1300 MWe PWR, a new step in full Mox core design

This paper presents a selection of the main preliminary results of a study program sponsored by COGEMA and currently carried out by FRAMATOME. The objective of the study is to investigate the feasibility of full MOX (Mixed Oxides) core loading in a 1300 MWe PWR (Pressurized Water Reactor) standard nuclear power plant. The investigation includes nuclear, thermal hydraulic and system aspects. The aim of the first phase of the program is to select an 18-month full MOX core loading strategy to serve as the basis for safety analysis. In this phase, several loading patterns have been investigated and a selection was made on the basis of conventional design criteria, such as thermal energy release, plutonium consumption, core peaking factor, thermal hydraulic performances and vessel irradiation. Only the main neutronic parameters have been evaluated. Due to the changes occurring in the values of these parameters with respect to conventional UO₂ cores, it has been necessary to improve the reactivity control devices: - Eight black rods have been added in the positions available on the vessel lid, - The boron carbide of the black rods has been enriched. - Enriched soluble boron has been adopted for normal operation. The actual ¹⁰B enrichment will be a feasibility study result. The feasibility study needs verifying compliance with technological constraints and safety criteria relative to a selection of design basis accidents. The next phase of the program deals with transient and safety analysis. The accidental transients that are most likely to be sensitive to the presence of high-content plutonium will be analyzed, mainly: - Main steam line break, which is very sensitive to soluble boron worth, moderator and Doppler coefficients, - Rod ejection, influenced by a reduced delayed neutron fraction and a smaller prompt neutron lifetime, - Loss of coolant (LOCA), - Extraction of a control group in sub-critical conditions. Due to the plutonium high content, the performance of heat exchangers of the residual heat removal systems will be carefully checked and improved, if necessary, and safety margins against void effect will be assessed. Experimental data on high-content plutonium are lacking, so that suitable design margins have to be kept to guarantee respect of a safe operation of the plant. A physical analysis based on mock-up experiments will be carried out to validate, and, if possible, to reduce uncertainties and to adjust design margins. Ali the above mentioned aspects will be analyzed in detail in order to demonstrate adequacy of the full MOX loading strategy