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Author: Per F. Peterson
Between 2007 and 2010, Congress must consider whether the United States needs a second repository for high-level radioactive waste.
Nuclear fission energy requires small inputs of natural resources compared to most other fossil and nonfossil energy technologies. When we consider net electricity generation (e.g., net electricity after subtracting consumption by internal plant loads and by uranium enrichment plants), the life-cycle resource inputs for nonfossil power sources are dominated by construction materials, most notably steel and concrete. The construction of existing 1970-vintage U.S. nuclear power plants required 40 metric tons (MT) of steel and 190 cubic meters (m3) of concrete per average megawatt of electricity (MW(e)) generating capacity.1 For comparison, a typical wind-energy system operating with 6.5 meters-per-second average wind speed requires construction inputs of 460 MT of steel and 870 m3 of concrete per average MW(e). Coal uses 98 MT of steel and 160 m3 of concrete per average MW(e) (Pacca and Horvath, 2002); and natural-gas combined cycle plants use 3.3 MT steel and 27 m3 concrete (Meier, 2002).
Because of this efficient use of natural resources, compared to other energy technologies, nuclear energy is an important candidate for the long-term, sustainable production of electricity and hydrogen.2 But any major role for fission will require practical approaches to spent-fuel management with environmental and public health impacts comparable to, or lower than, those of other sustainable energy technologies.
Of the resources required to produce fission energy, repository sites are arguably the only resource that has proven to be scarce. Given this scarcity, the allocation and efficient use of available repository capacity will require well informed technical and policy decisions. The 1982 Nuclear Waste Policy Act (NWPA), as amended in 1987, requires that Congress consider these questions between 2007 and 2010.3 Advanced fuel cycles (AFCs) cannot eliminate the need for repositories, but do have the potential to greatly increase repository capacity and improve performance (NRC, 1996). AFC optimization and economics will depend strongly on the licensing basis for repository sites and on the extent of AFC R&D performed prior to any large-scale deployment. In this article, I outline the major issues facing the United States in considering the role of AFC technology in nuclear waste management.
Regulatory standards to protect current and future public health, safety, and the environment are adopted through national policy-making processes. The regulatory criteria for Yucca Mountain require, among other things, that the groundwater below the Armagosa Valley near Yucca Mountain be protected for at least 10,000 years; the maximum radiation dose to an individual who drinks two liters of groundwater per day must be less than 4 mrem, in other words, less than 1.3 percent of current U.S. average natural radiation exposures. Like other regulated, engineered systems (e.g., aircraft), repository systems apply redundancy and diversity to meet these regulatory requirements with acceptable uncertainty. Uncertainty in the performance of individual repository barriers and processes are acceptable because redundancy and diversity reduce the uncertainty in total system performance. The isolation provided by deep geologic storage helps bound uncertainties by creating chemical, thermal, and hydrologic conditions that can be predicted with less uncertainty over long time scales than conditions near the surface. Indeed, the fate of geologic repositories may be one of the few important environmental and public health impacts from twentieth and twenty-first century energy production that we can predict with modest uncertainty over millennial time scales.4
In 2002, the United States selected Yucca Mountain in southern Nevada as the site for the nation’s first high-level waste repository, and Congress directed the U.S. Department of Energy (DOE) to prepare and submit a license application to the U.S. Nuclear Regulatory Commission (USNRC) by the end of 2004. This site-selection recommendation was based on the results of an integrated total-system performance assessment (TSPA-SR) model that included nine different barrier mechanisms. Repeated TSPA calculations, that varied the values of uncertain parameters in the model, generated statistical performance predictions showing that regulatory requirements for groundwater protection could be met by a factor of more than 100 with high confidence (DOE, 2002).
For the upcoming license application, improvements to the TSPA models, particularly in near-field transport and drift seepage, can be expected to reduce the need for conservative assumptions in many of the barrier models. At the same time, questions raised by the USNRC during the license review can be expected to require more conservative and wider uncertainty estimates for some barrier models. Recent "one-off" studies have shown that performance is relatively well distributed across the multiple barriers (Apted et al., 2002; NWTRB, 2002). Thus, to alter the recent TSPA-SR’s positive assessment of regulatory compliance, the USNRC would have to identify substantial deficiencies in the uncertainty and conservatism of a substantial fraction of the barrier models. This outcome appears unlikely.
The NWPA limits the capacity of the proposed Yucca Mountain repository to 63,000 MT of initial heavy metal in commercial spent fuel.5 The 103 U.S. commercial reactors currently operating will produce this quantity of spent fuel by 2014. Recently, the federal government has started to issue 20-year license renewals for U.S. nuclear plants, extending the permitted plant operating life to 60 years. As of May 2003, 16 U.S. plants had received renewals and 14 applications were under review; 22 more applications are expected in the next two years. Because of the low average production cost of nuclear electricity (1.69 cents per kilowatt-hour in 2002), it is anticipated that a substantial fraction of remaining U.S. plants will also seek renewals, thus increasing the total federal spent-fuel management obligation for current reactors to as much as 125,000 MT. Licenses for new plant construction would increase the total further.
Technical Capacity: Commercial Spent Fuel
The capacity of geologic repositories is set primarily by areal heat load limits for decay heat and by available footprint. At Yucca Mountain, spent fuel and high-level waste will be placed in corrosion-resistant canisters and emplaced horizontally in 5.5-m diameter drift tunnels. The TSPA-SR set a loading limit of 60 MT/acre, based on tunnels spaced 81 m apart and on the nominal characteristics of the first 70,000 MT of defense and commercial wastes planned to be sent to the repository. For spent fuel from pressurized-water reactors (PWRs), comprising 60 percent of commercial spent fuel, the TSPA-SR canister design permits a loading of 87 MT/acre. The boiling-water reactor (BWR) canister design permits 75 MT/acre.
With a relatively modest license amendment to increase the site capacity, an average areal loading of 75 MT/acre for commercial spent fuel is a reasonable assumption. This value has potential conservatisms that might be erased with subsequent, more aggressive license amendments. However, it is unlikely that increases in areal loading could exceed a factor of two or three for commercial spent fuel.
The maximum repository footprint at Yucca Mountain is correspondingly uncertain. An earlier viability assessment (TSPA-VA) concluded that, with a substantial new characterization, the total repository area could potentially be increased to somewhat more than 2,000 acres (8.0 km2). This suggests a minimum "technical" site capacity of approximately 75 x 2,000 = 150,000 MT of spent fuel, with a maximum site capacity greater by perhaps a factor of two or three. Thus any substantial construction of new U.S. nuclear power infrastructure in the coming decades will almost certainly create a technical requirement (perhaps as soon as 2030 to 2050) either for additional repositories or for the construction of infrastructure for recycling spent fuel.
Technical Capacity: Advanced Fuel Cycles
It is technically possible for AFCs to recycle and transmute almost all of the heavy actinide elements that contribute to decay heat, leaving only fission products and residual actinides for disposal. Only two of the fission-product isotopes- strontium(Sr)-90 and cesium(Cs)-137, both of which have 30-year half- lives - would contribute significantly to the remaining decay heat. Because these isotopes have relatively short half lives, it is technically possible to separate and manage them separately for the 200 to 300 years required for their nearly complete decay. Separation and separate management of Cs-137 and Sr-90 have already been demonstrated at large scale at the Hanford site in Washington state, where both cesium and strontium recovered from high-level waste are currently stored separately in sealed capsules.6
Without cesium and strontium, the remaining fission products and residual actinides that require geologic disposal have very small rates of decay-heat generation. Thus, it becomes relatively easy to estimate the capacity of the Yucca Mountain site. If the current canister design for defense high-level waste (capable of holding five 60-cm diameter cylinders of borosilicate waste glass) were used to hold fission products, the fission-product loading could be 500 kg/m of drift tunnel length;7 this is 7 times greater than the fission-product loading for current 21-assembly PWR canisters. A 1-GW(e) light-water reactor (LWR) (whether a BWR or PWR), which can produce energy for one million typical homes, also produces approximately 1,080 kg of fission products per year. Slightly more than two meters of Yucca Mountain drift could hold a year’s fission products from a plant this size.
At 2,000 acres, the Yucca Mountain site could have 100 km of drift tunnels spaced at 81 m. Without decay heat, the spacing could be reduced to 20 m, thus increasing the drift tunnels to 400 km. Using the existing defense-waste canister design, these drift tunnels could then hold 200,000,000 kg of fission products, the energy equivalent of burning one trillion tons of coal. This means that a single Yucca Mountain could replace 170 years of current, total, worldwide coal consumption.8
Separation and separate management of cesium and strontium would require management of these materials for one to three centuries, until radioactive decay reduces their heat output sufficiently to permit their disposal. However, there are alternative strategies for managing cesium and strontium decay heat, because it drops greatly over the time scales of surface storage and repository operation.
Because the Yucca Mountain repository is located above the water table, air can be circulated through the tunnels to remove decay heat. In the current design, ventilation will continue for 50 years after the final canister emplacement. As shown in Figure 1, the ventilation extracts most of the heat from cesium and strontium. For every 30 years of operation, the ventilation system regenerates roughly half of the repository’s capacity for holding cesium and strontium.
With ventilation, the heavy actinide elements, particularly americium(Am)-241, which has a 460-year half-life, drive Yucca Mountain’s postclosure thermal response. To eliminate the need for a second reposi-tory, one must therefore cap the total inventory of heat-generating actinides, particularly Am-241, within the thermal capacity of the site.
Relatively large inventories of Am-241 build up in the spent fuel of current LWRs because of successive neutron captures in U-238. Neutron capture in U-238 yields Pu-239, and fission of this Pu-239 provides a substantial fraction of the power output from LWRs (reaching 50 percent shortly before the fuel is discharged). But with the relatively low kinetic energy of neutrons in LWRs, a significant fraction of neutron reactions with Pu-239 are capture reactions that generate Pu-240, and