The approximately 1450 km2 Hanford Site in southeastern Washington State was commissioned for the production of special nuclear materials by the Manhattan District of the Army Corps of Engineers in 1943. Eight graphite-moderated, water-cooled reactors were constructed in five self-supporting complexes between 1943 and 1955. These reactors and their ancillary structures have been declared surplus and are in the process of being decommissioned by the U.S. Department of Energy (DOE) and its contractors.
The eight retired Hanford reactors will form the largest reactor decommissioning project undertaken anywhere in the world. This article describes the eight reactors to be decommissioned, the alternatives, estimated radiation exposure, project management approach, cost and schedules, and the experience leading up to this project.
Background
The eight surplus reactors and their supporting facilities are located at the Hanford Site within five operations areas situated a few kilometers apart on the south bank of the Columbia River.
The reactors operated between 1944 and 1971 and have since been shut down, each has been placed in safe storage and monitored under a maintenance and surveillance program. The reactors were operated between 1,900 to 4,400 MWt, depending on the specific unit. All eight reactors have similar design and operating characteristics.
In preparation for reactor decommissioning, planning was begun in 1974 to decommission the support facilities for the eight reactors. Actual decommissioning work began in 1976 on the auxiliary buildings and, to date, over 50 reactor support facilities have been decommissioned, including filter buildings, exhaust stacks, and laboratories. Extensive work has also been done on the clean-up of reactor fuel storage basins and the removal of asbestos from reactor buildings.
Reactor Building and Facilities
The surplus production reactors, which are graphite-moderated and water-cooled, were used to produce plutonium and other special nuclear materials. As shown in Figure 1, each reactor building contains inner and outer rod rooms, laboratory areas, fuel storage basins, and the reactor block. In many places, shielding and structural concrete in these areas exceed 1.25 meters in thickness.
A typical reactor building is a reinforced concrete and concrete block structure approximately 76 meters long by 70 meters wide by 29 meters high. The building has massive reinforced concrete walls (0.9 to 1.5 meters thick) around the reactor block to provide radiation shielding, with lighter construction above (either concrete block or corrugated asbestos-cement). The total estimated volume of concrete contained in each reactor facility is greater than 13,000 m3 (exclusive of the reactor block). Roof construction is primarily precast concrete slab or poured insulating concrete. Except for the reinforced concrete portions, these buildings can be classified as light, industrial structures.
The reactor block is located near the center of the building. Horizontal control-rod penetrations are on the left side of the reactor block (when facing the reactor front face), and vertical safety-rod penetrations are on the top of the reactor. Fuel discharge and storage basins are located adjacent to the rear face of the reactor. Experimental test penetrations are located on the right side of most of the reactors.
Figure 1. Cutaway of Typical Reactor Building
Figure 2. Construction Details of a Typical Reactor Block
Reactor Block. A typical reactor block (see Figure 2) consists of a graphite moderator stack encased in thermal shielding and a biological shield. The entire block rests on a massive concrete base and foundation. A typical reactor block assembly weighs approximately 8,100 tonnes. Overall, the reactor block dimensions are 14 meters high by 14 meters wide by 12.2 meters deep. The principal components of a production reactor block include:
- The reactor moderator stack, an assembly of graphite blocks cored to provide channels for process tubes, control rods, and other equipment
- Thermal and biological shielding, surrounded by a heavy, vault-like steel outer shell equipped with gas-tight seals for the reactor block penetrations
- The process tubes, which held the uranium fuel elements and carried the cooling water
- Horizontal control rods and vertical safety rods
- An emergency shutdown system, which dropped neutron-absorbing steel and boron balls into vertical safety channels for emergency reactor shutdown
- Monitoring, experimental, and test equipment.
The description in Figure 2 pertains to the F Reactor, but is considered typical of the B, C, D, DR, and H Reactors as well. The KE and KW reactors are similar but of larger dimensions.
Environmental Impact Statement
In preparation for the decommissioning of the eight reactors, an Environmental Impact Statement (EIS) [1] was prepared by DOE's contractors. The EIS provided a description of decommissioning alternatives, an analysis of environmental impacts, and engineering and cost considerations to assist the DOE in selecting a decommissioning method.
Several concepts or alternatives were studied and were included in the EIS:
1. Immediate one-piece removal
2. Seventy-five years of safe storage followed by deferred one-piece removal
3. Safe storage followed by deferred dismantlement
4. In situ disposal
5. No new action. (The no action alternative is one that must be evaluated for comparison purposes in all EISs.)
The final EIS and record of decision (ROD) have now been published. The DOE evaluated all options, their environmental and financial impacts, and chose the deferred one-piece removal alternative. However, the storage period will be less than 75 years in order to meet Hanford Site clean-up objectives.
One-Piece Removal of the Reactor Block
The one-piece removal of the reactor block could be accomplished immediately or after a safe-storage period. The majority of the technical aspects associated with a one-piece removal of the reactor block remain the same regardless of whether this option is immediately selected or performed at the end of the 75-year period.
Figure 3. Assembled Transporter in Route to Disposal Site
One-piece removal means that each reactor block will be transported, intact, on a tractor transporter from its present location to a specified low-level waste disposal facility located on the Hanford Site. The reactor block includes the graphite core, the thermal and biological shields, and the concrete base.
Contaminated areas of the associated fuel storage basins would be removed for disposal in the low-level waste disposal area along with other contaminated equipment and components in the buildings that house the reactors and fuel storage basins. Each reactor building would then be demolished and an excavation prepared around the reactor block. The weight of the reactor block would be transferred to I-beams that would be inserted through holes in the concrete base and then grouted in place. Figure 3 shows a conceptual depiction of the transporter system. If contaminated soil was identified during the excavation, it would be removed, packaged, and transported to the low-level waste disposal facility. The tractor transporter would then be coupled to the I-beams, and the block would be lifted from its remaining foundation by the transporter and carried intact on specially constructed haul routes to the waste disposal facility. Following removal of the reactor block, the site formerly occupied by the reactor would be backfilled, graded, seeded, and released for other use.
Radition Exposure to Decommissioning Workers and the General Public
A recent survey of one of the surplus Hanford reactors resulted in measured dose rates in normally accessible areas within the facility ranging from 0.0001 to 0.0028 Sieverts (Sv). Based on this dose-rate information, the occupational doses to decommissioning workers were estimated for immediate one-piece removal. The following assumptions were used as a basis for those dose estimates:
- Personnel engaged in decontamination and dismantlement operations would spend a maximum of six hours in a radiation zone during an eight-hour workday.
- Supervisors, radiation monitors, and other support personnel working in a radiation zone would be subjected to an average dose rate experienced by decommissioning workers.
These assumptions are believed to result in conservative occupational dose estimates. The occupational doses were estimated by multiplying the appropriate dose rate by the estimated worker-hours to complete each task, and then summing the products. The total occupational dose is estimated to be about 0.20 Sv for immediate one-piece removal of a “typical” reactor and about 1.59 Sv for all eight reactors. For the deferred one-piece removal the occupational dose would be less due to decay of the controlling radionuclides.
The location of the surplus reactors on the Hanford Site (isolated from the general public) and the contained nature of the decommissioning activities ensure that routine decommissioning operations would result in little or no radiation dose to the public.
Project Management
The decommissioning of the Hanford surplus reactors is a major project as defined in DOE Order 4700.1, Project Management System. It is recognized that the management system dictated by Order 4700.1 cannot be strictly applied to a decommissioning project. Efforts are under way at DOE Headquarters (EM-40) to issue a guidance document that will indicate a graded approach to project management as applied to large decommissioning projects such as the Hanford reactors. To implement this graded approach the execution of the reactor decommissioning project will utilize a systems engineering management approach. The systems engineering process is a sequence of activities and decision points that transform an identified mission need into a description of system performance parameters and a preferred system configuration. The systems engineering process considers all aspects of system requirements from the earliest stages of design through development, testing, and operation. The process supports project management by ensuring that technical control is on a level and integrated with funding, cost, schedule, and performance controls. Since the systems engineering approach identifies all activities necessary to meet the project objectives, it follows that unless an activity supports the meeting of a specific objective, that activity cannot be supported by the project.
Some of the areas where the graded approach is being implemented for the reactor project is in the project planning and design phases. The Project Plan and the Project Management Plan are being combined into a single document for the project, thus eliminating redundancy of documentation already prepared for the Hanford Site as a whole. Title I and Title II engineering are being combined into a single definitive design effort that will produce a set of activity specifications for all major portions of the decommissioning project from initial site preparation, through dismantlement of the reactor building, reactor block packaging, loading onto transporter, travel to burial facility, and burial of package, and all waste management activities. These activity specifications will then be the baseline activities for the systems engineering process.
Current planning is for the Environmental Restoration Contractor (ERC) to perform the definitive design, engineering services, and the decommissioning operations. Subcontractors will be used for design and construction of the transporter haul route and for placement of the steel cage around the reactor block, and concrete cutting operations.
Long-lead procurement items include the transporter system, the high-strength, steel-lifting I-beams, and the box steel structural support girders for the transporter interface.
Decommissioning Schedules, Costs, and Experience Base
Decommissioning schedules and costs were projected for each of the decommissioning alternatives as part of the EIS process. Safe storage followed by one-piece removal would involve a safe storage period after which all eight reactors would be decommissioned in 12 years; estimated cost: $615M (40 percent contingency, 1994 dollars).
Over 50 structures of varying size and complexity have been decommissioned at Hanford since 1976, when an official decommissioning program was first established. As part of the engineering and planning for these projects, several new technologies were developed and many innovative uses of existing technologies were used. As examples of this innovation, the following tools and techniques were developed at Hanford:
- A large-area, surface contamination monitor has been developed, capable of accurately measuring and recording surface soil contamination at a survey rate of up to 1 km2/h.
- A remotely operated sandblaster was designed and tested. This device was used to decontaminate the interior of a 70-meter-tall concrete exhaust stack.
- Tooling and equipment were designed to facilitate the clean-up of both loose and heavily compacted sludge and contamination from several fuel storage basins.
- Innovative metal cutting tools, including an “arc saw” capable of high-speed cutting of ferrous and nonferrous materials, have been developed and tested.
- A concrete scabbler capable of removing up to 2.5 cm of contaminated concrete surface has been tested for decontamination of concrete surfaces.
To ensure that decommissioning technologies are applied in the most cost-effective manner, while at the same time archiving information for future decommissioning projects, the DOE conducts a technical information exchange (TIE) workshop annually. The program is designed specifically to share decontamination and decommissioning information regarding the use of new or modified tools at each of the DOE's sites and any other decommissioning facility within the United States. Additionally, decontamination and decommissioning information is gathered and archived at the Remedial Action Program Information Center (RAPIC) at Oak Ridge National Laboratory and is available to organizations involved with any DOE decommissioning project. Decommissioning tools and techniques that have proven effectiveness are described in the Decommissioning Handbook [2] published by DOE in 1994.
The DOE is also involved in technology exchange with European and Far Eastern countries through the IAEA, the NEA, and various bilateral agreements. Through its international technology transfer program, the DOE shares its decommissioning experience, and at the same time benefits from the experiences of foreign governments in the areas of decontamination and decommissioning.
Through its national and international technology transfer efforts, the DOE is making sure that the experience base of today is applied to tomorrow's projects, to ensure that future projects are accomplished in a cost-effective and an environmentally safe manner.
Acknowledgments
Scott T. Spence of ICF Kaiser Hanford, Inc. provided valuable support to preparation of this article—in particular the information in the technology development area.