Transmutation strategies

However, some of the fuel cycle issues mentioned earlier remain open. The HLW conditioning into more compact and chemically stable form does not necessarily imply safer repository in the long term because long lived radioactive materials will be still present in the environment. Moreover, most of the potential fuel energy still remains un-recovered. Finally, proliferation resistance improvement due to less attractive Pu isotopics after its recycling may be offset by the increased proliferation risks due to the mere fact of Pu separation during SNF reprocessing which creates more opportunities for its diversion [Lowenthal M.D., 2002].

Transmutation strategies

Reduction in radiotoxicity of the nuclear waste intended for geological storage is clearly the most important goal for sustainable economic development and relying heavily on nuclear power as major non-polluting energy source. The objective is to reduce the nuclear waste radiotoxicity to a level below that of the uranium ore from which it originated within the time frame that engineering barriers preventing radioactive materials from leaking into the environment can be designed with high degree of reliability (less than 1000 years [DOE/NE, 2003], [OECD/NEA, 2002]).

Long lived radioactive nuclides can be transmuted to short lived or stable nuclides via neutron capture or fission reactions. By fissioning the actinides from the SNF the following goals can be achieved. Long lived nuclides are converted to generally short lived fission products reducing the long term radiotoxicity. Potentially weapons usable material (mainly Pu) is eliminated reducing proliferation risks. Finally, valuable energy potential is recovered extending uranium resources.

Numerous studies have been conducted on various options for radioactive waste transmutation since 1980’s. None of them was realized primarily due to economic factors. The suggested transmutation systems can be categorized as presented in Table 1.2.I. Each of the categories is discussed below in some details. Figure 1.2.3 schematically shows the proposed transmutation concepts.

Table 1.2.I. Technology options for transmutation systems and fuel cycles.

System feature Proposed options Neutron spectrum Fast, thermal, combination of two Type of coolant Water, gas (CO2, He), Liquid metal (Pb, Pb-Bi, Na) Type of fuel Metallic, Oxide, Nitride, combined (CERMET, CERCER) Fuel matrix Fertile free, Th, UO2 (MOX) Neutron source Critical, Accelerator driven (ADS) Fuel cycle Once through deep burndown, closed with multi-recycling Recycling reactors Single tier, multi-tier system Nuclides intended for transmutation Pu, Pu + MA, Pu + MA + LLFP Transmutation target type Homogeneous (mixed with fuel), Heterogeneous targets

Transmutation

Fast spectrum Thermal spectrum

Reactors ADS Reactors ADS

Na Pb/Pb-Bi He / CO2 LWR HTGR

Oxide Metal Nitride Fertile Free MOX Th Fertile Free

Pu Pu + MA

Figure 1.2.3. Transmutation system concepts