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Informing Severe Accident Management Guidelines for a Pressurized Water Reactor with MELCOR Simulations

Time: Wed 2022-04-13 09.00

Location: FA31, Roslagstullsbacken 21, Stockholm

Language: English

Subject area: Physics, Nuclear Engineering

Doctoral student: Nan Zhao , Kärnkraftssäkerhet

Opponent: Professor Jinho Song, Korea Atomic Energy Research Institute

Supervisor: Sevostian Bechta, Kärnkraftssäkerhet

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Severe accident management guidelines (SAMGs) play an important role in the hierarchical structure of the defense-in-depth (DiD) principle of reactor safety. Among different methods to verify and validate the effectiveness of SAMG on mitigating severe accident consequences, the approach of numerical simulations using best-estimate computer codes was extensively applied to evaluate the SAMG and SAM actions. 

In addition to a review on the previous works assessing SAMGs through numerical simulations, the present study is intended to examine and inform the effectiveness of SAMG and its actions for a Swedish pressurized water reactor (PWR) through numerical simulations of the MELCOR code. The research work is composed of i) development and qualification of MELCOR model for the PWR chosen; ii) evaluation of SAMG entry condition; and iii) assessment of operator actions in the SAMG (so-called SAM actions) under different accident scenarios. The SAM actions include depressurization (individual action) and primary-side bleed and feed (PBF) actions which are among the most important SAM actions. The risk-important accident scenarios selected in this study are station blackout (SBO), total loss of feed water (TLOFW), loss of coolant accident (LOCA), and their variations. 

The development and qualification of the MELCOR model for the Swedish PWR is conducted through nodal sensitivity studies which provide the impacts of the COR nodalization and CVH nodalization in the MELCOR model on simulation results. The qualified MELCOR model with achievable accuracy and computational cost is then adopted in the evaluation of SAMG and its actions through numerical simulations.

The interests of the numerical simulations for evaluating the SAMG entry condition and SAM actions are focused on the timing of events, accident consequences, negative/positive effects of SAM actions, etc. Based on the evaluation outcomes, the main points are concluded as follows:

-          The setpoint 650oC of the average core exit temperature (CET) is an effective entry condition of SAMGs (i.e., performing transition from EOPs to SAMGs at the onset of core damage), given the representative accident sequences as the main contributors to the core damage frequency (CDF) of the reactor chosen.

-          The PBF strategy is effective to cease the core relocation and prevent the RPV failure in both TLOFW and LOCA scenarios if the PBF actions are operated within respective grace periods which can be determined through the numerical simulations. 

-          The grace periods of PBF actions are not only dependent on the accident scenarios, but also affected by the timing of bleed/feed actions, RCS depressurization rate (opening of PORVs), injection flowrate, and their combinations.

-          The earlier RCS injection in the grace period can mitigate the hydrogen generation and radioactive release from the core, but a later RCS injection beyond the grace period will produce more hydrogen.

-          The RCS injection in the later stage of core degradation may also mitigate the release of fission products from primary circuits to the containment, since the injected water can scrub the aerosols generated from the core.