Spent fuel technology given the all clear

30 March 2001

Several methods of dealing with spent fuel have been proposed over the years. The US Nuclear Regulatory Commission has approved one method developed by the Argonne National Laboratory.

In a report with important implications for the future of Argonne’s nuclear technology programmes, a national panel of scientists approved the continued use of Argonne technology for treating spent nuclear fuel.

The Argonne process could recover useful uranium from spent fuel from the Department of Energy (DoE), saving millions of dollars in disposal costs. In addition to recycling the uranium, the process keeps the short-lived and long-lived waste products together, which disables the plutonium for use in weapons.

After tracking the progress of Argonne’s now completed electrometallurgical treatment programme for three years, the National Research Council (NRC) concluded that no technical barrier exists to prevent further use of the technology.

“The US Department of Energy (DoE) subsequently selected our technology for use in treating 26 tonnes of sodium-bonded fuel from Experimental Breeder Reactor II,” said Bob Benedict, who manages the development of Argonne’s electrometallurgical technology. Experimental Breeder Reactor-II was a research reactor that was shut down in 1994. DoE may also select the technology for use on an additional 34 tonnes of sodium-bonded fuel from another reactor.

Argonne developed electrometallurgical treatment specifically for sodium-bonded fuel. This is a metallic fuel that is bonded to its outer casing by sodium. “But sodium reacts chemically with air and water,” Benedict said, “so the fuel has to be treated to remove and neutralise the sodium before it can be disposed of.” The process converts the sodium into sodium chloride.

Argonne’s electrometallurgical process removes the useful uranium from the fuel, resulting in a significant reduction of the amount of waste produced. The waste takes one of two forms: a ceramic or a metal alloy. The ceramic waste, which is impervious to air and water, is produced by heating and compressing a composite of borosilicate glass and zeolite, a mineral that binds the fission products within its structure. The NRC report cites the expected performance of this ceramic under repository conditions as: “comparable to that of borosilicate high-level-waste glass.”

The metal alloy is made from the remains of the stainless-steel cladding that encased the fuel while it was in the reactor and noble or “non-reactive” metals produced as a by-product of the fission reaction.

The NRC report is significant beyond Argonne, Benedict said, because an agreement between former Idaho governor Phil Batt and the DoE requires all DoE spent fuel, including that at the Argonne-West site – home to Experimental Breeder Reactor-II – to be out of the state by 2035. Argonne’s fuel must be treated to stabilise it before it can leave the state.

The NRC panel also recommended that DoE consider the electrometallurgical treatment technology for use beyond sodium-bonded metallic fuel.

The electrometallurgical treatment process uses an electrorefining technique to separate uranium, inert materials, fission products and transuranic elements from spent nuclear fuel, greatly reducing the volume of high-level waste and placing disposal costs within the range of practicality. The process is currently being developed for application to all constituents of the DoE-owned spent nuclear fuel inventory. The flow sheet outlines the treatment process.

•The oxide reduction process converts oxide fuels into a metallic form which can be utilised in the electrorefiner.

•In the electrorefiner, the uranium is separated as a solid from the fission products and transuranic elements.

•The cathode processor converts the deposited uranium into a form suitable for interim storage.

•The waste products, whether metallic or in the molten salt are each treated either in the melting furnace or in the zeolite columns respectively to form stable high-level wastes for disposal.

The metal fuel element pieces are inserted into the electrorefiner, where the fuel is dissolved in molten salt electrolyte. The uranium is electrochemically deposited on a solid cathode mandrel that is removed, along with some adhering salt, for further treatment. The cladding hulls, containing only a small quantity of the less reactive fission products, are also removed for waste treatment. Most fission products and all the transuranic elements accumulate in the salt through repeated electrorefining cycles. Eventually, the electrolyte salt is pumped out of the electrorefiner for waste treatment and then returned as fresh salt.

The electrorefiner is about 13 feet tall to the top of the electrode insertion assemblies. There are two basket anodes and the two mandrel cathodes are immersed in a pool of molten salts. There is a stirrer assembly in the pool.

The cathode processor separates salt from uranium. This machine heats the cathode product to a high enough temperature to boil the salt away from the uranium product. The uranium is melted and then cooled into metal ingots. The salt vapour is condensed and collected for recycling back to the electrorefiner. A casting furnace melts and blends high-enriched uranium from the cathode processor with depleted uranium to produce a low-enriched ingot for storage.

The oxide reduction process is used to simplify the disposal of waste. There is a wide variance in the composition of DoE spent fuel. However, the Argonne electrometallurgical treatment technique can convert all of the spent fuel into a single set of waste forms, thereby simplifying their qualification for disposal. Oxide fuels, such as the core debris from the Three Mile Island 2 reactor, must first undergo a reduction step to convert the oxide compounds of the actinides to the metallic state suitable for electrorefining. The lithium reduction process accomplishes this pre-treatment, yielding the corresponding metals and lithium oxide. The reduced metal components become feed material for electrometallurgical treatment.

One of the objectives of the electrometallurgical process is to produce a minimum of waste forms. There are only two wastes, a metallic waste and a hot pressed waste.

The cladding hulls left behind in the anode baskets, the metallic particulate filters and zirconium are melted together in a furnace and cast into a corrosion-resistant metal-alloy waste form. The salt adhering to these materials is recovered and returned to the electrorefiner. The ceramic waste is loaded into a ribbed canister. It is then subjected to high temperature and high pressure, thus consolidating the waste.

Spent fuel storage and disposal

Argonne has developed new testing methods to assess the behaviour of commercial spent fuel and vitrified waste in storage and disposal environments.

Before spent fuel can be buried in a geological repository, its corrosion behaviour over extremely long time frames must be known. A series of tests were developed to determine the consequences of water leaking into spent fuel under conditions similar to those at the candidate repository site at Yucca Mountain.

Vitrified waste

A variety of laboratory tests with simulated and fully radioactive waste glasses are performed to obtain corrosion information. This work supports development and evaluation of ground disposal systems for high and low level waste. The laboratory tests are designed to simulate disposal conditions including those expected for the candidate repository site at Yucca Mountain. Interactions between humid air, groundwater, and various nuclear waste forms, such as high-level waste glasses and spent fuel are examined.

Before nuclear waste can be buried in a geological repository, its corrosion behaviour for long periods under respository conditions must be understood. Analytical capabilities at Argonne include the ability to conduct detailed experiments on and examination of radioactive waste forms, such as spent fuel and defence waste crystalline ceramics and glasses.

Argonne has developed a series of new tests to assess the behaviour of waste forms in repository environments, and determining the consequences of groundwater and water vapour coming into contact with spent fuel.

Following a few years’ exposure to test conditions, spent fuel begins to corrode, and a layer of uranium-based corrosion products precipitates on the surface of fuel fragments. The ground water at Yucca Mountain contains dissolved silica (as silica acid, H4SiO4) and sodium (Na+), and an important corrosion product formed in these tests is sodium boltwoodite, Na(UO2)(SiO3OH)(H2)1.5. The reaction of UO2 fuel with oxygen to form the corrosion products can be represented as:

UO2 + H4SiO4 + Na + 2O2 Na(UO2)(SiO3OH)(H2O)1.5 + H

This provides one example of how characterisation research at Argonne enhances the understanding of fundamental engineering and environmental problems. Tools used in the analysis of solids include optical microscopy, scanning and transmission electron microscopies, energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy, electron and X-ray diffraction, and bulk chemical techniques, such as inductively-coupled plasma-mass spectroscopy and alpha and gamma spectroscopies.

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