A number of small houses are proposed to be built around an old chemical plant. You are requested to conduct a quantitative risk analysis to assess the risk of fatality that the chemical plant facility poses to the nearby population.

Suppose that hazard identification process of the plant has identified only two hazardous incidents, which may occur in the facility:

Incident I ? An explosion resulting from detonation of an unstable chemical;

Incident II ? A release of a flammable and toxic gas resulting from failure of a vessel.

Assume the following conditions in the risk analysis:

• All hazards originate at a single point.
• Only two weather conditions occur. The atmospheric stability class and wind speed are always the same. Half of the time the wind blows from the northeast, and half of the time it blows from the southwest.
• Incident consequences are simple step functions. The probability of fatality from a hazardous incident at a particular location is either 0 or
• Explosion is centered at the center point of the facility.
• For Incident I, all persons within 200m of the explosion center are killed; all persons beyond this distance are unaffected.
• For Incident II, all persons within 100m of the explosion center are killed; all persons beyond this distance are unaffected.
• For Incident IIB, all persons in a pie shaped segment of radius 400m downwind are killed; all persons outside this area are unaffected.
• The frequency of Incident I is 1 x 10-6 events per year.
• The frequency of Incident II is 4 x 10-5 events per year.
• Ignition Probability of Incident II is 25%.
• The population distribution is indicated in the circle in each zone in Figure 2a.

Generate the Frequency-Number of Fatalities (F-N) data and present the societal risk in the F-N curve as in Figure 2b. Based on the finding of the societal risk, state whether the building development would be acceptable in Hong Kong.

102 Frequency (F) of accidents with N or More Fatalities (Per Year) 103 UNACCEPTABLE 10* 105 ALARP region 10-7 ACCEPTABLE 10 10 100 1000 10000 Number of Fatalities (N) Figure 2b Hong Kong Societal Risk GuidelinesNorth - E 100 50 90 C K 20 200m G A West - - 100m - - East H B - 90 D 400m 20 - F - 100 60 - - South Figure 2a Individual risk contour map and population distribution for the impact zones