Safety problems related to the use of MOX assemblies in PWRs

Curtailment of the LMFBR program along with the satisfactory performance of the La Hague reprocessing plant, with the consequent availability of large quantities of plutonium, provides Electricite de France (EDF) with the possibility of ''burning''mixed uranium and plutonium oxide fuel (MOX fuel) in the core of certain PWR power plant reactors, hence reducing enriched uranium fuel requirements. Design provision has in fact been made for this possibility on sixteen 900 MWe plant units and is explicitly authorized in the relevant authorization decrees. The feasibility and consequences of using this new type of fuel must be safely analyzed, with respect to the entire fuel cycle, i.e. from fuel fabrication, through utilization in the reactor, up to reprocessing and waste packaging. In this connection, EDF presented the safety authorities with a feasibility study, analysis of which resulted in authorization being granted for a first loading of MOX fuel in Saint Laurent B1. The program drawn up by EDF aims at the possibility of using this fuel in twelve of the sixteen 900 MWe authorized units in the proportion of 30% of the fuel assemblies in the core. The program began with the loading of 16 MOX assemblies in the Saint Laurent B1 reactor in Autumn 1987, and continued with progressive increases in the number of reactors partially loaded with MOX fuel with a view to attaining a reloading frequency of 12 core loads per year by about 1993-94, corresponding to the fabrication of 192 assemblies per year, containing 90 tons of heavy metal (U + Pu), including 4.8 tons of plutonium. Each refuelling operation is presently the subject of close surveillance, prior to attainment in 1989 of the first equilibrium core for the Saint Laurent B1 reactor (with the 3-batch refuelling mode adopted, two years are required to reach equilibrium and the core of this reactor will effectively contain 30% MOX assemblies in 1989, providing the third reload is authorized). (author)