Effect of buoyancy-induced natural convection heat transfer on thermal qualification of lead-shielded radioactive material transport packages

Radioactive materials (RM) are stored and transported in well-shielded packages that require certification by the nuclear regulatory authority. Certification is subject to successful demonstration of compliance to a series of tests. Among these, the thermal test is intended to simulate a 30-minute, fully engulfing fire, that may occur during a transport accident involving spillage of large quantities of hydrocarbon fuel. In the present work, the thermal response of a Type-B(U) transport package to fire exposure is investigated by applying CFD techniques. The phase-change processes are captured in detail and the effect of buoyancy-induced natural convection in enhancing the heat transfer and melt fraction is brought out. The duration of convection dominated heat transfer to the package is clearly demarcated from the conduction dominated heat transfer by identifying a cross-over point. The instantaneous heat transfer rates into the package are analysed to bring out the fraction of heat stored as sensible or latent heat. The effect of specifying fire boundary condition using HC-air curve, ISO-834 curve or a constant ambient temperature of 800 °C is also highlighted by means of parametric studies. Finally, some comments on the effect of RM package size on the heat transfer aspects is provided.