In-Situ y-PHA Measurements of the 285-3H Cooling Tower Components

S. R. Salaymeh and R. A. Dewberry
Westinghouse Savanah River Company
Aiken, SC 29808

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The Analytical Development Section of Savannah River Technology Center (SRTC) was requested by the Facility Disposition Division (FDD) to conduct in-situ gamma-ray pulse height analysis measurements to determine if the gamma activity present on the 285-3H cooling tower piping is naturally occurring radioactive materials (NORMS) or process related. Radiological surveys taken in support of Site Utilities showed beta – gamma contamination. The measurements’ main goal is to confirm that there is no process-related contaminants present on the associated piping of the cooling tower and to determine if the gamma is from NORMS or process related materials. To accomplish this, we have acquired 3 gamma-ray pulse-height analysis spectra. All acquisitions were made using a portable HPGe detector and EG&G Dart system that contains high voltage power supply and signal processing electronics. A personal computer with Gamma-Vision software was used to control the Dart MCA and to provide space to store and manipulate multiple 4096-channel g-ray spectra.

Our results showed that all the gamma peaks identified in the spectra are due to naturally-occurring radionuclides. This report will discuss the purpose of the measurements, the experimental setup, data acquisition, calculations and results, and a conclusion of the study.

The Analytical Development Section of Savannah River Technology Center (SRTC) was requested by the Facility Disposition Division (FDD) to conduct in-situ gamma-ray pulse height analysis measurements to provide input toward the decision to unconditionally release the 285-3H cooling tower. The 285-3H is a three-cell-cooling tower used to support H Area Operations and specifically Tritium. A new cooling tower (218-2H) was constructed in Tritium as part of a project to upgrade the HVAC system. Figure 1 shows a photograph of this cooling tower. When this was completed, the need for additional cooling water for process reasons was greatly reduced. The 285-3H Tower was constructed in 1968 and shutdown in late 1998 as a result of lower demands. The 285-3H cooling tower structure measures 28 ft. x 74 ft. for approximately 2072 ft².

The cooling tower structure is prioritized in the top ten for risk ranking of inactive facilities. During an inspection in May 2000, the general condition of the structure was categorized as condition Code S (No Value Except for Its Basic Material Content). The inspection also noted that the structure is deteriorating, and the amount of wood present in the tower presents a fire potential.¹

During a survey by Radiation Control Operation (RCO), beta-gamma contamination of the associated piping of the cooling tower was detected. Although the majority of associated cooling water piping probed 10,000 dpm beta-gamma, one elbow probed 25,000 dpm beta-gamma. FDD wanted to confirm that the cooling tower has no process-related contaminants prior to its unconditional release. The plan is to use the history of the units, data from RCO survey of the associated piping, g-PHA, and a sample plan to unconditionally release the cooling tower. The g-PHA measurements’ main goals are to confirm that there are no process-related contaminants present on the associated piping of the cooling tower and to determine if the beta-gamma is from naturally-occurring radioactive materials (NORMS).

FDD’s decision to treat the cooling tower and its associated piping as not contaminated with process-related contaminants will realize a considerable cost savings by not sending the components to the low-level burial site. In addition, WSRC Management Policy requires evaluation of treatment and disposal options that consider waste disposal, costs, and risks to dispose of large pieces of equipment rather than the burial option.²

To accomplish the measurements, we have acquired two g-ray pulse height analysis spectra and a background g-ray spectrum near the cooling tower. All acquisitions were made using a portable HPGe detector and EG&G Dart system that contains a high voltage power supply and signal processing electronics. A personal computer with Gamma-Vision software was used to control the Dart MCA and provide space to store and manipulate multiple 4096-channel g-ray spectra.³ We used a liquid-nitrogen-cooled portable germanium detection system that is described in Technical Report WSRC-TR-2000-00317.³ For all of the components we acquired holdup spectra in the close-field configuration as shown in Figure 2. We acquired spectra with the detector approximately 5 inches away from the object. At that range we could not observe all of the component, however we were able to obtain close coupled spectra from the points designated by RCO inspection as high in b-g activity. We believed that a diagnosis of the observed activity as naturally occurring radioactive material (NORM) would be sufficient to declare the complete batch of cooling tower components as free of process radioactivity. This reasoning is supported strongly by the RCO inspections and by the process knowledge of the tower components. SRS has used very similar reasoning to free release three water chiller units.^4,5 These acquisitions and the interpretations of the data are very similar to those of references 4, 5, and 6.

We obtained a local background spectrum with the detector isolated from the tower components. This background is shown in Figure 4. The spectrum shows several naturally-occurring g-ray peaks that are labeled on it. The background g-ray activity is composed of natural activity from ²³²Th daughters and natural ²³⁸U daughters with no contributions from ¹³⁷Cs or ⁶⁰Co. Using the peaks from the natural background, we were able to energy calibrate the detection system.

In both of the close-coupled component spectra we observed background activity only. Example spectra are shown in Figures 5 and 6. The count times were 598 seconds and 620 seconds. Note no process-related activity is observed in either spectrum. The photopeak areas where ¹³⁷Cs and ⁶⁰Co would appear on the two spectra are designated, and it is clear that neither of these two common process-related species contribute to the spectra. Therefore in no case was it necessary to calculate measured activity from our close-coupled configuration. It is necessary only to determine limits of detection for those nuclides that are process related.

The Table lists the measured counts for three of the naturally occurring gamma ray transitions (238.6 ²¹²Pb, 510.6 ²⁰⁸Tl, and 911.2 ²²⁸Ac). It also lists the process related gamma rays (185.6 ²³⁵U, 661.7 ¹³⁷Cs, and 1173.2 ⁶⁰Co). These gamma rays are the most likely to be observed at the Savannah River Site. We can determine the lower limits of detection (LLD) for each of the three process related g-rays from the background spectrum acquired. The LLD in units of cps is determined from

where 0.86 is the branching ratio, and 0.0045 is our measured close-coupled detection efficiency. The latter was determined experimentally in reference 5 and is in good agreement with the vendor’s specifications⁷ and with our previous efficiency calibration measurements.⁸

The value of 19 dps is equal to 0.5 nCi, and we have used this value as our limit of detection for ¹³⁷Cs in these measurements. Using estimated detection efficiencies of 0.0123 for the ²³⁵U transition at 186 KeV and 0.0027 for the ⁶⁰Co transition at 1173 KeV, we have determined limits of detection of 1 nCi for ²³⁵U and for ⁶⁰Co.

Observing the data of the table and comparing the spectra of Figures 4 – 6 it is evident that the naturally occurring radioactivity from ²¹²Pb, ²⁰⁸Tl, and ²²⁸Ac are above the background rate. By some mechanism these three species as well as other NORMS are present at enhanced levels on the cooling tower components. We have consistently observed this phenomenon is our g-PHA observations on SRS water chiller components and on SRS reactor building trolleys.^(4-6) Two mechanisms to explain these observations would be from use of Pb-based paint and as Th daughter products from welding activities. In previous measurements, and with several additional experimental tests on the reactor trolleys, we have been unable to confirm the source of enhanced NORMS on these components

Sample Number	186 KeV U-235 Area	238 KeV Pb 212 Area	511 KeV Tl 208 Area	662 KeV Cs 137 Area	911 KeV Ac 228 Area	1173 KeV Co 60 Area	Count Time (sec)
1/11/01Bkg	82± 29	104± 35	57± 16	35± 10	59± 14	8± 14	627
Tower	62± 34	385± 41	122± 22	39± 15	85± 14	-13± 11	598
Tower 2	144± 136	497± 45	157± 22	22± 13	67± 16	3± 12	620

ADS researchers have acquired two close coupled g-PHA spectra from the 285-3H cooling tower components. The acquisitions were obtained with a high purity, high-resolution germanium detector system from the two spots designated by RCO as highest in observed b-g activity. In both of the spectra obtained we observed multiple g-rays that come from the naturally occurring decay chains of ²³⁸U and ²³²Th. No process-related g-rays were observed. We were especially attentive toward process contamination from ²³⁵U, ¹³⁷Cs, and ⁶⁰Co, and we have set upper limits of content of 1 nCi total activity from each of these three species in both items. The entire batch of tower components can be eligible for free release based on the results of this report, RCO survey, and the results of the sampling plan.