AN EXPLORATORY SENSITIVITY STUDY WITH THE MACCS REACTOR ACCIDENT CONSEQUENCE MODEL

An exploratory sensitivity study with the MELCOR Accident Consequence Code System (MACCS) is presented. This study was performed to provide (1) an indication of the possible impact of consequence modeling uncertainties on the results of an integrated probabilistic risk assessment for a nuclear power station; (2) guidance on important parameters to direct data acquisition for future uncertainty analyses with MACCS; (3) a check that MACCS is operating as intended; and (4) insights on the effect of problem gridding on model resolution. The sensitivity results obtained in this study involve fatalities and injuries due to radiation exposure incurred within seven days of an accident and are based on the group of source terms (i.e. source term cluster) that made the largest contribution to early fatality risk in a probablistic risk assessment for the Peach Bottom Atomic Power Station. The following consequence measures were investigated: early fatalities, prodromal vomiting, lung impairment, population dose (0-500 miles), population dose (0-10 miles), mean early fatality distance, maximum early fatality distance, mean early fatality risk, probability of one or more early fatalities, and amount of central processing unit (CPU) time required to evaluate MACCS. The analysis used techniques based on Latin hypercube sampling and regression analysis. Results obtained in this study indicate that (1) the impact of consequence modeling uncertainty in an integrated risk assessment may be significant; (2) dominant variables involving early health effects include evacuation area, scaling for horizontal dispersion, population relocation time, dry deposition velocity, and groundshine shielding; (3) the models in MACCS related to early health effects are implemented correctly; and (4) model resolution may be improved by finer radial gridding.