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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 76 / 295Document ID.No.UKEPR-0002-162 Issue 04• when the water level in the steam generators becomes very low, after about20 minutes, the primary coolant pumps are stopped by operator action.These delays for reactor scram and for stopping of the pumps will result in a higher internalenergy of the RCP [RCS] and, thus, acceleration of the accident progression. Steamgenerators dry out at 44 minutes, and core uncovery occurs at 1 hour 17 minutes. Becausecore heat-up is very rapid, this scenario is studied only for the consequence of a delayeddepressurisation. The efficiency of the bleed capacity is also verified.2.2.3. Criteria for RCP [RCS] depressurisation and identification of availabletime periods2.2.3.1. Criteria for RCP [RCS] depressurisation2.2.3.1.1. Steps for RCP [RCS] depressurisationIf heat removal from the Steam Generators (SG) is unavailable, or the partial secondarycooldown and/or fast secondary cooldown fail, the operator must perform Feed & Bleed toprevent core melt.The principle of the accident management is the following:• SG depressurisation, action performed in Emergency Operating Procedures(EOP),• if not successful, primary Feed & Bleed which is performed by PDS actuation(first level of PDS actuation), action performed in EOP,• PDS valves actuation for mitigation of a consequence of severe accident, if notyet open (second level of PDS actuation), action performed in OperatingStrategies for Severe Accidents (OSSA).2.2.3.1.2. Criteria for primary Feed & BleedThe success of operator action to depressurise the RCP [RCS] depends on the availability ofplant state diagnostics, which may indicate to the operator the need for Feed & Bleedoperation. It is also assumed that the operator has completed preparations to perform theFeed & Bleed operation. The diagnostic criteria that indicate the need for Feed & Bleeddepend on the accident scenario as follows:• scenario with SG dryout and no RCS break (as LOOP with failure of 6 EDGs):low SG water level (3 m) for the 4 SGs,• small break loss-of-coolant accident (SB(LOCA)) scenario with SG feed andwithout secondary partial cooldown: low PZR pressure and high SG pressure willidentify failure of both partial cooldown and MHSI,• SB(LOCA) scenario with successful partial cooldown and operator failure toperform fast secondary cooldown: the strategy which relies on the longestallowed operator time for secondary depressurisation also reduces the availabletime for a manual primary Feed & Bleed.