Probabilistic safety assessment of flammable gas leakage in the HTTR-IS hydrogen production plant

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. The 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. The HTTR-IS system is one of the plants producing hydrogen with nuclear power. Such a system has high safety requirements. One of the safety studies to be conducted is the probabilistic safety assessment (PSA) study for concerned hydrogen explosion, which was not studied previously. Therefore, this study is conducted for the preliminary PSA of flammable gas leakage in the HTTR-IS system. First, a master logic diagram (MLD) was developed to identify initiating events. After identifying initiating events, event tree analysis (ETA) and fault tree analysis (FTA) 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 1/y. This frequency is higher than the criterion defined in this paper and unacceptable. Installments of additional components are investigated in order to reduce frequency. As a result, the average frequency of explosion is decreased to 7.9 × 10^-7 1/y.