Worker Dose Reduction during Removal of Tile Holes Containing Intermediate Level Radioactive Waste by Steel and Concrete Encapsulation

Transcription

1 Worker Dose Reduction during Removal of Tile Holes Containing Intermediate Level Radioactive Waste by Steel and Concrete Encapsulation A. Buchnea 1, M. Cleary 2, P. Szabo 3, M. Cahill 4 1 SCIMUS Inc., 18 Aneta Circle, Toronto, ON, Canada, M2M 3J2, buchnea@sympatico.ca 2 Ontario Power Generation, PO Box 7000, B23, Tiverton, ON, Canada, N0G 2T0 3 CH2M Hill Limited,180 King St. S., Suite 600, Waterloo, ON, Canada, N2J 1P8 4 Rankin Construction Inc., 222 Martindale Rd., Box 1116, St. Catherines, ON, Canada, L2R 7A3 Abstract. Elevated tritium concentrations in one of the perimeter groundwater monitoring wells at Ontario Power Generation s Radioactive Waste Operations Site #1 at the Bruce Nuclear Site prompted the removal of 23 in-ground Tile Holes containing intermediate level radioactive waste from the purification systems of the Douglas Point and Pickering nuclear reactors. This project involved the encapsulation and transfer of the Tile Holes from this site, which has not received waste since the mid 1970's, to a second facility, the Western Waste Management Facility, which is actively receiving waste. The incorporation of radiation protection considerations into all components of the project, from choice of methodology, equipment, design, training and execution resulted in worker radiation exposures that were very small. The strategy was to encapsulate the Tile Holes in situ with a steel casing and concrete shielding. The methodology used two nested casings, a hydraulic oscillator, a vacuum system to excavate soil, shielding collars, and a downhole video camera to minimize worker dose. High density concrete was used on some of the more radioactive Tile Holes. The Radiation Protection team worked together well with both the engineering and construction personnel and communicated through training sessions, daily briefings and on the job corrections. Although some of the radiation fields from the removed Tile Holes were as high as 0.5 Gy/h at 1 m, and in one instance part of the Tile Hole contents remained in the hole after the Tile Hole was removed, the total committed dose to the 15 personnel involved in the project was less than 2 msv. The maximum exposed individual received about 0.4 msv during the project. The careful planning and team-work during the execution of the project allowed the project to be completed well ahead of schedule, thereby contributing to the lower dose. 1. Introduction During the mid-seventies, intermediate level radioactive wastes, mainly in the form of ion exchange (IX) columns and filters from the Douglas Point Nuclear Reactor, were stored in 23 in-ground Tile Holes at the Radioactive Waste Operations Site #1 (RWOS1) at the Bruce Nuclear Power Site in Ontario, Canada. Use of this site was discontinued in 1976 and Ontario Power Generation (OPG) has monitored water quality in the groundwater system since then. The presence of moderately elevated tritium concentrations in one of the perimeter groundwater monitoring wells prompted several investigations. These have inferred one or more of the Tile Holes as the source. It was decided to remove the Tile Holes to prevent further leakage and transfer them to one of OPG s operating waste management facilities, the Western Waste Management Facility (WWMF). In order to minimize contamination spread and worker dose, it was decided to encapsulate the Tile Holes in concrete prior to removal. This paper describes the methodology highlighting the radiation protection (RP) features and effectiveness during the remediation. 2. Removal and Radiation Protection Methodology The Tile Hole removal sequence involved the following activities (Figure 1): Insert outer casing (or core barrel) concentrically around the Tile Hole using hydraulic oscillator (Figure 1a). Install a stabilizer on top of the Tile Hole and concrete shielding collar and cap. Remove the soil between the outer casing and Tile Hole to below an existing concrete base by vacuuming (Figure 1b). Remove the shielding cap and insert inner casing over Tile Hole (Figure 1c). Present Address: RR2, Allenford, ON, Canada, N0H 1A0

2 Replace the shielding cap, add permanent shielding concrete, lift the Tile Hole off its base, and allow to cure (Figure 1d). Remove shielding collar and cap and lift inner casing with Tile Hole encapsulated in concrete (Figure 1e). Insert into a base cap (Figure 1f) and prepare for transport to the WWMF. Remove outer casing for reuse and backfill. a b c d e f FIGURE 1. Steps in Tile Hole Removal Sequence.

3 Radiation protection considerations were key in the choice and design of the removal methodology. The methodology consisted of the following components some of which are novel: Use of a nested casing approach to avoid large-scale excavation. Use of a hydraulic casing oscillator to insert the outer casing 4 m into the ground. Use of a vacuum system to excavate soil around the Tile Hole. Use of a shielding collar and a downhole video camera to minimize worker dose. Vacuum Truck Shielding Collar Outer Casing Hydraulic Oscillator Key to the success of this methodology was the use of a hydraulic casing oscillator (Figure 2). This device is not common in Canada, however, it is used extensively in Asia and parts of Europe for the installation of piers for bridge abutments. It will insert a casing up to 2 m in diameter to depths of 60 m. It was suitable for inserting a 1.5 m diameter steel casing around a Tile Hole to a depth of 4 m into the ground with a minimum of disturbance. The adjacent Tile Hole was only about 0.5 m from the casing. FIGURE 2. Main Components of Removal System. The advantages of the nested casing approach, in conjunction with use of the hydraulic casing oscillator, included: Elimination of the need for open excavation, Minimization of ambient radiation fields in active work area by minimizing area with high radiation fields and allowing the use of a shielding collar as a work platform, Containment of any contamination during work, Improvements in site conventional health and safety, Reduction of risk of damage to the Tile Hole and adjacent Tile Holes thereby preventing release of contamination, Reduction of the potential to generate liquid radiological wastes, Reduction in the diameter of the encapsulated Tile Hole package and reduced costs associated with long-term storage. Using the vacuum system for excavating the soil allowed soil removal within a very confined space, thereby making possible the use of a shielding collar/cap to effectively reduce radiation fields. Worker exposure was further reduced by the use of high intensity halogen lamps and a downhole video camera during critical work procedures when radiation fields were high. It also allowed inspection of the Tile Holes in situ with the shielding collar in place. In order to keep ambient radiation fields as low as possible in the active work area, high density concrete was used for encapsulation of several of the Tile Holes exhibiting higher radiation fields. In addition, the designs of lifting devices, vacuuming heads, and various tools to make the excavation and encapsulation more efficient were designed to minimize exposure to radiation fields during the vulnerable periods in the removal. Standard radiation protection procedures such as the thorough training of workers; use of electronic personal dosimeters (EPD), time, distance, and shielding, to minimize worker dose; zoning, protective clothing, and contamination monitoring to control the spread of contamination; and strict supervision of workers by qualified radiation protection technicians were implemented throughout the removal project. There was a high degree of interaction between the RP team and the workers through daily work briefings and on the job communication. This team-work allowed the timely adaptation of procedures and the anticipation of and response to problems in order to minimize the worker dose.

4 3. Radiation Exposure During Tile Hole Removal The collective radiation dose budget for the Tile Hole Removal Project was 20 msv. The actual total collective dose for the project was 1.80 msv. The estimate for the maximum individual dose had been 2.70 msv, the actual dose was 0.39 msv. Table 1 summarizes the measured radiation fields on each Tile Hole and the total worker dose for each as estimated from the daily EPD readings. Table 1. Dose Statistics from Daily EPD Readings. Tile Hole Number Maximum Measured Field on Contact (mgy/h) Total Worker Dose by Tile Hole (EPD readings) µsv Per Cent of Total Dose 1 Removed during Pilot Test % % % % % % % % % % % % % % % <1% % <1% % <1% % <1% Totals % In Table 1, it can be seen that the Tile Holes with the highest dose rates resulted in the highest doses, as can be expected. Work was sequenced such that the Tile Holes with lower radiation fields were removed initially in order to refine the various procedures used in the removal operations and minimize the times required prior to working on those Tile Holes with higher radiation fields. Table 2 summarizes the total doses received during the various activities occurring during the Tile Hole removal. These were estimated from EPD readings taken from each worker involved before and after each activity by the radiation technician. These were used during the course of the project to refine procedures in order to minimize the worker exposure.

5 Table 2. Total Dose Per Major Activity. Total Worker Dose by Major Activity Activity (EPD readings) µsv Per Cent of Total Dose Outer Casing Installation 5 0.4% First Vacuum % Removal of Extender % Vacuuming with shielding cap % Insertion of Inner Casing, Stabilizer, and Lifting device % Video inspection of Tile Hole % Gamma survey of Tile Hole % Initial concrete Pours 4 0.3% Removal of Stabilizer, lifting device & final concrete pour % Lifting Encapsulated Tile Hole, Initial Survey and Grout % Paint and Stencil Tile Hole % Removal from RWOS % Gamma Survey of Encapsulated Tile Hole % Total % From Table 2, it can be seen that the activities with the Tile Hole with the sand shielding removed and prior to concrete addition (i.e. vacuuming, Tile Hole lifting, and surveys) resulted in the largest percentage of the dose (56%). Work on the encapsulated Tile Hole was next at 35%. Figure 3 shows the cumulative doses from the OPG official dose records (recorded in mrem, 1 mrem = 10 µsv, exposures greater than 10 µsv) for the 15 personnel involved in the Tile Hole removal project. As was stated, the total collective dose for the project was 1800 µsv (less than 10% of the dose budget and less than 20% of the estimated dose). The maximum exposed worker received 390 µsv, less than RWOS 1 Tile Hole Project - Cummulative- Dose by Day mrem Apr Apr Apr Apr Apr May May May May May May Jun Jun Jun Jun Jun Jun-02 FIGURE 3. Cumulative Radiation Dose during project (1 mrem = 10 µsv).

6 1% of the radiation worker annual dose limit and less than half of the annual dose limit for a member of the public. The total collective dose of 1800 µsv is less than the total dose calculated for the various activities in Table 1 and 2 since that dose estimate (1200 µsv) did not include the dose from ambient radiation fields in the work area. It is estimated that the ambient radiation fields are about 0.20 µgy/h and that about 12 workers are exposed to these fields for an average of 8 hours/day. Thus, for the duration of the project (50 days), a dose commitment of up to 960 µsv can be expected from the ambient fields. Since the actual hours spent in the radiation work area were less than the 400 hours assumed, this is consistent with the difference. The decrease in the cumulative dose at about May 23 in Table 3 resulted from the removal of a spurious incorrect dose commitment from the previously measured dose. The careful planning and team-work during the execution of the project allowed the project to be completed well ahead of schedule, thereby contributing to the lower than expected dose. 3. Contamination Control During Tile Hole Removal The methodology developed for the removal of the Tile Holes limited the potential for contamination to a small number of activities. For most of the Tile Holes, the surrounding sand that was excavated by vacuuming was known to be free from radioactive contamination and thus there was no need to implement contamination control measures for all but two of the tile holes, which had measurable levels of contaminated sand. Figure 4 shows the area tarped and cordoned off and the workers wearing Tyvec suits and gloves. Half face respirators were only worn during the vacuuming of that Tile Hole with the highest levels of contaminated sand. The inner chamber of the vacuum unit was cleaned and the filter bags were replaced after the contaminated sand was fully FIGURE 4. Contamination Control excavated. There was no measurable contamination in During Vacuuming. the unit after cleaning. There was no evidence of carryover of contamination into the secondary filters or HEPA filters of the vacuum unit. FIGURE 5. Monitoring Contamination on Tile Hole Base. Most Tile Hole bases remained in the excavated hole after the encapsulated Tile Hole was removed and were found to have no detectable contamination (Figure 5). There were a few bases in which fixed contamination up to an average of 2000 CPM over small portions of their upper surface. This was less than a criterion of 3000 CPM average that was established and thus, no decontamination of the bases was necessary. The Tile Hole Lifting method worked well for the most part. There were a few Tiles Holes whose contents dropped during the lifting, but all except for one were pinched by the concrete and came up with the encapsulated concrete. One of these Tile Holes, had measured radiation fields of 500 mgy/h from the bottom. The end cap of this Tile Hole was filled with high density concrete. One Tile Hole had evidence of tritiated water on its base, but this quickly evaporated. The bottoms of the encapsulated Tile Holes were covered in plastic as they were lifted into the end cap to prevent any of the inner contents from contaminating the work area during the lifting. The plastic was removed when the Tile Hole was suspended directly over its base.

7 FIGURE 6. Filter Left on Base of Tile Hole. In the case of the Tile Hole that was likely leaking as evident from the highest contamination levels in the surrounding sand, some of its contents (a filter) remained in the hole after the Tile Hole was removed. This was likely because the filter was surrounded with sand and this prevented the fresh concrete from pinching the filter. In addition, the outer shell of the filter was likely corroded away and the upper portion of the filter separated from the contents (Figure 6). The characteristics of the waste that remained in the hole after removal of the Tile Hole were as follows: The internal portion of a Pickering multi-element (8 elements) filter. o Radiation fields up to 10 mgy/h in the vicinity of the filter, o Possible residual ion exchange resin (from smell), o Tritium levels were up to 3 MPC a in hole initially, no above background tritium after removal. Sand backfill with contamination up to 50,000 cpm, about 8 drums volume, with maximum contact radiation fields of 0.3 mgy/h. First the sand was vacuumed and then the filter was encapsulated in concrete. Lifting devices were drilled into the four corners of the base and the encapsulated filter was removed for transport to the WWMF. Radiation fields were 0.15 mgy/h on contact with the encapsulated filter. 4. Summary FIGURE 4. Encapsulated Tile Hole Prepared for transport to WWMF. The Tile Hole Removal Project fulfilled OPG`s objective in preventing the release of radionuclides from RWOS1 into the groundwater system by removing the source. The removal proceeded in an efficient manner with a minimum of committed dose to workers although high levels of radiation were present. The RP input into the design and implementation of the project and the high commitment to a cooperative effort between the RP team, the engineers, and the workers contributed to the overall success of the project and the ability to effectively deal with the RP challenges that arose during the course of the project.