Risk-informed perspectives

A risk-informed perspective was added to the revised UGTA strategy, recognizing the twofold nature of the project goals. The first essential goal is to complete a sufficient level of characterization and modeling studies to establish a fundamental understanding of the processes of release and transport of test-produced radionuclides in groundwater. Second, this knowledge is applied to each CAU to identify the risk of radionuclide contamination to the public. Risk in this context is the likelihood and consequences of public exposure to contaminated groundwater and is mitigated by two factors. The first is natural attenuation or intrinsic reme­diation, the operation of natural processes that can reduce the concentra­tion of a contaminant in groundwater (National Research Council, 2007a). For groundwater flow at the NNSS, natural attenuation relies on the processes of dispersion, dilution, radionuclide retardation and radioac­tive decay to reduce the concentration of radionuclides in groundwater from their concentrations near test cavities. Second, access to contaminated groundwater is required to complete the pathway to public exposure sce­narios, the consequences portion of the risk definition. Public access to groundwater on the NNSS is restricted by the current institutional control policies. Assuming continuity of these policies, the likelihood of public access to contaminated groundwater is greatest where there is the potential for migration of radionuclides beyond the NNSS boundaries.

As noted previously, the approach to assessing the likelihood of the hazard part of the risk definition for radionuclide contamination of ground­water is developing probabilistic maps of exceeding the SDWA as specified in the FFACO agreement. The consequences of groundwater contamina­tion from underground testing are currently controlled through implemen­tation of worker safety protocols with respect to accessing contaminated groundwater and maintaining restrictions on public access to the NNSS. The uncertainties in these controls are the effectiveness and duration of active institutional control of the NNSS and the ability of NNSA/NSO to establish and maintain institutional controls for areas of groundwater contamination that extend off the boundaries of the NNSS.

Fig. 26.3 shows the location of sites of past underground testing on the NNSS for the major CAUs, and the expected directions of groundwater flow and radionuclide transport are shown in Fig. 26.2. The highest source term by activity in Curies is the Pahute Mesa CAU which contains less than 10% of the underground tests but more than 60% of the radiological source term (8.0 x 107 curies). The Yucca Flat CAU includes about 82% of the underground tests and 37% of the radiological inventory (5.1 x 107 curies). Slightly over 7% of the underground tests on the NNSS were conducted at the Rainier Mesa/Shoshone Mountain CAU which includes 0.7% of the radiological source term (8.9 x 105 curies). Finally, 10 underground tests were detonated at the Frenchman Flat CAU and these tests equal about

0. 14% of the radiological source term (1.9 x 105 curies).

Comparison of Fig. 26.3 and the above cited distributions of the radiologi­cal source term by CAU provide important risk perspectives. The Pahute Mesa CAU contains the highest underground inventory and the greatest potential for contaminant migration off of the NNSS. As noted previously, tritium contamination has already been detected in groundwater just south of the NNSS boundaries in western Pahute Mesa. By virtue of the high inventory and high likelihood of migration off of the NNSS, the Pahute Mesa CAU provides the greatest risk to the public. The Frenchman Flat CAU has the lowest inventory of the UGTA CAUs but the results of the transport modeling indicate a fair potential for offsite migration of radio­nuclides at the southeast boundary of the NNSS (Fig. 26.4). The Yucca Flat CAU includes the highest number of underground tests and a relatively high inventory, but sites of underground testing are more than 40 km away from the southern boundaries of the NNSS. Finally, the Rainier Mesa/ Shoshone Mountain CAU has both a relatively small inventory and a very long expected distance of transport of radionuclides to the southern boundary of the NNSS. From a risk perspective, it is the least hazardous of the testing areas on the NNSS.

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