Protecting Microreactors from Extreme External Threats

The deployment of Microreactors (MRs) must address protection against physical security threats, defined here as weapons detonations, wind-borne missiles, and aircraft impact. This paper presents an exploratory study on the robustness of one MR to resist such physical threats using simplified methods. The architecture of the MR studied here is the sodium-cooled, graphite-moderated thermal reactor (SGTR) developed at MIT, which is enclosed in a box-type structure composed of thick reinforced concrete walls. Three physical security threats are investigated in this paper: air blast, wind-borne missile impact, and aircraft impact. Earthquake shaking, although an external hazard, is not addressed here. Each threat is characterized for the purpose of analysis, but the loadings are not based on a site-or project-specific assessment. Indeed, these threats are neither endorsed by the authors nor based on regulatory criterion and warrant near-term definition by the nuclear industry. Three detonations are evaluated: a contact charge on the wall of the reactor building, and detonations of 20 kg and 450 kg of TNT at a standoff distance of 10 m from the wall. The local and global damage associated with the accidental impact of a Cessna Skyhawk 172 and a Falcon 7X is assessed. The impact of a hurricane-borne Schedule 40 pipe on the wall is also investigated, leveraging published data. These investigations employ simplified calculations complemented by preliminary nonlinear finite element simulations using ABAQUS. Although the findings of the study are directly applicable to the MIT SGTR, the processes followed could be applied to other MRs. The thick reinforced concrete radiation shield that serves as the enclosure for the MIT SGTR is highly effective at mitigating the effects of the considered physical threats and protecting the MR and its safety-class equipment from mechanical damage and preventing the release of radionuclides.