Development of a new steady-state thermal hydraulic and safety analysis code, OpenTHY, for a TRIGA MARK II research...

This work presents the development of a new powerful computer code OpenTHY for thermal hydraulic safety analysis of TRIGA type research reactor in the case of one-phase flow around the fuel pins. This code is used for preparing a very detailed thermal-hydraulic model for the Moroccan reactor to study the efficiency of core cooling in natural convection mode and to ensure that the reactor is operating in a safely manner respecting all safety limits. The OpenTHY code has also the possibility to take into account the presence of the Zirconium rod that is inserted into an annular fuel rod, which is essential to avoid cladding failure due to the hydrogen overpressure. The fuel element was discretized into multiple axial sections, with each section further subdivided radially into various subdivisions, covering four distinct regions: cladding, gap, active fuel, and zirconium rod. The active part was divided radially into a significant number of subdivisions allowing for detailed spatial distribution of the power peaking factors in both axial and radial directions. Then, we calculated different thermal-hydraulic safety parameters using the single channel model and specific correlations related to heat transfer coefficients and critical heat flux to determine the temperature profiles of coolant and fuel element in different axial and radial locations, the critical heat flux (CHF) and the departure from nucleate boiling ratio (DNBR) in each axial subdivision of the hottest channel of the core. Additional core configurations were evaluated and the results obtained by the OpenTHY code were validated through a comparison with experimental measures of temperature inside two instrumented fuel elements at various power levels. This study presents also the results calculated by PARET/ANL code. From these output data, the safety of the reactor is ensured in the conditions of natural convection-cooling and the maximum temperature profiles at the cladding outer surface and at the inner surface of the active fuel rod remain largely far from safety limits prescribed in TRIGA safety analysis report (SAR). In addition, the MDNBR presents a value widely higher than the design limit of 1.3 which guarantees the safe operation of the 2MW TRIGA MARK II under normal conditions.