Preliminary risk assessments of the small HTGR

Preliminary investment and safety risk assessments were performed for a preconceptual design of a four-module 250-MW(t) side-by-side steel-vessel pebble bed HTGR plant. Broad event spectra were analyzed involving (1) component damage resulting in unscheduled plant outages and (2) fission product releases resulting in offsite doses. The preliminary assessment indicates at this stage of the design that two categories of events govern the investment risk envelope: 1. Primary coolant leaks which release some circulating and plateout activity that contaminates the confinement. 2. Turbogenerator damage which involves extensive turbine blade failure. Primary coolant leaks are important contributors because associated cleanup and decontamination requirements result in longer outages that arise from other events with comparable frequencies. Turbogenerator damage is the salient low-frequency investment risk accident due to the relatively long outages being experienced in the industry. Thermal transients are unimportant investment risk contributors because pressurized core heatups cause little damage, and depressurized core heatups occur at negligible frequencies relative to the forced outage goal. Two accident families have been examined for safety risk: 1. Primary coolant leaks which release some circulating and plateout activity to the confinement. 2. Releases initiated by steam generator leaks which can release radioactivity directly to the environment. Primary coolant leaks dominate the upper portion of the safety risk envelope, while releases initiated by steam generator leaks govern the lower frequency region. These preliminary results demonstrate investment and safety risk goal compliance at this stage in the design process. Studies are continuing in order to provide valuable insights into risk-significant events to assure a balanced approach to meeting user and regulatory requirements. (author)