MCS Cycle Depletion Analysis and Validation of Excess Reactivity and Shutdown Margin for the Kartini Triga Mark II Research Reactor

Abstract

The Kartini Triga Mark II Research Reactor (Kartini TM2RR), one of the three research reactors in Indonesia, is a pool-type reactor with a thermal power of 100 kW that has been operating since 1979 and has legal operation licensing until 2019 from the regulatory body. As of 2018, the reactor is composed of 71 cylindrical fuel elements of height 66 cm in a hexagonal array, with most of the fuel elements remaining untouched from their first loading in the reactor in 1994. The reactor also comprises three rods for reactivity control and nine peripheral graphite block elements. The reactor is operated on a per-user-demand basis with irregular cycle lengths (at the maximum six hours operation per day) with an average operation time of 200 hours a year. A core safety analysis of the Kartini TM2RR is currently being prepared to complete requirements from the regulatory body. In this safety analysis, core excess reactivity and reactivity shutdown margin are the main parameters to be calculated precisely and compared to experimental data. The experimental data consists of measurements of the core excess reactivity and reactivity shutdown margin of the Kartini TM2RR that have been performed twice a year since 2010. The experiments were conducted at zero power by withdrawing one of three control rods step by step until the fully withdrawn position and repeated for each control rod separately. The modelling of the reactor and the subsequent depletion and criticality analysis are performed with the nuclear data library ENDF/B-VII.0 and the Monte Carlo code MCS under development at Ulsan National Institute of Science and Technology (UNIST) since 2013. The inner core and reflector area of Kartini TM2RR are first modelled with MCS from a MCNP input containing the reference geometry. The results of the MCS model with fresh fuel are verified against the results of the MCNP model with fresh fuel to guarantee the consistency of the two geometrical models. Then the depleted fuel compositions of each fuel element at the beginning of 2010 are computed with MCS depletion calculations of a single fuel element in an infinite hexagonal lattice (“jump-in” from the first loading of the fuel element to beginning of 2010). Those depleted fuel compositions are then inserted in the MCS whole core model and whole core depletion (one fuel element = one depletion cell) is performed with MCS, simulating the reactor operation from 2010 to 2018. Calculated values of core excess reactivity and reactivity shutdown margins are finally obtained with MCS and comparison analysis of the experimental and calculated values is conducted.