Once she has an understanding of release mechanisms and risk implications, the evaluator will next need to model potential releases for the risk assessment. This is often done by assigning a score to various release scenarios. To score the relative dispersion area or hazard zone of a spill or release, the relative measures of quantity released and dispersion potential can be combined and then adjusted for mitigation measures. When the quantity and dispersion components use the same variables, it might be advantageous to score the two components in one step.

As more and more variables are added to the assessment in order to more accurately distinguish relative consequence potential, the benefits of the scoring approach diminish. Eventually the evaluator should consider performing the detailed calculations estimating actual hazard zones using models for dispersion and thermal effects. As is noted in the introductory chapters of this book, the challenge when constructing a risk assessment model is to fully understand the mechanisms at work and then to identify the optimum number of variables for the model s intended use. For instance, Table 7.8 implies that overpressure (blast effects from a detonation) is not a consideration for natural gas. This is a modeling simplification. Unconfined vapor cloud explosions involving methane have not been recorded, but confined vapor cloud explosions are certainly possible.