Multi-scale, coupled reactor physics / thermal-hydraulics system and applications to the HPLWR 3 pass core

Abstract

Innovative reactor concepts, currently under investigation within the Generation IV International Forum, present features which may differ significantly from those of existing reactors and may go beyond the current state-of-the-art approach used for design and safety investigations requiring to develop new analyses tools. The High Performance Light Water Reactor (HPLWR) is an example of such a Generation IV reactor concept with additional requirements for advanced analyses tools; it is a thermal spectrum nuclear reactor cooled and moderated with light water operated at supercritical pressure. The pronounced water density reduction with the heat up, together with the multi-pass core design, results in a pronounced coupling between reactor physics and thermal-hydraulics core analyses which can not be neglected. The water density distribution within the core changes appreciably the reactivity and the leakage probability of the different core regions while the fuel temperature variations, and the associated actinides resonance broadening, results in 3D feedbacks distribution which modifies the power generation within the core. The steady state operative condition can be predicted only with coupled reactor physics / thermal-hydraulics analyses requiring the development of a new computational system. The coupled reactor physics / thermal-hydraulics analysis has been addressed using available stand-alone codes and expressing the coupling via data exchange among them. The envisioned steady state investigations do not raise any question on the time step selection, and an iterative procedure, in which the codes are run in series, has been chosen to achieve the coupling. The developed code-to-code interfaces, written in Perl language, are devoted to data extraction and input file preparation, they enable automation of the iterative procedure. The selected programming language allows simplicity and high flexibility of these code interfaces which have problem dependencies but can be easily modified to apply this system to a different HPLWR design or even to other reactor concepts. The multi-pass core design demands 3D models which have been built for the available stand-alone codes. The selected tools have been checked for the current applications by means of code-to-code comparison and inspection of the source code. After the initial testing of the coupled system, an approach to carry out whole core coupled analysis has been proposed and successfully applied obtaining promising results. These coupled analyses are based on a fuel assembly wise spatial representation of the core. The pronounced neutron flux gradients within the multi-pass core, together with the considerable changes in water properties with the heat up, challenges the accuracy of these average values obtained with the coupled system and hence the whole core has been investigated at sub-channel resolution extracting the boundary conditions from the predicted operative condition. A pin-power reconstruction technique has been introduced to produce reliable input data for the sub-channel investigations.