21 Nov (NucNet): Thermal nuclear reactors can burn minor actinides effectively and could play a valuable role in reducing the radiotoxicity of nuclear waste, a study by the Organisation for Economic Co-operation and Development’s Nuclear Energy Agency says.
The study, whose purpose was to review the literature to determine whether thermal reactors are a viable alternative to fast reactors as minor actinide burners, concludes that although thermal reactors may in some respects be more limited in what they can achieve, they could nevertheless be valuable in this role, either as an intermediate step towards a fleet of minor actinide burning fast reactors or as an end point in their own right.
As an intermediate step, minor actinide burning in thermal reactors could be used to demonstrate the technologies required, and possibly expedite the deployment of minor actinide burning fast reactors.
“Large numbers of thermal reactors are operational today and only a few would be needed to demonstrate the technology,” the study says. “In principle, there is no reason why an R&D programme on minor actinide burning in thermal reactors could not begin immediately, perhaps demonstrating the technology within a period of 10 to 15 years.”
Actinides (or actinoids) are those elements in the periodic table from actinium upwards. Uranium (U) and plutonium (Pu) are two of the principal elements in nuclear fuel that could be classed as major actinides. The minor actinides are normally taken to be the triad of neptunium (Np), americium (Am) and curium (Cm).
Current nuclear fuel reprocessing techniques only recover uranium and plutonium. Advanced processes aim to recover minor actinides and other long lived fission products for the purpose of transmuting them in reactors.
International Atomic Energy Agency statistics indicate that in 2006 about 110 tonnes of minor actinides were being contained in spent fuel storage worldwide, and an additional 40 tonnes were contained in high level waste products from reprocessing. In the absence of partitioning and transmutation, the amount will double by the year 2020, the IAEA said.
The NEA study says although the total minor actinide mass is relatively small – approximately 20-25 kg per year from a 1,000-megawatt light water reactor – it has “a disproportionate impact” on spent fuel disposal.
This is why there has been a longstanding interest in transmuting these actinides either by fission (to fission products) or neutron capture in order to reduce their impact on the back end of the fuel cycle.
The main problem is that in conventional reprocessing facilities, the minor actinides follow the fission products into the high-level waste stream, eventually being incorporated in vitrified high-level waste. Recycling minor actinides will require modifications to the normal reprocessing flow sheet to separate them from the fission products and route them separately or grouped together in some way.
The study, ‘Minor Actinide Burning in Thermal Reactors’, is online: