16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 80 / 295Document ID.No.UKEPR-0002-162 Issue 04• between the reactor vessel upper plenum and the steam generator inlet plenum,• between the inlet and outlet plena of the steam generator.Section 16.2.2.2 - Figure 17 shows the steam flow in each of these patterns. Naturalcirculation usually results in heat-up of the structural material, which will be subjected toelastic-plastic and creep behaviour.Creep behaviour occurs when the material is kept at a sufficiently high temperature forsufficient time. Creep strain increases with time and temperature until it reaches a criticalvalue at which creep rupture occurs.For a TLOFW scenario without opening of any PDS line, the heat-up of the primary loopstructures is very rapid. The maximum wall temperatures are [Ref]:• 1000°C for hot leg, and• 500°C for SG tubes, or 355°C if the SA valves are opened at the severe accidentcriterion of 650°C core exit temperature.In the EPR, the high ratio of the total cold surface of the SG tubes to the nominal corethermal power contributes to limiting the temperature in SG during loop natural circulation.The advantageous behaviour of SG tube material (Inconel 690) with respect to creep alsolimits the risk of tube rupture under high pressure scenarios. With delayed RCP [RCS]depressurisation, creep rupture would occur at the weakest point of the primary system.Studies on various parts of the RCP [RCS] which have the potential to fail (bimetallic welds inthe hot leg, pitting in the surge line, the lower part of the surge line, upper bimetallic welds inthe surge line, pressuriser, steam generator tubes and steam generator tube plates) haveshown that the weakest point is the bimetallic weld (between the austenitic and ferritic steels)of the hot leg, with no risk of SG tube rupture and hence no risk of containment by-pass.Even though the induced rupture of the steam generator tubes is unlikely, the related risk willbe evaluated for various core melt scenarios within the framework of the Level 2 probabilisticsafety assessment (PSA).