UCR1, 94222 PREPRINT IMPORTANT RADIONUCLIDES IN HIGH LEVEL NUCLEAR WASTE DISPOSAL: DETERMINATION USING A COMPARISON OF THE EPA AND NRC REGULATIONS Virginia M. Oversby This paper was prepared for submittal to Nuclear and Chemical Waste Management February 1986 This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. - QLJ Important Radionuclides in High Level Nuclear -Waste Disposal: Determination Using a Comparison of the EPA and NRC Regulations Virginia M. Oversby Earth Sciences Department Lawrence Livermore National Laboratory Livermore, CA 94550 Preprint - for submission to Nuclear and Chemical Waste Management ABSTRACT The performance objective for the engineered barrier system given in the NRC regulations (10CFR60) is used to determine a maximum release rate for each significant radionuclide for a generic repository containing PWR spent fuel. This release rate, integrated over the times during which release would occur, is then compared to the EPA requirements on limitation of total releases to the accessible environment. The amount by which the releases allowed under the NRC regulations exceeds the EPA requirements is an indication of the importance of the radionuclide for performance assessment purposes. Nuclides for which NRC-allowed releases from the engineered barrier system greatly exceed those allowed by EPA to the accessible environment will need to be controlled either by limiting their release at the EBS boundary to values that are lower than the NRC requirements or by reducing the amounts of these nuclides that reach the environment by processes that occur during transport. The simplest case, which assumed only the minimum performance required in 1OCFR60 on control of release rates, results in the identification of 17 chemical elements for which data on solubility and sorption would be needed for use in site performance assessment. Of these, americium and plutonium are by far the most important. The other actinides, carbon, and nickel are also important. If the assumption of congruent dissolution is imposed, with a 1% rapid release spike for cesium, iodine, carbon, and technetium, the list of elements reduces to 13, with iodine, cesium, selenium, and palladium being eliminated from the list. The importance of americium is greatly reduced in this case and plutonium becomes the most important element. A final analysis, which assumed a congruent dissolution rate of one part in 1,000,000 per year, results in a list of at most seven important elements. With reasonable assumptions the list can be narrowed to just americium and plutonium. All of the considerations point to the importance of understanding the behavior of americium and plutonium under conditions that are relevant to the waste form dissolution process and under processes that might pertain during transport from the repository to the accessible environment. Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. - 1 - INTRODUCT ION The Nevada Nuclear Waste Storage Investigations Project NNWSI) is studying the tuffaceous rocks at Yucca Mountain, Nevada, to evaluate their potential for use as a high level radioactive waste repository. As part of the NNWSI Project, Lawrence Livermore National Laboratory (LLNL) is responsible for the design of waste packages, the testing of waste package components under a range of possible repository environments, and the prediction of the performance of the waste package and its near field environment over the time periods required by the regulations related to high level waste repositories. LLNL is also responsible for development of EQ3/6, a geochemical modelling code that uses a collection of thermodynamic data to calculate the reaction progress between aqueous solutions and solids. For both of these tasks, an understanding of the individual regulations and their interrelationships is necessary to guide the direction of the work. The Environmental Protection Agency (EPA) has issued a final rule, 40 CFR Part 191, that provides generally applicable standards for the management and disposal of spent nuclear fuel and high level radioactive wastes. The long term requirements for performance of the repository are given in section 191.13 "Containment Requirements". These requirements limit the total quantities of radionuclides that are predicted to be released to the accessible environment over a period of 10,000 years. Cumulative releases from all significant processes and events must "have a likelihood of less than one chance in 10 of exceeding the quantities calculated according to Table 1" of 40 CFR 191 and "less than one chance in 1,000 of exceeding ten times the quantities calculated according to Table 1" (Reference 1). The values given in the EPA rule limit the releases of alpha-emitting radionuclides to 100 curies per 1000 metric tons of heavy metal (MTHM) except for thorium isotopes, for which the value is 10 curies. For radionuclides that do not decay by alpha emission, the general limit is 1000 curies per 1000 MTHM; exceptions are carbon-14 (100 curies), iodine-129 (100 curies), and technetium (10,000 curies). In addition to meeting the individual - limitations, the sum of the releases from all radionuclides must be such that - 2 - n E Ri 1 where Qi is the calculated release for radionuclide RLI is the Table 1 release limit for radionuclide I. and n is the number of radionuclides contributing to the sum The EPA rule also contains two "protection requirements", one related to individual dose limits and one related to special sources of ground water. Both of these limits apply to the first 1000 years after disposal and "undisturbed performance" of the disposal system. At present there is no source of water near the Yucca Mountain site that would qualify under the 40 CFR 191 definition of "special source of ground water." The individual dose limit will probably not be difficult to meet, given the long travel times that are calculated for movement of water through the unsaturated zone under the expected conditions of flux (Sinnock et al., 2). The Nuclear Regulatory Commission issued a rule for the licensing of geologic repositories for high level waste (10 CFR Part 60) in 1983, before the EPA rule had been finalized (Reference 3). This rule may need to be amended to reflect the content of the final EPA rule; however, for the purposes of identifying the radionuclides f most importance to the performance of a repository, the present NRC rule is adequate. The rule states in part that "the engineered barrier system shall be designed, assuming anticipated processes and events, so that ... (B) The release rate of any radionuclide from the engineered barrier system following the containment period shall not exceed one part in 100,000 per year of the inventory of that radionuclide calculated to be present at 1,000 years following permanent closure, or such other fraction of the inventory as may be approved or specified by the Commission; provided that this requirement does not apply to any radionuclide which is released at a rate less than 0.1% of the calculated total release rate limit. The calculated total release rate limit shall be taken to be one part in 100,000 per year of the inventory of radioactive waste, originally emplaced in the underground facility, that remains after 1,000 years of radioactive decay" (10 CFR 60, section 60.113 (a)(l)(ii)). - 3 - The NRC performance objective for the engineered barrier system, quoted above, can be used as the basis for evaluating which radionuclides will be the most significant in determining the performance of the repository system. We will first determine, for the case of average spent fuel, what the maximum allowed release rates would be for individual radionuclides. These release rates, integrated over the time of release, can then be compared to the EPA cumulative release limits to determine how much lower the release rates must be, or how much further control on releases to the accessible environment must be provided by the site characteristics, in order to achieve compliance with the EPA limits. These calculations can provide generic guidance that is independent of site characteristics or waste form performance models; the results are simply consequences of the EPA and NRC regulations taken together. Once the key radionuclides have been identified, researchers and designers for each individual site can determine whether the additional control of releases of specific important radionuclides will be best achieved through engineered barrier system design features or through retardation properties of the site geologic media. This information can then be used to set priorities for the research and development program for the site. - 4 - RELEASE RATES BASED ON THE NRC PERFORMANCE OBJECTIVE The Performance Objective for Release Rate The NRC statement of the performance objective for the engineered barrier system limits the release of individual radionuclides to one part in 100,000 per year of their inventory at 1,000 years after closure of the repository, or to 0.1% of the calculated total release rate limit. The calculated total release rate limit is based on one part in 100,000 of the "inventory of radioactive waste, originally emplaced in the underground facility, that remains after 1,000 years of radioactive decay." (Reference 3). A close examination of the wording shows that two different times are stated for the inventory calculations. The time for determining the calculated release rate limit is 1,000 years after emplacement, while the time for the individual isotope release rate calculations is 1,000 years after closure of the repository. No time period over which this performance objective would apply is stated in the rule.