Inert matrix innovative fuels for plutonium transmutation in LWRs

About 100 MT of excess plutonium are going to be originated from warheads dismantling under the START I and II agreements (50 MT by each side US and former Soviet Union), and another about 200 MT are already stockpiled from commercial spent fuel reprocessing. The civilian or reactor grade plutonium (RG-Pu), amounted at about 1000 MT total world inventory at the end of 1995, and is forecasted to attain some of 1600-1700 MT by year 2000. Therefore, there is a pressing need in finding novel and ever more safe methods to deal with all kind of excess plutonium in the aim of rendering it ultimately unusable for proliferation purposes. As to weapons Pu, one leading option (US and Russia) is to burn it in LWRs after having converted it to MOX fuel. However, among the possible types of fuel which can be envisaged to burn plutonium in LWRs, inert matrix fuel is a completely novel concept that finds justification since, especially for weapons Pu transmutation, the precisely required fuel is not immediately available and even for the LWR weapon-MOX, processes and facilities have to be set-up before reaching the operational stage. The dimension of such a delicate task and the perspective of finding even more effective and ultimate solutions from both security and safety standpoint, we think are sufficient reasons for putting attention on a new fuel concept such as IMF. Inert matrix fuel is an non-fertile oxide fuel consisting of PuO2, either weapon-grade or reactor-grade, diluted in a mixed compound of inert oxides such as stabilized ZrO2, Al2O3, MgO or MgAl2O4, and its primary advantage is the no-production of new plutonium during irradiation, because it does not contain uranium (U-free fuel) whose U-238 isotope is the departure nuclide for breeding Pu-239. This new fuel will have a plutonium relative content comparable to that one used in standard MOX fuel for PWRs. After discharge from reactor and adequate cooling time, the spent fuel is outlooked to be sent, as a HLW, directly to the final disposal in deep geological formations without requiring any further reprocessing treatment (once-through solution). Calculations related to inert matrix U-free fuel in a commercial PWR, operating in a once-through cycle scheme, show that IMF has a quite high Pu burning capability: >93% of fissile plutonium and 74/85% of total RG-/WG-Pu is burnt at end of fuel life. Moreover the fabrication tests on simulate fuel demonstrate that the nuclear-grade specifications can be met and a very limited solubility under the current fuel reprocessing techniques, coupled to the quality-poor residual Pu in the spent fuel, will make inert matrix fuel a potentially strong anti-proliferation product. Fabrication and irradiation of HEU and Pu oxide bearing fuel is expected to follow in the Halden Project context. (author)