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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 34 / 295Document ID.No.UKEPR-0002-162 Issue 04of the SG relief valves in order to depressurise the secondary side down to 60 bar in20 minutes (equivalent to a cooldown rate of 100°C/h).At 30 minutes after reactor scram, the operator initiates maximum depressurisation of thesecondary side by total opening of the main steam atmospheric dump valves (VVP [MSSS]).Due to the full opening of these valves and the size of the break, the primary pressure dropsto the accumulator threshold value before the onset of core uncovery. The accumulator waterdepletion happens 35 minutes into the LOCA scenarios with a small 5 cm break in the coldleg. Thus reflooding and subsequent core cooling give a delay before core uncovery.In all cases, when the temperature at the core outlet reaches 650°C, the dedicated severeaccident relief valve is opened, even if the primary system has already been depressurisedby the secondary cooldown.The 650 °C criterion is met only after drying out of the SGs and that the refill-up of the SGfeed-water tank until 100 hours is not taken into account, which is conservative.For evaluation of the calculation, as an example, selected parameters like water inventory,pressure, mass of molten material and maximum fuel rod temperature of a 5 cm break areshown in Section 16.2.2.1 - Figures 1 and 2.SGTR: core melt scenario [Ref]This has been analysed to identify the leak tightness functional requirements for thesecondary system consistent with the radiological source term targets. The scenario andassumptions are as follows:• an SGTR (a double-ended tube rupture) at the top of the tube plate,• no leak from the steam isolation valves,• emergency boron system (RBS [EBS]) unavailable,• failure of the (AAD [SSS]) and the (GCT [MSB]),• reactor trip and turbine trip on (RCP [RCS]) low pressure criterion,• at reactor trip, loss of the SEC [ESWS], the RRI [CCWS], RCV [CVCS], RIS[SIS] and Reactor Coolant Pumps,• manual isolation of the 4 steam generators on reactor trip. SEC [ESWS] initiatedon the 3 intact steam generators (4 ASG [EFWS] tanks available). Cooling partlyachieved on 4 SGs on the actuation of the safety injection signal (on RCP [RCS]low pressure). Cooling rapidly achieved on the 3 intact SGs at the safetyinjection signal + 30 minutes and rising of the VVP [MSSS] threshold of the SGto 96 bar. No re-filling of ASG [EFWS] tanks,• opening of the dedicated severe accident relief valve when the maximum coreoutlet temperature reaches 650°C.For evaluation of the calculation, selected parameters such as RCP [RCS] pressure, waterlevel in the core, masses of water in the primary circuit and pressuriser, temperature ofvapour in SG and pressure in SG are shown in Section 16.2.2.1 - Figure 7 to Figure 9.