Gas composition is also needed to assess the toxic effects of waste gas releases, and to design ventilation and stack monitoring systems. Knowledge of the gas composition could lead to a reduction in excessively conservative work controls. Also, knowledge of the actual composition will be required information for the analysis that is needed to close the USQ, and to resolve the safety issue. Gas composition from tank dome space gas monitoring must also consider waste gas composition, such as determined by the retained gas sampling system and gas composition can vary between steady state and acute release or gas release events. Similarly, the rate of generation and release of gases is required information for performing safety analyses, developing controls, designing equipment, and closing safety issues.
Separate work is currently underway to determine the degree of mixing in the tank domes.
Several factors are important for evaluating which tanks need additional continuous gas monitoring and detailed gas characterization for operational safety and for safety assessment. New continuous monitoring should be limited to those tanks with the higher potential gas release, pending development and application of new criteria and JCO approval. To a very rough approximation, this translates into a 10 -2 probability of exceeding the LFL if a gas release were to occur. In this table, the tanks are ranked by potential for maximum dome space gas concentration. On the other hand, new screening methodology and criteria are under development that use a risk-based approach, which might reduce the number of tanks to be monitored.