Safety considerations and countermeasures against fire and explosion at an HTGR-hydrogen production system. Proposal of safety design concept

Establishment of safety design concept and countermeasures against fire and explosion accidents is among key safety-related issues in an HTGR-hydrogen production system. We propose the different safety design concepts depending upon the origin of fire and explosion which may happen in the HTGR-hydrogen production plant. Against fire and explosion originated outside the reactor building (R/B), namely in the area of hydrogen production plant, the safety design concept is primarily to take a safe distance for preventing the damage on safety-related items or a proof wall if necessary. Because the hydrogen production plant is designed in the same safety level as a conventional chemical plant. The safe distance is proposed to limit an incident overpressure to 10 kPa so as not to suffer any damage on the items and to limit a wall-averaged temperature of concrete structures of the R/B to 175degC according to the current regulation. On the other hand, against a potential possibility of explosion originated inside the R/B, the safety design concept is to minimize the possibility of explosion low enough to assume no occurrence inside the R/B. That is, the measure is to exclude a simultaneous failure of a secondary helium piping and an endothermic chemical reactor. Furthermore, in severe accident condition in which the explosion may be postulated a priori, an incidental overpressure of explosion inside the reactor containment vessel (C/V) should be limited so as not to fail the C/V through restricting the amount of combustible gas ingress into the C/V by means of a combination of C/V isolation valve installed in the helium piping and emergency shut off valve in the process feed gas line. (author)