Cotter Corporation (N.S.L.) ( Cotter )

Transcription

1 COMPARISON OF BOUNDARY GAMMA EXPOSURE RATES AND AIR CONCENTRATIONS TO MODELED VALUES AND EXAMINATION OF HISTORIC ESTIMATES OF DOSE TO THE PUBLIC Prepared for: Cotter Corporation (N.S.L.) ( Cotter ) 7800 East Dorado Place Suite 210 Englewood, Colorado Prepared by: Two Lines, Inc. Radiation Risk Consultants 896 Overview Rd. Grand Junction, Colorado (970) Fax (309) May 1, 2013

2 TABLE OF CONTENTS 1.0 INTRODUCTION GAMMA EXPOSURE AT THE SITE BOUNDARY COMPARISON OF MILDOS-PREDICTED AND MEASURED AIR CONCENTRATIONS AT SITE BOUNDARY ESTIMATED DOSES FROM RADON DECAY PRODUCTS AT THE SITE BOUNDARY TOTAL ESTIMATED DOSES TO A MEMBER OF THE PUBLIC AT THE SITE BOUNDARY SUMMARY REFERENCES LIST OF TABLES Table 1 Annual average gamma exposure rate with background subtracted. 3 Table 2 Average ground level concentrations of Pb-210 in selected. 3 Table 3 Ratios of predicted to net measured radionuclide concentrations in air. 4 Table 4 MILDOS inhalation doses from radon and decay products. 8 Table 5 MILDOS estimated total effective dose equivalent (TEDE). 9 LIST OF FIGURES Figure 1 Location of boundary air samplers surrounding the Cotter milling facility. 2 Figure 2 Predicted vs. gross measured U-nat air concentrations. 6 Figure 3 Predicted vs. gross measured Th-230 air concentrations. 6 Figure 4 Predicted vs. gross measured Ra-226 air concentrations. 7 May 1, 2013 i Page i

3 1.0 INTRODUCTION In a letter dated December 6, 2012, James Grice of the Colorado Department of Public Health and Environment stated that for Cotter to discontinue conducting the MILDOS public dose assessment, the following must be achieved: A demonstration is required, using past 3 years data, along with the submission of the Environmental Monitoring Protocol document by February 28, 2013, to show: (1) the Regulations, has been met; and (2) a comparison and correlation between MILDOS dose assessment and boundary air monitoring results and (3) an estimation of the environmental doses, including external exposure and internal intakes, resulting from the anticipated decommissioning activities. Even though the proposed license amendments have been revoked, the purpose of this memorandum is to respond to those concerns. Air samplers and environmental gamma dosimeters are located at five locations around the boundary of the site as shown in Fig. 1. Air sampler locations beyond the perimeter are also shown. 2.0 GAMMA EXPOSURE AT THE SITE BOUNDARY Gamma exposures are measured at four locations on the boundary of the Cotter Milling Facility site using environmental TLDs for quarterly measurements. The four boundary locations are AS-202 (east), AS-203 (south), AS-204 (west) and AS-206 (north). Gamma measurements are also taken on the mill entrance road (AS-209). A background location in Cañon City (CC-2) is measured quarterly along with other community locations. For each sampling location shown in Table 1 for the years , the annual Cañon City background gamma exposure rate was subtracted to produce net of background exposure rates. The mean annual average exposure rate for the 5 sampled locations in 2.6, 2.7, 4.2, 2.2 and 7.2 ur/hr, for AS-202, AS-203, AS-204, AS-206 and AS-209, respectively. For each TLD location, the 5-yr mean was converted to rem/hr or rem/yr assuming that 1 rem = 1 R. None of the boundary locations exceeds the rem//hr limit specified in regulation (2) on an average basis. Further, assuming continuous (8760 hr/yr) occupancy at the sampled location, none of the boundary locations exceeds the 0.05 rem/yr limit. AS-209, the detector located by the mill access road exceeds the limit in section (2) by approximately 25%. However, as with all the perimeter locations, it should be noted that there is no actual occupant at the location and the AS-209 is closer to the actual mill location than the other boundary locations. 1 May 1, 2013 Page 1

4 Figure 1 Location of boundary air samplers surrounding the Cotter milling facility. May 1, 2013 Page 2 2

5 Table 1 Annual average gamma exposure rate with background subtracted. Location AS-202 AS-203 AS-204 AS-206 AS-209 Net μr/hr yr mean rem/hr 2.5E E E E E-06 rem/yr COMPARISON OF MILDOS-PREDICTED AND MEASURED AIR CONCENTRATIONS AT SITE BOUNDARY Quarterly air samples are taken at the same locations at which gamma exposures are measured. Samples are analyzed for natural uranium, Th-230, Ra-226 and Po-210 as well as Pb-210 and Th-232. The four quarterly samples are averaged to create an annual average for each nuclide and sampler. The MILDOS-AREA model (ANL 1998) is used annually to calculate potential dose to members of the public from mill operations (Cotter 2012). For the past several years, MILDOS-generated air concentrations have been compared to sampled air concentrations for the nuclides at the same locations. Because MILDOS does not consider background, background has been subtracted from measured prior to the comparison. In this analysis, Pb-210 and Th-232 are not considered. Lead-210 results at sampled locations are controlled by global Rn-222 concentrations. Radon-222 emanates from the soil and is dispersed through the atmosphere. The Rn-222 decay products build in as the parent decays. The short lived decay products of Rn-222 attach to dust particles and are carried long distances with the air. Pb-210 is the longest-lived of the Rn-222 decay products. The Pb-210 concentration in air varies with location. The average ground level concentrations in selected states are shown in Table 2 (NCRP, 1992). Table 2 Average ground level concentrations of Pb-210 in selected. State California Illinois Ohio Massachusetts Air concentration (μci/ml) 1.6E E E E-14 3 May 1, 2013 Page 3

6 As shown in Figure EA-8A (Cotter 2013) the average annual Pb-210 concentration in air at the Cotter boundary locations is far less variable through time than natural uranium, Th- 230 or Ra-226. This is because the majority of the Pb-210 in air results from global contributions rather than the Cotter facility. Further, the measured Pb-210 values are, on the average since 1979, a few percent of the effluent concentration limit. MILDOS is designed to model uranium decay chain constituents and Th-232 is not one of them. Air concentrations for Th-232 are not calculated directly by MILDOS. Doses from Th- 232 are calculated by ratioing dose conversion factors of Th-232 and Th-230. For the other three nuclides, Unat, Th-230 and Ra-226, there are several different methods that may be used to estimate how well the predictions fit the actual measurements. One is to calculate a ratio of predicted to net measured (P/NM) for each sample or set of samples. In that case, a model that perfectly predicts the measurements would have a mean value of 1.0. As can be seen in the results summarized in Table 3, this is rarely the case for the sample locations and years presented. Table 3 Ratios of predicted to net measured radionuclide concentrations in air. Predicted/Net Measured U-nat Mean AS AS AS AS AS Th-230 AS AS AS AS AS Ra-226 AS AS AS AS NA AS A value >1 for the P/NM indicates that the model overpredicts the measurement and a value <1 indicates that the model underpredicts the measurements. There is no clear cut trend for the ratio. What confounds the examination of the P/NM is that, as mentioned 4 May 1, 2013 Page 4

7 above, background has been subtracted from the measurements. So, for measurements that are lower than background, the net measurement result is expressed as a negative number. For example, the large negative values for U-nat in for AS-203 (2008) and AS-206 (2011 ) indicate that the measured value was slightly lower than background, so subtracting background results in a very small number. When this small negative denominator is divided into the positive prediction, the result is a large negative value. Similarly, for Th-230 and Ra-226, some large negative values exist. For all three radionuclides, the model overpredicts the annual average more often than it underpredicts. For U-nat, Th-230 and Ra-226, the number of P/NM ratios is >1 in 14, 16, and 16 of the 25 measurements. Further, the mean value of the ratio is generally above 1, if the negative values are neglected. Exceptions to this pattern are AS-206 for U-nat and AS- 204 for Th-230. Another method of examining how well a model predicts is to do a regression analysis of the prediction versus the measurements. A perfect model would result in a regression with a slope of 1.0, an intercept of 0.0 and a correlation coefficient of 1.0. For each nuclide, predictions were plotted vs. measurements for the various sample locations and years as shown in Figures 2-4. The difference in the data used in these figures from those in Table 3 is that the measurements presented have not been adjusted for background, so there are no negative values. For U-nat, the model tends to over predict the measured concentrations by approximately 30% on average, as shown by the slope of 1.3 (Fig. 2). At a measured concentration of 5E- 16 μci/ml, the regression line would predict a value of 1.3 * = 6.62 E-16 μci/ml or 32% above the measured value. At low concentrations, that trend would hold true as evidenced by the low intercept value of It is also important to note that the coefficient of determination (r 2 ) of 0.50, indicates that only 50% of the variance in the data is explained by the regression line. If all points were directly on the line, R 2 would equal 100%. Regardless, the correlation coefficient, r = 0.71, is significant at p < 0.05, which means that the line is a statistically significant fit. As shown in Fig. 3, for Th-230, the model also tends to over predict the measured concentrations slightly, in spite of a positive slope and a negative intercept. At 6E-16 μci/ml, the model would predict 8.42E-16 μci/ml or 40% above the measured value. At 2E-16 μci/ml, the overprediction drops to 17%. As with the U-nat data, the regression line accounts for only 50% of the variance in the data, but it is still statistically significant at P = For Ra-226, (Fig. 4), relative to U-nat and Th-230, MILDOS does a poorer job of predicting concentrations in air. The coefficient of determination, r 2, indicates that only 3% of the variance in the data is represented by the line. The corresponding regression coefficient of 0.17 is not significant, which indicates that the regression line is not a valid representation of the data. The high measurement of 7.9 greatly influences the slope of the line. A regression performed with that point removed did improve the correlation, but did not result in a statistically significant fit. Since Th-230 is the parent nuclide of Ra-226, it is not 5 May 1, 2013 Page 5

8 clear why MILDOS does such a poor job of predicting the Ra-226 concentrations, relative to Th-230 concentrations. Figure 2 Predicted vs. gross measured U-nat air concentrations. Figure 3 Predicted vs. gross measured Th-230 air concentrations. 6 May 1, 2013 Page 6

9 It is important to put the measured and predicted values in context. Effluent limits in CDPHE Regulation 4 Table 4B2 for U-nat, Th-230 and Ra-226 are 9E-14, 2E-14 and 9E-13 μci/ml, respectively. These limits represent values that would result in 50 mrem if breathed continuously for a year. The values measured at the samplers listed are no more than a few percent of the applicable limits. The maximum concentration at any of the samplers for the 5-year period examined are 7.33E-16, 7.21E-16 and 7.92E-16 μci/ml for U-nat, Th-230 and Ra-226, respectively. Assuming that a person resides at the location of the samplers continuously and taking in to account that the effluent limit represents 50 mrem, the resulting dose from inhalation of the maximum concentration equals 0.41 mrem, 1.8 mrem, and 0.04 mrem for U-nat, Th-230 and Ra-226, respectively. These are far lower than the limit of exposure to a member of the public from mill operations. Figure 4 Predicted vs. gross measured Ra-226 air concentrations. 4.0 ESTIMATED DOSES FROM RADON DECAY PRODUCTS AT THE SITE BOUNDARY MILDOS calculates receptor doses from radon and radon decay products, but includes them in the over inhalation dose and does not display radon dose results explicitly. Rather, doses from radon and radon decay products can be surmised by two methods. First the 40CFR190 effective dose is, by definition, excludes radon. The Total Effective Dose Equivalent (TEDE) includes dose for both inhalation of radon and radon decay products as well as radioparticulates. Therefore, subtracting the 40CFR190 effective dose from the TEDE will give the dose from radon and radon decay products. Second, inhalation dose from radon and radon decay products may be calculated by multiplying the bronchial dose 7 May 1, 2013 Page 7

10 by 0.06, which is the organ weighting factor. Estimates of effective dose from inhalation of radon and its decay products for are compiled in Table 4. As with all MILDOS estimates, these values are overestimates because they conservatively assume that a receptor is present at that location continuously. Table 4 MILDOS inhalation doses from radon and decay products (mrem/yr). Location AS E E E E E+01 AS E E E E E+00 AS E E E E E+00 AS E E E E E+00 AS E E E E E TOTAL ESTIMATED DOSES TO A MEMBER OF THE PUBLIC AT THE SITE BOUNDARY Estimates of annual dose to members of the public are made each year using MILDOS and reported in the Cotter annual report. Doses at the perimeter locations are shown in Table 5 by pathway. Food chain pathways, including the milk pathway, are not included in the analysis, but typical experience is that doses from the food chain represent at most a few percent of the total effective dose equivalent to a member of the public. External doses result from direct irradiation from ground contamination (ground) and submersion in air containing radioactive materials (cloud). These along with inhalation doses from radon and particulates comprise the doses to receptor locations. At the boundary locations shown in Table 5, the majority of the dose arises from inhalation and the majority of that is from radon and its decay produts. In general for the past five years, the dose to a presumed member of the public who was continuously present at a boundary location has been far below both the 10 CFR 20 regulatory limit of 100 mrem total effective dose equivalent and the 40 CFR 190 limit of 25 mrem effective dose without radon exposure. 6.0 SUMMARY Measured annual average gamma exposure rate at the boundary locations is far below any regulatory limits. The same is true of MILDOS-generated estimates of inhalation and external doses from submersion in contaminated air (cloud) and exposure to particulates deposited on the ground. 8 May 1, 2013 Page 8

11 Table 5 MILDOS estimated total effective dose equivalent (TEDE) (mrem/yr). Location Inhalation (particulates + radon) AS E E E E E+01 AS E E E E E+00 AS E E E E E+00 AS E E E E E+00 AS E E E E E+01 External from Ground AS E E E E E-01 AS E E E E E-02 AS E E E E E-02 AS E E E E E-02 AS E E E E E-01 External from Cloud AS E E E E E-02 AS E E E E E-02 AS E E E E E-02 AS E E E E E-02 AS E E E E E-02 Total Effective Dose Equivalent AS E E E E E+01 AS E E E E E+00 AS E E E E E+00 AS E E E E E+00 AS E E E E E+01 The correlation of MILDOS-generated air concentrations with measured concentrations of U-nat and Th-230 (Figs. 2 and 3) was statistically significant (p < 0.05) if data from all five years and boundary locations were considered jointly. The same was not true for Ra-226 (Fig 4). Because MILDOS is based on an annual average Gaussian plume model it is not surprising that correlations between the model results and measured results, while significant for some, were generally poor. The meteorological data are fed into the MILDOS simulation are from the weather station located near the administration building of the site. Given the locations of the boundary sampling sites, it is likely that different microclimates exist at those locations in comparison to the weather station location. Hence, each boundary location would like behave to some extent differently from the weather station location. In that regard, measurements of air concentration and gamma exposures are likely more accurate of the annual average dose that might be received by a member of the public than MILDOS estimates. Further, given that both measurements and MILDOS estimates are all 9 May 1, 2013 Page 9

12 well below any dose limit, it seems reasonable to remove the requirement that MILDOS modeling be performed annually. It is also important to note again, that for measured air concentrations, measured gamma exposure rates and MILDOS estimates, there is no reduction in dose taken for occupancy at the specific location. If an actual receptor were in place at the measured or modeled location, it would be reasonable to limit the dose relative to the amount of time spent at the location to account for time away for work, school and other activities. Allowance would also be made for shielding from time spent indoors. Given those constraints the actual dose to a receptor based on either measured or modeled doses would be even lower than those described in this memo. 7.0 REFERENCES Argonne National Laboratory (ANL), MILDOS-AREA User s Guide (Draft). Environmental Assessment Division, September. Cotter Corp., Environmetal and Occupational Performance Report, ALARA Review & Annual Report on Remedial Action Plan Activities. October 5, Cotter Corp., Semiannual Effluent Report. February 28, National Council on Radiation Protection and Measurements (NCRP) NCRP Report No. 94, Exposure of the Population in the United States and Canada from Natural Background Radiation. NCRP. Bethesda, MD. 10 May 1, 2013 Page 10