WO2001003144A2

WO2001003144A2 - Drying nuclear fuel materials

Info

Publication number: WO2001003144A2
Application number: PCT/US2000/018214
Authority: WO
Inventors: Robert W. Mouk; Albert E. Abel; Clyde Frank
Original assignee: Commodore Applied Technologies, Inc.
Priority date: 1999-07-01
Filing date: 2000-06-29
Publication date: 2001-01-11
Also published as: AU6064900A; WO2001003144A3

Abstract

Methods are provided for the use of ammonia as a drying agent to reduce water and/or moisture content in contaminated materials including nuclear fuel materials, such as nuclear fuel assemblies and rods, nuclear fuel containers, pool sludge contaminated by nuclear fuel components, spent nuclear fuel and combinations thereof.

Description

DRYING NUCLEAR FUEL MATERIALS

TECHNICAL FIELD

This invention relates generally to the drying of contaminated materials and more particularly to reducing water and/or moisture content in nuclear fuel materials by contact with ammonia.

BACKGROUND OF INVENTION

Nuclear fuel assemblies, containing nuclear fuel rods, consolidate nuclear fissionable material into a geometric and physical form that is suitable for use in a nuclear reactor. The fuel rods are designed so that the fissionable material is encapsulated within a sheath material for permanent sealing. However, during extended operation of a nuclear reactor, the cladding of fuel rods may develop leaks thereby allowing water from the reactor system to invade the interior of the rods. Water penetrating the nuclear rods can be problematic during use in the reactor, long term storage and/or in fuel reprocessing.

Fuel rods are normally removed from nuclear reactors after economically optimized periods of use and before all the fissionable materials has been completely consumed. The remaining content of valuable unfissioned material can be reclaimed by chemical reprocessing. However, before the reprocessing commences, a period of storage must occur during which radioactive short-lived isotopes decay. During the decay period, the spent fuel rods are usually stored in "wet" cooling fuel pools on nuclear utility sites.

Spent fuel storage facilities at nuclear reactor sites were originally designed with the expectation that spent fuel rods would be transported off-site for reprocessing or permanent disposal within a short period of time. However, because reprocessing in the nuclear industry does not currently exist in the United States and permanent disposal sites have not opened, there is a need to store spent fuel rods on site in "wet" fuel pools for longer periods than initially planned. This long term storage has presented additional problems relating to defects in the cladding, particularly non-Zircaloy cladding owned by the United States government, that causes a breaching of the tube with the concomitant inclusion of water into the interior of the fuel rods. Further breaching of the cladding only exacerbates the storage problem by producing additional radioactive contaminated materials, generally in the form of fuel pool sludge.

The Department of Energy (DOE) is under increased pressure to open a permanent repository for commercial fuel and to stabilize fuels owned by the agency. To date, spent nuclear fuel rods remain in "wet" storage while the DOE completes work on a permanent waste repository in moisture free geological formations. If spent fuel assemblies are to be disposed of as radioactive waste and stored in these deep geological formations they first must be conditioned in such a way to substantially remove any water and/or moisture from within the fuel assembly before final containment in a waste package. Removal of moisture or water is necessary to reduce possible adverse chemical reactions and possible corrosion of waste packaging during interment in long term storage. Consequently, a drying stage is important prior to final containerization of the fuel rods. Several methods have been implemented to remove water from fuel rods including drip drying, cold vacuum drying and hot vacuum drying. The first method is used by private industry and usually cannot be applied to the damaged fuel assemblies. Vacuum drying is expensive and time consuming. Moreover, hot vacuum drying can also cause the cladding to expand thereby producing additional damage to the casing with the concomitant release of fission products and/or oxidation of the fissionable material. Accordingly, an improved method is needed which will substantially reduce and/or remove water and/or moisture from nuclear fuel materials without the disadvantages of damaging fuel rod casings, driving off fission products or oxidizing uranium fuel.

SUMMARY OF THE INVENTION

For purposes of this invention, the terms and expressions below, appearing in the specification and claims, are intended to have the following meanings:

"Water" as used herein includes water in the liquid and/or vapor phase.

"Fuel rod" or fuel element as used herein means a fuel container that can be any geometric form including but not limited to a rod, plate, tube, microsphere and a combination thereof. "Wet liquid ammonia" as used herein means ammonia that contains water in an amount greater than the amount contained before contacting the nuclear fuel materials.

"Fuel assembly" as used herein means a collections of fuel rods in a designed array which is used by commercial electrical utilities, the Federal Government, Universities and other entities.

An object of the present invention is to provide an effective and more efficient method for drying nuclear fuel assemblies in a temperature range that reduces temperature related safety hazards.

Another object of the present invention is to reduce the likelihood of pressure buildup in sealed containers due to radiolytic decomposition of water by reducing water and/or moisture content in spent nuclear fuel rods. A further object of the present invention is to provide a method for reducing the moisture content in newly fabricated fuel rods to adhere to federally regulated and required specifications before final sealing.

A still further object of the present invention is to provide a method to remove water from spent nuclear fuel in a reaction system that is totally sealed thereby precluding inadvertent discharges to the atmosphere. Yet another object of the present invention provides for a method of drying fuel assemblies, spent nuclear fuel and/or wet and nuclear contaminated materials using equipment that is commercially available via "off-the-shelf" purchase. These and further objectives are accomplished by the methods of the present invention which comprise contacting nuclear fuel materials containing water with a sufficient amount of ammonia to substantially reduce the water content within the nuclear fuel materials. The nuclear fuel materials may include but are not limited to nuclear fuel assemblies and rods, containers and pool sludge contaminated by nuclear fuel components, spent nuclear fuel and combinations thereof.

This process may be performed using liquid and/or gas ammonia, including anhydrous ammonia and solutions of ammonia comprising small amounts of water. Preferably, anhydrous liquid ammonia having less than 1% water content is used.

Contact time of the ammonia with the nuclear fuel materials may range from seconds to several days depending on the amount of included water. The amount of additional water within the rod or assembly may be determined by any means known in the art including weighing the fuel assembly or fuel rod to determine the increase amount of weight over that of the original specifications. In one embodiment of the present invention, water and/or moisture is reduced in nuclear fuel assemblies by a method comprising: a) introducing the nuclear fuel assembly into a closed vessel; b) introducing a sufficient amount of anhydrous ammonia into the closed vessel to contact the nuclear fuel assembly; and c) maintaining the nuclear fuel assembly in the closed vessel for a sufficient time to substantially reduce the water and/or moisture content therein.

This embodiment may further comprise venting the vessel under conditions suitable for evaporation of the ammonia. In another embodiment of the invention, the method comprises: a) introducing at least one nuclear fuel rod into a closed vessel, the nuclear fuel rod having a casing with at least one opening therein; b) introducing anhydrous liquid ammonia into the closed vessel in a sufficient amount to immerse the fuel rods therein; c) elevating pressure within the closed vessel to force the anhydrous liquid ammonia into the interior of the fuel rod through the at least one opening; d) maintaining the fuel rod in the closed vessel under elevated pressure and temperature conditions for a sufficient time for the anhydrous liquid ammonia to extract the water from the interior of the fuel rods; e) withdrawing the liquid ammonia and extracted water from the closed vessel while maintaining elevated temperature and pressure; and f) venting the vessel to atmospheric conditions thereby allowing residual ammonia and extracted water to evaporate.

In the practice of this embodiment, multiple rods are maintained in the closed vessel at temperatures ranging about -30°C to about 100°C, and more preferably, above the freezing temperature of water. Pressure within the vessel may be increased and preferably not exceeding about 250 psig to maintain the anhydrous ammonia in a substantially liquefied state. It is believed that the increased pressure may facilitate entry of the ammonia through perforations and/or openings to access the nuclear fuel. It may be advantageous to fluctuate pressure in a cycle mode, such as increasing the pressure to force ammonia into the nuclear fuel (inhale) and then decreasing the pressure to allow the ammonia laden with water to escape therefrom (exhale) . The duration of exposure and treatment may range from hours to days, depending upon the amount of water that must be extracted from the nuclear fuel material.

Optionally, the methods disclosed herein further contemplate practicing the invention wherein ammonia recovered from the vessel is recycled for recirculation and reuse. The recovered ammonia may be reprocessed by conventional methods known in the art which can include drying and recompression, or distillation methods. Accordingly, recovered wet ammonia can be processed to reduce water content to produce anhydrous ammonia for the purpose of reintroduction into the system. This object may be accomplished by the following method comprising the steps of: a) introducing at least one nuclear fuel rod into a closed vessel wherein the nuclear fuel rod comprises a casing having nuclear fuel contained therein, the casing having at least one perforation; b) introducing anhydrous liquid ammonia into the closed vessel in a sufficient amount to immerse the nuclear fuel rod therein; c) elevating pressure within the closed vessel to force the anhydrous liquid ammonia into the interior of the fuel rod through perforation (s) ; d) maintaining the fuel rod in the closed vessel under elevated pressure and temperature for a sufficient time for the anhydrous liquid ammonia to extract the water from the interior of the fuel rods thereby generating a wet liquid ammonia; e) draining the wet liquid ammonia from the closed vessel while maintaining the elevated temperature and pressure within the vessel; f) dewatering the wet liquid ammonia thereby providing an anhydrous liquid ammonia for reentry into closed vessel; and g) venting the vessel to atmospheric conditions thereby allowing residual wet liquid ammonia to evaporate.

Once the wet liquid ammonia is drained and dewatered it may be recirculated into the closed vessel for further extraction of water from within the interior of the fuel rods.

Dissolved fission products may also be separated from the dewatered ammonia. Dewatering of the wet liquid ammonia can be accomplished by any method known in the art including distillation, absorption, adsorption and preferably, passing the wet liquid ammonia through quicklime (CaO) . More preferably, removing water and/or moisture from ammonia can be accomplished by the formation of metal hydroxides using solvated electrons in solution that react with the water and/or ammonia. The formation of solvated electron solutions by the addition of an alkali or alkaline earth metal, such as sodium, in anhydrous liquid ammonia is well known and shown in the following formula:

dissolve in NH₃

Na ° ^■* Na⁺ (solvated) + e^~ (solvated)

It has long been known that a solvated electron solution forms NaOH and H₂ when brought in contact with water. But, the use of ammonia as a drying agent in conjunction with solvated electrons as a mechanism for the removal of water and/or moisture from ammonia to form a continuous operating system, has not heretofore been taught or referenced in the literature.

It is believed that the use of solvated electrons as a drying agent to remove water from ammonia reacts according to the following formulas:

2Na+ + 2e^" + 2H₂0 → 2NaOH + H₂

2Na⁺ + 2e^' + 2NH₃ → 2NaNH₂ + H₂

NaNH₂ + H₂0 → NaOH + NH₃

Water penetration into the spent fuel rods will likely occur via small pin hole openings. Although ammonia will easily enter these openings, the rate of exchange of ammonia from within the fuel rod to the ammonia surrounding the rod will be restricted by the size and number of openings. By introducing solvated electrons as a mechanism for removing moisture from the wet liquid ammonia, it is believed that moisture will be drawn more rapidly into the anhydrous ammonia surrounding the rod and that solvated electrons, by virtue of their extremely small size will be drawn into the rod casing. During "wet" storage of nuclear fuels that contain uranium metals and other fissionable metals, corrosion of the uranium metals can occur upon prolonged exposure to moisture. This corrosion can be extensive causing further breaching of the cladding which may result in buildup of sediments in the fuel pool, such as metal hydrides. Also, the fuel rods themselves have been found to contain significant levels of metal hydrides, such as uranium hydride. Unless these hydrides are destroyed, they may pose a risk of a pyrophoric event if the nuclear fuel is exposed to air or another oxidizer. As such, another object of the present invention is to reduce metal hydrides from the nuclear fuel assemblies and/or sediment from the fuel pool comprising the steps of: a) introducing the nuclear fuel assembly or nuclear fuel pool sludge into a closed vessel; b) introducing anhydrous ammonia into the closed vessel in a sufficient amount to immerse the fuel assembly or the sludge therein; c) maintaining the fuel assembly or sludge in the closed vessel for a sufficient time for the anhydrous ammonia to react with the metal hydrides therein; and d) venting the vessel to atmospheric conditions thereby evaporating residual ammonia.

It is believed that the chemical process for destroying and/or reducing of the metal hydrides is as follows:

2MH + 2NH₃ → 2MNH₂ + H₂

where M is the metal of interest, MH is the metal hydride and MNH₂ is a metal amide formed as an end product of the reaction.

Optionally this reaction may be enhanced by the introduction of an oxidizing agent or an active metal that produces solvated electrons, such as alkali and alkaline earth metals. This process may be performed using both liquid and/or gas ammonia, and preferably anhydrous ammonia. A sufficient time for treatment will be determined by the quantity of metal hydrides contained within the rod or sludge. As such, timing could range from a few moments to several days.

Concerns exist regarding the possibility of corrosion to the fuel cladding by microbial activity. It is well known that microbes, although capable of adapting to even the most extreme conditions are readily destroyed by abrupt, severe changes in their environment including loss of water. The processes of the present invention can remove a sufficient amount of water to inhibit and/or eliminate future microbial growth. This is another significant benefit to the ammonia drying processes of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings:

FIG. 1 is a diagrammatic view of an apparatus used in the practice of the disclosed methods in accordance with this invention; and

FIG. 2 is a flow diagram showing the process for ammonia drying of contaminated matrices and/or materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The processes of the present invention are directed towards reducing water and/or moisture from nuclear fuel materials including nuclear fuel assemblies containing spent nuclear fuel.

Methods of the present invention can be demonstrated by reference to Figure 1 which shows a closable reaction vessel 12 constructed to withstand increased pressures. Within the closable vessel 12 , at least one nuclear fuel rod 14 is placed either as a single unit or in a bundle containing multiple rods . When multiple rods are introduced into the reaction vessel, the rods may be spaced sufficiently from each other to provide access to openings or perforations for introducing ammonia into the exterior of the fuel rod.

The nuclear fuel rods placed within the closable vessel are not completely sealed containers due to several reasons which include: breaching of the cladding tube due to defects and/or corrosion during operation of a reactor; the head has been removed for reprocessing of unspent nuclear fuel; and/or the rod is newly fabricated and has yet to be hermetically sealed. Rod failure may occur under the extreme conditions imposed on the fuel cladding in the reactor core and this can result in small openings across the cladding wall sufficient in size to permit egress of fission products and/or fuel, and/or ingress of water coolant. For purposes of this invention, any crack and/or pinhole may provide access into the interior of the fuel rod for removing and/or driving off any water therein and/or destroying unwanted metal hydrides. In some instances, there will not be sufficient openings through the exterior of the fuel rod for performing the process in a timely fashion. In such situations, it may be advantageous to provide additional openings into the interior of the rods by removing the head of the fuel rod or by drilling additional holes through the casing.

In practicing the methods of this invention, the fuel rod 14 is placed in the closed vessel 12 and anhydrous liquid ammonia is transferred to the reactor vessel by opening the valve 18 between the ammonia storage tank 20 and the vessel 12. The anhydrous liquid ammonia is pumped into the vessel in a sufficient amount to submerge the rod in the anhydrous liquid ammonia and/or in an amount that exceeds the amount of water to be removed from the interior of the fuel rod. Preferably, the amount of ammonia may be at least four times greater than that of the water to be removed from the fuel rod. The amount of additional water within the rod or assembly may be determined by any means known in the art including weighing the fuel assembly or fuel rod to determine the increase amount of weight over that of the original specifications .

Anhydrous liquid ammonia and water are iscible thereby forming ammonium hydroxide according to the following formula:

NH₃ + H₂0 <^■► NH₄OH For drying purposes, the ammonia acts as a simple chemical transport mechanism where moisture is trapped in ammonia and transferred to a separation area where the ammonia is removed from the ammonium hydroxide according to the following equations: NH₄OH -^► H₂0 + NH₃ T

Anhydrous liquid ammonia is extremely well suited to this task because of its fluidity. It can easily penetrate the small openings in fuel rod cladding thereby quickly gaining access to the interior of the rods. Additionally, when introduced in a pressurized system, the ammonia will generate driving pressure to force itself back out of the rod as the pressure in the vessel is reduced.

It has been found that the present method for reducing water or moisture in a nuclear fuel assembly is most effective when the temperature ranges from -30°C to about 100°C, and more preferably at a temperature that maintains water in a substantially non-frozen state. Pressure within the vessel may be elevated and preferably not exceeding about 250 psi. Preferably, the temperature and/or pressure in the reaction vessel is controlled to maintain the anhydrous ammonia in a liquefied state.

The rods are retained in the reaction vessel 12 for a sufficient time to substantially remove water and/or moisture from the interior of the rod and/or chemically destroy unwanted metal hydrides within the fuel assembly. Generally, optimum removal of water and/or metal hydrides is achieved in a time period ranging from about 12 to about 72 hours. If the fuel assemblies are intact, the processing time may be greatly reduced.

After the water removal process is complete and/or metal hydrides destroyed and while maintaining the elevated temperature and pressure within the vessel, the liquid ammonia containing any water extracted from within the fuel rod may be drained from the vessel at valve 22 for storage in the ammonia storage tank 20, recirculation and/or drying. Any remaining liquid ammonia may be vented from the vessel at valve 26 to allow residual ammonia to evaporate. Evaporation may be facilitated by applying a vacuum to vessel 24. The vented ammonia may be contaminated with fission products and as such a suitable scrubber 28 may be implemented for removing fission products thereby preventing escape of same to surroundings.

A further variant of the drying method according to the present invention comprises dewatering of "wet liquid ammonia" removed from the reaction vessel 12 after the drying process is progressing through a fuel rod drying cycle and/or is completed. One significant advantage of using anhydrous ammonia lies in the fact that separation of the water from the ammonia is a well-defined, existing technology. Simple distillation of the "wet ammonia" produces a substantially dry or anhydrous ammonia which is suitable for reuse as a drying agent. Any desiccant and/or drying agent that has the ability to extract and/or absorb water from ammonia may be used in the present invention. Preferably, dewatering of "wet ammonia" is accomplished by contacting the same with quicklime (CaO) , and more preferably, using an alkali and/or alkaline earth metal that generates solvated electrons in liquid ammonia.

The dewatering process may be carried out in a separate vessel. As such, the wet liquid ammonia may be drained from the vessel at valve 22 and subsequently directed into a drying vessel 24 for contact with a desiccant or drying agent. The moisture laden ammonia may be circulated through a bed of quicklime (CaO) for removal of water and then may be recirculated into the vessel to extract additional moisture. In this fashion the ammonia can be dried and the system equilibrium remains in favor of the moisture removal from the interior of the fuel rod.

Alternatively, and preferably, in another embodiment of the present invention, the drying of wet liquid ammonia is performed by treating it with an alkali and/or alkaline earth metal. It has been found that combining the ammonia with an active alkali metal constitutes a powerful drying agent capable of dewatering the wet liquid ammonia and thereby allowing the same to be recirculated into the vessel for further drying of the fuel rods. The method comprises the formation of a drying agent which includes the dissolution of an active metal, such as sodium, in the wet liquid ammonia to generate "solvated electrons". The active metal dissolved in the liquid ammonia for generating solvated electrons may be selected from one or a combination of metals found in Groups IA and IIA of the Periodic Table. Largely for reasons of availability and economy, it is most preferred that the active metal be selected from the group consisting of Li, Na, K, Ca, and mixtures thereof. In most cases, the use of sodium, which is widely available and inexpensive, will prove to be satisfactory.

When sodium and other alkali or alkaline earth metals dissolve in an ammoniacal liquid, solvated electrons are chemically generated. The sodium becomes a cation by losing a valence electron as illustrated in the following equation:

dissolve in NH₃

Na ° ^■* Na⁺ (solvated) + e^~ (solvated)

The ammonia molecules of the solvent surround and stabilize the electrons and permit them access to the entire liquid volume thereby allowing the electrons to act as a reducing agent, to liberate hydrogen atoms from ammonia and water molecules.

2e^" + 2H₂0 → 20H^" + H₂

2e^'+ 2NH₃ → 2NH +H₂

Also the solvated electrons need not react directly with a water molecule.

NH₂ ^_ + H₂0 → OH^" +NH₃

The solvated electrons may be created in situ within the reactive vessel 12 by introducing sodium directly into the reaction vessel through valve 18 from an auxiliary container 16. The sodium is introduced at a controlled rate to facilitate dissolution of sodium in the anhydrous liquid ammonia under stirring conditions. The amount of sodium should be in a sufficient amount to generate a stoichiometric amount of cations and solvated electrons to react and/or combine with any excess water contained in the wet liquid ammonia. Alternatively, the active metal may be predissolved in liquid ammonia in an auxiliary chamber 16 and then introduced into the reaction vessel 12. The course of the reaction involving solvated electrons can be followed by monitoring the blue color or conductivity of the reaction mixture which is characteristic of solutions of nitrogenous base and active metal. When the blue color disappears, it is a signal that the solvated electrons have been consumed, and more active metal or solution containing solvated electrons may be added, if necessary.

During the process, anhydrous liquid ammonia can be slowly vented from the reaction vessel to maintain a working temperature within the reaction vessel especially if the need arises to counteract any heat generated.

The vented ammonia may be scrubbed in scrubber 28 which will remove any hydrogen gas that may form during the reaction and the cleaned anhydrous ammonia can be stored or reused.

In another embodiment the wet liquid ammonia may be drained from the reaction vessel 12 at valve 22 for introduction into drying chamber 24 wherein the active metal may be added. The dewatering process is kept separate from the drying of the fuel rod process, and therefore, the hydroxide precipitate can be easily removed from the dewatering chamber without disturbing the drying process within the reaction vessel 12.

This invention has been explained using nuclear fuel assemblies and rods as representative examples, however it should be recognized that the disclosed methods may also be utilizes with many matrices and materials containing excessive or unwanted quantities of moisture. These matrices and materials may include soils, sludges, sediments, etc contaminated with hazardous, toxic and/or radioactive materials such as nuclear fuel materials including radioactive isotopes. As shown in Figure 2 this invention may be practiced by treating contaminated matrices and materials with ammonia to remove unwanted moisture and/or water. The methods of this invention can be carried out batch-wise in a sealed pressure vessel equipped with heating/cooling, stirring, etc. It is also possible to carry out the process on a continuous basis. In such a system, any formed precipitate due to dewatering of wet liquid ammonia is continuously removed while the reaction vessel is continuously charge with anhydrous liquid ammonia. The invention will be more clearly perceived and better understood from the following example.

EXAMPLE 1

A porous oxide material, that being magnesium oxide (MgO) , was investigated as surrogate spent nuclear fuel. The porous oxide is being used as a representative of metallic fuel corrosion products, waterlogged U0₂ pellet rods, and many of the sludges found in spent nuclear fuel containers and storage pools.

Water uptake was measured on a MgO rod having the dimensions of 2 cm in diameter and 5 cm in length for a volume of approximately 16 cm. Upon immersion in water, vigorous bubbling occurred over the entire specimen surface as air inside the pores was displaced by water. Approximately 4 milliliters (4 grams) of water was adsorbed by the MgO specimen.

The waterlogged specimen was then placed in a glass pressure vessel with approximately 500 milliliters of liquid ammonia. Because the pressure vessel was rated for operation at 60 psig, experiments could be performed up to 5°C. In actual practice, the relief valve was set to release at 58 psig, corresponding to 3°C to create an additional safety margin.

The specimen remained in the ammonia soak for approximately one hour and then was removed to air dry for approximately two hours. During this time it was expected that most of the ammonia would evaporate without significant evaporative loss of moisture from the specimen. The specimen was then placed in a vessel and heated to drive off any remaining water to be collected on a desiccant. The desiccant was a 3-Angstrom molecular sieve (potassium zeolite) that was advertised to reject ammonia and accept the slightly smaller water molecules. The amount of remaining water in the specimen was then determined by weighing the weight gain on the 3- Angstrom molecular sieve. This experiment produced an apparent water extraction efficiency of approximately 96 percent.

EXAMPLE 2

To reconfirm the reliability and efficiency of the water extraction methods of the present invention, another MgO specimen was prepared according to the method of Example 1. The water and ammonia soaked specimen was prepared and weighed to provide a "wet weight". It was then heated to 105°C in a flow of helium gas for four hours. The helium carrier gas was bubbled through boric acid solution to capture the ammonia and water vapor. Finally, the amount of ammonia in this solution was established by titrating sulfuric acid to the ammonia endpoint using EPA Method 350.2. The post-heating specimen was reweighed to provide a "dry weight", and the difference between the "wet weight" and "dry weight" provided a numerical value for the weight of absorbed liquid. The weight of ammonia determined by the titration method was subtracted from the "wet weight" to establish the amount of residual water after the ammonia soak. The net result of this determination was an extraction efficiency of 90%, effectively confirming the drying results of Example 1. These results also indicate that an ammonia molecule, which is nearly the same size as a water molecule, can penetrate and remove water from extremely small spaces in spent nuclear fuel rods.

EXAMPLE 3

A spent nuclear fuel rod containing nuclear fuel in a cladding tube, is removed from a "wet storage" pool on the cite of a nuclear power plant and weighed to determine the approximate amount of excess water contained within the clad tubing. Before introduction into a closable and pressurizable vessel, the head of the fuel rod is removed and/or additional openings drilled into the cladding of the tube. The fuel rod is placed within the closable vessel and sealed therein. Anhydrous liquid ammonia is transferred into the closed vessel in a sufficient amount to submerge the fuel rod in the liquid ammonia and preferably at least four times the stoichiometric amount of water contained within the rod. The temperature within the vessel is maintained at about 5°C to insure that any water contained within the fuel rod remains in a substantially non-frozen state for easier removal. The pressure within the vessel is periodically increased and decreased ranging from about 58 psig to about 40 psig, respectively, to force the anhydrous liquid ammonia into the rod and then subsequently allow the wet ammonia to escape from within the rod.

The fuel rod remains in the pressurized vessel for a sufficient time to transfer the water from within the rod to the ammonia. The vessel is drained of wet ammonia and the pressure in the vessel is reduced thereby allowing any ammonia remaining within the rod to evaporate. The dried fuel rod may be taken to ultimate storage or for further reprocessing.

Claims

We claim:

1. A method for reducing water content in a nuclear fuel assembly, the method characterized by the steps comprising: a) introducing the nuclear fuel assembly into a closable vessel; b) introducing a sufficient amount of ammonia into the closable vessel to contact the nuclear fuel assembly and to substantially reduce the water content within the nuclear fuel assembly.

2. The method according to claim 1 characterized in that the nuclear fuel assembly comprises at least one nuclear fuel rod, the nuclear fuel rod having at least one opening.

3. The method according to claim 1 characterized in that the ammonia is anhydrous and the amount of ammonia is at least four times the amount of water within the nuclear fuel assembly.

4. The method according to claim 2 characterized in that the nuclear fuel rod is maintained in the closed vessel under elevated pressure and temperature.

5. The method according to claim 3 characterized in that the anhydrous ammonia extracts water from the nuclear fuel assembly thereby generating a wet liquid ammonia.

6. The method according to claim 5 characterized in that the wet liquid ammonia is dewatered by contacting with a drying agent, said drying agent selected from the group consisting of CaO and an alkali metal and alkaline-earth metal.

7. The method according to claim 4 characterized in that the pressure is cycled up and down repeatedly.

8. A method for reducing water content in a nuclear fuel material characterized by the step which comprises contacting the nuclear fuel material with a sufficient amount of ammonia to substantially reduce the water content therein.

9. The method according to claim 8 characterized in that the ammonia is anhydrous liquid ammonia.

10. The method according to claim 9 characterized in that the ammonia is in an amount of at least four time the stoichiometric amount of water content in the nuclear fuel.

11. A method for removing water from a nuclear fuel rod, the method characterized by the steps comprising: a) introducing the nuclear fuel rod into a closable reaction vessel wherein the nuclear fuel rod comprises a casing having at least one perforation; b) introducing anhydrous liquid ammonia into the reaction vessel in a sufficient amount to immerse the fuel rods therein; c) elevating pressure within the reaction vessel to force the anhydrous liquid ammonia into the interior of the fuel rod through the at least one perforation; d) maintaining the fuel rod in the reaction vessel under elevated pressure for a sufficient time for the anhydrous liquid ammonia to extract the water from the interior of the fuel rods thereby generating a wet liquid ammonia; e) draining the wet liquid ammonia from the reaction vessel while maintaining the elevated pressure within the vessel; f) dewatering the wet liquid ammonia thereby providing an anhydrous liquid ammonia for reentry into the closed vessel; and g) venting the vessel to atmospheric conditions thereby allowing residual liquid ammonia to evaporate.

12. The method of claim 11 characterized in that the step of dewatering the wet liquid ammonia includes introducing an active metal into the reaction vessel in a sufficient amount to react with and recover water from the wet liquid ammonia.

13. The method of claim 12 characterized in that the active metal is selected from the group consisting of Li, Na, K, Ca, and mixtures thereof.

14. The method of claim 11 characterized in that the dewatering step occurs in the reaction vessel.

15. The method of claim 11 characterized in that the dewatering step occurs in a separate drying compartment and dry ammonia is reintroduced into the reaction vessel.

16. The method of claim 15 characterized in that the separate drying compartment contains a drying agent selected from the group consisting of quicklime and an active metal.

17. The method of claim 11 characterized in that the reaction vessel is maintained at a temperature above the freezing point of water.