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Partitioning and Durable Waste Forms for Highly Radiotoxic Isotopes

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Partitioning and Durable Waste Forms for Highly Radiotoxic Isotopes PARTITIONING AND DURABLE WASTE FORMS FOR HIGHLY RADIOTOXIC ISOTOPES Bruce E. Kirstein, 1,* and Sue B. Clark2 1Staff, U.S. Nuclear Waste Technical Review Board, ** 2300 Clarendon Blvd, Suite 1300, Arlington, VA 22201 2Board Member, U.S. Nuclear Waste Technical Review Board and Regents Professor of Chemistry at Washington State University in Pullman, WA 99164 *Correspondence concerning this paper should be addressed to Kirstein ([email protected]) **The views expressed in this paper are those of the authors and do not necessarily represent the views of the U.S. Nuclear Waste Technical Review Board (www.nwtrb.gov). Partitioning light-water reactor spent nuclear research and development needed to bring the fuel offers the opportunity to dispose of the more separations technologies to economic, full-scale troublesome radioisotopes in durable waste forms for operation. A suitable waste form for the partitioned the purpose of providing long-term isolation. These isotopes and all elements in the various separated more troublesome radioisotopes can be identified by waste streams can then be identified. However, a their radiotoxicity to determine which should be geologic repository will still be required for all disposed of in a repository with long-term elements, except possibly for recovered uranium. An performance assurance. The purpose of this paper is important consideration then is the relative masses of to investigate the partitioning of isotopes into two these partitioned elements in final waste forms and groupings: one requiring long-term repository the size in terms of disposal masses of the resulting performance assurance with respect to disposal, and repositories. It is the purpose of this paper to consider the other that does not. Once grouped in this way available information to accomplish partitioning and applicable separation technologies and durable disposal, and to compare disposal masses for direct waste forms are identified, and a material-balance disposal of spent nuclear fuel, partitioning of the analysis is used to estimate the disposal requirements spent nuclear fuel mass and subsequent disposal, and of these groupings in terms of masses. Disposal limited recycle of plutonium and uranium in light masses are estimated for the cases of direct disposal, water reactors where a reduction in the disposal mass partitioning, and limited recycle where recovered of plutonium can be realized. plutonium and uranium are recycled once. Recovered uranium with adequate decontamination II. PARTITION BASIS is expected to be disposed of or stored as the oxide. This investigation is not intended to imply that spent Considerable discussion on partitioning and nuclear fuel should be partitioned solely for the transmutation to improve the utilization of a geologic purpose reducing the mass disposed in a single repository and protect human health and safety can repository, but to illustrate the relative disposal be found in the peer-reviewed literature.1,2 Two masses for the identified components. measures determine which radioisotopes should be partitioned, or separated, and these measures are 1) I. INTRODUCTION curies, and 2) radiotoxicity. Consideration of curies is described in the Yucca Mountain Total System Some long-lived troublesome radioisotopes Performance Assessment for License Application.3 could be partitioned from light-water reactor (LWR) Based on this criterion, cesium, strontium, spent nuclear fuel so that they can be disposed of in a americium, plutonium would be partitioned because repository with long-term performance assurance. they are the major contributors to the total curie These radioisotopes requiring partitioning from spent inventory of spent nuclear fuel out to 10,000 years. nuclear fuel can be put into durable waste forms that After this time period, the curie inventory has provide for robust isolation. By isolating these decayed to approximately one percent of the starting radioisotopes, the time frame of repository inventory. performance assurance for the remaining radioisotopes is reduced. Once those radioisotopes Conversely, radiotoxicity of an isotope is the that require long-term disposal are identified, inventory weighted by an appropriate human dose appropriate separations technologies to accomplish conversion factor.4 For a mixture of isotopes, such as the partitioning can be identified along with further spent nuclear fuel, radiotoxicity is summed over all IHLRWMC 2013, Albuquerque, NM, April 28-May 2, 2013 581 radioisotopes. Despite having units of dose, The Yucca Mountain Repository as presented in radiotoxicity does not provide a measure of potential the Total System Performance Assessment for the dose from any nuclear waste material because it does License Application3 for an oxidizing geologic not account for the quantity of the inventory causing environment provides an example of dose derived exposure to humans or escaping into the from geologic specific characteristics. The results of environment. Calculation of dose requires this work show that 239Pu, 237Np, 129I, and 226Ra knowledge of the interplay between waste generally dominate the mean annual dose for the management techniques and geologic-site postclosure period from 100,000 to one million years. characteristics. Hedin5 provides a discussion of Greneche et al.,1 derive evaluations of the radiotoxicity and accessibility (mobility) to illustrate radiological dose impact for specific fuel-cycle cases the concept of risk in the context of the long-term of the disposal of high-level radioactive waste in safety of a deep repository by taking into account repositories sited in granite, clay, and salt, all of waste management techniques and geologic-site which are reducing environments. In this same characteristics. Grambow6 describes the transport of discussion it is shown that the contribution to spent mobile fission products and activation products in fuel toxicity due to fission products is quite small clay and the impact of the disposal environment on after about 500 years. The predicted doses derived repository safety. by Greneche et al.,1 fall far below the dose constraint of 0.3 mSv/a (30 mrem/year) recommended by the Westlén7 bases the radiotoxicity discussion on International Commission on Radiological half lives of important actinides and long-lived Protection.10 Also concluded is that the impact of fission products along their respective dose partitioning for all cases is that the maximum dose is conversion coefficients. Westlén derives a plot of the rather limited because it is essentially due to mobile time evolution of the radiotoxicity from spent nuclear long-lived fission and activation products. The fuel and also the radiotoxicity in this same fuel for mobile long-lived fission products are 129I, 79Se, individual actinides along with the total due to the 135Cs, 99Tc, and 126Sn. Actinides contribute very little actinides. The main observation from Westlén’s to the dose due to the reducing conditions of the results shows that the radiotoxicity of spent fuel selected geologies. approaches that of natural uranium at about 107 years and that the transuranics are the reason for this very III. IDENTIFICATION OF CANDIDATE long time frame. RADIOISOTOPES FOR PARTITIONING The shapes of the total radiotoxicity curves for Considering radiotoxicity, the following spent nuclear fuel as a function of time described by radioisotopes are candidates for partitioning to reduce Piet4 and Westlén7 are approximately the same; both the long-term radiotoxicity of spent nuclear fuel decrease with respect to time in about the same independent of the repository geologic medium: 79Se, manner and show a slight change in slope at around 93Zr, 99Tc, 107Pd, 126Sn, 129I, and 135Cs (Ref. 7). The 100,000 years. The shapes of the fission-product fission products identified by Piet4 are (also) 93Zr and curves for each are also approximately the same, 126Sn. Based on radiotoxicity, the fission-product rapidly decreasing until about 500 years followed by elements identified as candidates for partitioning are little change with the final estimated radiotoxicity selenium, zirconium, technetium, palladium, tin, being below that of natural uranium. The shapes of iodine and cesium. The transuranics identified by the transuranic (TRU) curves are also approximately Westlén that are the major contributors to spent-fuel the same; the TRUs account for the major portion of radiotoxicity are plutonium, americium and curium; the radiotoxicity out to about 100,000 years. The neptunium is not a major contributor. The contribution of individual TRUs, or actinides, to the transuranics identified by Piet4 are also the major total actinide radiotoxicity as derived by Westlén contributors to spent fuel radiotoxicity. Based on show that plutonium and americium are the dominant radiotoxicity, all of the transuranic elements are contributors. Neptunium is never a major contributor candidates for partitioning. to the total radiotoxicity and its contribution is always less than the radiotoxicity of natural uranium. Using the criterion of repository-specific geology Other references recognize radiotoxicity as the with respect to the long-term dose contributors rather preferred criterion for determining which than radiotoxicity, different isotopes are identified for radioisotopes should be isolated; some of these are partitioning. For a repository sited in oxidizing tuff, Nishirhara et al.,8 Gombert,9 and Greneche et al.1 the major dose contributors identified by McNeish et al.,3
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