Hydrogen is regarded as a clean fuel since it does not pollute when burned with air. The issue is how to produce substantial and inexpensive hydrogen for commercial use. A utilization of nuclear power to produce hydrogen from water has many merits. Firstly, nuclear power emits no carbon-dioxide. Secondly water contains hydrogen and exists everywhere. HTTR-IS system is one of the plants producing hydrogen with nuclear power. Such system needs high safety. One of safety issues to be conducted is Probabilistic Safety Assessment (PSA) study for concerned hydrogen explosion which has not been studied previously. Therefore, the objective of this study is the preliminary PSA of flammable gas leakage in the HTTR-IS system. At first, a Master Logic Diagram (MLD) was performed to identify initiating events. After identifying initiating events, ETA (Event Tree Analysis) and FTA (Fault Tree Analysis) were performed to quantify the average frequency of an accident. The initial result of the PSA shows that the nominal frequency of explosion is 2.5 ×10~(-4) I/year. This frequency is higher than the criterion defined in this study and unacceptable. Installments of additional components are investigated to reduce frequency. As a result, the average frequency of explosion is decreased to 1.3×10~(-6) I/year.
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