A safety assessment of rotary mode core sampling in flammable gas single shell tanks: Hanford Site, Richland, Washington

This safety assessment (SA) addresses each of the required elements associated with the installation, operation, and removal of a rotary-mode core sampling (RMCS) device in flammable-gas single-shell tanks (SSTs). The RMCS operations are needed in order to retrieve waste samples from SSTs with hard layers of waste for which push-mode sampling is not adequate for sampling. In this SA, potential hazards associated with the proposed action were identified and evaluated systematically. Several potential accident cases that could result in radiological or toxicological gas releases were identified and analyzed and their consequences assessed. Administrative controls, procedures and design changes required to eliminate or reduce the potential of hazards were identified. The accidents were analyzed under nine categories, four of which were burn scenarios. In SSTS, burn accidents result in unacceptable consequences because of a potential dome collapse. The accidents in which an aboveground burn propagates into the dome space were shown to be in the ``beyond extremely unlikely`` frequency category. Given the unknown nature of the gas-release behavior in the SSTS, a number of design changes and administrative controls were implemented to achieve these low frequencies. Likewise, drill string fires and dome space fires were shown to be very low frequencymore » accidents by taking credit for the design changes, controls, and available experimental and analytical data. However, a number of Bureau of Mines (BOM) tests must be completed before some of the burn accidents can be dismissed with high confidence. Under the category of waste fires, the possibility of igniting the entrapped gases and the waste itself were analyzed. Experiments are being conducted at the BOM to demonstrate that the drill bit is not capable of igniting the trapped gas in the waste. Laboratory testing and thermal analysis demonstrated that, under normal operating conditions, the drill bit will not create high enough temperatures to initiate a propagating reaction in the waste. However, system failure that coincides in a waste layer with high organic content and low moisture may initiate an exothermic reaction in the waste. Consequently, a conservative approach based on the current state of the knowledge resulted in limiting the drilling process to a subset of the flammable-gas tanks. Accidents from the chemical reactions and criticality category are shown to result in acceptable risk. A number of accidents are shown to potentially result in containment (tank liner) breach below the waste level. Mitigative features are provided for these accidents. Gas-release events without burn also are analyzed, and radiological and toxicological consequences are shown to be within risk guidelines. Finally, the consequences of potential spills are shown to be within the risk guidelines.« less