Fires, Explosions, and Venting in Nuclear Reactors

A brief historical review covers salient reactor fires and explosions, principally centred around the use of graphite as a neutron moderator, and the high temperature generation of hydrogen in reactions of steam and zirconium. An alternative to uncontrolled, excessive, build-up of pressure, followed by uncontrolled explosion, is the provision of a buffer vessel, in which there is separation of hydrogen from radioactive products in permeable membrane separators. The hydrogen is then flared. Possible rates of production of hydrogen are compared, along with the rates at which it can be separated and flared in lifted jet flames, which give the highest burn rates. Cross winds can result in a transition to rim attached, downwash and wake-attached flames, all with a signicantly reduced burn rate, or complete flame extinction. The performance of lifted jet flames of C3H8, CH4 and C2H4, when exposed to increasing air cross winds velocities, are presented. These provide a basis for synthesising the performance of H2 flames, also in cross flows. The H2 relationship is rather different from that of the hydrocarbons, on account of the higher chemical reactivity of hydrogen, its small laminar flame thickness, reduced air requirement, higher acoustic velocity, and minimal flame lift-off distance. Destruction of hydrogen lifted jet flames by the cross flow of atmospheric air is significantly less likely than it is for propane jet flames. Flaring with micro-tubes might be advantageous for integrating flaring with membrane hydrogen separation, whilst high mass flow rates can be achieved with large diameter flares in the lifted flame, supersonic regime.