Analysis of Station Blackout for the LDR-50 SMR in SPECTRA and Apros

In the last couple of years, there has been a growing interest in small modular reactors (SMRs). These innovative designs aim to enhance passive safety and simplify reactor construction, while also presenting new challenges in demonstrating operational safety. This paper discusses calculation results for the LDR-50 reactor: a 50 MW_th Small Modular Light Water Reactor (SM-LWR) engineered by VTT in Finland since 2020, and tailored specifically for district heating applications. The LDR-50 SMR heavily relies on passive systems both for operational reliability and in accidental conditions. This paper addresses analyses conducted on the LDR-50’s response to a station blackout (SBO) scenario, since this transient is one of the most challenging and interesting scenarios for the LDR-50. These analyses, facilitated by the utilization of two system codes, SPECTRA (developed by NRG) and Apros (developed by VTT and Fortum), allowed for a comprehensive evaluation of the reactor’s performance under extreme conditions, highlighting the effectiveness of its passive safety systems. The results of the two system codes are compared, with focus on the primary system behavior and subsequent heat removal through the reactor vessel walls into the containment pools over a time span of 22 days. In addition to the SBO transient analysis, sensitivity calculations were performed with the SPECTRA code to assess the flexibility of the LDR-50 reactor in higher-power and lower-power modes.