MOSES special report: Hydrogen behaviour in Olkiluoto BWR reactor building during a severe accident

Hydrogen behaviour in Olkiluoto reactor building during a severe accident has been assessed. The objective of the work was to investigate, if hydrogen can form flammable and detonable mixtures in the reactor building, evaluate the possibility of onset of detonation, assess the pressure loads caused by detonation, and study the integrity of walls and pipe penetrations in the reactor building. The initial accident sequence was station blackout with depressurisation of the reactor coolant system. A conservative assumption of 100% zirconium oxidation in a core was made. The hydrogen was assumed to leak from the containment into the reactor building from the pipe penetration in the upper part of the containment. Three different rooms in the reactor building and two different leakage sizes were considered. The analyses were made with the MELCOR (initial conditions), FLUENT (hydrogen accumulation, deflagration, flame acceleration), DETO and DET3D (detonation loads) and ABAQUS (structural analysis) codes. The analyses indicated strong accumulation of hydrogen to the upper parts of the reactor building rooms under consideration. Combustible conditions existed in all analysed cases somewhere in the room. The simulations and semi-empirical analyses indicated that the criteria for the onset of detonation are reached in some, but not in all cases considered. The possibility of deflagration-to-detonation transition (DDT) in the reactor building could not, therefore, be excluded. First results of three-dimensional detonation studies for reactor building room B.60.80 showed that the highest pressure spikes of about 10 MPa occurred in the corners of the room. Corresponding pressure impulses were about 30-35 kPa-s. Structural analyses showed that the concrete wall of the room may survive the detonation peak transient, but the relatively slowly decreasing static-type pressure after the peak detonation damaged the wall much more severely than the short detonation peak pressure. Further work consists of the analyses of integrity of the containment pipe penetration in the room B.60.80 under detonation loads.