Feasibility of plutonium use in BWR reactors. A way to dispose of the spent fuel

To assess the convenience of a closed fuel cycle, preliminary calculations have been done to evaluate which option will be the most attractive to follow from an economic point of view. Currently in Mexico, there is no defined policy for high level waste, so it is necessary to perform several studies to help define a possible strategy focused on the spent fuel. The calculations shown here indicate that from the economic point of view, recycling could be an expensive solution or at least more expensive than the once-through option. 1. Introduction. The BWR reactors of Laguna Verde Nuclear Power Plant have an electrical output of 654 MWe each, and the core contains 444 fuel assemblies. To reach the 18-month cycle currently established for operation, it is necessary to load around 112 fresh fuel assemblies (1/4 of the core, approximately) after each operation cycle, resulting in 112 spent fuel assemblies being discharged from the reactor. The BWR fuel assembly (FA) contains approximately 180 Kg of heavy metal (uranium). After discharge and reprocessing, the amount recovered will be 94% uranium and 1% plutonium, which means 169.2 kg of uranium and 1.8 Kg of reactor grade plutonium. If a once-through cycle is considered for both reactors, the amount of fuel assemblies through their entire life of operation will be 112 fuel assemblies/cycle multiplied by the number cycles minus one plus the initial load of the reactor. This produces 3244 assemblies for each reactor, resulting in a total of 6488 fuel assemblies or 1622 ton of high radioactive waste. When recycling the spent fuel of both reactors, practically all the fuel discharged will be reprocessed except for the last four cycles (if the plant is planning to close and there is no license extension). This would result in 1448 UOX assemblies plus 612 MOX fuel assemblies as spent fuel from both reactors, or the equivalent to 515 ton of high radioactive waste. So, when using recycling, the amount of spent fuel is reduced to around 32% of the original amount produced without recycling. Once the MOX fuel is loaded into one of the reactors (in fact the MOX can be loaded into the two reactors dividing the amount of MOX available), the amount of UOX fuel will be reduced, and less spent fuel will be available for reprocessing. Consequently, the plutonium production decreases and the amount obtained from the reprocessing will be enough for 27 MOX assemblies. Here, the load of MOX assemblies is reduced and the number of uranium assemblies loaded will be higher, increasing the uranium spent fuel discharged. Now the plutonium will be enough for the manufacturing of 28 MOX reaching the equilibrium point. For the once-through cycle, we will have 6488 spent FA for direct disposal, while for the recycling option, we need to reprocess 4412 uranium spent fuel assemblies to get the necessary plutonium to manufacture 628 MOX fuel assemblies. However, as the core will be mixed, 5860 uranium fuel assemblies are necessary, resulting in 1448 UOX FA plus 628 MOX FA for disposal. 2. Economics. To assess the economics for each option, the parameters shown in the Table 1 were used. The costs for uranium and services correspond to the spot prices reported for UxC Consulting Company during the last week of September 2005, and the enrichment and Burnup corresponds to technical data of LVNPP. Direct disposal data: To estimate the cost for direct disposal of spent fuel, the prices for Sweden reported in the OECD study were taken as a basis to evaluate the costs in the calculations. So for transport and storage, a cost of 230 USD/Kg HM was assumed and 610 USD/Kg HM for encapsulation and final disposal. Recycling option data: The OECD study reported the PWR fuel cycle unit prices, using as a reference USD 1991. Those data assign a cost of 860 USD/Kg HM to the recycling option. Those prices include transport, reprocessing and waste disposal. 3. Results. The results obtained (after several calculations using electronic sheets developed for the fuel cycle costs, including the front and back end of the cycle) f or once-through and partial-recycling options are shown. To calculate the reprocessing, disposition, and MOX manufacturing costs. The OECD study was used. The methodology applied corresponds to constant money calculations, to make a direct comparison between once-through and recycling schemes. However, the costs for final conditioning and disposition have uncertainties attached and the use of costs reported in the OECD study should be considered generic. Taking this into account, the costs for the recycling option can be higher. 4. Conclusions. The main result from this study, under the scenarios proposed, is the fact that even with the current higher costs of uranium, the recycling option is more expensive than the once through option. The results show that the recycling option is around 7.5% more expensive, and the reduction of high level waste will be approximately 68% which is significant. Another possible advantage will be that the spent fuel storage pools will be almost empty except at the end of life for the plant