Abstract The high temperature coolant of nuclear power plants (NPPs) has been investigated for the hydrogen production, which could be a major role of a green energy promotion. An accident of the high temperature gas cooled reactor (HTGR) is modeled for the stabilized hydrogen production using nuclear energy. For the clean energy resource pursuit in preventing the climate change, the hydrogen is one of very attractive energy sources. The non-operational data could be produced by the fuzzy set theory which is one of non-linear complex algorithms. So, the result can show the possibility of the event happening instead of the exact solutions. The random numbers are generated for membership numbers of the fuzzy function. The event manipulation is done by new membership numbers for the propagations. The final result is 1.0 in 8 times during 100 months. So, the frequency is 0.08, or 8% of successful long-term cooling by conduction.