The review of Westinghouse’s validation evidence for the effectiveness of the AP1000 passive reactor heat removal system

1.1 In 2007, Westinghouse (WEC) submitted its AP1000 reactor design to ONR for a phased Generic Design Assessment (GDA). This culminated in 2011 with ONR publishing a suite of Step 4 technical assessment reports, an interim Design Acceptance Confirmation (iDAC) and the 51 outstanding GDA Issues which must be resolved before ONR will consider granting a “full” DAC. On reaching this point, WEC paused its GDA related activities. In 2014, WEC requested to resume GDA to support the Moorside project. 1.2 In the event of a plant transient, the AP1000 is designed such that core decay heat removal is normally accomplished by the (active) startup feedwater system. However, if that system is not available (for example, due to a loss of all ac electrical power), emergency core decay heat removal is provided by the Passive Residual Heat Removal (PRHR) heat exchanger. The PRHR heat exchanger is a C-tube heat exchanger connected, through inlet and outlet headers, to the reactor coolant system. The inlet to the heat exchanger is from the reactor coolant system hot leg, and the return is to the steam generator outlet plenum. The heat exchanger is located above the core to provide natural circulation flow when the reactor coolant pumps are not operating. The In Containment Reactor Water Storage Tank (IRWST) provides the heat sink for the heat exchanger. After the IRWST water reaches saturation (in about two and half hours), steam starts to vent to the containment atmosphere. The condensation that collects on the containment steel shell (cooled by the passive containment cooling system) returns to the IRWST, maintaining fluid level for the PRHR heat exchanger heat sink. The extant safety documentation submitted to ONR during the initial GDA period (Ref 1, Section 15.2.6) made a claim that by these means, the PRHR heat exchanger, in conjunction with the passive containment cooling system, would be able to keep the reactor coolant subcooled indefinitely.

1.3 ONR fault studies inspectors investigated this claim during GDA Step 4 (Ref 2, notably section 4.2.3.3). They found that Ref 1 did not include a design basis assessment of the passive containment cooling system for such plant transients or a substantiation of the efficiency with which re-condensed water is collected and recycled back to the IRWST. As a result, amongst the 51 GDA Issues is GI-AP1000-FS-06 (Ref 3) which requires WEC to provide validation evidence showing that the IRWST is functionally capable of cooling the PRHR during intact circuit faults for 72 hours.