Nuclear power plants can provide CO2-free electricity and heat, supporting the European net-zero emissions objectives. The Polish energy transformation strategy includes the development of large-scale pressurized water reactors, as well as small modular reactors (SMRs). Both state-owned and private companies consider SMRs as a potential source of electricity and beyond, i.e. district heating, industrial process heat, and hydrogen production. Over the past few years National Centre for Nuclear Research, Poland (NCBJ) has been involved in several high temperature gas-cooled reactor (HTGR) projects at both the national (Gospostrateg-HTR and HTR-MEiN) and European level (Gemini Plus and Gemini for Zero Emission).

At the national level, a small-scale, prismatic-type research HTGR of 30 MW(thermal), named the HTGR-POLA, is being considered to be built at the NCBJ site. Its main mission would be to serve as a demonstrator of HTGR technology for Polish industry. This paper investigates the reference HTGR-POLA plant design (v1), the resulting optimized core configuration (v2), and their impact on the reactor safety performance during selected design-basis accidents (DBAs).

The proposed optimization of the core, from the initial to the optimized configuration, was successful, with the aim of improving the overall safety of the plant. The introduced capability of the coupled neutronic/thermal-hydraulic phenomena simulations for the reactor core over the whole fuel cycle is a promising approach, as it allows for the approximation and identification of the most relevant safety issues without relying on overly conservative assumptions.

The results of the calculations performed using the MELCOR 2.2. code are presented for selected accident scenarios, depressurized loss-of-forced circulation and pressurized loss-of-forced circulation, which represent different types of postulated initiating events for HTGRs. Specifically, these scenarios correspond to a pipe break and a primary blower stop, respectively.

The presented core volume fraction, as a function of fuel temperatures, shows that during most severe scenarios, the fuel temperature remains more than 500°C below the safety limit of 1600°C, at which the fission product release rate from the fuel significantly increases. These results, which will support the future probabilistic safety analyses of the HTGR-POLA reactor by assessing the accident consequences, will constitute a significant component of the preliminary safety analysis report (PSAR). The PSAR is a regulatory and legal requirement for newly built nuclear power plants and research reactors in Poland.