抄録
As a part of a development of the risk assessment methodologies against external hazards, a new methodology to assess forest fire hazards is being developed. Frequency and consequence of the forest fire are analyzed to obtain the hazard intensity curve and then Level 1 probabilistic safety assessment is performed to obtain the conditional core damage probability due to the challenges by the forest fire. "Heat", "flame", "smoke" and "flying object" are the challenges to a nuclear power plant. For a sodium-cooled fast reactor, a decay heat removal under accident conditions is operated with an ultimate heat sink of air, then, the challenge by "smoke" will potentially be on the air filter of the system. In this paper, numerical simulations of forest fire propagation and smoke transport were performed with sensibility studies to weather conditions, and the effect by the smoke on the air filter was quantitatively evaluated. Forest fire propagation simulations were performed using FARSITE code. A temporal increase of a forest fire spread area and a position of the frontal fireline are obtained by the simulation, and "reaction intensity" and "frontal fireline intensity" as the indexes of "heat" are obtained as well. The boundary of the fire spread area is shaped like an ellipse on the terrain, and the boundary length is increased with time and fire spread. The sensibility analyses on weather conditions of wind, temperature, and humidity were performed, and it was summarized that "forest fire spread rate" and "frontal fireline intensity" depend much on wind speed and humidity. Smoke transport simulations were performed by ALOFT-FT code where three-dimensional spatial distribution of smoke density, especially of particle matters of PM2.5 and PM10, are evaluated. The snapshot outputs, namely "reaction intensity" and "position of frontal fireline", from the sensibility studies of the FARSITE were directly utilized as the input data for ALOFT-FT, whereas it is assumed that the active fire area is along the frontal fireline and all the smoke is blown to leeward direction of the nuclear power plant site. The time-dependent change of particle matter density is utilized to evaluate a cumulative amount of particle matter at the air filter of the decay heat removal system. The total amount of particle matter at the air filter is around several hundred grams per m^2 which is well below the operational limit of the air filter of 15 kg/m^2.
