Adequacy of bleed condenser/degasser condenser relief valves capacity in the CANDU operating plants to mitigate consequences of a total loss of heat sink accident

Prepared by: CNSC Staff

Abstract

In 2003, while conducting their own reviews, CNSC staff asked nuclear power plant licensees to review the scenario of a total loss of all heat sinks. Subsequently in 2004, the discharge rate of the bleed condenser/degasser condenser relief valves (BCRV/DCRV) was assessed in order to confirm that the relief capacity requirements were satisfied. In addition, as part of the Fukushima action item related to evaluation of the BCRV/DCRV relief capacity, the issue was reassessed to confirm that the relief capacity requirements are still satisfied and the overpressure in the primary heat transport system is well below the failure pressure for the boiler tubes. Meeting this criterion would ensure that there would be no boiler tube failure during a total loss of all heat sinks, a scenario that is categorized as a beyond-design-basis accident.

Therefore, based on the CNSC’s assessments of the licensees’ analyses and testing, CNSC staff draw the following conclusions:

  • The BCRV/DCRV can provide adequate relief capacity for both design-basis and beyond-design-basis events.
  • The relief valve design and testing meet requirements of the American Society of Mechanical Engineers.
  • Addition of the recently installed emergency mitigating equipment provides another layer of defence in depth to the existing mitigating measures.
  • Even if an event involving the total loss of all heat sinks happens and no mitigating action is taken, significant radiological releases are extremely unlikely to occur, since boiler tube failure and the consequential containment bypass are not expected to occur.

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