International collaboration on modeling of Boiling Water Reactors (BWR)
Adrian Tentner, Argonne National Laboratory, USA
Andrey Ioilev, VNIIEF / Sarov Laboratories, Russia
Simon Lo, CD-adapco, UK

In the extreme environment of the reactor core of a Nuclear Boiling Water Reactor (BWR), the ability to accurately predict the different boiling regimes is crucial. The boiling regime determines the rate of heat transfer from the fuel assemblies to the water, and subsequently the plant performance. Consequently, detailed understanding through simulation with STAR-CD is highly desirable. Modeling these phenomena in STAR-CD is the subject of a collaborative project between Argonne National Laboratory (USA), Sarov Laboratories (Russia), the Russian Federal Nuclear Centre (VNIIEF) and CD-adapco.

The project, sponsored by the US Department of Energy’s Initiative for Proliferation Prevention Program, brings together experts in CFD and the thermal-hydraulic phenomena in BWRs. Its goal is to develop an analysis methodology for studying the two-phase boiling flows in BWR fuel assemblies. Ultimately, the models and experience from the project will be encapsulated into an “Expert System” (es) solution, to enable others to carry out such analyses.

The boiling flow phenomena in BWRs are very complex. In simple terms, the reactor core consists of many fuel assemblies; each containing an array of nuclear fuel pins. The coolant water flows between these fuel pins from bottom to the top. As the coolant water rises, heat from the fuel pins is transferred to the water. The water boils and turns into a mixture of water and steam in the upper section of the reactor: undergoing a phase change. In addition, the boiling goes to bubbly flow, slug flow, annular flow and mist flow.

In terms of heat transfer between the coolant and the fuel pins, the coolant goes from single-phase convective heating to subcooled boiling, and saturated boiling heat transfer. Each of these “flow” and “boiling” regimes require special models that describe the mass, momentum, and energy transfer between the liquid and vapor phases, as well as the heat transfer between each phase and the fuel pins.

After the first year of the project, new models for bubbly flow and sub-cooled boiling have been developed, implemented and validated in STAR-CD. Several validation cases were carried out. Each case gives very satisfactory results and demonstrates that the models developed are capable of modeling the bubbly and subcooled boiling flow. Figures 1 and 2 show schematics and comparisons to STAR-CD for one of the validation cases involving boiling of sub-cooled water in the heated section of the pipe followed by condensation in the unheated section. Full details of the models and the validation cases can be found in Ref [1].

Further work for the project includes development of a flow regime prediction method, prediction of critical heat flux and extension of the models to all flow regimes found in the BWRs.

So far, the combined expertise of the partners has delivered validated models for a number of the complex phenomena in a BWR. This marks a significant milestone on our road to simulating the extreme environment of a BWR.

Reference
1 A. Tentner, S. Lo, A. Ioilev, M. Samigulin and V. Ustinenko “Computational fluid dynamics modeling of two-phase flow in a boiling water reactor fuel assembly” Mathematics & Computation, Supercomputing, Reactor Physics & Nuclear & Biological Applications, Palais des Papes, Avignon, France, September 12-15, 2005.

Figures
01: Scheme of experimental section for study of boiling and condensation
02: Average void fraction as a function of distance along the pipe