Development and application of a conceptual approach for defining high-level waste

This paper presents a conceptual approach to defining high-level radioactive waste (HLW) and a preliminary quantitative definition obtained from an example implementation of the conceptual approach. On the basis of the description of HLW in the Nuclear Waste Policy Act of 1982, we have developed a conceptual model in which HLW has two attributes: HLW is (1) highly radioactive and (2) requires permanent isolation via deep geologic disposal. This conceptual model results in a two-dimensional waste categorization system in which one axis, related to ''requires permanent isolation,'' is associated with long-term risks from waste disposal and the other axis, related to ''highly radioactive,'' is associated with short-term risks from waste management and operations; this system also leads to the specification of categories of wastes that are not HLW. Implementation of the conceptual model for defining HLW was based primarily on health and safety considerations. Wastes requiring permanent isolation via deep geologic disposal were defined by estimating the maximum concentrations of radionuclides that would be acceptable for disposal using the next-best technology, i.e., greater confinement disposal (GCD) via intermediate-depth burial or engineered surface structures. Wastes that are highly radioactive were defined by adopting heat generation rate as the appropriate measure and examining levels of decay heat that necessitate special methods to control risks from operations in a variety of nuclear fuel-cycle situations. We determined that wastes having a power density >200 W/m3 should be considered highly radioactive. Thus, in the example implementation, the combination of maximum concentrations of long-lived radionuclides that are acceptable for GCD and a power density of 200 W/m3 provides boundaries for defining wastes that are HLW