An Object Oriented Approach to Simulation of TRIGA Mark II Dynamic Response

This paper deals with the development of a model for the nuclear research reactor TRIGA Mark II operating at University of Pavia. The purpose of the modeling is to reproduce the dynamic behavior of the reactor on the entire operative power range, i.e. 0÷250 kW, using the object oriented approach, implemented by the Modelica language. The main advantage is the a-causal formulation of the model, based on equations instead of statement assignment. Equations do not specify which variables are inputs and which are outputs, thus the causality in the model is unspecified and is fixed only when the corresponding equation system is solved. In this way, equations can be solved according to the data flow context in which the solution is computed. The model describes the entire plant, including the heat removal system. The component representing the reactor core contains a series of subcomponents linked together through rigorously defined interfaces: in this way, it is possible to consider the interactions between the different physical aspects of the system. Equations governing natural circulation have been implemented in a component which defines the mass flow rate through the core, according to the temperature difference at the ends of the channels. Secondary and tertiary cooling loops are modeled using a simplified heat exchangers configuration: concentric tube version is adopted, which allows recreating the heat exchange dynamics without a great modeling effort. The developed a-causal model has been validated through the comparison with experimental data collected on the site, concerning three different power transients at 100 kW, 50 kW and 1 kW. A corresponding causal model has been referenced as concerns fuel and coolant temperatures evolution during the transients. The predictions of the two main approaches to dynamic modeling have been compared. A very satisfying accordance is found as discrepancies observed on the coolant temperature are comprised between 0.5% and 1% for the three transients.