A fault tree analysis consisting of assumption of accident scenarios, construction of fault trees, and performing of analyses is given for a loss of cooling accident in a HALW (High Activity Liquid Waste) tank of a typical reprocessing facility. A systematic method is developed to avoid `missing scenarios`, and `confused scenarios` which are major problems in these analyses. Model plant data, basic failure frequency data and a fault tree analysis code named FTL have been introduced from NUKEM GmbH, Germany. The analyzed occurrence frequencies for the assumed scenarios of the loss of cooling accident are discussed to show a design improvement measure to attain a more reliable system. These results are compared with general requirements to be selected as DBE (design basis event) used in the safety assessment of the facility design. DBE is an event which is considered to have relatively high occurrence probabilities and considerable effects on environment. (author).
Nomura, Yasushi 
- Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment