Comparison of ALOHA and EPIcode for Safety Analysis Applications

EPIcode (version 7.0) and ALOHA (version 5.2.3) are two of the designated toolbox codes identified in the Department of Energy's Implementation Plan for DNFSB Recommendation 2002-1 on Software Quality Assurance issues in the DOE Complex. Both have the capability to estimate evaporation rates from pools formed from chemical spills and to predict subsequent atmospheric transport and dispersion. This report provides an overview of the algorithms used by EPIcode and ALOHA to calculate evaporation rates and downwind plume concentrations. The technical bases for these algorithms are briefly discussed, and differences in the EPIcode and ALOHA methodologies highlighted. In addition, sample calculations are performed using EPIcode and ALOHA for selected chemicals under various environmental conditions. Side-by-side comparisons of results from sample calculations are analyzed to illustrate the impact that the different methodologies used by EPIcode and ALOHA have on predicted evaporation rates and downwind concentrations. It is recommended that the safety analyst explicitly evaluate the strengths and limitations of any code before selecting it for a specific application. User skill and expertise can often outweigh most of the differences between ALOHA and EPIcode. Recognizing that EPIcode is inherently a scoping tool, while ALOHA is based on more detailed models, the user is recommended to perform a parameter sensitivity study to determine major dependences in the applied model and to check code output with independent techniques, such as a hand calculation, alternative computer code application, or spreadsheet techniques. A multi-tiered approach of this type will provide better confidence in overall results than to unilaterally use one code alone without questioning.