GB2064201A

GB2064201A - Hydrogen Resistant Nuclear Fuel Container

Info

Publication number: GB2064201A
Application number: GB8035163A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1979-11-26
Filing date: 1980-10-31
Publication date: 1981-06-10
Also published as: ES8200501A1; FR2470430B1; IT8023707A0; ES493246A0; IT1131763B; SE444367B; SE8005404L; CH648430A5; DE3028057A1; BE884341A; FR2470430A1; DE3028057C2; JPS5676087A

Abstract

An improved nuclear fuel container of a composite construction having components providing therein a barrier system for resisting hydrogen diffusion, and a unique method of producing same. The composite nuclear fuel container disclosed includes a casing of zirconium or alloy thereof with a porous copper cladding overlying an oxidized surface portion of the zirconium or alloy thereof.

Description

SPECIFICATION Hydrogen Resistant Nuclear Fuel Container This invention relates broadly to an improvement in containers for nuclear fuel for service in nuclear fission reactors, and nuclear fuel elements comprising the improved containers having therein a body of nuclear fuel materials such as compounds of uranium, plutonium and thorium, and a method of producing such nuclear fuel containers and elements.

Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which can have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or sheath. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.

The container serves several purposes and two primary purposes are: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present, or both if both the coolant and the moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator, or both if both the coolant and the moderator are present. Common container materials are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium (columbium), certain magnesium alloys, and others. The failure of the container, i.e., a loss of the leak tightness, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.

Problems have been encountered in the manufacture and in the operation of nuclearfuel elements which employ certain metals and alloys as the container material due to mechanical or chemical reactions of these container materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent nuclear fuel containers since they have low neutron absorption cross sections and at temperature below about 7500C (about 3980C), are strong, ductile, extremely stable and nonreactive in the presence of demineralized water or steam which are commoniy used as reactor coolants and moderators.

However, fuel element performance has revealed a problem with the brittle splitting of the container due to the combined interactions between the nuclear fuel, the container and the fission products produced during nuclear fission reactions. It has been discovered that this undesirable performance is promoted by localized mechanical stresses due to fuel-container differential expansion (stresses in the container are localized at cracks in the nuclear fuel).

Corrosive fission products are released from the nuclear fuel and are present at the intersection of the fuel cracks with the container surface. Fission product are created in the nuclear fuel during the fission chain reaction during operation of a nuclear reactor. The localized stress is exaggerated by high friction between the fuel and the container.

Within the confines of a sealed fuel element, hydrogen gas can be generated by the siow reaction between the container, and the residual water inside the container may build up to levels which under certain conditions can result in localized hydriding of the container with concurrent local deterioration in the mechanical properties of the container. The container is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide over a wide range of temperatures.

The zirconium container of a nuclear fuel element is exposed to one or more of the gases listed above and fission products during irradiation in a nuclear reactor and this occurs in spite of the fact that these gases and fission product elements may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from the ambient atmosphere during manufacture of the container and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and other compositions used as nuclear fuel, release measurable quantities of the aforementioned gases and fission products upon heating, such as during fuel element manufacture and further release fission products during irradiation.Particulate refractory and ceramic compositions, such as uranium dioxide powder and other powders used as nuclear fuel, have been known to release even larger quantities of the aforementioned gases during irradiation.

These released gases are capable of redacting with the zirconium container containing the nuclear fuel.

Thus in light of the foregoing, it has been found desirable to minimize attack of the container from water, water vapor and other gases, especially hydrogen, reactive with the container from inside the fuel element throughout the time the fuel element is used in the operation of nuclear power plants. One such approach has been to find materials which will chemically react rapidly with the water, water vapor and other gases to aliminate these from the interior of the container, and such materials are called getters.

A number of other approaches to this problem have been enumerated in some detail in U. S.

Letters Patents Nos. 4,022,662, issued to Gordon and Cowan May 10, 1977; 4,029,549 issued to Gordon and Cowan June 19, 1977; and 4,045,288, issued to Armijo August 30, 1977, each assigned to the same assignee of this application for patent. The contents of the disclosure of said Letters Patent Nos. 4,022,662, 4,029,545 and 4,045,288 are accordingly incorporated by reference in this application for patent However, a recently proposed approach to the problem of fuel container or element failures attributable to deleterious interactions between container casings composed of zirconium or zirconium alloys and the nuclear fuel and/or the fission products thereof, has been to provide a copper metal cladding or barrier layer on the inside surface of such fuel casings.A layer of copper cladding or plating is generally considered to primarily function as a chemical barrier impeding destructive fission products, such as cadmium, cesium, iodine and the like, from contacting and attacking the zirconium or zirconium alloy of the fuel element casing or container.

Although a copper cladding or plating on the inner surface of zirconium or alloys thereof of a fuel casing or container has demonstrated promising performance in many aspects regarding the aforesaid destructive interactions, it appears that such a construction may be wanting or not sufficiently effective with respect to resisting hydrogen and in turn hydriding conditions likely to be encountered upon the occurrence of a defective fuel container or element. For example, steam entering into a defective or damaged fuel container or element would react with uranium dioxide fuel producing a wet hydrogen atmosphere within the casing of zirconium or alloy thereof.Studies have indicated that zirconium or alloys thereof will readily hydride and thus embrittle in such a hydrogen-containing environment when the arrival rate of steam or oxygen thereto falls below that value required to provide and maintain a protective oxide film over the exposed surface of the zirconium or its alloys.

This value has been determined to be as low as about 0.002 psig (0.1 mm Hg) partial pressure of steam. Copper cladding or plating of the zirconium or alloy thereof surface of the casing or container appears to restrict or impede the steam or oxygen arrival to the underlying zirconium or its alloy, whereby the zirconium or alloy thereof is highly vulnerable to hydriding and in turn embrittlement since hydrogen can preferentially diffuse through or across a metal cladding or plating such as copper.For instance, the diffusion coefficients for hydrogen and oxygen in copper (Dg2=1 .5x 10-5 VS D2u=2.95x 10-10 cm2/s at 4000C) indicate that the occurence of a significantly accelerated rate of hydriding of copper covered zirconium or its alloy with respect to the rate of oxidation is most probabie.

Embrittlement of the fuel casings attributable to hydriding of the zirconium or alloy thereof, greatly accelerates the deterioration and ultimate total failure of defective or damaged fuel elements.

This invention comprises a composite nuclear fuel container for nuclear fission reactor service, and method of producing same, comprising a casing or fuel sheath of zirconium or its alloy having a gas porous cladding of deposited copper superimposed over the inside surface of the zirconium or alloy of the casing or sheath, and a layer of oxide of the zirconium or alloy thereof formed on the said inside surface of the casing or sheath and intermediate thereof and the porous deposited copper cladding.

The term zirconium throughout the balance of the application is used and understood to include alloys of zirconium as well as the pure zirconium metal itself.

The figure comprises a cross-sectional view of one embodiment of a container for nuclear fuel according to this invention.

This invention is primarily based upon and comprises a composite construction or combination of material components including a porous copper layer or cladding which is permeable to steam as well as hydrogen and is superimposed overthe surface of zirconium or its alloy constituting a substrate and the primary structure of a nuclear fuel casing or container. The capability of steam to readily penetrate and pass through the porous copper cladding or layer enables for the in situ formation of an oxide of zirconium on the underlying surface of the zirconium structure subsequent to the application of the overlying porous copper cladding or layer thereon.This sequence of first applying the copper to the surface of the metallic zirconium substrate provides for greater versatility and effectiveness in performing this step and enable the use of the most expedient application means therefor, such as electrolytic deposition of the copper on the zirconium or its alloys.

Moreover, in addition to the invention enabling the formation of an oxide phase of zirconium on the surface underlying the previously applied copper cladding for the purpose of initially producing a protective oxide barrier intermediate the copper and zirconium to preclude any interdiffusion of the zirconium and copper and the adverse effects thereof, the invention further provides for the thereafter continuous access for steam to pass through the overlying porous copper cladding to the underlying surface of the zirconium or oxide thereof.

Thus, upon the occurrence of a defect or damage in a nuclear fuel element of the construction of this invention whereby the fuel container of a copper clad zirconium casing is accidently exposed to the water or steam coolant and the resulting wet hydrogen environment, steam can also penetrate the copper cladding to the underlying surface and the ingressing steam will thereupon continuously replenish and maintain the intermediate oxide phase on the zirconium providing a protective oxide barrier, thereby preventing access of the hydrogen to the zirconium and any damaging embrittlement thereof due to hydriding.

Referring to the drawing, there is shown a nuclear fuel element in cross-section including a container constructed according to this invention.

The container 10 comprises a casing or sheath 12 of zirconium, or an alloy of zirconium, with a layer of oxide of the zirconium or alloy thereof 14 on its inside surface, and a porous cladding 1 6 of copper that is permeable to steam.

A body of nuclear fuel 1 8, for example pellets, of an oxide of uranium, plutonium and/or thorium is retained within the container 10 to isolate the fuel from the nuclear reactor's coolant medium.

The nuclearfuel containers 10 of a preferred embodiment of this invention can be produced by depositing copper cladding 1 6 in a thickness of less than 10 microns, and preferably of about 0.5 to about 5 microns, whereby it is highly permeable to steam, directly on the inside surface of a nuclear fuel casing or sheath of zirconium or its alloy. The porous copper cladding 16 can be applied to the zirconium substrate by any apt method or process, including electrolytic or electroless deposition techniques such as described in U. S. Letters Patent Nos. 4,017,368, 4,137,131; and 4,093,756.

Following the depositing of the porous copper cladding 1 6 overlying the surface of the zirconium casing 12, the copper clad zirconium unit is then subjected to de-aerated steam at temperatures of about 3000C to about 4000C, for example, autoclaved in 4000C steam at 10 psi for 24 hours. The dry steam penetrates through the overlying porous copper layer and reacts with the underlying surface of zirconium or its alloy, oxidizing the same and thereby forming a body or layer of an oxide of zirconium intermediate the zirconium casing substrate and the overlying cladding of copper.

Test specimens of fuel casings composed of zirconium alloy tubing (Zircaloy-2 alloy--see U.S.

Pat. No. 4,164,420) were provided with copper claddings plated on the inside surfaces thereof in several different thickness, namely, about 0.5-1 micron; about 1-2 microns; and about 10 microns. The clad specimens were autoclaved with de-aerated steam at 4000C for 24 hours.

Like samples of the zirconium alloy casings with copper cladding of each of the specified thicknesses were subjected to a hydrogen pickup test consisting of subjecting the samples to the potentially hydriding conditions of exposure to one atmosphere of wet hydrogen for a period of 72 hours or 300 hours.

The sample with a 1-2 micron copper cladding after a 72 hour exposure to wet hydrogen showed a hydrogen pickup in the zirconium alloy of about 1 50 ppm whereas the sample with a 10 micron cladding of copper showed a hydrogen pickup about 1000 ppm with the same time of exposure.

The samples having had 300 hours of exposure to one atmosphere of hydrogen were subjected to a neutrographic evaluation and the samples with either 0.5-1 micron or 1-2 micron thick copper cladding all had hydrogen contents within the zirconium alloy less than the amount detectable by the neutrographic technique, namely about 500--800 ppm. The reference samples having a 10 micron thick copper cladding over the zirconium alloy showed hydrogen contents in the alloy of several thousand ppm or more. These results demonstrate the pronounced effects and significance of the porosity of the copper cladding the penetration of steam therethrough upon hydrogen permeation of the zirconium alloy substrate in accordance with the invention.

Claims

1. A container for nuclear fuel for service in nuclear fission reactors, comprising: a) a casing of zirconium or a zirconium alloy; b) a porous cladding of deposited copper superimposed over the inside surface of said metal casing; and, c) a layer of oxide of zirconium or a zirconium alloy on the inside surface of said metal casing and intermediate said inside surface and the porous deposited cladding of copper superimposed over said inside surface.

2. A container for nuclear fuel as claimed in claim 1, wherein the porous cladding of deposited copper superimposed over the inside surface of the metal casing is up to 5 microns thick.

3. A container for nuclear fuel as claimed in claim 1, wherein the porous cladding of deposited copper superimposed over the inside surface of the metal casing is 0.5 to 5 microns thick.

4. A container for nuclear fuel as claimed in any one of claims 1 to 3, wherein said container includes a body of nuclear fuel material selected from uranium compounds, plutonium compounds, thorium compounds, and mixtures thereof.

5. A container for nuclear fuel for service in nuclear fission reactors, comprising: a) a casing of zirconium or a zirconium alloy; b) a gas porous cladding of deposited copper metal superimposed over the inside surface of said metal casing in a layer 0.5 to 5 microns thick; and, c) a layer of oxide of zirconium or a zirconium alloy formed in situ on the inside surface of said metal casing and intermediate said inside surface of the metal casing and the gas porous deposited cladding of copper superimposed over said inside surface.

6. A container for nuclear fuel as claimed in claim 5, wherein the layer of oxide of the metal of the casing is 0.5 to about 1 micron thick.

7. A container for nuclear fuel for service in nuclear fission reactor, comprising: a) a casing of zirconium or a zirconium alloy; b) a steam porous cladding of deposited copper metal superimposed over the inside surface of said metal casing in a layer 2 to 5 microns thick; c) a layer of oxide of zirconium or a zirconium alloy about 0.5 to about 1 micron thick formed in situ on the inside surface of said metal casing and intermediate said inside surface of the metal casing and the steam porous deposited cladding of copper superimposed over said inside surface.

8. A method of producing a container for nuclear fuel for service in nuclear fission reactors, comprising the steps of: a) providing a casing of zirconium or a zirconium alloy; b) depositing a porous cladding of copper superimposed over the inside surface of said metal casing; and, c) thereafter oxidizing the inside surface of said metal casing to thereby provide a layer of oxide zirconium or a zirconium alloy intermediate said inside surface of the metal casing and the porous cladding superimposed over the said inside surface of the metal casing.

9. A method of producing a container for nuclear fuel as claimed in claim 8, wherein the inside surface of the metal casing is oxidized with steam.

10. A method of producing a container for nuclear fuel as claimed in claim 8, wherein the inside surface of the metal casing is oxidized with de-aerated steam.

11. A method of producing a container for nuclear fuel as claimed in any one of claims 8 to 10, wherein the inside surface of the metal casing is oxidized with steam at a temperature of 3000 to 4000 C.

12. A method of producing a container for nuclear fuel as claimed in any one of claims 8 to 11, wherein the porous cladding of copper is deposited superimposed over the inside surface of the metal casing in a thickness of up to 5 microns.

13. A method of producing a container for nuclear fuel as claimed in any one of claims 8 to 11, wherein the porous cladding of copper is deposited superimposed over the inside surface of the metal casing in a thickness of 0.5 to 5 microns.

14. A method of producing a container for nucelar fuel as claimed in any one of claims 8 to 13, wherein the gas porous cladding of copper is deposited superimposed over the inside surface of said metal casing in a thickness of 2 to 5 microns.

1 5. A method of producing a container for nuclear fuel as claimed in any one of claims 8 to 14, wherein the steam for oxidizing the inside surface of the metal casing is applied through the gas porous cladding of deposited copper metal superimposed over the inside surface of the metal casing.

1 6. A method of producing a container for nuclear fuel as claimed in any one of claims 8 to 15, wherein the inside surface of the metal casing is oxidized to provide an oxide layer of 0.5 to 1 micron thick.

1 7. A method of producing a container for nuclear fuel as claimed in claim 8, substantially as hereinbefore described.

18. A container when produced by a method as claimed in any one of claims 8 to 17.

1 9. A container for nuclear fuel as claimed in claim 1, substantially as hereinbefore described.