A Methodology of Risk Assessment, Management and Coping Actions for a Nuclear Power Plant Hit by High-Explosive Warheads

Introduction: The research effort was focused on: detailed loss of coolant accident (LOCA) scenario analysis in AP1000 NPP (Nuclear Power Plant). The vulnerability scenario is based upon near miss hit of guided bomb unit (GBU-28), including blast loading and fragments’ impact. A basic model for the assessment of the AP1000 NPP vulnerability due to in-structure shock caused by explosion was developed based on hydro-code simulations.

Methods: A comprehensive risk assessment methodology composed of four phases was developed. Since the in-structure shock influences the safety systems at standoff distances greater than the NPP shielding wall perforation distance of 1 m., it was the main hazard investigated in the risk management model. The methodology is comprised of: (I) System Analysis, (II) Hazard Analysis (the warhead blast and fragments loading and penetration and the in-structure shock), (III) Damage Assessment (deflections, deformations, stresses, forces, accelerations and their consequences); and (IV) Risk Analysis of the in-structure shock consequences. Using fragility curves (as it is represented in the AP1000 technical applicant documents to the NRC) for analysis of the expected failure modes according to explosion events faces difficulties. The difficulties lie in the fact that explosions have a unique feature of short duration of dynamic structural loading in comparison with the earthquake duration of loading. Therefore, several adjustments were carried out in the risk analysis and assessment due to explosion events. Because of the lack of technical data the nuclear engineering institute (NEI) data of NPP systems resistant to airplane crash and equipment was used, even though the crash versus blast duration and amplitude are not equal.

Results: The findings reveal that the risk expectancy of LOCA given the reference event of GBU-28 can reach a range of between $2.197B and $6.262B. Probabilities of failure of the critical components: pressurizer, cooling pumps and valves are high (>1∙10-4) and indicate that the critical components of the AP1000 NPP are highly vulnerable to the scenario. The number of casualties and injuries in the threat scenario (hit at distance of 3m from the shielding structure) is 75.

Conclusions: The AP1000 is vulnerable to the threat scenario and should be protected against the threat, for example, by adding shielding layer on structure top at least up to 1m from the shielding structure`s walls and isolating or strengthening the safety systems. Lowering the risk to an acceptable range will enable the development of the nuclear energy power source as a viable alternative. Further research is recommended in order to explore protective solutions for the critical AP1000 components.