Acknowledgment and Disclaimer





Assessments for Disposal of
Radioactive Waste Materials
Stored at DOE Facilities



Project Description and Significance

The U.S. Department of Energy (DOE) needs to find a way to dispose of approximately 12,000 metric tons of defense-related waste. (This amount is expressed as metric tons of heavy metal-MTHM.) High-level radioactive waste (HLW) from Hanford's single-shelled tanks, foreign fuel, and decomissioning of DOE research reactors could increase that amount. Sandia National Laboratories and the DOE are searching for alternatives through their study of possible treatment and disposal of waste stored at the Idaho National Engineering Laboratory (INEL). In this short-term study, Sandia used the performance assessment (PA) methodology developed for the Waste Isolation Pilot Plant (WIPP) Project to generate information that DOE could use in its decisions on future waste disposal.


Because about 50% of the volume and fissile mass of spent nuclear fuel within the DOE complex is located in Idaho (see table), the concept for the program originated at INEL, in particular, at the Idaho Chemical Processing Plant ("Chem Plant") where spent fuel and high-level waste have been stored since 1953. In 1963, the Chem Plant began processing the liquid high-level wastes and some hazardous wastes stored there to form a dry granular powder called "calcine" that can be stored more safely than liquid. However, the decision in 1992 by the national administration to discontinue reprocessing of spent nuclear fuel has left about 300 MTHM of waste in spent fuel in storage with unspecified plans for future disposal.
Location (% of total) of DOE Spent Fuel*

TotalsINELHanford Savanah River Other
No. of Storage Units (48,736) 30 20 24 26
Volume of Spent Fuel (1373 m3) 54 18 12 16
Fissile Mass (78 MT) 50 35 9 6
Heavy Metal (2675 MT) 11 80 7 2
Uranium Mass (2600 MT) 9 82 8 1
Total Mass of Spent Nuclear Fuel (4506 MT) 27 53 12 8

*Does not include high-level waste.


Sandia's Contribution

In early 1993, the DOE sought guidance on ways to prepare the waste (such as treatment, conditioning, and packaging) for eventual disposal, choosing a performance assessment as the means of obtaining technical input. Sandia's WIPP Performance Assessment Department (6749) is assisting the DOE in evaluating possible treatment and disposal methods for spent graphite nuclear fuel, more than 90 experimental spent nuclear fuels, and radioactive high-level waste stored at INEL and elsewhere in the DOE complex.


In the first part of the study, several waste treatment options and two generic disposal sites were considered-one in salt and the other in saturated granite. In the second part of the study, the hypothetical repository was in unsaturated tuff, similar to the potential repository at Yucca Mountain, Nevada, so that the effects of disposal of DOE spent fuel in a similar repository could be examined (Figure 1). (Yucca Mountain is under consideration for disposal of commercial spent fuel and a portion of the high-level defense waste in the United States.)




Figure 1. Layout of waste disposal rooms and access drifts in tuff for
hypothetical repository containing DOE spent fuel and high-level waste.


As part of an examination of the response of an unsaturated tuff repository containing DOE spent fuel and high-level waste, Sandia studied the possibility of a nuclear criticality condition developing in one or several containers of highly enriched uranium spent fuel. The technical aspects of determining how a criticality can occur and its effects are complicated in a geologic repository because­unlike a nuclear reactor­the range of possible thermal-hydraulic conditions cannot be defined over time.


The analysis used the methodology established by Sandia National Laboratories for assessing the long-term performance of a geologic disposal system, used data collected from past assessments of disposal systems, and examined several levels of simplification in the consequence models to develop an understanding of uncertainty in the results. For each treatment option considered, the Performance Assessment Department used computer simulations of the waste type in the hypothetical disposal systems.


The INEL PA considered the containment and individual protection requirements of the Environmental Protection Agency's (EPA) regulations for disposal of spent nuclear fuel and radioactive wastes (40 CFR 191) and the performance criteria for containers and wastes as defined in the Nuclear Regulatory Commission's 10 CFR 60 (although these regulations are being reviewed and are not expected to be repromulgated until 1998). The combined studies offered the DOE important information on methods of disposal and treatment of the waste. For example, the spent fuel did comply with the 40 CFR 191 regulatory criteria and in general behaved similarly to fuel from commercial pressurized water reactors (PWR). Some spent fuels performed slightly better than commercial fuels, some slightly worse when evaluated against the containment requirements of 40 CFR 191; (see Figure 2). As expected, the high-level waste from reprocessing performed better than the spent fuel because uranium had been removed. Besides the type and integrity of the cladding and the original radionuclide inventory, the differences in behavior were caused by temperature-sensitive corrosion rates of the containers. Although they are not shown, differences in behavior also depended on the geologic medium in which the waste was placed. Although the criticality calculations were preliminary, they supported the belief that it would be difficult to encounter conditions that would provide enough water from infiltration in a 10,000-year regulatory period to generate a criticality in a tuff repository.


Future Work

The INEL study is an example of performance assessment methodology being applied early in a program to generate information for decision making on program direction. The preliminary studies on criticality have created a basis from which to pursue this issue further, which is of interest to agencies involved with either defense-related or commercial disposal systems.



Figure 2. In general, high-level waste performs better than spent
nuclear fuel, but all DOE spent fuel complies.


For further information, contact:

Rob P. Rechard
Sandia National Laboratories, MS-1328
Albuquerque, NM 87185-1328
Phone: (505) 848-0691
e-mail: rprecha@sandia.gov

or

D.R. (Rip) Anderson
Sandia National Laboratories, MS-1328
Albuquerque, NM 87185-1328
Phone: (505) 848-0692
e-mail: drander@sandia.gov

Submitted July 1995
Layout design by Wanda Mar.