Fig. 8-3 Burnup dependence of plutonium composition in a PuO2-ZrO2(Y,Gd)-Al2O3 fueled PWR obtained by the 2-dimensional core calculation.

The Japanese long-term program for nuclear energy development and utilization states that no excess plutonium shall be stored. In this context, a once-through burning process for plutonium has been proposed as one of the processes for plutonium annihilation. In this process, rock-like fuels having multiphases are fabricated based on conventional MOX fuel technologies. They are irradiated in light water reactors (LWRs) to generate electricity. As the resulting spent fuels are geologically stable, it is possible to dispose of them as high-level waste (HLW) after 30 ~ 50 years of cooling without further processing. Accordingly, this process is expected to meet the triple objectives of denaturing the plutonium (nonproliferation), generating electricity (economy), and forming stable HLW (environmental safety). From the previous study of this process, two multicomponent systems of PuO2-ThO2-Al2O3-MgO and PuO2-stabilized ZrO2-Al2O3-MgO were selected. They exhibit good phase relationships and chemical stabilities. At present, an experimental study is ongoing to further examine the phase relationships of the fuel materials, their properties, the distribution of fission products, the dimensional stability due to irradiation, and the chemical stabilities of these spent fuels. From reactor burnup calculations using simplified cell models, it has been shown that the 239Pu in weapons plutonium is almost completely burned and more than 80% of the total plutonium can be transmuted in a conventional LWR. It also has been shown that the void and Doppler reactivities estimated for a zirconia-type fuel (PuO2-stabilized ZrO2-Al2O3) LWR are much smaller than those of a conventional LWR. Two modified fuel systems that have ThO2 or W added are being studied for the improvement of these reactivities. Fig. 8-3 shows the plutonium transmutation characteristics estimated by the 2-dimensional core calculation. As much as 83% of the total plutonium and 98% of the 239Pu are transmuted by about 1,400 days of burnup. The quality of plutonium becomes very poor in the spent fuels. A total of 0.87 tonne of plutonium would be denatured every year assuming the use of a 1 GWe PWR operating at 80% availability.

Reference H. Akie et al., A New Fuel Material for Once-Through Weapons Plutonium Burning, Nucl. Technol., 107, 182 (1994).

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