Ron Smith
U S ENERGY Corporation
227 Gateway Drive, Suite 201
Aiken, SC 20894
(803) 652-2311

Jim McNeil
Life Cycle Environmental
4360 Corporate Road, Suite 100
Charleston, SC 29405-7445
(803) 744-7110 Ext. 520

Guidelines were developed for acceptable levels of residual radioactivity in the Heavy Water Components Test Reactor (HWCTR) facility at the conclusion of its decommissioning. The HWCTR is a test reactor located at the Savannah River Site (SRS). Housed in a steel-domed containment structure, the HWCTR was operated in the 1960s to test experimental fuel assemblies for heavy water power reactors. It has been planned to dismantle the reactor and release the two-acre site for unrestricted use. For this project, the U. S. Department of Energy (DOE) plans to use the release criteria of DOE Order 5400.5, Radiation Protection of the Public and the Environment.¹ These criteria require that exposure from residual radioactivity be as low as reasonably achievable (ALARA), but no more than 100 millirem per year. Using source terms developed from data generated in a detailed 1996 characterization study², RESRAD and RASRAD-BUILD residual radioactivity computer codes were utilized to calculate guidelines for radionuclides in the facility. These guidelines, expressed as derived concentration guideline levels (DCGLs), were substantially higher than concentrations of radionuclides measured during the 1996 characterization program. The calculations showed that the present inventory of residual radioactivity in the facility (not including radioactive equipment to be removed) would produce less than one millirem per year above background to a hypothetical individual on the property.

This paper describes how the RESRAD and RESRAD-BUILD residual radioactivity computer codes were used to calculate guidelines for residual radioactivity in the HWCTR facility at the SRS at the conclusion of its decommissioning. The HWCTR decommissioning entails an unconditional release of the site property.

For this project, the DOE plans to use the release criteria of DOE Order 5400.5, Radiation Protection of the Public and the Environment¹. These criteria require that exposure to members of the public from residual radioactivity be less than 100 millirem per year for all exposure pathways, and as low as reasonably achievable below this value. To implement the release criteria, guidelines are required that relate the amounts of different radionuclides remaining in the facility to the annual exposure limit. These guidelines are expressed in terms of dpm/100 cm² and pCi/g.

DOE Order 5400.5 specifies that authorized limits for each property shall be set equal to generic or derived guidelines. According to the order, the authorized limits (i.e., the derived guidelines) are to be approved by the DOE headquarters program office.

The study focused on the reactor containment building. It produced residual radioactivity guidelines based on two different criteria: 100 mrem per year above background and 15 mrem per year above background to a hypothetical individual exposed to radioactivity on the site property. The 15 mrem per year value corresponded to the criterion of the Nuclear Regulatory Commission's proposed radiation cleanup rule, at the time the study was performed. It is considered appropriate for a reference value corresponding to as low as reasonably achievable (ALARA), given the limited amounts of radioactive contaminants in the HWCTR facility.

The study also produced two estimates of the maximum annual radiation exposure to a hypothetical individual on the property. One estimate assumed that the radioactivity associated with the biological shield, the spent fuel basin, contaminated concrete and embedded radioactive piping would remain in place. The other assumed that that this radioactivity would be left in place along with all radioactivity in installed equipment, with the exception of the reactor vessel assembly and the steam generators.

This project marked the first time that such computer modeling had been used at SRS in connection with decommissioning of nuclear reactor facilities. It also was the first time that RESRAD-BUILD had been used at the site. The results provided considerable insight in planning for the decommissioning.

The HWCTR facility is located on approximately two acres in the northwest quadrant of the SRS, on property known as U-Area. U-Area lies three miles from the nearest SRS property boundary and about two and one-half miles from any major nuclear materials production facilities on the site. The elevation of the HWCTR site is approximately 288 feet above sea level. No stream runs near the property. The water table at the site is some 50 feet below the lowest point in the reactor containment building.

The federally-owned SRS reservation is located near Aiken, South Carolina. The DOE Savannah River Operations Office manages the SRS. The Westinghouse Savannah River Company operates the site under contract to DOE.

The HWCTR was a pressurized heavy water test reactor used to test candidate fuel designs for heavy water power reactors. The nominal reactor power of 50 megawatts (thermal) was dissipated to the atmosphere through a muffler, which lies approximately 110 yards east of the reactor building.

The containment building is 70 feet in diameter. The steel dome structure rises approximately 65 feet above the ground; the floor of the lowest level is approximately 52 feet below grade. The below-grade part was constructed of pre-stressed concrete. The containment building houses the reactor and coolant systems, the refueling machine, the spent fuel basin and the reactor instrumentation. The reactor building is illustrated in Figure 1.

The reactor vessel has an overall height of approximately 37 feet (including the monitor pin nozzles which protrude from the vessel bottom), a diameter of about eight feet and weighs approximately 100 tons. Two test loops, isolated from the primary cooling system, provided special test conditions.

Also in the reactor building is a small spent fuel basin made of reinforced concrete lined with stainless steel. This basin, 9 feet by 13 feet wide, extends from the ground floor to the minus 27-foot level. It contains a 54-inch by 54-inch shipping cask pit that extends 15 feet below the basin floor. A system of floor drains leads to a 350-gallon sump tank located in the monitor room at the minus 52-foot

The HWCTR was operated from March 1962 to December 1964 to test fuel elements and other reactor components of potential use in heavy water moderated and cooled power reactors. During operation, 36 different fuel assemblies were tested. Ten of these experienced cladding failures as operational capabilities of the different designs were being established, releasing fission products, uranium and transuranics into the primary cooling system. Spills of radioactive water from the primary cooling system and other systems occurred frequently in most areas of the reactor building during the operating period.

In December of 1964, operations were terminated and the facility was placed in a standby condition. The facility condition since then has approximated safe storage or protective confinement.

Additional work on the HWCTR decommissioning project began several years ago. In Fiscal Year 94, four auxiliary buildings on the HWCTR site were demolished. In early Fiscal Year 95 detailed radiological contamination surveys of the reactor building were performed. These showed that removable contamination levels in the facility were low.

In early 1996, the control building was demolished. During the summer of 1996 a detailed characterization study of the facility was performed².

The characterization study served as the basis for the source term calculations. It provided detailed information on the radioactive contaminants in the facility. The following data from the 1996 characterization is most pertinent to this study:

• Induced radioactivity in the reactor biological shield, based on laboratory analysis of five cores. These cores were drilled through the concrete shielding at five different elevations. The radial depth of detectable activation in the region which saw the highest neutron flux was 41 inches from the shielding inner surface. The samples showed that Eu-152 with an inventory of approximately 0.0329 curies in the activated concrete opposite the active core region. In the lower shield which contains barite as an aggregate, Ba-133 predominated at approximately 0.103 curies. The most activity in the shielding, however, is Co-60 at 0.620 curies, most of which is associated with a lower axial shield directly below the reactor bottom head. This shield contains concrete with 90 percent steel shot as aggregate.
• Radioactivity imbedded in concrete floor surfaces, based on 49 concrete samples analyzed in a laboratory. Only low concentrations of radioactivity were found to be embedded in concrete floors. The highest Co-60 value measured was 0.9 pCi/g. Cs-137 concentrations ranged up to 220 pCi/g. The highest values of Pu-239 measured in the facility was 0.06 pCi/g.
• Radioactivity inside of floor drains and other imbedded piping. Beta-gamma scans at floor drain openings showed up to 380,000 dpm/100 cm²; alpha scans showed up to 20 dpm/100 cm². Water samples in drains on the minus 52-foot level showed low levels of H-3, Co-60 and Cs-137.
• Transferable surface contamination based on the results of 970 smears taken on external surfaces in the facility. Only two smears taken on facility surfaces showed levels above limits of 1000 dpm/100 cm² beta-gamma and 20 dpm/100 cm² alpha. The results of these surveys were consistent with the extensive removable radioactivity surveys made in 1994 which also showed very low levels.

The surveys also showed that most areas of the facility had radiation levels well below one millirem per hour

The decommissioning process will involve dismantlement of the facility. Equipment contaminated with radioactivity will be removed, along with hazardous materials. The steel containment dome will be removed. The underground concrete structure of the reactor building will be removed to five feet below grade. A final radiation status survey will be accomplished. After it has been confirmed that residual radioactivity levels meet cleanup guidelines, concrete rubble produced during the decommissioning will be placed at the bottom of the building cavity. Clean earth will be used to fill the cavity. Then the property will be graded and seeded with grass, and made available for any suitable use.

The decision that dismantlement would be undertaken emerged from an engineering study of alternatives for the decommissioning. This study is described elsewhere in these proceedings. (See Alternatives for Decommissioning of the Heavy Water Components Test Reactor at the Savannah River Site, by Jim McNeil and Jeff Lee.)

The DOE Decommissioning Resource Manual³ states that site-specific guidelines for radioactivity in soils and remaining structures are to be developed using the DOE material code RESRAD, employing a realistic pathway analysis. Procedures for using RESRAD appear in the Manual for Implementing Residual Radioactive Material Guidelines Using RESRAD⁴.

RESRAD is a computer code developed by Argonne National Laboratory that can be used to model the transport of radionuclides in soil and determine the resulting radiation exposure to humans. Outputs include tables and graphs which show annual radiation exposure for periods as long as 10,000 years. Also, radionuclide guideline levels that equate to particular annual exposures can be produced by the code.

The basic RESRAD exposure pathway scenario entails a family farm. The family living on the property could receive radiation exposure from residual radioactivity through eight different pathways, such as direct exposure to contaminated soil, eating plant foods grown on the property and drinking milk from on-site livestock. The model includes provisions for contamination from soil (or for the HWCTR case, buried structures) leaching into the ground water and then reaching the environment of those living on the property.

RESRAD requires a variety of inputs. A source term that describes the concentrations of different radionuclides present is one key input. The geometry of the source, especially with respect to ground water, is another. Then there are numerous other inputs such as characteristics of the soil and the rates of intakes of foodstuffs.

When a building remaining on a decommissioned site may be occupied in a realistic future scenario, a related computer code may be used to calculate site-specific guidelines. This code is RESRAD-BUILD. It is described in RESRAD-BUILD, A Computer Model for Analyzing the Radiological Doses Resulting from the Remediation and Occupancy of Buildings Contaminated with Radioactive Material. RESRAD-BUILD also may be used to establish guidelines associated with possible occupancy of a portion of a buried contaminated structure.

RESRAD-BUILD is a pathway analysis computer code developed by Argonne National Laboratory to evaluate the potential radiological dose to an individual who lives or works in a building contaminated with radioactive material. Outputs include tables of exposures associated with building occupancy. Radionuclide guideline levels associated with a particular annual exposure can also be calculated. Unlike RESRAD, the program does not produce graphs.

The program makes use of an air quality model that considers the transport of radioactive material from one compartment in a building to another. Six exposure pathways are considered, including external exposure from the source and inhalation of airborne radioactive particles. Like RESRAD, the program requires a variety of inputs, such as the radionuclide concentrations, the source geometry and the building description .

Two potential exposure scenarios were considered in this study. The potential radiation exposures that could be associated with these scenarios and DCGLs for the HWCTR facility were calculated over a 10,000-year period using RESRAD and a 30-year period using RESRAD-BUILD.

Scenario A assumed residential use of the site as a family farm. This is typically the worst case scenario³.

A hypothetical resident was assumed to spend 50 percent of his time indoors in a house built on the remediated HWCTR property. Twenty-five percent of his time would be spent outdoors on the property, and the remaining 25 percent of time away from the site. The resident was assumed to eat homegrown produce and eat meat and drink milk from livestock fed with forage grown onsite. Groundwater drawn from a well located on the property would be the only source of water for drinking, household use and irrigation.

Note that RESRAD analyses also can model radiation exposures associated with eating fish from an onsite pond. This pathway was not considered plausible for the HWCTR site, given the small size of the property.

Another plausible use for the HWCTR property would involve constructing a building on the site. Scenario B assumes that an administrative building with a basement four meters deep is built on the property.

It is assumed that the building basement incorporates as one of its walls an existing wall of the HWCTR underground structure. Under this scenario, a hypothetical individual was assumed to work eight hours per day inside the building basement, five days per week for 50 weeks per year.

The family farm scenario is a basic RESRAD exposure pathway scenario. It has been used in other RESRAD studies at the SRS, such as the study of the Solid Waste Disposal Facility in E-Area⁵. Even though the present SRS future-use plan does not accommodate residential use, such use is plausible in the long term. Because the HWCTR property is not located in the central region of the site near highly-contaminated nuclear facilities, it would be more likely to eventually become a residential area than would property located in the central part of the SRS. The administrative building scenario also would be a plausible use for the HWCTR property. The assumption of a basement is plausible as well. This assumption opens up exposure pathways beyond those in the RESRAD analysis, as one can see by comparing Table 2 to Table I.

The administrative building basement assumption is consistent with exposure pathway scenarios used in connection with decommissioning of the Shippingport Atomic Power Station in Pennsylvania⁶. The Shippingport project entailed dismantlement of that facility and unrestricted release of the site property. Some underground concrete structures with low levels of residual radioactivity remained in place. These were filled with building rubble and clean earth, as will be the case for the HWCTR project.

As mentioned previously, the predominant radionuclide in the biological shield source term is Co-60 at approximately 620 mCi. Other radionuclides are Ba-133 at 103 mCi, Eu-152 at 57 mCi and Eu-154 at 3.58 mCi. The total amount of radioactivity in the shielding was calculated to be approximately 785 mCi. This amount includes radioactivity inside the reactor vessel lower 10-inch piping which passes through the shielding.

In the concrete floors of the facility, a total of approximately 6.6 mCi was calculated to be present. Cesium 137 was the predominant radionuclide at 4.5 mCi, followed by H-3 at 1.3 mCi, C-14 at 0.34 mCi, Sr-90 at 0.30 mCi and Co-60 at 0.05 mCi. These quantities include radioactivity inside embedded piping and the spent fuel basin.

Fig. 2. RESRAD Models

To determine the effect of leaving installed radioactive equipment in the facility, a modified source term was calculated. In this source term the radioactivity associated with all installed equipment except the reactor vessel assembly and steam generators is included. Also included is induced activity in the reactor biological shield The part of the source term associated with radioactive systems is 0.0771 curies. This value is somewhat larger than the source term for contaminated concrete (rubble) and smaller than the one for the biological shield.

For the RESRAD-BUILD calculation, the source term used was that which would produce 15 mrem per year from each of the radionuclides found during the 1996 characterization program to be present in significant quantities.

Figure 2 shows the source geometry with respect to the building structure which will be buried during the decommissioning.

1. Department of Energy Order 5400.5, Radiation Protection of the Public and the Environment, U.S. Department of Energy, Washington, D.C.
2. Heavy Water Components Test Reactor Facility Characterization Report, U. S. Department of Energy, Savannah River Site, Aiken, SC 29808 (September 1996).
3. Decommissioning Resource Manual, U.S.Department of Energy, Office of Environmental Management Manual DOE/EM-0246, August 1995.
4. Yu, C., et. al., Manual for Implementing Residual Radioactivity Material Guidelines Using RESRAD, Version 5.0, Argonne National Laboratory, ANL/EAD/LD-2 (September 1993).
5. Hsu, R. H. and Cook, J. R., Soil Release Guidelines - Expanded RESRAD Study, WSRC-RP-95-287, Westinghouse Savannah River Company, Savannah River Technology Center, Aiken, SC 29808 (March 1, 1995).
6. Eger, K.J., Final Consolidated Implementation Plan for the Shippingport Station Decommissioning Project Site Release Criteria, KJE87-144a, Revision 1 (July 13, 1988).

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