- 151 - 5.2.1 Loss of the primary heat sink In its specifications, ASN asked EDF to study the induced losses of safety systems, and loss of the alternate heat sink in particular. Initially, EDF will analyse each facility or installation individually; in a second phase it will be assumed that all the installations or facilities (reactors, pools, etc.) on a given site are affected simultaneously. For the reactors having several heat sinks (namely the Flamanville 3 EPR reactor), the successive loss of the heat sinks must be considered. For each of these situations, indicate the time for which the site can remain in this situation without external aid, before damage to the fuel becomes inevitable. The situation of total heat sink loss is called "H1". This situation can affect either a single reactor or all the reactors on a site, and in the latter case it is referred to as a "whole-site H1". Total loss of the natural heat sink leads to loss of the cooling functions of the core and spent fuel pool in the fuel building (BK37). It is detected in the ESWS system by appearance of the low flow alarms which will lead to first one, then two SEC channels being declared unavailable in succession. Total loss of the heat sink renders the feedwater function and the essential service water system (ESWS) unusable. This is followed by gradual heating of the component cooling system (CCWS). The following systems gradually become unavailable : the component cooling system (CCWS), the residual heat removal system (RHRS), the reactor cavity and spent fuel pool cooling and treatment system (PTR), the primary pumps (loss of cooling of the bearings, motor and thermal barrier), the safety injection system (SIS) and the containment spray system (CSS). The measures taken with equipment immediately present on the site enable the following functions to be ensured for the time necessary to restore the heat sink: maintaining of one charging pump necessary for injection at the primary pump seals. It allows make-up of borated water and reactor depressurising by auxiliary spraying; the thermal inertia of the primary system borated water reserve (PTR tank) is then used as a backup heat sink under an operating procedure devised for this purpose. In the long term the component cooling system (CCWS) no longer cools the auxiliaries correctly. It is stopped manually and declared unusable when the fluid temperature exceeds its maximum operating temperature (temperature at heat exchanger output exceeding 50 or 55°C depending on the sites); replenishing of the auxiliary feedwater system reserve (EFWS tank) to allow removal of residual power by the steam generators in the longer term, if the residual heat removal system (RHRS) becomes unavailable. Evaluation of the impact of an H1 situation on the reactors (affecting first one, then all the reactors of a site) EDF has identified 4 possible configurations: Primary system closed and residual heat removal system (RHRS) not connected Primary system closed and residual heat removal system (RHRS) connected Primary system just open Primary system sufficiently open Case 1: H1 situation affecting a single reactor The thermal inertia of the primary system borated water reserve (PTR tank) is used in the event of loss of the essential service water system (ESWS). It allows the following to be kept in service: one of the primary system pumps, normal spraying and letdown (CVCS). The reactor is thus taken through to shutdown status following a procedure similar to a normal reactor shutdown. In the primary system closed states, a cliff-edge effect in a situation of total heat sink loss ("H1" situation) is associated with the exhaustion of the feedwater reserves (EFWS + SER). On the basis of the SER water volumes required by the technical specifications (TS), the site has an autonomy of several days (100 hours). The SER tanks are 37 BK: Nuclear fuel storage building
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