- 243 - The shutdown command is given when 2 of the 3 triaxial accelerometers positioned at 120° to one another on reactor level B exceed the 0.01 g threshold; for emergency shutdown of the reactor, via the reactor safety system; for second barrier containment and containment isolation via a 2-out-of-3 voting system and relay circuitry. Conformity of the facilities with the current frame of reference At the end of the conformity review conducted as part of the CSA, the ILL identified a number of deviations. Two are particularly important with regard to the CSA: firstly, the safety valves designed to stop loss of the primary cooling water inventory in the event of rupture of an experimental canal flux thimble are not seismic-qualified, and secondly, some of the seismic requirements for the gaseous effluents discharge system are not satisfied. The ILL proposes to address the conformity deviations taken as a whole by the end of 2012, and some of them during the 2011-2012 winter shutdown, notably as regards the two major conformity deviations mentioned above. The exercise has also shown that the main difficulty lies in verifying that all the work carried out further to the last periodic safety review complies with the specifications. In its CSA report, the ILL gave a commitment to do this by the end of 2012. Lastly, a substantial task remains in defining the safety requirements. Assessment of the safety margins In the complementary safety assessments the licensee assessed the "plausibility" of the ground acceleration levels producing cliff-edge effects for the "RFS 2001" earthquakes and "sediments" soil type conditions in the sense of RFS 2001-01. The licensee then assessed the margin corresponding to the transition from the MHPE to the SSE with respect to the standard deviation σ associated with the seismic movement prediction model used in RFS 2001-01. At the end of its analysis the licensee proposed adopting as the "ultimate plausible" earthquake the SSE +σ level for "sediments" soil type as used in RFS 2001-01. Regarding the civil engineering, ILL's analysis of the earthquake behaviour of the structures is based on the results of the design studies carried out in the facility reassessment presented to the Advisory Committee in 2002 and a complementary assessment of their behaviour beyond the design-basis conditions. This latter assessment is based essentially on expert opinions. The seismic behaviour scenario for the HFR building can be summarised as follows: up to an earthquake intensity equal to 1.2 SSE, the reactor building behaviour basically remains in the elastic range, and the predictable containment damage characterised by reversible cracking of the wall would in this case result in a very slight loss of leak-tightness; up to an intensity of 1.3 SSE, the resistance of the HFR building is maintained. The damage should remain very slight, with a deterioration in leak-tightness of a scale difficult to quantify; up to an intensity of 2 SSE, the resistance of the building is maintained . At this stress level the damage should remain slight, but would result in a significant deterioration in leak-tightness of a scale difficult to quantify. For the reinforced concrete reactor containment, the ILL considers that the containment damage resulting from the SSE and up to 1.3 SSE would correspond to a 10-fold increase in the leakage rate (corresponding to the "small breach in reactor containment" situation). From 1.3 SSE to 2 SSE (also corresponding to SSE + σ), the ILL considers that the containment damage would correspond to a 100-fold increase in the leakage rate (corresponding to the "moderate breach in reactor containment" situation). For the central core and the canal 2 civil engineering structures, the stability of these two key SSCs would be assured up to an intensity equivalent to 2 times the SSE. This margin is therefore sufficient for these elemens not to create a cliff-edge effect below this earthquake intensity. The hot cell would remain stable and would not represent a threat for the equipment situated in the concrete containment. Regarding the equipment items, the earthquake behaviour analysis of the mechanical structures is based on the results of design calculations presenting the reinforcements carried out between 2003 and 2006 and on the complementary assessment of their behaviour beyond the design-basis conditions. This latter assessment is based essentially on the opinions of experts who estimated the margins for those equipment items that had not been designed to earthquake design standards. This experts' report is to be published.
RkJQdWJsaXNoZXIy NjQ0NzU=