Fig. 1. SL-1 burial ground.
Chris M. Hiaring and Douglas K. Jorgensen
Lockheed
Martin Idaho Technologies
Joseph S. Rothermel and Gregory B. Cotten, P.E
Parsons
Infrastructure & Technologies Group Inc.
ABSTRACT
The SL-1 burial ground at the Department of Energy (DOE) Idaho National Engineering Laboratory was constructed in 1961 to dispose of radioactively-contaminated debris, soils, and gravel generated by the destruction of a small nuclear reactor. The INEL was officially placed on the National Priorities List in November 1989. Consequently, remediation of the SL-1 burial ground falls under the purview of Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).
The SL-1 burial ground consists of three excavations, in which a total volume of 99,000 cubic feet of contaminated material was deposited. The excavations ranged from 8 to 14 feet in depth, and at least 2 feet of clean backfill was placed over each excavation. Shallow mounds of soil over the excavations were added at the completion of cleanup activities in September 1962.
Results of the baseline risk assessment indicate that the direct human exposure pathway dominates the overall risk for the burial grounds. The primary contributor to this risk is cesium-137 and its progeny. Based upon consideration of the requirements of CERCLA, on detailed analysis, and on public comments, the DOE, the Environmental Protection Agency (EPA), and the State of Idaho have selected containment by capping with an engineered long-term barrier comprised primarily of natural materials as the preferred alternative. The cover will be designed to maintain effective long-term isolation of contaminants. The number and thickness of layers designed in the cover were dependent on local climatic and geographic conditions, including precipitation rate, freeze depth, indigenous plant and animal species, and local topography. The engineered lifetime of the cap is a minimum of 400 years. Surface water diversion measures, including contouring and grading, will be used as necessary to direct runoff away from the burial ground and into nearby naturally-occurring drainage formations.
The cover system design provides:
The capping system will be combined with institutional controls consisting of access and land use restrictions to discourage intrusion into the SL-1 burial ground. The DOE would be responsible for establishing and maintaining land use and access restrictions for at least 100 years. Access restrictions in the form of fences, warning signs, and permanent markers would be used to deter unauthorized entry into the burial grounds.
BACKGROUND
The Stationary Low-Power Reactor No. 1 (SL-1) was a small nuclear power plant designed by Argonne National Laboratory to generate electric power and space heat for the U.S. Army at remote arctic installations. Argonne National Laboratory operated the reactor until February 1959 when Combustion Engineering, Inc., commenced operations. They operated the reactor as a test, demonstration, and training facility for Army personnel. By the time the reactor was shut down for the 1960 Christmas holiday, the reactor had expended approximately 40 percent of its core life.
On the evening of January 3, 1961, a three-man crew was preparing the SL-1 reactor for startup. The crew was reinstalling the control-rod drive mechanisms, which required lifting the rods a very short distance to make the connection. One of the crew, for unknown reasons, lifted the control rod 20 inches rather than the required 1/4 inch, which caused the reactor to go critical.
A tremendous amount of energy was immediately added to the cooling system of the reactor which caused uncontrolled boiling of the coolant. This, in turn, pushed the column of water above the core and hammered the water into the lid of the pressure vessel head causing the reactor shield plugs to be expelled along with much of the top head shielding. There were three casualties as a result of this excursion and the reactor vessel and building were severely damaged and highly contaminated.
This incident was followed by a massive cleanup operation to dismantle and dispose of the reactor and building. Initially, the material from the cleanup operations was sent to the Radioactive Waste Management Complex for disposal in the subsurface disposal area. However, this effort resulted in a spread of contamination between the sites. Once material was being recovered from the building itself, which was highly contaminated, there was concern about transporting uncontained waste on a public highway. It was decided that a disposal area near the SL-1 site would be the best way to minimize spread of contamination.
The SL-1 burial ground was constructed approximately 1,600 ft northeast of the SL-1 reactor site. In October 1961, a trench (Trench 1) 6 ft wide, 500 ft long, and between 6 and 10 ft deep (due to shallow bedrock) was excavated. A pit (Pit 1), 12 ft wide, 466 ft long and 10 ft deep was excavated parallel to the trench. It soon became apparent during the cleanup operations that Pit 1 and the trench would not be adequate to accommodate the volume of waste. In January 1962, a second pit (Pit 2) was constructed between Pit 1 and the trench. Pit 2 was 20 ft wide, 400 ft long and 10ft deep.
Approximately 99,000 ft3 of radioactively-contaminated waste and debris were placed in the burial ground. The waste and debris included the reactor building materials, various pieces of equipment, and contaminated dirt and gravel from the roads and the facility grounds. Backfill was placed over the waste in the pits and trench until radiation levels were near background. Concrete monuments were then placed at each corner of the pits and at the ends of the trench. Finally, a 600-by-300-foot area around the burial ground was fenced and radioactive contamination signs were placed along the fence to limit access into the area.
Numerous radiation surveys of the surface of the burial ground have been performed since the SL-1 accident. During these surveys, the activities were found to range from 0.1 to 50 milliroentgen (mR)/hour. Several soil sampling events have also been performed to determine the nature and extent of the soil contamination area. The sample results ranged between 2.5 to 360 pico Curies/gram (pCi/g).
Based on the original source of surface contamination (aerial distribution of contaminants during demolition and cleanup of the SL-1 reactor) and the limited mobility of radionuclides in the soil at the INEL, it is believed that contamination is restricted to the upper 0.5 foot of soil. Potential pathways for contaminant migration at the SL-1 burial ground are limited by site conditions. The SL-1 site is fairly isolated, is gently sloped, is in a desert climate, and has a great depth to groundwater (approximately 667 feet). Although there is surface contamination at the site, the majority of contamination is subsurface. In general, the potential pathways for contaminant migration include atmospheric transport and transport via surface water and groundwater.
There is a potential for windblown migration of radionuclide present in the surface soil at the SL-1 burial ground, although the presence of a thick grass cover minimizes mobilization of dust and its dispersion by wind.
No surface water migration pathway exists at the site, and there are no surface water features. The SL-1 burial ground is in a topographic low, minimizing the chance for significant erosion due to surface water but increasing infiltration from precipitation. Flooding of the Big Lost River is not a concern at SL-1 because of topography, distance from the river, and the INEL's flood diversion system.
The SL-1 site was originally identified as a potential hazardous waste release site in September 1986. It was included in the Idaho National Engineering Laboratory (INEL) Consent Order and Compliance Agreement (COCA) in 1987 with the Department of Energy - Idaho Operations Office (DOE-ID), Environmental Protection Agency (EPA) - Region 10, the Idaho Department of Health and Welfare and the United States Geological Survey. The site was later included in the INEL Federal Facility Agreement and Consent Order (FFA/CO), which superseded the COCA. The SL-1 burial ground was evaluated under a Remedial Investigation/Feasibility Study (RI/FS) and a baseline risk assessment was conducted to evaluate current and future potential risks to human health. Radionuclides were identified as the only contaminants of concern at the SL-1 burial ground. Cesium-137 and progeny posed unacceptable risks from direct exposure at the burial ground. The baseline risk assessment also indicated that the direct exposure risk would not decrease to an acceptable 1 in 10,000 for 400 years.
In January 1996, the Record of Decision (ROD) was approved by the agencies who selected containment by capping with an engineered long-term barrier, comprised primarily of natural materials. An engineered barrier can effectively isolate contaminated materials, inhibit migration of contaminants from the burial ground, and allow time for radioactive decay of the primary contributor, cesium-137.
The remedial action objectives (RAOs) for the SL-1 Burial Ground are defined in the ROD and are summarized below.
ENGINEERED BARRIER DESIGN
The cover has been designed to meet the following RAOs as outlined in the ROD: provide shielding from penetrating radiation, prevent animal and insect intrusion, inhibit inadvertent human intrusion, have longevity (400 years) using naturally-occurring materials, contain contaminated surface soils, and require low maintenance.
The original concept for the cap design required that both the biotic barrier and the inadvertent human intrusion barrier be constructed over the entire 600-by-300-foot area which would cover all the soil contamination areas. However, because of the remote location of the site and material availability, a method to reduce cost and material, while meeting the requirements of the ROD, needed to be found.
During the RI/FS, a study was performed, to assess the shielding ability of proposed cap designs. The shielding ability of each cap design is dependent on the elemental composition, density, and thickness of the materials specified. Since the thickness of each layer of material in the cap designs is subject to modification during the remedial design phase, each material is individually assessed for shielding performance. The computer code Microshield was used to determine the thickness of cap construction materials required to reduce the dose rate above the burial grounds to background levels, assuming a single layer of the material. The computed thickness results indicate that an approximate thickness of 0.1 m (0.3 ft) at SL-1 for any one material used in the capping design is sufficient to satisfy the RAO of preventing exposure to penetrating radiation. The shielding ability of each cap is therefore not considered to be a significant design requirement of the containment alternatives, as the thickness of each cap design is more than adequate for reducing the dose rate above the burial ground to background levels.
It was known from previous sampling efforts that the entire 300-by-600 foot SL-1 area was not contaminated over the action levels, rather there were several "hot spots." It was decided that the area to be shielded could be substantially reduced by identifying and consolidating contaminated soil to the pit and trench areas, and then capping just that area. Therefore, a more aggressive sampling effort was conducted which utilized a grid sampling system over the 300-by-600 foot area. If a single sampling point within the grid is found to be over the action level of 16.7 pCi/g, the entire grid area soils will be consolidated in the capping material. The areas of soil contamination which are above the action levels were excavated and consolidated between trench 1 and pit 2 (Fig. 1). The biobarrier and the inadvertent human intrusion layer are only then placed over the consolidated contaminated soil and the waste trenches and pits.
Fig. 1. SL-1 burial ground.
The engineered barrier that was placed over the SL-1 burial ground consists of a biotic barrier and an inadvertent human intrusion layer (Fig. 2). The configuration of the biotic barrier used inthe SL-1 design is based on studies previously and currently conducted by The Environmental Science and Research Foundation concerning the effects of biotic barriers against insects and small mammals. While these studies have primarily been directed at eliminating precipitation pathways to buried waste by prohibiting insects and small mammals from penetrating the barrier, conversely, this study would prove the barrier effective against these insects and small mammals bringing contaminated materials to the surface. The results of these studies suggest that an effective biotic barrier is a gravel/cobble/gravel sandwich of the following dimensions from top to bottom:
The inadvertent human intrusion layer was placed over the biotic barrier. This layer consists of a basaltic rip rap layer a minimum of 24 inches thick (Fig. 2). Inadvertent human intrusion is discouraged by the physical size of the rock armor and the obvious deliberate mound construction. Basalt angular boulders or "rip rap" was chosen for this layer and is available at various locations at the INEL as a result of past excavation of the material for building foundations. Available basaltic rip rap ranges in size gradations from 6 to 36 inches.
Fig. 2. SL-1 Burial ground.
To assist the rip rap in deterring inadvertent human intrusion, a permanent marker was installed which identifies the contents of the mounds. Design for a permanent marker was based on a study done for the Waste Isolation Pilot Plant (WIPP).
The permanent marker consists of a granite edifice similar in size and shape to a large tombstone, roughly 4 ft. high (above grade), by 3 ft. wide by 10 in. deep. This edifice has three symbols engraved on it for the buried reactors: the universally-recognized symbols indicating radiation (Tri-Foil), Hazardous Substances (Skull and Cross Bones), and Do Not Enter (Circle with a Bar). This edifice has been installed on a reinforced concrete footing, designed to withstand the forces of nature for the design life of the project (400 years). The edifice is placed at the four corners of the barrier area.
COST-SAVING INITIATIVES
Since the INEL and SL-1 are at least 50 miles from any commercial operation providing a variety of materials in a large assortment of size and configuration, it was vitally important to use local material in consideration of cost and schedule. Therefore, the design was based on the flexibility to utilize locally available naturally-occurring materials.
It was hoped that forethought during the design phase also would lead to cost savings during implementation by specifying material size gradations that are locally available. The design team researched the INEL and, using the size criteria specified by design, found material stockpiles that closely matched the size criteria. These stockpiles were specified in the construction documents as the borrow source. The cost during construction for sieving, sorting, crushing, or blasting has been virtually eliminated due to research and identification of borrow sources of these materials.
Another effort to reduce the size and cost of the barrier was the field sampling effort aimed at identifying contaminated soil and reducing the overall barrier size.
CONSTRUCTION
The construction of this barrier was accomplished through a firm fixed-price subcontract with a fixed performance period. Qualified contractors were identified through a pre-bid, pre-qualification process which required documented work experience in work activities which were similar to those to be encountered on the SL-1 project. At bid time, qualified contractors were subject to a two-part Request for Proposal, a technical proposal and a cost proposal. The technical proposal graded safety record, personnel experience, and proposed work execution method. A short list of contractors scoring highest on technical proposal then had their cost proposals opened. The low bidder was then chosen from the short list.
Anticipated areas of greatest risk during construction were greater-than-expected quantities of radiologically-contaminated soil requiring consolidation and cover, contamination of personnel or equipment causing health and safety concerns or requiring greater-than-expected decontamination efforts, weather delays caused by extreme weather conditions experienced on high mountain deserts areas in Southeastern Idaho, and the constructability of the barrier.
Of the risk concerns, two had some impact on the project. Contaminated soil requiring consolidation was greater than expected. There was 30 percent more contaminated soil requiring consolidation than initially planned. Sampling efforts to quantify contaminated soil prior to construction were limited in their ability to identify contamination sources anywhere but in the immediate vicinity of the sample point. Because the source of radiological contamination was primarily a discreet particle affixed to various building materials which were subsequently drug to the burial trenches, soil sampling efforts to identify contamination were difficult unless hitting right on the source. The other difficulty encountered was having real-time field testing that could read through background noise and provide accurate data. Real-time field testing was also made more difficult because of the nature of the radiation source it was seeking. Devices available could not describe the exact location of the particle, rather they described an area where the particle might exist. This meant excavating much larger quantities of soil simply to capture a minute particle.
Weather delays for the project were less drastic, however the project did experience delays due to high winds. To counter windy conditions and subsequent wind-blown conditions, water trucks were employed with projecting sprayers to keep the soil moist. This method of water application did not allow the water to be applied in a very uniform manner which caused heavily saturated areas and areas that did not get enough cover. A preferred method would be the use of a sprinkler system. This was not an option because of the lack of a water source.
The cost risk associated with the constructability of the barrier was mitigated by the contracting method of specifying a firm fixed-price and schedule scheme, while soliciting the expertise of the subcontractor through the technical proposal to find the best way to place the various layers of the barrier. The overall project schedule was accomplished within a reasonable margin of the specified performance period. This means that during the initial planning stages of the project the designer and planners accurately forecasted the difficulty of barrier construction utilizing ordinary construction practices. The subcontractor installing the barrier did not have to apply extraordinary construction practices to install the barrier. The successful completion of the project required good common sense, upfront planning, and coordination.
The construction project ran as planned for the most part. Change orders to the project for unforeseen activities were less than 10 percent of the original bid, with the biggest extra cost coming from the discovery of additional contaminated soil requiring recovery and cover.
LAND USE RESTRICTIONS
The DOE will limit land use near the SL-1 burial grounds to industrial applications for the duration of INEL operations, after which the DOE will notify the U.S. Department of Interior, Bureau of Land Management, to impose similar land use restrictions. Access restrictions in the form of fences, warning signs, and permanent markers will be used to deter would-be trespassers. Surface water diversion measures, including contouring and grading, will be used as necessary to direct runoff away from the burial grounds and into nearby, naturally-occurring drainage formations. The DOE would be responsible for establishing and maintaining land use and access restrictions for at least 100 years. The adequacy, effectiveness, and necessity of institutional controls would be evaluated during each five-year review of the remedial action.
Cap integrity monitoring and radiological survey programs will be established to ensure the functionality of the containment systems and provide early detection of potential contaminant migration. Cap integrity monitoring for cracks, erosion, and any observable degradation will be conducted to identify maintenance requirements. Monitoring requirements will be the responsibility of the DOE and will be evaluated for adequacy, effectiveness, and necessity during each five-year review of the remedial actions.
REFERENCES