Bounding severe water ingress analysis in modular high-temperature gas-cooled reactors

A major safety advantage of modular high-temperature gas-cooled reactors (MHTGRs) is the slow response to most postulated accident transients. Because of its high-quality ceramic fuel, significant fission product release resulting from fuel failure can only occur if the fuel is exposed for an extended time period to temperatures significantly above 1600 degree C. The only conceivable mechanistic scenario for early and significant fuel failures would be a very severe and rapid water ingress into the core, which could result in a significant reactivity insertion and fuel heatup. During normal operation, ∼160 kg/s of helium are circulated, entering the core at ∼260 degree C and exiting at ∼690 degree C. Coolant transit time through the loop is ∼10 s. If a severe break in the steam generator were to occur, water would enter the primary loop, mix with helium, and enter the core as vapor. The severity of any such ingress transient would depend on how fast the vapor could enter the core, and in what concentration. This would be primarily a function of the break location and size. The approach taken here is to evaluate, as an upper bound, the temperature transient for the most severe, yet physically possible, water ingress scenario, namely, that the water vapor concentration in the core rises from 0 to 100% within a very short time, where the rise time is to be varied parametrically