Tank Wastes Planned for On-Site Disposal at Three Department of Energy Sites: The Savannah River Site: Interim Report (2005)

In the Ronald Reagan National Defense Authorization Act of 2005 (Section 3146 of Public Law 108-375), Congress directed the Department of Energy (DOE) to request a study from the National Academies¹ evaluating DOE’s plans to manage radioactive waste streams from reprocessed spent fuel that “exceed the concentration limits for Class C low-level waste as set out in Section 61.55 of Title 10, Code of Federal Regulations;^2,3 DOE plans to dispose of on the sites specified below rather than in a repository for spent nuclear fuel and high-level waste; and are stored in tanks at the Savannah River Site, South Carolina; Idaho National Engineering and Environmental Laboratory, Idaho; and the Hanford Reservation, Washington.”

Congress asked the National Academies to assess the following:

1. DOE’s knowledge of the physical, chemical, and radiological characteristics of the waste in the tanks;

2. actions that DOE should consider to ensure that management plans comply with the performance objectives for land disposal facilities;

3. DOE’s monitoring plans to verify compliance with the aforementioned performance objectives;

4. existing technology alternatives for waste management;

5. technology gaps for waste retrieval and management; and

6. any other matters that the committee considers appropriate and directly related to the subject matter of the study.

The full statement of task and the performance objectives referred to in element (b) of the task can be found in Appendix A.

Task element (f) was reinforced by Representative John Spratt (5^th District of South Carolina) and House Armed Services Committee staff, who presented the charge to the committee at its first meeting in March 2005. They asked the committee to interpret the charge broadly to include any relevant matters of importance, with emphasis on any portion of the tank waste that would be disposed at the sites.⁴ Congress asked for an interim report on task element (b) for the Savannah River Site in six months (see below) and a final report on all of the sites in twelve months. This report fulfills the first requirement.

The National Defense Authorization Act states that the interim report shall address “any additional actions the Department should consider to ensure that the Department’s plans for the Savannah River Site, including plans for grouting of tanks, will comply with the performance objectives [of Part 61 of Title 10, Code of Federal Regulations] in a more effective manner …” (Section 3146 (e)(A)). The committee worked with DOE, the South Carolina Department of Health and Environmental Control (DHEC), the U.S, Nuclear Regulatory Commission (USNRC), DOE’s contractors, and others to obtain the information needed for the study. To this end, the committee obtained a large number of documents and held three public meetings to obtain information from experts and interested members of the public. Some data and analyses were not available to the committee (not yet collected, not yet calculated, or not yet made public⁵), and some plans had not yet been formulated or finalized by the time this report entered the National Academies report review process in early June 2005. Appendix B describes the main documents to which the committee had access and the missing pieces of information needed to assess DOE plans fully with respect to the performance objectives of 10 CFR 61.

Although the committee was unable to evaluate fully what, if any, actions are needed for DOE to comply with the dose limits in the performance objectives of the regulation, in this interim report the committee evaluates and recommends actions that it believes could (1) reduce the quantity of waste left on-site, and (2) increase DOE’s understanding of other factors that reduce dose and risk—namely, the long-term performance of waste forms and other barriers to the release of radionuclides to the environment. The committee judges that these actions will increase DOE’s ability to comply with the performance objectives in general and will help DOE fulfill its requirement to take actions to make releases of radioactivity to the environment as low as reasonably achievable (ALARA),⁶ with economic and social considerations taken into account.

Because some information was not yet available, as noted previously, and because the committee may gain insights from later meetings and site visits in Idaho and Washington State, the committee may choose to extend its comments on the Savannah River Site in its final report.

In 2003, the Secretary of Energy asked Congress to grant DOE explicit authority to dispose of some of the waste stored in high-level waste (HLW) tanks on-site. DOE’s impetus for requesting congressional action was that recent lawsuits and court rulings⁷ threatened its cleanup schedules, in particular, at the Savannah River Site. DOE is taking actions that it characterized as implementing an “aggressive action to accelerate risk reduction,” which entails an accelerated schedule for retrieval of waste and closure of tanks (DOE, 2004). In October 2004, Congress passed legislation that responded to DOE’s request.

In addition to requesting a study from the National Academies, the Ronald Reagan Defense Authorization Act of 2005 (Section 3116) gives DOE the authority it sought for the Savannah River and Idaho sites (the Hanford Site is explicitly excluded from Section 3116, but is included in this study). Specifically, the act states that the term “high-level radioactive waste” does not include radioactive waste resulting from the reprocessing of spent nuclear fuel that “the Secretary [of Energy], in consultation with the Nuclear Regulatory Commission, determines (1) does not require permanent isolation in a deep geologic repository for spent fuel or high-level waste; (2) has had highly radioactive radionuclides removed to the maximum extent practical; and (3) disposal complies with the performance objectives of Subpart C of 10 CFR 61 and is pursuant to a State-approved closure plan or State-issued permit.”⁸ In addition, if the waste exceeds concentrations for Class C low-level waste (LLW), as set out in 10 CFR 61.55, DOE must dispose of the waste pursuant to plans developed by DOE in consultation with the USNRC.

Disposal actions for wastes that DOE determines under Section 3116 not to be high-level waste are to be monitored by the U.S. Nuclear Regulatory Commission in coordination with the host state. DOE already develops its plans for Savannah River Site tank waste disposition subject to the approval of the South Carolina Department of Health and Environmental Control under the Savannah River Site Federal Facility Agreement (FFA), and with input from the South Carolina Governor’s Advisory Council.

The Savannah River Site has 51 underground tanks⁹ that are used for storing 138,000 cubic meters (36.4 million gallons) of hazardous and radioactive waste. The Savannah River Site started generating the waste in 1953 when a large chemical processing facility, called the F Canyon, was brought into service to separate uranium and plutonium from irradiated targets and spent nuclear fuel from reactors on the site to support the U.S. nuclear weapons program. None of the reactors is operating today and plutonium production has ceased at the site, but related operations in a second chemical processing facility, the H Canyon, and other waste processing operations continue to generate relatively small amounts of waste.

Each canyon facility piped liquid waste from the spent nuclear fuel processing operations to a set of tanks located in its area: the F-Area Tank Farm has 22 tanks and the H-Area Tank Farm has 29 tanks.¹⁰ The tanks range in size from about 2,850 cubic meters (m³) to 4,900 m³ (750,000 to 1.3 million gallons) and are approximately 23-26 meters (75-85 feet) in inner diameter and 7.5-11 meters (24.5 to 35 feet) in height from the inner tank floor to the ceiling. They are buried at a shallow depth (1 to 3 meters below the land surface), with four nearly submerged in the saturated zone (below the water table).

Access to the interior of the tanks is attained through access portals, called risers, which rise from the top of the tank through the ground cover to the land surface. The number of risers in each tank ranges from 15 to 40, and the diameters of most of the apertures range from 58 to 107 centimeters (23 to 42 inches) depending on tank type. Some risers are larger: the center riser of a Type IV tank is approximately 2.7 meters (9 feet) (Fogle, 2002).

Most of the tanks have a carbon steel inner wall and an outer wall constructed of concrete, with an annular space between them. If the outer wall has a metal liner, then the liner provides what is called a secondary containment (i.e., a tank inside a tank). If the outer liner rises only partway up the outer wall, it provides only partial secondary containment. Eight of the tanks have no annulus or secondary containment (Type IV tanks), 16 have partial secondary containment (Type I and II tanks), and 27 have full secondary containment (Type III and IIIa tanks). Figure 1 illustrates the four general tank types. Only the Type III and IIIa tanks with full secondary containment are considered “compliant tanks” under the site’s Federal Facility Agreement,¹¹ which regulates management of hazardous waste at the site and uses the Resource Conservation and Recovery Act (RCRA) requirements for wastes stored in tanks (40 CFR 264.193 (b)). The “noncompliant” tanks are generally past their 30-year design life, and many (13, at last reporting; DOE, 2005b) have a history of cracks or leakage (either from the tank into the annular secondary region or from the surrounding media into the tank),¹² although only one tank is believed to have leaked a small quantity of waste to the environment. Waste levels have been lowered below the location of known leaks, and at present DOE believes that there are no active leaks.

FIGURE 1 Diagrams of tank types at the Savannah River Site (not drawn to scale). There are twelve Type I tanks, four Type II tanks, twenty-seven Type III (and IIIA) tanks, and eight Type IV tanks in the tank farms. Risers are not depicted in most of these diagrams but are present on each tank.

All but the Type IV tanks contain dense networks of vertical and horizontal cooling pipes, referred to as cooling coils, which circulate chilled water. These cooling coils are used to remove heat produced from radioactive decay in the waste to prevent it from reaching high temperatures.

Waste from the canyons contains acids and other chemicals used in the separation processes, as well as the radionuclides (fission products such as cesium-137 and actinides such as neptunium-237) separated during recovery of plutonium and uranium. The wastes in the tanks exist mainly in three physical forms: sludge, supernatant liquid, and saltcake. Together, the supernate and saltcake are referred to as salt waste. To prevent corrosion of the carbon steel tanks, sodium hydroxide was added to neutralize the acid and make the waste alkaline before it was pumped to the tanks. This caused metals and most radionuclides to precipitate as an insoluble sludge,¹³ which settled to the bottom of the

tanks. The liquid remainder, or unconcentrated supernate, contains soluble salts and is referred to as a salt solution. If concentrated by evaporation, much of the salts initially in solution will precipitate to form a solid saltcake.

To conserve tank space, most of the salt solutions have been processed through an evaporator (a heated tank that evaporates water from waste) to produce saltcake, leaving relatively small volumes of concentrated supernate solution. Although there is disagreement about the contents of the individual tanks, the totals of the individual radionuclides and chemicals in the tank systems are relatively well known. The total radioactivity in each physical form is shown in Figure 2 and Table 1, which also lists the radioactivities for other wastes on- site.

The supernate contains more than 90 percent of the inventory of soluble radioactive species, mainly cesium-137. The saltcake is a solid material composed of more than 99 percent of salts, such as sodium nitrate, which contains lower (by approximately a factor of 10 to 20) concentrations of soluble and insoluble radioactive constituents. The waste in the tanks (see Figure 2) contains approximately 1.6 × 10¹⁹ becquerels (426 million curies [MCi]) of radioactivity; approximately half of the radioactivity is in the sludge and half in the salt waste. Most of the volume is in the salt waste, approximately 128,000 m³ (33.8 million gallons), whereas the sludge represents approximately 9,800 m³ (2.6 million gallons).

More than 95 percent of the radioactivity in the waste comes from cesium-137 (and its short-lived decay product barium-137) and strontium-90 (and its short-lived decay product yttrium-90). Both the cesium and the strontium isotopes have half-lives of approximately 30 years. The cesium poses a particular hazard for people working near the waste because it emits penetrating radiation (gamma rays). Other radioactive constituents in the waste are of

FIGURE 2 Aggregated volume and radioactivity distributions among the tank waste phases in all tanks at the Savannah River Site as of December 2004. SOURCE: DOE (2005a).

concern for other reasons: DOE has concluded that carbon-14, selenium-79, technetium-99, iodine-129, tin-126, and neptunium-237 dominate the long-term risk to the public from disposed waste because of their long half-lives and their mobility in the environment (Cook, 2005). The actinide isotopes, including isotopes of plutonium and americium, decay into a series of other radioactive substances (together referred to as a decay chain) and also constitute long-term hazards, particularly for inadvertent intruders who may disturb the waste.

DOE’s plan to manage the waste retrieved from the tanks is to separate the radioactive from the nonradioactive components, the latter of which make up nearly the entirety of the waste volume. This processing generates two waste streams: (1) a high-activity waste stream, which will be immobilized and disposed off-site in a high-level waste repository, and (2) a low-activity waste stream, which is to be disposed on-site as low-level radioactive waste. Figure 3 illustrates the waste flows that DOE has described for tank wastes at the Savannah River Site. The reader should note that the wastes planned for repository disposition are not the subject of this study, because they are not planned for onsite disposal. They are included here because management of the tank wastes must be considered as a whole system of interconnected parts. For example, any chemical agents used to clean the tanks must be compatible with the processing of at least one of the two waste streams.

For nearly 10 years, DOE has been retrieving sludge from the tanks at the Savannah River Site for immobilization in glass. After retrieval from the tank, the sludge is transferred to a dedicated waste tank where it is “washed” to remove soluble salt constituents that will interfere with the glass-forming process and to reduce the volume of material that is sent to the Defense Waste Processing Facility (DWPF) for vitrification into logs of waste glass. The

logs are to be disposed off-site in a high-level waste repository. The wash water and a low-activity liquid waste stream from the DWPF are sent back to the tanks (see Figure 3).

DOE is still developing facilities to process the salt waste. Three progressively more sophisticated and effective separation processes are to be brought into service for processing different batches of salt wastes: DOE proposes to use two “interim” processes (described in Section II) for what it calls “low-activity salt,” that is, salt waste that contains what DOE considers to be “low concentrations of radionuclides,” until the Salt Waste Processing Facility (SWPF) begins operations. The SWPF is scheduled to begin operation in 2009. The SWPF’s processing capability may be supplemented by the interim processing facilities or they may be used to treat unique waste streams. Tank wastes are to be processed to concentrate the radionuclides into a high-activity waste stream that will be vitrified at the DWPF. The other separated fraction, consisting mainly of the nonradioactive salts and other constituents with low concentrations of radionuclides that make up the less contaminated, low-activity waste stream, is to be conditioned in the Saltstone Production Facility—an operation that mixes liquid waste with grout¹⁴ to create a waste form referred to as saltstone, which is disposed on-site as a monolith in concrete vaults. Until now, the Saltstone Production Facility has handled very low activity waste. The higher radioactivity

FIGURE 3 Waste flows in the Savannah River Site waste management plans. Note that the sizes do not necessarily scale with the sizes of the waste flows.

anticipated in the liquid waste that DOE plans to send to the facility prior to SWPF start-up has required DOE to reconfigure the equipment and facility as well as add additional shielding in certain areas.

One of the functions of the saltstone is to stabilize some of these species (e.g., technetium and neptunium isotopes) by establishing reducing conditions.¹⁵ The cementitious materials are a mix of ground granulated blast furnace slag, portland cement, and fly ash. Briefly, each material was selected for the following main properties: slag creates the reducing conditions, portland cement enables the waste form grout to set (solidify) and gain strength in a reasonable amount of time, and fly ash (a by-product of coal-fired power plants) helps minimize thermal cracking by limiting the heat generated by the grout in the saltstone during the curing process. The specific proportions of cementitious materials in the grout are modified to be better suited to the liquid that is being stabilized, based on an analysis of the waste. The saltstone grout is pumped directly into a concrete vault constructed on the site, which has a concrete roof and will eventually have an engineered cap (a physical barrier that sits atop the disposal site). The engineered cap retards water intrusion into the vault, thereby reducing the mobility of the radionuclides in the saltstone.

After waste is removed from a tank, DOE plans to “close” the tank. The closure plans involve the emplacement of layers of engineered grout in each tank. If DOE uses the same approach it used for previous closures, the lowermost layers would be intended to partially encapsulate the residual sludge on the bottom of the tank with a reducing grout and to bind any remaining liquid as water of hydration (water absorbed by the grout as it sets). The middle layer would be a controlled low-strength material designed to lend structural stability to the tank to prevent collapse. The top layer, used only in the Type IV tanks, would be a stronger material designed as an intruder barrier. DOE plans to clean out and close the tanks one or a few at a time. Two Type IV tanks, Tanks 17 and 20, have already been grouted and closed in this manner, and DOE intends to close two neighboring tanks, Tanks 18 and 19, next.

Tank waste at the Savannah River Site is regulated by different agencies. DOE regulates the radioactive component of this waste through the Atomic Energy Act and Section 3116 of the Ronald Reagan Defense Authorization Act of 2005; the state regulates the hazardous component of the waste through the South Carolina wastewater treatment and hazardous waste regulations (SCDHEC, 2004a, 2004b); and the U.S. Nuclear Regulatory Commission reviews waste determinations and monitors compliance in pursuing the cleanup of the tanks under Section 3116. The U.S. Environmental Protection Agency is involved indirectly through the Federal Facility Agreement for the Savannah River Site (DOE, 1996a).

In conjunction with the regulatory documents listed above, DOE has applied criteria from the following laws, regulations, and DOE orders in decision making regarding cleanup of wastes at the Savannah River Site (DOE, 1996a):

• Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) criteria described in 40 CFR 300.430(e)(9).¹⁶

• Resource Conservation and Recovery Act Section 3004(u) and (v),¹⁷ and

• DOE Order 435.1, Waste Management.¹⁸