Numerical Analysis of in-vessel Melt Retention of CANDU Reactors During a Severe Accident

Abstract of the technical paper presented at:
14th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics,
Operation, and Safety (NUTHOS-14)

Prepared by: S. Gyepi-Garbrah, F. Mehdi Zadeh, B. Moore, T. Nitheanandan
Canadian Nuclear Safety Commission

Abstract:

Corium, the molten mixture of nuclear fuel and reactor debris, presents significant challenges in the safety analysis of CANDU reactors. Understanding corium behaviour near the end shields and crust formation on the inside surface of the calandria vessel is crucial for assessing the safety strategy known as In-Vessel Retention (IVR). The heat dissipated through the reactor vault water around the calandria must not exceed the critical heat flux to prevent the calandria vessel failure and molten corium pool from overheating. Therefore, understanding three-dimensional effects in corium hydrodynamics near the end shields and crust formation on the inside surface of the calandria is essential for designing mitigation measures and prevent reaching the critical heat flux on the external surface of the calandria vessel during a severe accident.

The IVR strategy aims to prevent the release of corium into subsequent barriers (i.e., containment and the site boundary of CANDU reactors) while maintaining the integrity of the reactor vessel. Accurate simulation of corium behaviour is vital for evaluating the performance of IVR structures, predicting corium relocation patterns, and assessing potential breaches in containment.

This paper investigates the behaviour of three-dimensional corium flow, encompassing complex phenomena related to multiphase flow, solidification, and heat transfer during crust formation. We employ CFX-Solver to study the flow behaviour in a 1/5 scaled-down model of the reactor's corium pool and observe its progression. Experimental data obtained under representative conditions from the Canadian Nuclear Laboratories (CNL) are used to validate the CFX model.

The findings from this study offer valuable insights into corium hydrodynamics near the end shields and crust formation, contributing to the enhancement of IVR safety strategies and efficient accident management in CANDU reactors. This study underscores the significance of ongoing research and development in corium behaviour modelling and simulation techniques.

Our understanding of corium hydrodynamics and heat transfer mechanisms will evolve as further research progresses, leading to more precise predictions and assessments. The integration of advanced computational tools and experimental validation holds promising potential for refining IVR safety strategies and ensuring the dependability and efficacy of such measures.

The Canadian Nuclear Safety Commission is contributing to the International Atomic Energy Agency’s (IAEA) Cooperative Research Project (CRP) on In-Vessel Melt Retention (IVMR) to ensure collaboration with international stakeholders in emphasizing IVR's role in maintaining containment integrity by performing benchmark exercises, including adopting a realistic modeling approach to confirm the effectiveness of IVR in CANDU reactors and new build water-cooled reactor.

Keywords

CFX, severe accident, IVR, severe accident management

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