Conceptual study on air ingress mitigation for VHTRs

Abstract

An air ingress accident following a postulated pipe break is considered a critical event for a very high temperature gas-cooled reactor (VHTR) safety. Following helium depressurization, it is anticipated that air will enter the core through the break leading to oxidation of the in-core graphite structures. Under extreme circumstances and without mitigation features this accident may lead to exothermic chemical reactions between graphite and oxygen depending on the accident scenario and the design. Under extreme circumstances (beyond design basis), a loss of structural integrity may occur in some core structures and lead to elevated release of radiological inventory for the fuel matrix.This paper discusses various air ingress mitigation concepts applicable for the VHTRs that would prevent core damage even in the most extreme scenarios. The study begins with identifying important factors (or phenomena) associated with the air ingress accident using root-cause analysis. By preventing main causes of the important events identified in the root-cause diagram, the basic air ingress mitigation ideas were conceived and developed.Among them, two concepts were finally evaluated as effective candidates. One concept is to inject helium directly into the lower plenum (direct in-vessel injection); the other concept is to enclose the reactor with a non-pressure boundary with an opening at the bottom (ex-vessel enclosure). Computational fluid dynamics (CFD) methods were used to evaluate these concepts for proof of these principles. Results indicate that both concepts can effectively suppress air ingress. In the first concept, the buoyancy of the injected helium replaces the air in the core and the upper part of lower plenum. This prevents air from moving into the reactor core and shows the most potential for mitigating graphite oxidation within the vessel. In the second concept, air ingress is limited by molecular diffusion through the opening at the enclosure bottom, a very slow process that allows sufficient time for the core to cool. Existing VHTR concepts can be modified to accommodate either of these mitigation concepts.