Proceedings of the Ninth International Seminar on Fire and Explosion Hazards (ISFEH9), pp. 809-818 Edited by Snegirev A., Liu N.A., Tamanini F., Bradley D., Molkov V., and Chaumeix N. Published by Saint-Petersburg Polytechnic University Press ISBN: 978-5-7422-6498-9 DOI: 10.18720/spbpu/2/k19-9 809 Fires, Explosions, and Venting in Nuclear Reactors Palacios A. 1, *, Bradley D. 2 1 Universidad de las Americas, Puebla, Department of Chemical, Food and Environmental Engineering, Puebla, Mexico 2 University of Leeds, School of Mechanical Engineering, Leeds, UK *Corresponding author’s email: adriana.palacios@udlap.mx ABSTRACT 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 significantly reduced burn rate, or complete flame extinction. The performance of lifted jet flames of C 3 H 8 , CH 4 and C 2 H 4 , when exposed to increasing air cross winds velocities, are presented. These provide a basis for synthesising the performance of H 2 flames, also in cross flows. The H 2 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. KEYWORDS: Hydrogen, jet flames, reactor venting, cross flow. NOMENCLATURE B molar fuel/cross flow air rate ratio C molar fraction of air in combined molar flows of fuel and air into lift-off volume C c critical value of C for reduction in U b * by cross flow C p constant pressure specific heat (J/kg·K) C SL values of C, at the equivalence ratio for maximum laminar burning velocity, S L D pipe diameter (m) D o pipe external diameter (m) f ratio of fuel to air moles in fuel-air mixture for S L k thermal conductivity (W/m·K) L flame lift-off distance (m) u j mean fuel flow velocity at the exit plane of pipe for subsonic flow. For ratios of atmospheric pressure to P i equal to, or less than the critical pressure ratio, or choked sonic velocity after isentropic expansion from P i (m/s) U* dimensionless flow number for choked and unchoked flow, (u j /S L )( k δ /D) 0.4 (P i /P a ) U δ * Value in Eq. (1) with δ =ν/S L in expression for U* Greek k δ laminar flame thickness, (m) (k/C p ) To /ρ j S L φ SL equivalence ratio for maximum laminar