14.7 - Fault and Protection Schedule - EDF Hinkley Point

PRE-CONSTRUCTION SAFETY REPORT
CHAPTER 14: DESIGN BASIS ANALYSIS
SUB-CHAPTER : 14.7
PAGE : 3 / 48
Document ID.No.
UKEPR-0002-149 Issue 02
The main subsequent additions to the FRATEC 01 approach were linked to changes in “design
rules” (e.g. SFC, i.e. redundancy requirement at the component level) shifting to “safety analysis
rules” (e.g. the requirement to assume an “aggravating failure” i.e. a redundancy requirement at
the function level). Other examples were extension of the scenario duration through the operator
action phase, and the introduction of additional event combinations involving Loss Of Offsite
Power (LOOP).
Changes to the list of events were made related to the experience feedback and changes in
system design options.
EPR specific options led to transients being added or excluded from the Design Basis
Assessment (DBA) list that was used for in service French Nuclear Power Plants (NPP). The
changes were in three main categories as follows:
1) Changes resulting from EPR constitutive options, such as:
• consideration of transients occurring in the different plant states led to PCC events
for outages being introduced as design basis accidents (these were considered as
beyond design basis accidents in previous French NPPs, and consequently
analysed using less stringent rules);
• consideration of transients in auxiliary buildings, which led to the introduction of fuel
pool transients, and transients in the effluent treatment building and the nuclear
auxiliary building.
2) Changes due to design improvements in the EPR that had a direct impact on the list of
design basis faults. An example was application of the break preclusion principle that led to the
exclusion of 2-A LOCA from the PCC list. LOCA outside the containment was added to the DBA
list to address the specific EPR design of the residual heat removal system function, which is
provided by the safety injection system.
3) Changes due to the application of PSA early in the design process, which impacted on the
justification of the list itself and on the way transients were arranged into the different PCC
categories.
The use of PSA also impacted the list of RRC-A events considered, as mentioned below.
1.3. CORE-MELT RISK REDUCTION CATEGORY (RRC-A EVENTS)
The origin of the list of scenarios involving multiple failures (called RRC-A sequences in the EPR
approach) was a French Safety Authority (ASN) request for consideration of design extension
conditions representative of the total loss of main redundant systems (power supply, heat sink,
SG feedwater, long term RIS [SIS] and reactor trip through Anticipated Transients Without
Scram). PCC-4 criteria were imposed, but less stringent rules were accepted.
Later on, other scenarios were added at the request of the ASN, generally to illustrate the
robustness of the defence in the EPR design (drainage of two SGs, initially corresponding to the
SFC exception on main steam isolation valves; multiple SGTRs combined with Main Steam Line
Break).