Evaluation of the Fuchs-Nordheim model including heat capacity variation for a TRIGA® reactor

Resumen

The TRIGA^® (Training, Research, Isotope General Atomics) are reactors that, currently, are utilized for staff training, radioisotope production and for researching in many areas of science; nowadays, more than 20 countries own TRIGAs^®, including Brazil. These reactors have two operating modes: steady state (where it operates within constant neutron flux and its parameters also remain constant) and pulsed mode, in which a single control rod (named transient rod) is quickly withdrawn from the core, increasing reactivity and leading to a prompt supercritical state, which causes an exponential increase in power and temperature. However, due to the negative temperature coefficient of reactivity of the fuel used to feed the reactor, the power reaches a peak, and it rapidly decreases until shutting down fission reactions, in a process that is inherently safe. The pulses that are generated during reactor operations can have its parameters described by mathematical models, such as Fuchs-Nordheim modelling, that equate the behavior of reactivity, power and temperature during the lifespan of the pulse. However, the current model has some basic limitations, mainly because it considers that parameters such as heat capacity remain constant during the whole process, and it is known that, in practical cases of nuclear reactor operations, heat capacity is a parameter that is a function of temperature. Therefore, this article aims to evaluate the impact that the introduction of heat capacity as a linear function of temperature has on final results, comparing data with and without this assumption.