GB2545934A

GB2545934A - Single stage reprocessing of spent nuclear fuel

Info

Publication number: GB2545934A
Application number: GB1600030.9A
Authority: GB
Inventors: Richard Scott Ian
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-01-02
Filing date: 2016-01-02
Publication date: 2017-07-05
Also published as: GB201600030D0

Abstract

A molten salt electrolysis cell comprising a cathode permitting intermittent melting of dendritic metal such as uranium allows or other metals. This offers an efficient single stage recovery of useful nuclear fuel, such as the actinides from spent nuclear fuel. The cathode may be made of steel or preferably tantalum or tungsten. When sufficient uranium has been deposited the cathode may be heated rapidly to allow the uranium to melt and then the cathode is left to freeze and electrolysis can be restarted. The heating may be performed using induction heating. The anode may consist of graphite rods or a steel mesh basket containing the spent nuclear fuel. Once the Uranium has been removed from the waste, further electrolysis may take place to remove plutonium and the higher actinides from the sample.

Description

Single stage reprocessing of spent nuclear fuel Technical field

The present invention relates to a superior method of reprocessing spent nuclear fuel to recover and reuse fissile isotopes

Background

Reprocessing of spent nuclear fuel is in the long term interests of the environment and energy supply as it can greatly extend the power available from uranium resources while reducing the environmental burden of long radioactive lifetime spent nuclear fuel. Reprocessing today is almost entirely carried out using variations of the PUREX process originally developed to produce weapons grade plutonium. As a result the process carries major proliferation concerns. Its complexity also makes it expensive and as a result the contribution of reprocessed fuel to today's nuclear fuel supply is minimal.

Pyroprocessing is an alternative to the PUREX process with several inherent advantages, lower cost, less ability to produce weapons usable material and smaller size. Progress in implementing it has been slow however. The most developed methods are electrochemical processes in molten salts where uranium is first extracted by electrochemical deposition on an iron electrode. Unfortunately, the form in which the uranium is deposited is highly dendritic, causing problems with recovery of the uranium from the electrode and necessitating a distillation process to remove residual electrolysis salt from the uranium before it can be melted and cast into ingots. The result is a relatively complex process.

There is therefore a need to develop a superior method of managing the dendricity problem. Summary A novel refractory metal electrode system has been discovered that allows periodic melting of dendritic uranium without removing the electrode from the molten salt electrolysis cell. This allows a single process to take as input spent nuclear fuel and produce as output pure uranium metal ingots, an actinide free fission product mixture requiring only 300 years of secure storage and a mixed uranium/higher actinide/lanthanide mixture either as a metal ingot or as a bismuth or cadmium alloy which is suitable for halogenation for use as fresh nuclear fuel.

Detailed description of the invention

Instead of using an iron cathode inserted from the top of the electrochemical cell, as is usually done, the cathode is formed as an open topped vessel at the base of the electrochemical cell. The electrochemical cell is filled with a suitable molten salt electrolyte and provided with an anode suspended in the molten salt electrolyte. This anode can be of any type known to those skilled in the art including noble metals, graphite or a basket containing metallic or oxide based spent fuel. Where the electrochemical reaction produces halogen at the anode it is often desirable to carry that gas away in a stream of noble gas such as argon to prevent it dissolving in the electrolyte and diffusing to the cathode.

The cathode is provided with a powerful heating system. This can be direct resistive heating or induction heating by coils surrounding the cathode. The cathode is provided with a mechanism to allow the removal of molten metal from within the cathode, for example by using a drain valve or a pressure driven dip tube.

The cathode is formed from an electrically conductive material capable of containing molten uranium (>1132°C) and chemically compatible with such molten metals. High temperature steels, particularly with high molybdenum content are suitable, as are high molybdenum alloys. Both of these do however alloy to a small degree with molten uranium resulting in minor contamination. Tantalum or tungsten are therefore preferred materials.

Operation of the system is by passing electric current between the electrodes. As uranium is deposited in dendritic form in the cathode, the space within the cathode fills with the dendritic uranium. When sufficient uranium has been deposited, the electrolysis current is stopped and the cathode is heated rapidly to cause the uranium to melt. When the uranium dendrites have melted the cathode heating is stopped and the uranium allowed to freeze. Electrolysis is then restarted. This cycle can be repeated until the cathode is adequately filled with uranium at which point the molten uranium is removed from the cathode through the drain valve or dip tube or other mechanism.

This process can be continued until the ratio of plutonium to uranium halide in the electrolyte exceeds approximately 5 (the exact limiting composition being determined by factors including current density, electrolyte composition and temperature). The process can be continued further but pure uranium will no longer be deposited.

Recovery of plutonium and higher actinides from the electrolyte, after most of the uranium has been recovered can be by any of the several methods described in the literature. These include but are not limited to

Continued electrolysis with deposition of the mixed actinides in the cathode, optionally with intermittent melting to overcome dendricity problems

Addition of bismuth, cadmium or other low melting point metal to the cathode with the deposited actinides forming molten alloys as electrolysis continues

The above process followed by vigorous mixing of the bismuth and salt phases so as to bring the distribution of actinides and lanthanides between the phases to equilibrium Replacement of the cathode with a cathode formed from aluminium which forms solid alloys with the actinides

Addition of aluminium to the cathode either in the solid state or in the molten state followed by continuing electrolysis

Chemical reduction of the electrolyte with addition of a bismuth/Group 1 metal alloy followed by vigorous mixing of the salt and metal phases to allow equilibrium to be reached

The heating system is of relatively high power so that it can melt the uranium before heating the entire electrolyte volume to high temperatures. Optionally a removable barrier can be placed above the cathode to prevent convective heat transfer into the electrolyte, thereby allowing use of a lower power heating system.

Efficient electrolysis requires mixing of the electrolyte. This can be achieved mechanically, but use of a cooling apparatus in the upper wall of the electrolysis cell with heating by the cathode heaters can produce efficient convective mixing of the electrolyte without use of moving parts.

Example 1

Figure 1 shows a diagram of the electrolysis cell.

The base electrolyte is a eutectic mixture of NaCI and mixed actinide trichlorides prepared by carbo-chlorination of spent oxide nuclear fuel. The cathode is a tantalum cylinder open at the top and electrically insulated from the steel wall of the electrolysis cell. It is surrounded by induction heating coils and is fitted with a tantalum needle valve. The upper part of the electrolysis cell is provided with water cooling coils. The anode is an array of graphite rods with a mechanism to bubble argon gas over the electrode surface.

Electrolysis is commenced resulting in dendritic uranium deposition within the cathode cylinder. As uranium is deposited and chlorine evolved from the anode fresh actinide trichloride mixture is added to the electrolyte to maintain its volume. When sufficient uranium has been deposited, electrolysis is stopped and the uranium melted using the induction heater. After cooling electrolysis is restarted and the cycle repeated until the cathode is half full of uranium after which the uranium is bled out of the needle valve though an argon atmosphere into moulds where it sets into ingots.

The process is continued until the plutonium to uranium concentration in the electrolyte is 5:1. Uranium is drained from the cathode and electrolysis continued but without addition of any further mixed actinide trichlorides. Electrolysis continues until the desired depletion of actinides from the electrolyte has been achieved. In order to achieve high levels of americium removal from the electrolyte it is necessary to also remove substantial amounts of certain lanthanides, particularly neodymium.

The mixed actinide/lanthanide deposit is melted using the induction heater and drained from the cathode for use as nuclear fuel.

The remaining electrolyte may be reused with a fresh addition of mixed actinide trichlorides and the process repeated. This repetition can be continued until accumulation of fission products in the electrolyte reaches levels where it interferes with the process. The electrolyte then undergoes a final electrolysis stage to remove final traces of higher actinides (especially americium) along with a fraction of the remaining lanthanides. The resulting actinide free electrolyte is then removed for disposal, leaving the residual actinide and lanthanide metals in the cathode. A fresh charge of NaCI/actinide trichloride eutectic mixture is then added to the electrolysis cell and the cathode heated to melt the residual actinide/lanthanide metal mixture which then reacts with the uranium trichloride producing uranium metal and actinide/lanthanide chlorides. When that reaction is complete, the cathode is cooled and electrolysis commenced as above.

Example 2

Figure 2 shows a diagram of the electrolysis cell

The base electrolyte is a KCI/LiCI eutectic mixture. The cathode is as described in example 1 but the anode is a steel mesh basket containing metallic spent nuclear fuel. This can be either actual metallic fuel or oxide fuel that has been electrochemically reduced to a metallic form.

Electrolysis is commenced and results in dissolution of uranium and actinides from the metallic spent fuel into the electrolyte. Uranium is selectively deposited at the cathode while the plutonium, higher actinides and other chlorides accumulate in the electrolyte. The process is continued with intermittent melting of the dendritic uranium in the cathode as required until the plutonium to uranium ratio in the electrolyte rises to 5. The anode basket is then removed and replaced with a graphite/argon anode as described in example 1. Electrolysis is continued as in example 1 to recover the plutonium and higher actinides and generate an actinide free waste product.

Claims

Claims 1) A molten salt based electrolysis cell for recovery of actinides from spent nuclear fuel which uses a cathode permitting the intermittent melting of dendritic uranium or other metals during the electrolysis process