11
What Should be Done: INEEL HLW Calcine
Because the sodium-bearing waste (SBW) and the existing calcine are different classes of waste [mixed transuranic (TRU) low-level waste (LLW) and high-level waste (HLW), respectively] with differing characteristics, they should not be treated in the same way, processed in the same equipment, or disposed of at the same disposal site. Indeed, because disposal and transportation options differ for these two waste types, adopting the same treatment method, waste form, and disposal site is almost certainly not optimal. Therefore, treatment and disposal strategies for SBW and calcine are considered separately, with calcine discussed in this chapter and SBW in Chapter 12. Whether either of these wastes should be processed in the near future is also discussed, since the apparent risks associated with the current storage of SBW and the HLW calcine differ.1
The chapter considers what should be done with the Idaho National Engineering and Environmental Laboratory (INEEL) HLW calcine in light of
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constraints of regulations and cost (discussed in Chapter 9),
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as-yet undefined boundary conditions that impact the ultimate disposition of INEEL wastes (discussed in Chapters 9 and 10), and
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legitimate opportunities for future relief from milestones of a nonregulatory nature.
To expand on the third point, given that the year 2035 target deadline is not a regulatory constraint and is subject to renegotiations (in fact, the Settlement Agreement makes explicit provision for this), the committee, in addressing its Statement of Task, considered technical treatment options that would meet this target as well as options that would not. The result is shown below, as the committee position of what option should be chosen.
As discussed in Chapters 9 and 10, the committee believes that no clearly acceptable pathway for ultimate disposal of the HLW calcine has been defined, and therefore, more information than is currently available is needed to make an informed technical decision among the various processing options. The ultimate disposal location is uncertain, the form in which it would be acceptable at that location may differ from current specifications developed for the first repository, and the transportation pathway to get there (with its regulatory requirements)
is similarly an open question.2The committee believes that aggressive efforts should continue to define these three points, including the development of a regulatory solution for the Resource Conservation and Recovery Act (RCRA) constituents (e.g., delisting petitions, and confirmation that organics do not survive the calcining process, if this information is needed).
In the meantime, it is the committee's strong recommendation that the processing of calcine be deferred for a time limited to the duration within which the bins maintain their integrity, with modifications to the bins made as necessary to ensure safe storage. If and when during this period an acceptable pathway for ultimate disposition is established, as described above, an evaluation should then be made as to whether to pursue that route or continue storing the calcine in the bins.
If calcine were stored for a few decades, the concentrations of the radioactive species would be reduced to a level of activity within the regulatory limits for ''remote-handled" (RH) TRU waste.3 For longer time frames, the TRU activity would be reduced even further. For example, in approximately 500 years, the TRU activity would be reduced by approximately a factor of ten,4 if calculations (Garcia, 1997) of initial inventories of the isotopes 238Pu and 241Am are correct, as could be confirmed by subsequent measurements. After approximately 500 years, the Cs and Sr activity would decay by about five orders of magnitude to approximately Class A LLW limits (see Figure 11.1). The consequence of this decay is that the residual activity (several hundred nCi/g of TRU elements, with additional activity from residual fission products) in the calcine, coming from fission products such as 99Tc, 93Zr, and 93mNb and actinides such as 239Pu, 240Pu, and 241Am (Garcia, 1997: Tables 9 and 10), would represent a level of activity within the regulatory limits for "contact-handled" (CH) TRU waste (Garcia, 1997: Tables 9 and 10).
As these calculations show, the calcine could be processed with less technical risk and less potential radiological exposure if the processing is deferred beyond the near term. This processing could segregate selected species to generate a low-activity fraction for disposal on-site (meeting regulatory limits such as Class A LLW limits) and a smaller TRU fraction for disposal in a suitable repository. The radiation hazard associated with retrieval and processing of the calcine will lessen over time. According to Engineering Design File "EDF-BSC-002" of Dahlmeir et al. (1998), the radiation levels to be expected during calcine removal are in excess of tens of Roentgens (R) per hour, and possibly hundreds or thousands of R per hour.
The only near-term action should be improved confinement and stabilization of the bins, ventilation system, and associated equipment. The hygroscopic content of the calcine requires that there be adequate protection from atmospheric moisture if "caking" is to be avoided. A risk assessment is strongly recommended to identify which of these and other actions are most important to reduce risk. The risk assessment should also substantiate that this
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That is, it is not certain that the nation's first repository will be the one to contain the HLW calcine. Moreover, the waste acceptance criteria (WAC) for waste forms to qualify for disposal in the first repository are subject to future change, and the WAC for a second repository are unspecified (see discussion of these issues in Chapters 9 and 10). Because of these uncertain conditions, the full set of specifications on any treatment process to produce a final waste form cannot be determined at present. |
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In support of this statement, the WIPP waste acceptance criteria for RH-TRU wastes specify a maximum radioactivity concentration of 23 Ci per liter (23,000 Ci per cubic meter), which Figure 11.1 shows is attainable for the HLW calcine after a few decades of decay after reprocessing. |
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This factor of 10 is primarily due to the decay of the initial inventories of 238Pu (T1/2 = 88 years) and 241Am (T1/2 = 432 years). Some 241Am is produced over time via beta decay of the (small) initial inventory of 241Pu. Much of the HLW calcine originated from reprocessing of Navy SNF enriched in 235U. While in reactors, successive neutron captures and beta (β) decays [specifically, 235U(n, γ)236U, 236U(n, γ) 237U, 237U(β-)237Np, 237Np(n, γ)238Np, and 238Np(β-)238Pu)] produced the predominant amount of 238Pu (another, much more minor source of 238Pu is from alpha decay of 242Cm), which decays with a relatively short half-life compared to other TRU isotopes. This significant fraction of 238Pu in the waste means that INEEL calcine, more than other DOE HLW inventories, can be remediated via natural radioactive decay while in interim storage (Figure 11.1). |
Figure 11.1 This graph shows decay in activity of INEEL HLW calcine over time. The longer-lived radionuclide constituents, which are actinides and long-lived β emitters listed in Table I of 10 CFR 61, are reduced by approximately a factor of 10 after 500 years. The shorter-lived radionuclides, which are the fission products Cs and Sr and other species listed in Table II of 10 CFR 61, are reduced by approximately five orders of magnitude in 500 years. Also shown are the Class C LLW limits that Tables I and II provide for these two categories of radionuclides. From Berreth (1988).
storage of a mixed HLW with significant TRU content in a near-surface environment (a storage which would probably require regulatory relief) poses a low risk at present and in the foreseeable furore to workers, the public, and the environment. This recommendation does not challenge the general strategy of geologic disposal for HLW, which is an issue outside the scope of this study, but emphasizes that decisions on the ultimate fate of the INEEL HLW should be postponed pending the resolution of waste management issues noted above and pending the results of adequate risk analyses,5
In this recommendation to defer processing of HLW calcine until site(s), route(s), and waste form specifications are firmly established, no time period is specified for the duration of interim bin storage. Limitations to this time period could come from
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a technical assessment of bin integrity over time, and/or
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regulatory and other requirements.
To expand on (1), the information provided to the committee does not specify the failure mode (e.g., corrosion or seismic stability) that is the most limiting for bin integrity, and does not indicate whether 500 years signifies a mean time to failure or another design criterion. The committee recommends that during any period of interim bin storage, continuing verification of bin integrity is essential. To expand on (2), if the Licensing Requirements for the Independent Storage of Spent Nuclear Fuel and High-Level Radioactive Waste (10 CFR 72) were to apply to INEEL bin storage, then a regulatory license could be granted for up to 20-40 years, with any renewals for time periods beyond that contingent upon sufficient technical justification to satisfy the requirements for a license extension. Such matters of regulatory strategy were not examined in detail by the committee, and would require attention in the event that resolution of ultimate disposal site(s), route(s), and waste form specifications is not attained in the near future.
PERSPECTIVE ON OTHER HLW INVENTORIES
To expand upon this last point, HLW inventories being processed or involved in plans for processing at other DOE sites are in configurations much less stable than the INEEL HLW calcine. At the Hanford Reservation, for example, liquid HLW is contained in carbon steel tanks, many of which are single-shell tanks known to have leaked. At the Savannah River Site, the liquid HLW is stored in single-and double-shell carbon steel tanks that are located close to ground water. These conditions appear to pose a greater hazard or potential for release than solid calcine stored in stainless steel bins at INEEL. Since the current calcine configuration seems to be one of low risk, the need for action is correspondingly less, and a rush to select a long-term treatment option unwarranted.
