Initial demonstration of the NRC`s capability to conduct a performance assessment for a High-Level Waste Repository

Branch technical position on the use of expert elicitation in the high-level radioactive waste program

“…Dewispelare et al (1993) subsequently applied the formal process of elicitation to the prediction of future climate, with associated parameter distributions, at Yucca Mountain, and to the estimation of corresponding probabilities of occurrence. As part of the NRC staff's independent Iterative Performance Assessment efforts to develop a performance assessment review capability, the staff has relied on informal elicitations to identify and screen scenarios (see Codell et al (1992) and (1994) who identified situations where the use of expert judgment might be appropriate in the HLW program. From this review, the staff was able to verify that lessons learned from these direct experiences had relevance for broader applications.…”

Review of scenario selection approaches for performance assessment of high-level waste repositories and related issues.

“…The radioactive waste management literature is quite abundant with treatises on the subject which will not be repeated here; the interested reader is encouraged to peruse more in-depth discussions in, for example, Nuclear Energy Agency (1987), Buxton (1989), Nuclear Energy Agency (1989a), Davis et al (1990), Gallegos and Bonano (1993), Codell et al (1992), and Thompson and Sagar (1993), among others. Conventionally, the sources of uncertainty affecting a PA are classified into three major categories: (i) uncertainty in the future states of the system, where state is defined as the boundary conditions and forcing function under which the system will evolve; (ii) uncertainty in conceptual models; and (iii) uncertainty in parameters of mathematical models.…”

“…As part of conducting Phase 1 of its Iterative Performance Assessment (IPA) effort (Codell et al, 1992), the NRC adopted a methodology for the selection of scenarios, the basic difference of which compared to other scenario approaches is the type of the events and processes it considers. For example, the NRC only considers EPs, the source of which is outside the accessible environment.…”

“…The NRC also defined the term "scenario" as a description of one of various ways in which the repository system may evolve over time (Codell et al, 1992).…”

“…Probabilities of the EPs were not evaluated; therefore, the screening of the EPs was primarily based on lack of physical reasonableness and insignificant consequences. The last three steps in the methodology (construction of scenarios, probability of scenarios, and screening of scenarios) were not implemented (Codell et al, 1992). In Phase 1 , 20 EPs were considered, but only two were used to construct scenarios: pluvial conditions and drilling.…”