Plutonium processing Transmutation products

Nuclear power reactors cause the transmutation of chemicals (uranium and plutonium) to fission products using neutrons as the catalyst to produce heat. Fossil furnaces use the chemical reaction of carbon and oxygen to produce CO2 and other wastes to produce heat. There is only one reaction and one purpose for nuclear power reactors there is one reaction but many puiposes for fossil-burning furnaces there are myriad chemical processes and purposes. 

Plutonium-239 and tritium for use as military explosives are the two major transmutation products. The nuclear process for Pu-239 production is the same as for energy generation, but there are some differences (a) metallic natural uranium clad with aluminum facilitates later dissolution for plutonium recovery, and the reactor operates at a relatively low temperature because of the aluminum clad and better heat transfer (due to the metallic natural uranium) (b) the irradiation cycle is limited to a few months to minimize the Pu-239 conversion to Pu-240 and Pu-241 and (c) a carbon or a heavy water moderator is used to increase the neutron efficiency. 

The classical Purex process was designed to produce nearly pure uranium and plutonium. The Chemical Engineering Division of Argonne National Laboratory has demonstrated UREX+, an advanced aqueous process with five extraction trains that split commercial reactor spent fuel into five streams 1) a nearly pure uranium stream (95.5% of the heavy metal in the spent fuel) 2) technetium sent to transmutation (0.08 /o) 3) Pu/Np converted to MOX fuel for LWR fuel and Am/Cm for transmutation or fast-flux reactor fuel (0.962 /o) 4) Cs/Sb decay heat producers sent to interim decay storage (0.017 /o) and 5) a mixed fission product stream (3.44 /o) composed of gases and solids incorporated into a waste form for geological repository disposal.f The percentages shown are computed from Table 1. 

The aim of the present improvement work on the PUREX process is to make the separations more selective and to create effluent streams of high purity. Thus, modifications are performed to make neptunium end up in a fraction for later transmutation in a reactor or accelerator-driven system. This can be achieved by a better control of redox conditions in the process. Today neptunium is partially co-exlracted with plutonium and uranium. There are also suggestions to withdraw product streams with Tc and respectively, i.e., long-lived nuclides that might be of interest for transmutation. 

A variation of the PUREX process is being proposed by the US Department of Energy as a possible alternative partitioning scheme for the transmutation of wastes. This aqueous process called UREX, only removes uranium from spent commercial LWR fuel and leaves plutonium in the HLW stream with the other minor actinides and fission products. Nonaqueous pyroprocessing, a variation of the ANL electrorefining process, is then proposed to be used to separate both the plutonium and minor actinides so that they can be transmuted in an ADS. For further information related to potential modifications of this process for other accelerator transmutation of waste applications, see ANL-99/15 (1999). 

Each of these elements may be used for production of nuclear fuel or other purposes. The recovery efficiency for uranium is reported as 99.87% and for plutonium 99.36%-99.51% (NEA 2012). The extended PUREX includes separation of neptunium and technetium as well as recovery of americium and curium that are also separated from each other by additional extraction stages as given in detail in the flowsheet (NEA 2012). The advanced UREX-i-3 process generates six streams after separation uranium for re-enrichment Pu-U-Np for mixed oxide fuel c for managed disposal Am-Cm to be used as burnable poisons and for transmutation high-heat-generating products (Cs and Sr) and a composite vitrified waste with all other fission products. Some fuel types may require preliminary steps like grinding to enable their dissolution. 

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