Optimisation of accident management measures to reduce iodine releases during SGTR

Steam generator tube rupture (SGTR) accidents create a bypass of the containment of a pressurised water reactor (PWR) and can therefore result in the release of primary system coolant to the atmosphere via the steam relief or safety valves. In general, primary system coolant will transport radionuclides such as iodine131. Accident management strategies for SGTR accidents therefore aim to reduce releases to the environment while ensuring core cooling. The Downhill Simplex algorithm is used in this paper to optimise the timing of accident management measures during a SGTR accident. The secondary system steam relief and safety valves (SRV) are assumed to fail in the stuck open position at the first opening. Depressurisation of the primary system by opening the pressuriser pilot operated relief valve (PORV) and keeping the primary pressure low by shutting down two of the three trains of the high pressure injection system (HPIS) is assumed as the accident management procedure. The success of the measures is evaluated by a Relap5-3D simulation, which calculates the thermal hydraulic behaviour of the system. One of the key parameters used to assess success is the amount of iodine-131 released into the environment. The algorithm varies the timing of a set of three operator actions - opening the PORV and shutdown of HPIS trains one and two. In addition to iodine release, two other parameters are evaluated - reactor core coolant level and primary system pressure. Three normalisation functions are used to convert these parameters into a single target value, which is low when the core is covered and both primary system pressure and iodine release are low. The simplex algorithm then modifies the timing of operator actions to achieve a local minimum of the target value. The results show that the Downhill Simplex algorithm can be used to optimise the timing of operator actions. Although timing cannot be directly implemented in Accident Management Procedures (AMPs), it is important to be aware of time sensitivity when designing AMPs. In addition, the algorithm can be adapted to optimise design parameters such as valve sizes, hydro accumulator nominal pressure levels. The work was performed within the EURATOM R2CA project.