel block 400. This thermal contact aids in the transfer of heat between the fuel assemblies 100, further assisting in managing peaking power and adding significant thermal inertia to the core 600 during transient events.

[0110] Also, in the disclosed configuration, balancing of gas flow to optimize cooling rates is significantly simpler and can be implemented for each fuel assembly chamber 404 using displacer elements 300 of various sizes.

[0111] The size of the annular cooling channel 230 may be optimized on every fuel assembly chamber 404 and positioned within the fuel block 400 to optimize pressure drop and heat transfer based on the profile of the fuel block 400.

[0112] Additionally, with the fuel compacts 200 in intimate contact with the fuel blocks 400, the thermal resistance to removing decay heat is lower and the fuel blocks operate closer to the fuel temperature.

[0113] In the configuration of the present disclosure, only fuel assembly chamber 404 need be provided in fuel block 400. That is, separate coolant and fuel assembly holes, as may be provided in related art examples, are not required, which allows for a smaller fuel block 400 and, therefore, a smaller nuclear fission reactor system. For example, a nuclear fission reactor system according to the present disclosure may be 30% smaller than related art examples.

[0114] A further important advantage of the disclosed apparatus is that the temperature profile of the fuel block 400 can be tailored by varying the material and / or geometry of the fuel assembly 100.

[0115] Thus, a nuclear fission reactor system according to the present disclosure may include multiple fuel assemblies 100 having the same external shape, some of the fuel assemblies having different features and / or characteristics than other fuel assemblies to provide different thermal output capabilities from the fuel assembly locations. Thus, by varying the thermal output capabilities from the fuel assemblies, the fuel block, and therefore the fuel core 600, can be configured to have an optimal temperature distribution.

[0116] The presented and proposed new fuel block design reduces fuel block manufacturing costs, significantly improves heat transfer, and offers the potential for higher fuel loadings while providing comparable coolant reactivity performance.

[0117] Additionally, the disclosed configuration allows the power density profile of the fuel blocks to be locked into the design during initial assembly for optimal power flux. Thus, the reactor system does not require refueling, saving maintenance costs and eliminating the need to transport fissile material to and from maintenance locations.

[0118] Attention is directed to all references and documents related to this application, filed contemporaneously with or prior to this specification, and open to public inspection herewith, and the entire contents of such references and documents are incorporated herein by reference.

[0119] All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive.

[0120] Each feature disclosed in this specification (including any accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each disclosed feature is only one example of a common series of equivalent or similar features.

[0121] The invention is not limited to the details of the foregoing embodiments, and extends to any novel one or any novel combination of features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one or any novel combination of steps of any method or process so disclosed.

Claims

1. 1. A nuclear fission reactor system comprising a reactor unit having a fuel block with a central axis extending along a length of the fuel block between a first surface and a second surface, the fuel block defining a plurality of fuel assembly chambers extending from an inlet opening on the first surface of the fuel block to an outlet opening on the second surface of the fuel block, and a plurality of fuel assemblies; Each fuel assembly is a fuel compact defining a flow path extending from an inlet at a first end of the fuel compact to an outlet at a second end of the fuel compact, the flow path defining an interior surface of the fuel compact; a displacer element disposed within the coolant flow path and mounted such that a clearance is maintained between the inner surface of the fuel compact and the displacer element to define an annular cooling flow path; each of the plurality of fuel assemblies is positioned in a respective one of the plurality of fuel assembly chambers; 1. A nuclear fission reactor system, wherein at least one fuel assembly of the plurality of fuel assemblies is configured to transfer heat to a coolant in its annular cooling channel at a first rate, and at least one other fuel assembly of the plurality of fuel assemblies is operable and configured to transfer heat to the coolant in its annular cooling channel at a second rate.

2. The nuclear fission reactor system of claim 1 , wherein said fuel assembly chambers are evenly distributed within said fuel block.

3. 3. The nuclear fission reactor system of claim 1, wherein in some regions of the fuel block, there are more fuel assembly chambers per unit volume than in other regions of the fuel block.

4. 4. The nuclear fission reactor system of claim 3, wherein a central region of said fuel block has more fuel assembly chambers per unit volume than a remainder of said fuel block.

5. 5. The nuclear fission reactor system of claim 1, wherein the annular cooling channel of one of the fuel assemblies has a different flow area than the annular cooling channel of at least one other fuel assemblies.

6. 6. The nuclear fission reactor system of claim 5, wherein the annular cooling channel of at least one of the fuel assemblies has a constant flow area along its length.

7. 6. The nuclear fission reactor system of claim 5, wherein the annular cooling channel of at least one of the fuel assemblies has a flow area that decreases along its length.

8. 8. The nuclear fission reactor system of any one of claims 1 to 7, wherein the displacer element is provided with a heat transfer mechanism, the heat transfer mechanism of one of the fuel assemblies configured to transfer heat at a different rate than the heat transfer mechanism of at least one of the other fuel assemblies.

9. 9. The nuclear fission reactor system of claim 8, wherein said heat transfer mechanism is provided as a roughened surface of said displacer element.

10. 10. The nuclear fission reactor system of claim 9, wherein the heat transfer features are provided as fins, pedestals, axially extending ribs, and / or circumferentially extending ribs extending from the displacer element.

11. 11. The nuclear fission reactor system of any one of claims 1-10, wherein at least one of the displacer elements of the plurality of fuel assemblies comprises a first material, and each of the remainder of the displacer elements of the plurality of fuel assemblies comprises one of a second material, a third material, or a fourth material, each of which is different from the first material.

12. 12. The nuclear fission reactor system of claim 11, wherein the first material is a neutron moderator and a neutron multiplier.

13. 13. The nuclear fission reactor system of claim 11 or 12, wherein the first material is beryllium oxide.

14. the second material is a neutron moderating, absorbing material; the third material is transparent to neutrons; 14. The nuclear fission reactor system of any one of claims 11 to 13, wherein the fourth material is a neutron poison.

15. the second material comprises graphite, boron carbide, and / or silicon carbide; the third material comprises zirconium, zirconium hydride, and / or a low neutron capture metal; 15. The nuclear fission reactor system of claim 14, wherein the fourth material comprises boron carbide particles and / or gadolinium oxide particles in a graphite matrix.

16. 16. A land-based, air-based or sea-based power plant comprising a nuclear fission reactor system according to any one of claims 1 to 15.

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