Optimizing Filtered Containment Vent System Performance for CANDU Reactors

Abstract of the technical paper presented at:
SWINTH-2024
June 17-20, 2024

Prepared by:
Mohamed Shawkat,
Canadian Nuclear Safety Commission

Abstract:

In this study, a new approach to explore the symptom-based parameters that can be used as indicators to activate the Filtered Containment Vent System (FCVS), considering the design constraints of the system, and FCVS set the ON/OFF limits based on the inter-correlation between these parameters to optimize the FCVS performance. The considered operating parameters are containment pressure, temperature, hydrogen content, and radioactive source term. The flexibility of changing the ON/OFF activation conditions during the multiple venting cycles depending on the relation between the identified parameters is investigated. The current practice of using the containment pressure as the only indicator to activate the FCVS has the benefit of simplifying the system operating documents which may be beneficial for the operation perspective. However, during severe accident progression, the dependence on pressure signal as a single input parameter to activate the FCVS does not ensure effective and optimum performance of the system due to the following reasons:

1. The integrity of the containment is mainly jeopardized by containment pressure, temperature, and hydrogen quantity. Using only the pressure value as an indicator to activate the FCVS implicitly assumes that during all the severe accidents both the temperature and hydrogen are directly proportional with the containment pressure, and they will not reach a threatening level prior to reaching the system opening pressure limit.
2. The FCVS has its own design constraints (i.e., maximum flow, overheating due to fission product accumulation, risk of flooding due to steam condensation (in case of using dry filters), risk of hydrogen ignition within FCVS during venting…etc.). With the general knowledge that severe accident management is symptom-based, these constraints may change during the multiple venting cycles and the dependence on only the pressure will not provide an effective method to capture them.
3. The venting cycles, after the first cycle, may result in a high radiation field at the isolation valves station or around the path to this station which raise habitability concerns that may hinder the ability of the operator to continue activating the system if more venting cycles are required.

The above reasons will require the operator to arbitrarily choose low ON/OFF pressure limits to operate the FCVS which will result in early venting of the containment and a longer duration of the venting cycles which may be unnecessary.

As a case study, multiple MAAP-CANDU simulations were performed for several severe accidents in a generic CANDU-6 NPP to examine the ranges of proposed operating parameters and the relation between them. A hypothetical FCVS design was proposed with pressure and temperature design constraints that were determined based on the ability to mitigate Station Blackout (SBO) consequences. The response of the containment following FCVS venting based on the intercorrelation between the proposed operating parameters was compared to the response due to venting based on only the containment pressure to illustrate the impacts of each approach.

In the future, the new approach can be used to automate the operation of the FCVS with the assumption that it will be powered by a separate reliable power source.

This paper demonstrates the CNSC’s commitment to research and development by investigating possible opportunities to optimize the performance of FCV beyond its classical operating fashion, to further lower the risk of severe accident consequences. Publishing the paper in a conference specialized in severe accidents meets CNSC mandate in disseminating CNSC scientific activities and expose the research findings to scientific community scrutiny for further improvements through discussion.

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