High-temperature control rod for light water reactor

Abstract

To provide materials of control rods for use in nuclear reactors.SOLUTION: A high-temperature control rod (22) for a nuclear fuel assembly includes: a neutron-absorbing material (24) with a melting point of greater than 1500°C that does not form a eutectic with a melting point of less than 1500°C, and may further include a cladding material having a melting point of greater than 1500°C. The cladding material is selected from the group consisting of silicon carbide, zirconium, a zirconium alloy, tungsten, and molybdenum. The absorbing material (24) is selected from the group consisting of Gd2O3, Ir, B4C, Re, and Hf. The metal cladding or the absorbing material (24) may be coated with an anti-oxidation coating (26) of Cr with or without a Nb intermediate layer (28).SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] Statement Regarding Government Rights This invention was made with government support under Contract No. DE-NE0008222 with the Department of Energy. The United States Government has certain rights in this invention.

[0002] The present invention relates to nuclear reactors and, more particularly, to materials used in the manufacture of control rods.

Background Art

[0003] A typical nuclear reactor core contains a number of fuel assemblies, each consisting of a plurality of elongated fuel rods or similar elements. The fuel rod array contains guide tubes for receiving control rods. Above the fuel rod assembly is a control rod assembly containing a number of control rods, which in some design examples can be positioned in alignment with, and moved from, guide tubes located between some of the fuel rod clusters. FIG. 1 shows an exemplary nuclear fuel assembly including control rod guide tubes and a control rod assembly.

[0004] In use, the control rod assembly can, as required, lower the control rods to strategic positions within different guide tubes depending on the operating requirements of the fuel assembly at a given point in time.

[0005] For example, current control rods with Ag-In-Cd alloy rods in stainless steel cladding tubes have functional durability to withstand environments up to 1168°C. The temperature of the control rods is approximately 200°C lower than that of the fuel rods, and the fuel rods are maintained below 1200°C, which is the limit for current design basis accidents. Therefore, current control rod materials adequately meet normal operating conditions. However, in the event of an accident that exceeds the design temperature of the nuclear reactor (i.e., beyond design basis accident), the temperature of the control rods can reach approximately 1700°C. At such a temperature, current control rod materials will melt and corrode the fuel rods.

[0006] Events such as the tsunami that damaged the nuclear power plant in Fukushima, Japan, indicate that although rare, beyond-design-basis accidents can occur, and it is necessary to consider the occurrence of such events when designing the equipment of nuclear power plants. Therefore, it is necessary to address the possibility of the control rods melting down.

Summary of the Invention

[0007] The following summary is provided to facilitate understanding of some innovative features specific to the embodiments disclosed in this application and is not intended to be a complete description. To fully understand the various aspects of these embodiments, it is necessary to consider the entire specification, claims, drawings, and abstract of this application as a whole.

[0008] To address the issue of the accident tolerance of control rods, improvements have been made to the materials constituting the control rods.

[0009] In this application, a material for a nuclear reactor control rod that maintains stability until at least 1668 °C beyond the design basis accident conditions and meets the conditions of accident-tolerant nuclear fuel is described. Control rods configured by several combinations of cladding and neutron absorbers are described. Also, control rods configured with a coating on the neutron absorber are described.

[0010] Examples of control rods include high-temperature control rods for nuclear fuel assemblies comprising a cladding material with a melting point exceeding 1500 °C and a neutron absorber with a melting point exceeding 1500 °C, and neither the cladding material nor the neutron absorber forms a eutectic with a melting point below 1500 °C with each other or with other materials of the fuel assembly.

[0011] Examples of control rods include high-temperature control rods for nuclear fuel assemblies comprising a solid, rod-shaped, coated metallic neutron absorber with a melting point exceeding 1500 °C and not forming a eutectic with a melting point below 1500 °C with the coating material or other materials of the fuel assembly. This metal rod may comprise an oxidation-resistant coating.

[0012] An example of a control rod used in an atomic fuel assembly may include a solid neutron absorber selected from the group consisting of Ir, Re, and Hf, and an oxidation-resistant coating covering the absorber.

[0013] In various aspects, a combination provided to enhance the accident tolerance of control rods in a nuclear reactor includes a coating material selected from the group consisting of silicon carbide, zirconium, zirconium alloy, tungsten, tungsten alloy, molybdenum, and molybdenum alloy, and a neutron absorber.

[0014] In various aspects, the absorber has an effective neutron absorption cross-section of 8 barns or more.

[0015] In a particular aspect, the coating material may be a metal covered with an oxidation-resistant coating that does not form a eutectic with a melting point below 1500°C. The metal coating material can be selected from the group consisting of zirconium, zirconium alloy, tungsten, tungsten alloy, molybdenum, and molybdenum alloy.

[0016] In various aspects, the oxidation-resistant coating is an outer chromium coating. In a particular aspect, an intermediate niobium layer is located between the outer layer and the coating material, or between the outer layer and the solid neutron absorber. Molybdenum and tantalum can also be used as intermediate layers.

[0017] The non-metallic coating material may be SiC.

[0018] In a particular aspect, the absorber is selected from the group consisting of B 4 C, Re, Ir, and Hf.

[0019] The coating material may be a tube that receives the absorber. The absorber may be in the form of either ceramic pellets or metal slag contained within the tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features and advantages of the present invention will be better understood by referring to the accompanying drawings.

[0021]

Figure 1

[0022]

Figure 2

[0023]

Figure 3

[0024]

Figure 4

[0025]

Figure 5

DETAILED DESCRIPTION OF THE INVENTION

[0026] As used in this application, the singular forms preceded by "a", "an" and "the" include the plural forms as well, unless the context clearly indicates otherwise. Accordingly, the articles "a" and "an" used in this application refer to the fact that the grammatical referents of the articles are one or more (i.e., at least one). For example, "an element" means one element or a plurality of elements.

[0027] As non-limiting examples, the directional terms such as top, bottom, left, right, below, above, front, rear, and their variants used in this application are related to the geometric arrangement of the elements shown in the accompanying drawings and do not limit the scope of the claims of this application unless otherwise specified.

[0028] Including the claims, in this application, unless otherwise indicated, it should be understood that all numbers representing amounts, values or characteristics are modified by the term "about" in all cases. Therefore, even when the term "about" is not explicitly stated together with a number, it can be read as if there is the term "about" before the number. Therefore, unless otherwise indicated, all numerical parameters described in the following description may vary depending on the desired characteristics that the compositions and methods according to the present invention are directed to. As a minimum, and without intending to limit the application of the doctrine of equivalents to the claims, each numerical parameter described in this application should be interpreted by taking into account at least the number of significant figures reported and applying ordinary rounding techniques.

[0029] Also, any numerical range recited in this application is intended to include all fragmentary parts subsumed therein. For example, the range "1 to 10" is intended to include any fragmentary part between the recited minimum value 1 and maximum value 10 (including the minimum and maximum values). That is, the minimum value is 1 or more and the maximum value is 10 or less.

[0030] The accident-tolerant nuclear fuel is characterized in that the cladding can withstand much higher temperatures than the zirconium (Zr) alloy currently in use. For example, the highest temperature that SiC can withstand is 2545 °C, and a coated cladding such as applying a coating of Cr and Nb to a Zr cladding can withstand temperatures up to 1668 °C. However, the design of nuclear fuel components for obtaining accident tolerance is carried out assuming the minimum amount of injected water in response to off-design accidents where the fuel temperature reaches about 1700 °C. The control rod must maintain stability at a temperature of at least 1668 °C, preferably above that, in order to acquire tolerance to off-design accidents.

[0031] In various aspects, the material for providing an accident-tolerant nuclear reactor control rod has an effective cross-sectional area equal to or preferably greater than 8 barns (1 barn = 10 -28 m 2 ). The effective nuclear reaction cross-sectional area is the weight percentage of the absorber, the nuclear reaction cross-sectional area and the density (g / cm 3) is calculated by dividing the product by the molecular weight (g / mol) of the absorbent. The effective cross-sectional area of the currently used Ag80% / In15% / Cd5% control rod is about 16. When used as an absorbent, Gd 2 O 3 The effective cross-sectional area of is 867 based only on gadolinium because oxygen does not play a role in absorbing neutrons.

[0032] Referring to the drawings of the present application, FIG. 1 shows a conventional fuel assembly 10 including a control rod assembly 12, an upper nozzle 14, a lower nozzle 16, and a grid assembly 18. The nozzles 12, 14 and the grid assembly 18 align the fuel rods 20 and the control rods 22.

[0033] As shown in FIG. 2, the control rod 22 described in the present application has an antioxidant coating 26 on the neutron absorber 24. As shown in FIG. 3, in a particular aspect, an intermediate layer 28 exists between the neutron absorber 24 and the oxidation-resistant layer 26 of the control rod 22.

[0034] In a particular aspect, the neutron absorber 24 has a coating layer 30 as shown in FIG. 4. The neutron absorber 24 has the shape of a solid tube as shown in FIGS. 2 to 4 or the shape of pellets or slag 32 as shown in FIG. 5.

[0035] In various aspects, the coating material for providing an accident-tolerant reactor control rod has a melting point exceeding 1500°C. The current atomic fuel reactor core model with the minimum water injection volume suggests that it can maintain a temperature of about 1700°C indefinitely. Since the temperature of the control rod is about 200°C lower than that, if there is a coating that can withstand a temperature exceeding 1500°C, the geometry of the control rod can be maintained.

[0036] In various aspects, the melting point of the neutron absorber of the control rod preferably exceeds 1500°C. When the melting point is higher than 1500°C, the absorber does not melt even if the coating of the control rod is damaged, so it is not diluted or displaced by another molten substance.

[0037] To provide an accident-tolerant nuclear reactor control rod, the cladding material and the absorber material of the control rod must not form a eutectic that melts together with each other or with other materials in the fuel assembly and has a eutectic point (the lowest melting point that the composite material can reach) below 1500 °C.

[0038] In various scenarios, the cladding and absorber materials of this control rod are designed to withstand a fluence equivalent to 20 years of operation under various usage patterns (base load / load following / load regulation). This function of the control rod, i.e., this ability which is an advantage of the improved control rod design, is not impaired by corrosion and neutron absorption over these 20 years.

[0039] This absorber material should be reasonably priced, and it is preferable that the manufacturing cost of the control rod is not prohibitively high. Understanding that the material cost and the labor cost of control rod processing can vary over time, the cost factor in selecting materials in line with the objectives of the present invention is not a limiting factor. However, the tendency to select / non-select any given material may change depending on market trends at any given point in time.

[0040] Table 1 lists various materials that meet the desired specifications and the related properties of each material. As a result of applying the above-mentioned specifications, in various scenarios, the combinations of cladding / absorber materials and the coated absorber materials listed in Table 1 were determined to meet the requirements for an accident-tolerant nuclear fuel assembly control rod. [Table 1] [Table 2] [Table 3] [Table 4]

[0041] Examples of cladding materials in combinations of materials used for control rods in various scenarios include the following. 1. A zirconium (Zr) alloy coating having an outer coating of chromium (Cr) and an intermediate layer of niobium (Nb) between the Cr layer and the Zr layer. The eutectic point of this coating is 1668 °C between Cr and Nb. 2. A tungsten (W) alloy coating with an outer coating of Cr. The eutectic point of this coating is 1863 °C. 3. A molybdenum (Mo) alloy coating with an outer coating of Cr. The eutectic point of this coating is 1858 °C. 4. A silicon carbide (SiC) coating. The decomposition temperature is about 2545 °C.

[0042] In various aspects, the zirconium alloy is a coated ZIRLO® manufactured according to the procedure disclosed in U.S. Patent No. 4,649,023, the relevant parts of which are incorporated herein by reference. ZIRLO® is an alloy consisting essentially of zirconium with niobium 0.5 - 2.0% by weight, tin 0.7 - 1.5% by weight, iron 0.07 - 0.14% by weight, at least one of nickel and chromium 0.03 - 0.14% by weight, the total of iron, nickel and chromium at least 0.12% by weight, carbon up to 220 ppm. This alloy preferably contains 0.03 - 0.08% chromium and 0.03 - 0.08% nickel. Those skilled in the art will understand that other zirconium alloys suitable for the desired applications can be used.

[0043] In various aspects, examples of absorbers used in control rods include the following. 1. Gd 2 O 3 has a melting point of 2420 °C and a very high neutron absorption rate. Gd 2 O 3 is relatively costly, so it is not currently ideal to use this material alone. However, since this material has a very large thermal neutron cross - section, it can be diluted by up to about 50 times with other oxides with high melting points and low cost such as Al 2 O 3 (melting point 2072 °C) or CaO (melting point 2575 °C) to reduce the final cost. 2. B 4C has a high melting point of 2763 °C and a moderately high neutron absorption rate. Gd 2 O 3 Unlike this, it does not cost an exorbitant amount. In a certain specific aspect, B 4 C is a useful absorber. 3. Rhenium (Re) has a melting point of 3180 °C and a moderate neutron absorption rate. This material is also relatively expensive. In a certain specific aspect, Re is a useful absorber. 4. Hafnium (Hf) has a melting point of 2222 °C and a moderate neutron absorption rate. This material is also relatively expensive. However, in a certain specific aspect, Hf is a useful absorber. Other metal absorbers shown in Table 1 include osmium (Os), tantalum (Ta), and niobium (Nb). However, since the effective cross-sectional areas of these materials are less than 8 barns, it is not preferable to use them in a nuclear fuel assembly that meets the specifications to withstand design basis accidents.

[0044] The absorber may be in the form of pellets or slag inserted into a hollow cladding tube. The term slag used here refers to a metal absorber, and the term pellet refers to a pressure-sintered ceramic absorber. The pellet-shaped absorbers listed in Table 1 may contain Gd 2 O 3 and B 4 C. The slag-shaped absorbers listed in Table 1 may contain iridium (Ir), rhenium (Re), and hafnium (Hf). Whether it is in the form of slag or pellets, the absorbers used generally have the same geometric shape and will fit snugly inside the hollow of the cladding tube. The absorber can be cylindrical or other suitable shapes (such as cubes or rectangular parallelepipeds) according to the shape of the hollow cavity of the cladding tube. The absorber may be a single unit or a plurality of absorbers in the form of stacked cylinders or stacked blocks.

[0045] The absorber may be a solid metal rod covered with a coating material. For example, referring to Table 1, absorbers of metals such as Ir, Re, and Hf may have a configuration like a solid rod. There is an outer coating of Cr as the coating material on the rod to prevent oxidation of the lower metal rod during normal operation and accident conditions, but there are cases where an intermediate coating of Nb is applied and cases where it is not. Molybdenum and tantalum can also be used as the intermediate layer. In this embodiment, no coating material is used.

[0046] The high-temperature control rod may include, for example, a coating material of a metal or metal alloy with a melting point exceeding 1500 °C, and an oxidation-resistant coating that covers such a metal but does not eutectic and has a melting point below 1500 °C. The neutron absorption cross-section of this control rod is large. Another example of a high-temperature control rod includes SiC and a neutron absorber that has a melting point exceeding 1500 °C and does not form a eutectic with the SiC with a melting point below 1500 °C. The neutron absorption cross-section of this embodiment of the control rod is also large.

[0047] The metal coating material can be selected from zirconium (Zr), tungsten (W), molybdenum (Mo), and their alloys. These alloys can be selected from suitable alloys known to those skilled in the art for use in the high-temperature environment of a nuclear reactor. In various aspects, the coating material may be in the shape of a tube configured to hold absorber pellets or slag.

[0048] In various aspects, the cladding tube has a coating of an oxidation-resistant material. This coating consists of an outer coating of chromium (Cr), but there are cases where it further includes an inner coating of niobium (Nb) and cases where it does not. Other combinations of coatings can also be used. For example, there is a choice between an outer layer of Cr and an intermediate layer of Mo or Ta.

[0049] These combinations of coatings for the control rod and neutron absorbers are cost-effective and relatively easy to manufacture combinations for enhancing the accident tolerance of nuclear fuel assemblies.

[0050] The tube bodies, rods, and metallic slags described in this application can be machined by any method known to those skilled in the art. Due to the small tolerances for size, composition, molecular weight, and other properties such as those listed in Table 1, as well as other properties known to be relevant in the nuclear industry, precise manufacturing methods should be used.

[0051] Pellets can be formed by pellet manufacturing methods known in other commercial fields. For example, pellets of powdered or particulate ceramic absorbents can be formed by first homogenizing the particles so that the particle size distribution and surface area are relatively uniform. In certain aspects, additives such as lubricants, combustible absorbents, and pore-forming agents can be added. The formation of pellets from the particles can be carried out by compressing the particle mixture with a suitable commercially available mechanical or hydraulic press so as to obtain the desired "green" density and strength.

[0052] Basic presses incorporate a single-acting die platen, while the most complex presses may have a multi-acting platen so as to be able to form "multi-level" components. There are various presses over a wide range of pressing capabilities. The pressing capability required to compress the powder into the desired compact pellet shape is determined by the product of the projected surface area of the component and the load factor determined by the compression characteristics of the powder.

[0053] To start the process, the die is filled with the particle mixture. The die filling rate is mainly based on the fluidity of the particles.

[0054] Once the die is filled, the punch is moved towards the particles. The punch applies pressure to the particles to compress them into the geometry of the die. In certain pellet-forming processes, a load of several hundred MPa is applied to the particles supplied to the die for biaxial compression to form cylindrical pellets.

[0055] The compressed pellets are heated and sintered in a controlled atmosphere (usually containing argon) in a heating furnace. The heating temperature varies depending on the material during sintering. Sintering is a thermal process that densifies the as-formed pellets by converting the mechanical bonds formed between particles during compression into stronger bonds, thereby significantly enhancing the strength of the pellets. The compressed and sintered pellets are cooled and machined to the desired dimensions. The diameter of exemplary pellets is about 1 centimeter or slightly less, and the length is 1 centimeter or slightly greater.

[0056] The oxidation-resistant coating can be applied using a thermal deposition method such as the cold spray method. When there are two layers, the inner layer can be applied first, ground and polished, and then the outer layer can be applied and ground and polished.

[0057] In the cold spray method, the carrier gas is sent to a heater and heated to a sufficient temperature so that the temperature after expansion in the nozzle is maintained at a desired value (for example, 100 to 500 °C). In various scenarios, the carrier gas is preheated to a temperature of 200 to 1200 °C at a pressure of, for example, 5.0 MPa. In a particular scenario, the carrier gas is preheated to a temperature of 200 to 1000 °C. Also, in a particular scenario, it is preheated to a temperature of 300 to 900 °C, and in other scenarios, to a temperature of 500 to 800 °C. This temperature varies depending on the Joule-Thomson cooling coefficient of the specific gas used as the carrier. Whether the gas cools when its pressure changes during expansion or compression depends on the value of the Joule-Thomson coefficient. When the Joule-Thomson coefficient has a positive value, the carrier gas cools, so it is necessary to preheat the carrier gas to prevent excessive cooling that may affect the performance of the cold spray method. A person skilled in the art can determine the degree of heating to avoid excessive cooling using well-known calculation methods. For example, if the carrier gas is N 2In the case where the inlet temperature is 130 °C, the Joule-Thomson coefficient is 0.1 °C / bar. When a gas with an initial pressure of 10 bar (about 146.9 psia) and a final pressure of 1 bar (about 14.69 psia) impinges on the tube body at 130 °C, it is necessary to preheat the gas to about 130.9 °C, which is about 9 bar × 0.1 °C / bar (i.e., about 0.9 °C) higher.

[0058] For example, when using helium gas as the carrier gas, the gas temperature is preferably 450 °C at a pressure of 3.0 - 4.0 MPa. Also, the temperature of the nitrogen carrier gas is 1100 °C at a pressure of 5.0 MPa, but it may be 600 - 800 °C if the pressure is 3.0 - 4.0 MPa. Those skilled in the art will understand that the temperature and pressure variables change depending on the type of equipment used, and the temperature, pressure, and volume parameters can be adjusted by modifying the equipment.

[0059] Suitable carrier gases include inert or non-reactive gases, and in particular, gases that do not react with Cr or Nb particles, or the tubular or rod-shaped substrates to be coated. Examples of carrier gases include nitrogen (N 2 )), hydrogen (H 2 ), argon (Ar), carbon dioxide (CO 2 ), and helium (He).

[0060] There is a fair degree of freedom in the selection of the carrier gas. A mixed gas may be used. The selection of the gas is determined by both physical properties and economy. For example, a gas with a small molecular weight can increase the speed, but maximizing the speed should be avoided because the number of particles adhering due to particle rebound will be reduced.

[0061] In an exemplary cold spray process, high-pressure gas fed into a heater through a conduit is rapidly and substantially instantaneously heated there. The gas heated to the desired temperature is directed towards an instrument such as a gun. Particles of the desired coating material (here Cr or Nb) held in a hopper are, after being released, directed towards the gun, forced through a nozzle by a compressed gas jet, and directed towards a rod-shaped or tubular substrate. The sprayed Cr or Nb particles adhere to the surface of the rod or tube, forming a coating consisting of the particles.

[0062] The cold spray process is based on expanding a heated carrier gas under control to propel particles and deposit them on a rod-shaped or tubular substrate. The particles collide with the substrate or the already deposited layer and undergo plastic deformation by adiabatic shear. Successive colliding particles stack up to form a coating. To facilitate the deformation, the particles may be heated to a temperature between one-third and one-half of the melting point of the powder on the Kelvin absolute temperature scale before being introduced into the carrier gas. The entire area to be coated or the area where material buildup is required is scanned by a nozzle (i.e., sprayed linearly from one end of an area to the other, from the topmost to the bottommost).

[0063] In order to remove surface contaminants and improve the adhesion and distribution of the coating, some surface treatment such as grinding or chemical cleaning of the surface of the tube or rod may be necessary.

[0064] The particles are sprayed at a rate sufficient to form a dense, impermeable or substantially impermeable coating layer. In various scenarios, the jet spray speed is 800 - 4000 feet per second (about 243.84 - 1219.20 meters per second). The Cr or Nb particles are deposited on the surface of the tube or rod at a rate sufficient to obtain the desired production rate at the commercial or research level.

[0065] A person skilled in the art can easily calculate the deposition rate based on the equipment used in this process, and can adjust the deposition rate by changing the components of the equipment that are the determining factors of the deposition rate. In certain aspects of this method, the deposition rate of the particles will be up to 1000 kg / hour. The acceptable deposition rate ranges from 1 to 100 kg / hour, and in various aspects it ranges from 10 to 100 kg / hour. However, higher or lower rates (such as 1.5 kg / hour) are also used and good results have been obtained.

[0066] This method can include a step of further annealing the coating after the coating is deposited. Annealing modifies the mechanical properties and microstructure of the coated tube. In annealing, the coating is heated in the range of 200 to 800 °C, preferably in the range of 350 to 550 °C.

[0067] The coated substrate may be treated by grinding, sanding, polishing, or any other known method to finish it to a smoother surface after the coating formation or annealing step.

[0068] Although the present invention has been described based on several examples, none of the examples are limiting in all respects but are illustrative. Therefore, the present invention can have many variations that a person skilled in the art with ordinary skills can derive from the description of this application in detailed embodiments.

[0069] All patents, patent applications, publications or other disclosure materials mentioned in this application are incorporated herein by reference in their entirety as if each reference was explicitly incorporated herein by reference. All references and any materials or portions thereof mentioned as being incorporated herein by reference are incorporated herein only to the extent that they do not conflict with the existing definitions, statements or other disclosure materials described in this application. Therefore, the disclosure matters described in this application supersede, to the extent necessary, the materials incorporated herein by reference that conflict with it, and the disclosure matters explicitly described in this application have the right of determination.

[0070] The present invention has been described with reference to various exemplary embodiments. The embodiments described in this application are to be understood as illustrating exemplary features at various levels of detail of the various embodiments of the disclosed invention. Thus, unless otherwise specified, to the extent possible, one or more features, elements, components, ingredients, structures, modules, and / or aspects in the disclosed embodiments can be combined, separated, substituted, and / or reconfigured with one or more other features, elements, components, ingredients, structures, modules, and / or aspects in the disclosed embodiments without departing from the scope of the present invention. Accordingly, those of ordinary skill in the art will understand that various substitutions, changes, or combinations are possible in any of the exemplary embodiments without departing from the scope of the present invention. Those skilled in the art will also, upon considering this application, recognize many equivalents to the various embodiments of the invention described herein, or will be able to confirm such equivalents simply by routine experimentation. Thus, the present invention is limited not by the description of the various embodiments, but by the claims.

Claims

1. a cladding material (30) selected from the group consisting of tungsten, tungsten alloys, molybdenum and molybdenum alloys; an oxidation-resistant coating (26) covering the coating material (30); a layer (28) of a material selected from the group consisting of niobium, molybdenum and tantalum between the oxidation resistant coating (26) and the cladding material (30); a neutron absorbing material (24); The neutron absorber (24) is Gd ２ O ３ and an oxide, and the cladding (30) is a tube that receives the neutron absorbing material (24). A control rod (22).

2. The control rod (22) of claim 1, wherein the oxidation resistant coating (26) comprises chromium.

3. G.D. ２ O ３ The control rod (22) of claim 1, wherein said oxide is diluted up to 50 times.

4. The oxide is Al ２ O ３ The control rod (22) of any of claims 1 to 3, wherein said Fe is selected from the group consisting of CaO and CaO.

5. The control rod (22) of any of claims 1 to 3, wherein the neutron absorbing material (24) is in the form of one of ceramic pellets (32) or metal slugs (32) contained within the tube (30).

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