EdF must install containment pressure sensors at all 58 of its PWRs by the end of 2005, following a decision that has been made by nuclear safety authority DGSNR.
The sensors, which have already been installed on about 10 of EdF's 900MWe PWRs, will allow better management of core damage accident sequences by letting operators know how close a containment is to a dangerously high internal pressure.
The sensors are aimed at optimising use of the U5 sand filters to which EdF reactor containments are connected and through which containment air would be filtered before being vented to the atmosphere after a severe accident. The U5 filters are supposed to be opened no sooner than 24 hours after initiation of an accident, once containment pressure reaches a threshold of 5 bar(a). The time lag is designed to give authorities time to implement measures to protect the public, as well as to allow some reduction of radioactive aerosols within containment before discharging anything to the atmosphere.
In addition, recourse to the U5 filters implies contamination of circuits and potentially higher worker doses, so EdF is motivated to find more ways to avoid their use.
In January 2000, EdF asked to extend the threshold for opening the filter to 7 bar for its 900MWe PWRs, and to 6 bar for its 1300MWe and 1450MWe PWRs, in order to give more time for containment depressurisation. The safety authorities agreed, but on the condition that sensors be installed to measure containment pressure between 0 and 7.5 bar.
This latest decision ends discussions between EdF and DGSNR in which EdF tried to postpone installation of the sensors on some 900MWe units until their 20-year outages, which would fall in the second half of this decade. DGSNR insisted that all the reactors be equipped by 2005.
