Examination of Hypothetical Transportation Accidents for Transportable Nuclear Power Plants

The reintroduction of Transportable Nuclear Power Plants (TNPPs) opens the doors for potential accidents which have not been previously considered under current licensing standards. TNPPs transported via truck, rail, aircraft, or boat may be involved in various hypothetical transportation accidents. 10 CFR 71, Packaging and Transportation of Radioactive Material, regulatorily addresses various accident scenarios. However, when applying logistics and design constraints alternative methods to license and transport these TNPPs will be required. These alternative methods require the use of Probabilistic Risk Assessments, modeling of accident scenarios, and experimental data to demonstrate an equivalent level of safety to 10 CFR 71. This study examines various hypothetical transportation accident scenarios to determine the worse-case outcomes. The initial accident scenario was a large impact of a reactor module that would lead to a rearrangement of the core into a potentially more critical geometry. Each rearranged model was also submerged and immersed in both salt and freshwater to examine the effects. To observe the most conservative scenarios, reactivity control materials, such as control rods or drums and burnable poisons, were absent. These core rearrangement accidents were modeled in the continuous-energy Monte Carlo code Serpent and the point kinetics equations analyzed the resulting transient. A radiation source term was developed with inputs from both the transient and release mechanisms outlined in PNNL-33607. The health physics code HotSpot was then used to generate a visualization of potential releases. Analysis of these highly conservative accidents scenarios and rearrangements demonstrated that TRISO failure due to a transient was likely limited to incidents with a submerged core and in leakage of water. In these cases, core rearrangement was not a prerequisite for TRISO failure as the control design of the reactor yielded the worst-case result in terms of maximum fuel temperature. When examining radiation releases, initial scoping analysis shows that emergency planning zones could be as small as 10s of meters based on 10 CFR 100, Reactor Siting Criteria, for accidents which occur on land. This analysis demonstrates the safety of expected TNPP reactor designs in that, with conservative assumptions, relatively small releases can be expected.