The security of nuclear power plants becomes an ever more important issue after the tragedy of the World Trade Center of New York City on September 11, 2000. Utilities around the world have significantly increased the resources used to enhance the security of nuclear power plants. In this study, a mathematical model based on Monte Carlo simulations is developed to calculate the probability of invaders being located in a building in the vital area, on a floor of the building, and in a region of the floor when invaders break into the vital area successfully.

In the model, it is assumed that invaders have no specific target to sabotage and they have no prior knowledge of the layout of the plant. Invaders are running around randomly in the vital area, and Monte Carlo simulations are used to trace their paths on a floor. The results can help the security of a nuclear power plant locate invaders in the vital area.

If it is assumed that the damage caused by the invaders is similar to the damage induced by a fire, the product of the probability of invaders being located in a region and the fire conditional core damage probability of the region ijk (CCDPF,ijk) from a fire hazard probabilistic safety assessment can be used to prioritize the importance of the region to the risk of terrorist sabotage and to allocate resources that enhance physical protection. The summation of the product for all the region is the conditional core damage probability of terrorist attack (CCDPS).

The surrogate plant used in the present study is a typical General Electric–designed Boiling Water Reactor-6 with Mark III containment. The vital area in this study has six entrances and three buildings: turbine, control, and auxiliary. Among the 196 regions considered in the analysis, the majority (between 85% and 92% for different entrances) have the Pijk of locating terrorist probability of less than 10−2. The top three average Pijk regions are located in the turbine building. The eight regions with CCDPF,ijk equal to 1.0 of the plant contribute 86.8% of the total average CCDPS. Region AUX56, which is the corridor on the third floor of the auxiliary building, contributes 33.9% of the risk of core damage. Region AUX14, which is the corridor on the first floor of the auxiliary building, contributes 17.1% of this risk.