Simulation of Iodine Spiking phenomenon during Design Extended Condition type A conditions at VVER-1000 and Generic PWR

After the Fukushima Daichi accident in 2011 design extension conditions (DEC) came into focus. Most regulatory regimes now require that design extension conditions are considered in the design of new reactors, while existing reactors should try back fitting safety enhancements to be able to deal with design extension conditions as far as reasonably achievable. DEC are divided in categories A and B, where DEC-A scenarios are characterized by multiple failures of safety systems but with the reactor core remaining intact. A phenomenon that plays a significant role in DEC-A scenarios involving a containment bypass is Iodine Spiking (IS). In this paper we analyse a PRImary to SEcondary leaking accident (PRISE) with the assumption of a stuck open atmospheric relief valve at the first opening at two different reactor designs (VVER-1000, Generic PWR). For the thermal hydraulic best estimate simulation we use RELAP5-3D, for the transport of iodine we use the Relap5-3D radionuclide transport model. Additionally, to analyse the Iodine Spiking (IS) phenomenon for the examined accident scenario, we created an empirical IS model which is based on the existing NRC IS model to estimate how much iodine is released to the primary system from the fuel. The results of this study show that the Relap5-3D radionuclide transport model is capable of providing a bounding analysis on the iodine released to the environment. The analysis also shows that good results can be achieved for the analysed scenarios using an empirical model to estimate the iodine released to the primary system. The analysis also shows that during said scenario significant quantities of radioactive fission products are released into the environment.