The effect of central flux trap on-line sampling on Tehran Research Reactor(TRR) core safety parameters

Abstract

Introducing the Neutron Flux Trap Channel to research reactors is an effective method to increase the neutron flux utilization for conducting the irradiation experiment and radioisotope production. The maximum thermal neutron flux could be achieved due to increased moderation in the flux trap area surrounded by the fuel elements, while all safety parameters and criteria must be met during the operation of the reactor. Regarding the TRR operator’s report about induced positive reactivity due to the entrance of some samples during reactor operation, it is predicted that this region is over-moderated, and the moderator reactivity coefficient is positive locally. Hence, this study aims to analyze experimental observations and the reactor safety parameters during online sampling into the flux trap channel. In this analysis, TRR Core’s different operating cycles were studied, and static, kinetic, and dynamic parameters of the reactor core with or without a flux trap channel were evaluated. For this purpose, some experiments were conducted to determine the safety parameters resulting from the sample placement, and then calculation results were validated against the experimental ones. Results indicate that void and temperature reactivity coefficients in the flux trap channel are locally positive; for example, the temperature and void reactivity coefficients for operating cycles with 33 fuel assemblies are +3.60 pcm/°C and +18.39 pcm/%, respectively. If 56.7% void is created in this channel, about 1.080$ positive reactivity will be induced to the core.Finally, accident analysis was conducted during various scenarios of reactivity insertion of different samples, and the variation of reactor power, reactivity, and fuel and clad temperatures were estimated in different transients. It is indicated that the maximum fuel and cladding temperatures in the worst case will be 183 °C and 144 °C, respectively, less than the melting point of aluminum meat of fuel.