Analysis of loss of cooling accidents in the spent fuel pools using system thermal hydraulics and computational fluid dynamic codes.

• The models of single basket with spent fuel assemblies were developed and simulations were performed by employing ANSYS FLUENT (computational fluid dynamic) and RELAP5 (one dimensional) codes. • The model, developed using one-dimensional code, allows to describe coolant mixing and received with its results fits well with computational fluid dynamic code calculations. • The loss of cooling accident in the spent fuel pools of Ignalina NPP was performed using one-dimensional code RELAP5. The interest in the safety of on-site storage of Spent Fuel Assemblies (SFAs) has been increased after the accident on 11 March 2011, in Fukushima Daiichi Nuclear Power Plant. It is important to analyse the behaviour of not only integral effects in spent fuel pools, but also the local effects during heat removal from the SFAs. Usually, for the integral analysis the one-dimensional system computer codes were used. These codes allow to analyse whole pools and cover long time, with a relatively low cost of computational resources. The Computational Fluid Dynamic (CFD) codes could perform very detailed analysis, but this requires large computational resources. The strong side of CFD codes is the possibility to assess local effects which could occur. In the current work the analysis of heat transfer from the SFAs, placed in special baskets was performed using ANSYS FLUENT 17.2 and RELAP5 codes. The models of single basket with SFAs were developed using both codes. The comparison of calculations allowed to evaluate capabilities of both codes. It is shown, that using one-dimensional system thermal hydraulics code developed model allows to describe coolant mixing inside basket sections and received results fits well with CFD code calculation results. Based on gained experience, the loss of cooling accident in the spent fuel pools of Ignalina NPP was analysed using the model, developed by employing RELAP5 (one-dimensional system thermal hydraulics) code. The calculation results were used for safety evaluation of Ignalina NPP facilities. [ABSTRACT FROM AUTHOR]