An Evaluation of the Hydrogen Flammability and the Hydrogen Concentration in the Reactor Building for Wolsong 2

Following a core uncover, the core will heat up and the fuel pin cladding will oxidize in the presence of steam. The reaction follows a parabolic rate law, and a significant cladding oxidation does not occur until the cladding temperature reaches approximately 1800 deg. F. One of the products of the cladding oxidation reaction is hydrogen, which can accumulate in the PHTS if the PHTS is intact or in the reactor building if a pathway exists from the PHTS. If the hydrogen concentration reaches 4 to 6% in the reactor building (which is very possible following a significant core damage), the hydrogen can ignite and cause a spike in the reactor building pressure and temperature. If enough hydrogen is allowed to build up in the reactor building, the ignition of the hydrogen can lead to a pressure spike that exceeds the reactor building ultimate design pressure, thereby jeopardizing the reactor buildings integrity. Even if reactor building integrity is not jeopardized by a global hydrogen burn, the fact that a global hydrogen burn could occur will prevent the TSC from exiting the SAMG. Hydrogen generation and flammability is a concern throughout most of the guidelines in the Wolsong 2 (CANDU 6 type reactor) SAMG. The purpose of this paper is to define whether the hydrogen in the reactor building atmosphere is flammable, and to estimate the hydrogen concentration in the reactor building atmosphere based on an estimated oxidation percentage. For the Wolsong 2 this CA, four sets of hydrogen figures were developed for the case of no venting (no core/concrete interaction), a 15% venting (no core/concrete interaction), a 30% venting (no core/concrete interaction), and no venting (core/concrete interaction for 24 hours resulting in a 114 deg. F superheat and a addition of non-condensable gases). (authors)