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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 4 / 295Document ID.No.UKEPR-0002-162 Issue 04It should be noted that the approach to decay heat removal from the containment in severeaccidents only makes use of the EVU [CHRS]. This approach is adequate, as exposure ofthe concrete basemat to core melt is prevented by the core melt stabilisation system. Theinteraction between melt and concrete and resulting gas release into the containment istherefore limited to the ablation of the sacrificial concrete in the reactor pit and core catcher.Only if unlimited core concrete interactions take place a venting system needed forcontainment pressure relief. With regard to venting, a significant advantage of the EPRsolutionis that heat removal in a severe accident is performed with a closed containment. Incontrast, a venting system would require the opening of a valve for containment pressurerelief.In combination with the maximum containment leak rate of 0.3% by volume per day atcontainment design pressure of 5.5 bar, pressure and temperature control ensures that theradiological objectives as defined in section 0 of this sub-chapter are met.The analysis of the EPR response in severe accidents employs representative and boundingscenarios. Representative scenarios are used for the design of severe accident mitigationsystems and the analysis of their efficiency, while bounding scenarios involve pessimisticassumptions and are used to show that there is no sudden escalation of consequences justbeyond the design basis.The analyses employ best-estimate assumptions, codes and methods in order to exhibit themargins involved in the safety design of the plant. Corresponding codes and models haveundergone validation against representative experiments. These validated codes then allowthe extrapolation of experimental findings to reactor scale. In effect, the severe accidentanalyses are backed-up by representative experiments.1.2. SPECIFIC DESIGN MEASURES1.2.1. Primary System DepressurisationA failure of the reactor vessel at high pressure is a significant potential contributor to earlycontainment failure (missiles created by the pressure vessel movement, melt dispersalincluding direct containment heating). Even though such a failure is physically unlikely, giventhat the primary loop is assumed to fail prematurely, the Technical Guidelines clearly requirea design objective of transforming high-pressure core melt scenarios into low-pressure coremelt scenarios with high reliability, so that reactor vessel failure under high pressure can bepractically eliminated.For the EPR, this is achieved through two dedicated severe accident depressurisation valvetrains that are part of the primary depressurisation system (PDS) but independent of thepressuriser safety valves. The 2 x 100% design philosophy is followed to provideperformance margin. The valves are DC-powered and as such connected to the 12 hourbatteries. Section 16.2.1 - Figure 1 shows the primary system depressurisation system.Each depressurisation train has a discharge capacity of 900 t/h of saturated steam at designpressure. This capacity ensures that RCP [RCS] pressure at time of breach of the RPV iswell below 20 bar for representative scenarios and does not exceed 20 bar for boundingscenarios. Depressurisation will eventually be activated by the operator when the core outlettemperature reaches 650°C. The coolant is discharged into the pressuriser relief tank (PRT),which itself is protected by rupture disks and connected to 2 of the 4 reactor coolant pumprooms. The rupture disks are designed to burst at a pressure difference of 20 bar.