Plutonium and minor actinide recycling in light water

Consideration has been given in the recent past to using separated plutonium and MAs as a feedstock for FR fuel for the accelerated incineration of TRUs. This option requires special reactor core designs (IFR, CAPRA), advanced fuel types (oxides, metals, nitrides) and new or advanced reproc-essing techniques (e.g. pyrochemical reprocreproc-essing), all of which require the development of adapted waste processing facilities. Figure 8 shows the LWR UO₂–Purex–TRU–IFR fuel cycle.

Co-processing of plutonium and MAs further increases the necessary (fissile) plutonium enrichment, the specific alpha activity of the fresh recycled fuel and the decay heat of the discharged spent fuel. The waste issues

U natural

FIG. 8. The LWR (UO)–Purex–TRU–IFR fuel cycle.

associated with the use of oxide fuel are very similar to those to be encountered with FR MOX fuel discussed in Section 4.1.3. However, the use of new types of fuel (metal, nitride, etc.) calls for a specific discussion of the issues involved.

4.1.4.1. Metal fuel fabrication for advanced liquid metal reactors and advanced fuels for burner reactors

In the framework of the Integral Fast Reactor project [18], a specific fuel fabrication technology has been developed and tested on the cold (and hot) pilot scale. At the EBRII facility, metal fuel was recycled by casting a uranium–

plutonium–zirconium alloy on the laboratory and hot pilot scale. It is obvious that these processes are still in the exploratory stage and cannot be considered as proven technology, but their potential should be investigated since metal alloy fuel permits very high burnups and has good material and neutronic characteristics for transmutation of TRUs. Uranium–plutonium–zirconium–

MA alloy has been fabricated for property evaluation, and it is planned to irradiate it in an FR [26].

Attention has been drawn recently to the potential of nitride and carbide fuels [27] for FBuRs. Nitride TRU fuel containing macroscopic quantities of MAs can be produced by a combination of an internal gelation method and a carbothermic synthesis. These nitride fuels can be reprocessed by electrore-fining methods similar to the technology developed for metal fuel.

Much technological experience has been accumulated during 30 years of R&D on fast breeder reactors worldwide, which can be transferred to FBuR technology.

Reprocessing of metal and nitride fuel relies on the use of pyrochemical processes and is followed by pyrometallurgical fuel fabrication for recycling in FRs or ADS systems.

4.1.5. Plutonium and minor actinide recycling in light water reactor UO₂ and MOX and accelerator driven system transuranic elements

The reservation of certain governments to allow the separation of plutonium during reprocessing operations has led to the development in the USA of a scheme [28] in which the TRUs are kept together during the reproc-essing and transferred to ADS facilities for transmutation–incineration. In this option 78% of the reactor fleet continues to be operated by LWRs, either LWR UO₂ or LWR MOX, depending on the availability of aqueous reprocessing.

However, the remaining 22% of the electricity grid has to be fed by new reactor types, which are still in the design phase. This option (see Fig. 9) is obviously a

long term solution of the TRU issue, relying principally on the availability of dependable ADS facilities capable of operating on a 24 h/d, 300 d/a basis.

In the supposition that such reactor fleets can be set up over the next 50 or 60 years, the question arises of what are the fuel cycle and waste management implications of this option:

(a) Aqueous reprocessing in its modified form (Urex) must be available for the majority of the spent fuel to eliminate uranium and some LLFPs (¹²⁹I,

99Tc). See Section 2.4.1 for an analysis.

(b) The separation of TRUs from the bulk of the fission products has to be performed by a pyrochemical process producing a concentrate of TRUs in metallic form ready to be recycled as solid fuel (metal or nitride) in the ADS reactor. Complete incineration of the TRU inventory implies the multiple recycling of TRUs in the ADS reactor and the effective separation of MAs from bulk rare earth fission products.

(c) Alternatively, an ADS reactor with a thermal neutron spectrum could be conceived with a molten salt core and continuous separation of TRUs from fission products. However, this option involves the development of a full pyrochemical reprocessing cycle with much improved TRU

ADS

All mass flow in kg/ TW·h(e)

ADS

FIG. 9. Advanced reactor and fuel cycle scenario with an LWR (UO₂)–Purex–ADS–TRU fuel cycle.

separation yields. Experience was gained in the 1960s with a molten salt reactor, and a thorough review of the operation challenges and issues encountered with this reactor type should be made before making the proper choice.

(d) Waste management problems have not yet been fully assessed and only laboratory experience has been gained.

The geographic dispersion of the fuel cycle facilities with the LWR UO₂/ ADS TRU option is one of the main factors for public acceptance. If 22% of the reactor fleet is converted into ADS facilities with pyrochemical essing plants in the immediate vicinity of the ADS reactor, the aqueous reproc-essing section (Urex) should be downscaled and associated with the location of the reactor and pyrochemical facilities. The multiplication of collocated reactor and recycling facilities throughout the territory of a State or continent repre-sentative of a 100 GW(e) reactor capacity is the major challenge for this option.

However, if the sites for such activities can be selected and collocated with the waste disposal facilities, much less transport of spent fuel and waste concen-trates will be required. The LWR UO₂/ADS TRU option favours a dispersed location of relatively small nuclear energy complexes coordinated with collocated waste disposal facilities.

4.1.6. Plutonium and minor actinide recycling in a combined double strata