Nuclear Power Plant (NPP) Vulnerability to Near Miss Munition Explosions- Dynamic Resistance and In-Structure Shock Analysis

Introduction: Nuclear power plant (NPP) might be exposed to direct or near miss hits of high explosive munitions due to war activities. Direct hits of guided bombs, can cause major damage up to total destruction of the reactor and radioactive materials dispersion to the surrounding environment. This research was focused on the resistance of a Westinghouse AP1000 NPP, including its internal systems, against guided bomb unit GBU-28 near miss explosions. The technical information of the facility and the munition was based upon published unclassified data with complementary assumptions. Two main NPP failure mechanisms were postulated. The first- if the nuclear reactor shielding structure composed by a 90 cm (3 foot) steel-concrete-steel wall is perforated by the blast and fragments than the NPP should stop operating immediately since a leak of radioactive gas might occur. The second- failure of the internal systems due to strong in-structure shock might cause loss of coolant accident (LOCA) with a possible core melt down. The combined blast and fragment impulses at various standoff distances from the AP1000 shielding structure were defined and the wall`s dynamic response was analyzed. The maximum standoff distance in which the wall might be perforated was found to be about 1 m.

Methods: A dynamic analysis of the shielding structure exposed to blast and fragments of the near miss explosions was carried out, based on finite elements model using Abaqus CAE 2017 software. The results were compared to empirical data of reinforced concrete elements breaching and were found to be at similar order of magnitude. The whole shielding structure including its main systems exposed to a nearby explosion was modeled with the MSC/Dytran 2013.0.2 code. Presently, the model includes the shielding structure with its concrete base and the supporting structures for the reactor and the steam generators.

Results: The shielding wall is breached in two areas of 0.25m² due to the dynamic loading of blast and fragments in a 1m standoff distance. The in-structure shock due to a 3m standoff explosion includes high accelerations of the order of 12g in a short time span, of the order of 0.1s. These accelerations are compared to the Reactor Cooling System capacity to withstand LOCA initiating shock.

Conclusions:. The AP1000 shielding wall might be breached due to near miss external hits. The internal systems might be damaged because of the in-structure shock that may initiate LOCA.