Материал: Апсе ENVIRONMENTAL PROTECTION 2014

3.3.2. Non-aqueous (dry) technologies for SNF reprocessing

The pyrochemical gas-fluoride technology

Main mission of the gas-fluoride technology is to provide SNF reprocessing without application of any liquid reagents (dissolvents, extractants and so on) and, as a consequence, without large volumes of liquid HLW. The gas-fluoride technology is based on different boiling temperatures, different volatilities and different abilities to be adsorbed by some adsorbents of uranium, plutonium and FP fluorides. At normal atmospheric pressure, uranium hexafluoride begins boiling at 560С, plutonium hexafluoride – at 62 0С, i.e. the boiling temperatures differ insignificantly. At these temperatures, main mass of fission products can form only non-volatile or weak-volatile fluorides.

The gas-fluoride technology includes the following main stages:

1.Thermal melting of fuel claddings at 16000С.

2.The SNF fluorination by gaseous fluorine-nitrogen mixture (20% F2

and 80% N2 for corrosion protection of technological vessels and pipelines) at 4000С:

(U,Pu)O2 + 4 F2 + 3 H2 → (U,Pu)F6 + 2 HF + 2 H2O.

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Main mass of FP fluorides (up to 85%) remains in the non-volatile sediment while well-volatile fluorides of uranium, plutonium and some fission products together with gaseous fission products (Xe, Kr, I) go out from spent fuel.

3.Freezing of FP fluorides in the fore-condensers at 270С. The forecondenser is a cylindrical vessel into which the gas flow is introduced at an angle to vertical axis of the cylinder. Solid particles can strike against the cylinder wall and drop out of the gas flow. Weak-volatile fluorides of some fission products (Cs, Ru, Zr, and Nb) can be removed.

4.The gas flow passes through the column filled up with solid granules of sodium fluoride NaF at elevated temperature. At this stage, different sorption ability of NaF granules in respect of uranium, plutonium and

FP fluorides is used to separate them. Uranium, neptunium and technetium fluorides are preferentially sorbed by NaF granules at 1000С. Plu-

tonium, ruthenium, zirconium and niobium fluorides are preferentially sorbed by NaF granules at 4000С.

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5. Desorption of uranium and plutonium hexafluorides from the surface of NaF granules by gaseous mixture (10% F2 and 90% N2) at 4000С.

The following main drawbacks of the gas-fluoride technology can be mentioned:

1.Incomplete purification of uranium hexafluoride UF6 from some FP fluorides. About 99,5% of uranium is extracted from spent fuel but uranium content in the recovered uranium hexafluoride flow equals 96% only. Thus, technological vessels and pipelines are contaminated with the remaining 3,5% of uranium.

2.Plutonium volatilization takes place with the lower efficiency than uranium volatilization. So, plutonium can contaminate technological equipment units too.

3.The gas-fluoride technology is not able to reprocess spent MOX-fuel because of large plutonium content.

FLUOREX technology

Japanese specialists have developed a new original FLUOREX technology that combined some stages of aqueous solvent-extraction PUREX technology with some stages of non-aqueous gas-fluoride technology.

The FLUOREX technology includes the following main stages:

1.Dismantling of fuel assemblies and chopping of fuel rods.

2.Decladding of fuel rods by alternating the oxidation reaction with oxygen and the reduction reaction with hydrogen. Uranium dioxide transforms into uranium octa-oxide via the oxidation reaction and returns into its initial state via the reduction reaction. The alternation continues until the extended fuel meat throws the weakened cladding off and becomes powder-like material,

3.Fluorination of spent fuel powder. Main uranium mass converts into gaseous uranium hexafluoride and escapes spent fuel together with some volatile and gaseous FP fluorides. Uranium (partially), practically full plutonium amount, main mass of fission products and minor actinides remain in the non-volatile sediment.

4.Uranium hexafluoride is separated from the accompanying sub-

stances by adsorption on NaF granules with high values of the decontamination factor (106 ÷ 10 7). Final product of this stage is a pure uranium hexafluoride separated from any impurities. Such uranium

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hexafluoride can be then used to produce low enriched uranium fuel for light-water and heavy-water power reactors. Also, uranium hexafluoride can be re-enriched up to any desirable level.

5. Residuals of the fluorination process (small uranium quantity, almost full plutonium amount, fission products and minor actinides) are dissolved in nitric acid followed by the extraction - re-extraction treatment, quite like in traditional PUREX technology. Final product of this stage is the co-extracted uranium and plutonium which can be used to produce MOX-fuel pellets.

The pyrometallurgical technology for SNF reprocessing

The pyrometallurgical technology was initially intended for reprocessing of spent mixed metal uranium-plutonium fuel discharged from advanced fast breeder reactors with high breeding gain.

Till recently, the research fast reactor EBR-II was operated in Argonne National Laboratory (USA). The reactor was loaded with metal U-Zr fuel enriched up to 50% 235U. The pyrometallurgical electrochemical refining technology was worked out just to reprocess SNF discharged from the EBR-II reactor. General scheme of the electrochemical refining facility is presented in Fig. 3.2.

The electrochemical refining facility represents a cylindrical vessel filled up with liquid cadmium in the bottom part and molten salts (mixture of potassium, sodium, calcium and barium chlorides) above the liquid cadmium layer (anode). From the top part, iron rod (cathode) is introduced into the molten chloride layer.

The electrochemical refining process includes the following main steps:

1. Spent fuel rods are chopped into short pieces and loaded into a perforated graphite basket.

2.The graphite basket with spent fuel pieces is loaded into the liquid cadmium layer.

3.Spent fuel is dissolved by liquid cadmium. Fuel claddings and some insoluble fission products can be removed for further treatment as solid radioactive wastes.

4.The dissolved SNF and fission products are distributed in layers of liquid cadmium and molten salts by such a way:

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The cathode deposition is periodically taken off and melted down into a fresh nuclear fuel. The vacuum melting and casting of fuel rods are used at this step. The molten U-Pu-Zr alloy is poured into a cylindrical central hole of a quartz glass block. Upon completion of the cooldown phase, the quartz glass and the metal rod can be easily separated, and the metal rod is ready for manufacturing of a fuel element.

The finer purification of mixed uranium-plutonium fuel can be achieved by using the halide-slagging process. The following chemical reaction of the cathode deposition with magnesium chloride

2(U,Pu) + 3MgCl2 → 3Mg + 2(U,Pu)Cl3;

can transform metal uranium and plutonium into their chlorides. Then, the uranium and plutonium chlorides can be returned into the molten salt layer, and the electrochemical refining is repeated. Even if the hal- ide-slagging process is applied, the decontamination factors in respect of some undesirable FP can be increased up to 102-103 only via 106-108 in the solvent-extraction PUREX-technology.

The Integrated Fast Reactor concept

Nuclear specialists from Argonne National Laboratory (USA) have developed the project of a modular fast reactor with the integrated nuclear fuel cycle, where the pyrometallurgical electrochemical refining technology could be used for SNF reprocessing. The project was named Integrated Fast Reactor (IFR).

The IFR project can be characterized by the following specific features:

1.Modular small-power (170 MWe) fast reactor with metal U-Pu-Zr fuel and liquid-metal (sodium) coolant.

2.Small sizes of the reactor core and small fuel volume with increased uranium enrichment.

3.Co-allocation of NPP and pyrometallurgical facility for SNF reprocessing in a single site.

Advantages of the IFR project:

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