Hazard Categorization Modern Dosimetry Threshold Quantities. Mr. William C. Walker Oak Ridge National Laboratory UT-Battelle, LLC; *

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1 Mr. William C. Walker Oak Ridge National Laboratory UT-Battelle, LLC; * walkerwc@ornl.gov Introduction Hazard Category (HC)-2 and HC-3 Threshold Quantities (TQs) were first published by the Department of Energy (DOE) in The TQs were derived via the use of effective dose methodologies published by the U.S. Environmental Protection Agency (EPA) and the U.S. Nuclear Regulatory Commission (NRC). These effective dose methodologies utilized contemporaneous dosimetry data for the derivation of HC-2 and HC-3 TQ values for approximately 100 radionuclides, as published in DOE-STD (1) The Los Alamos National Laboratory (LANL) subsequently published a fact sheet in the mid-1990 s which expanded the HC-3 TQs to include 757 radionuclides. (2) In 2011, the National Nuclear Security Administration (NNSA) issued a supplemental directive (SD) which provided guidance associated with the calculation of updated HC-2 and HC-3 TQs (3) using modern dose coefficients (DCs). Similar to DOE-STD-1027, the NNSA SD published updated HC-2 and HC-3 TQs for approximately 100 radionuclides based on the provided guidance. For isotopes that do not have threshold values supplied in the SD, threshold values may be selected with appropriate justification by applying the methodology, as described in the SD. Starting in 2013, a team comprised of DOE and NNSA contractor personnel engaged in a task to calculate updated TQs for additional radionuclides. This paper intends to: (i) summarize the rational in the selection of input dosimetry to be used for the calculation of updated TQs, (ii) review a comparative analysis of updated TQs with the original TQs, and (iii) explore the possibility of future revisions to dosimetry data. The Evolution of Inhalation and Ingestion Dose Coefficients Published by the International Commission on Radiological Protection ( ) The calculation of HC-2 and HC-3 TQ values utilized dosimetry intake coefficients which were published in the late 1970 s and early 1980 s. The International Commission on Radiological Protection s (ICRP) (a.k.a. the Commission) report Publication 30 was the primary source of inhalation and ingestion DCs utilized in the HC-3 TQ calculations. (4) The DCs utilized for the calculation of HC-2 TQs originated from DOE published reports which adopted many of the recommendations set forth by the ICRP. (5) (6) * Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. Page 1 of 12

2 The DCs published by the ICRP and the DOE were based on recommendations set forth by the Commission though the issuance in 1977 of Publication 26. In Publication 26, the Commission quantified the risks of stochastic effects of radiation and proposed a system of dose limitations. (7) Almost concurrent with the issuance of the original HC-2 and HC-3 TQs, the Commission revised its recommendations with the issuance of Publication 60 in The Commission s revised recommendations on radiation protection standards were developed to take into account new biological information related to the detriment associated with radiation exposures and supersede the earlier recommendations in Publication 26. As a direct result of the revised recommendations in Publication 60, revised DCs were published for workers in Publication 68 (1994), (8) and for members of the public in Publication 72 (1996). (9) The Evolution of ICRP Nuclear Transformation Data ( ) In addition to inhalation and ingestion DCs, the derivation of HC-3 TQs requires the evaluation of nuclide specific photon emission data with regards to the potential direct external dose exposure effect. The primary source for nuclide specific data originates from basic nuclear decay information contained in Evaluated Nuclear Structure Decay Files (ENSDF), as maintained by the National Nuclear Data Center (NNDC). The Commission compiled ENSDF information with the release of Publication 38 in (10) The information in Publication 38 represented the radionuclide transformation data used by the Commission in computing the inhalation and ingestion DCs in Publication 30. In 2007, the Commission once more revised its recommendations with the release of Publication 103 in 2007 (11). As acknowledged in Publication 103, the nuclear transformation data of Publication 38 are the same as have been used in ICRP publications since In Publication 103, the Commission committed to preparing a database of nuclear decay data to replace Publication 38 for use in future calculations of DCs. In 2008, the Commission fulfilled this commitment with the release of revised nuclear transformation data in Publication 107. (12) The Commission also noted that the updated nuclear transformation data would be utilized for revising the DCs in Publications 68 and 72. As of the summer of 2014, the Commission has not yet published revised DCs based on Publication 107. Therefore, Publication 107 data is only utilized for radionuclides not listed in Publications 68 and 72. Hierarchy of Non-ICRP Dosimetry Data Sources As previously noted, the DCs from ICRP Publications 68 and 72 were developed using Publication 38 nuclear transformation data. Publication 38 was developed from an ENSDF data file spanning 839 radionuclides. However, Publications 68 and 72 only provided inhalation and ingestion DCs for radionuclides with half-lives or 10 minutes and greater (approx. 700 radionuclides). Publication 107 was developed from an ENSDF data file spanning 1,252 radionuclides with 922 radionuclides having a half-life equal to or greater than 10 minutes. The differences between Publications 38 and 107 highlighted the need for inhalation and ingestion DCs for almost 200 radionuclides not addressed by Publications 68 and 72. Page 2 of 12

3 In order to address the lack of DCs for radionuclides not addressed by Publications 68 and 72, other data sources were utilized, which included: FGR-12, External Exposure to Radionuclides in Air, Water, and Soil. (13) This report, published by the Environmental Protection Agency (EPA), tabulates air-immersion dose coefficients based on the dosimetry recommendations of Publication 60. A DOE report documenting public external dose coefficients (5) had been previously utilized as the sole data source for air-immersion DCs for the calculation of the original HC-2 TQs. However, the DOE external DCs were developed based on dosimetry recommendations prior to Publication 60. Since Publication 60 superseded previous Commission recommendations, the DOE report is considered outdated, and as such superseded by the EPA external dosimetry report, FGR-12. DOE-STD , Dose Concentration Technical Standard. (14) In 2011, the DOE issued a report which computed DCs in manner similar to Publication 72. The DOE report deviated from Publication 72 by using revised dosimetry parameters from Publication 89, and revised nuclear transformation data from Publication 107. As such, this data source was only utilized for isotopes not listed in Publication 72 (for inhalation DCs), and for isotopes not listed in the EPA report, FGR 12 (for airimmersion DCs). JAERI-Data/Code , Dose Coefficients for Radionuclides Produced in High Energy Proton Accelerator Facilities: Coefficients for Radionuclides Not Listed in ICRP Publications. (15) Published by the Japan Atomic Energy Research Institute (JAERI), this document calculates worker and public DCs for radionuclides not listed in ICRP publications. This reference was used in the calculation of both HC-2 and HC-3 TQs as it addresses DCs for both worker and public and to obtain half-lives for radionuclides not listed in Publications 38 or 107. The JAERI report was cited for radionuclide DCs which are have not been published in any of the previously discussed non-icrp dosimetry data sources. Consistent with the criteria used for the publication of radionuclides in Publication 68 and 72, only radionuclides from the JAERI report with half-lives greater than 10 minutes were considered for evaluation. Comparative Analysis: HC-3 TQs Following the hierarchy of input dosimetry data as previously described, updated HC-3 TQs have been calculated for 1,252 radionuclides. Table 1 summarizes a comparison between the current HC-3 TQs and the updated HC-3 TQs. The comparative analysis of Table 1 groups the radionuclides by their respectively assigned limiting exposure pathway. As confirmed in Table 1, the limiting pathway designation remains essentially unchanged for the 757 radionuclides with a current HC-3 TQ. The inclusion of nuclear transformation The methodology involved in establishing a TQ value is based on the determination of the quantity (in terms of activity) necessary to result in a 10-rem dose for four exposure pathways. The limiting exposure pathway requires the least amount of material to yield a 10-rem dose. Page 3 of 12

4 data from ICRP-107 and dose coefficients from other non-icrp dosimetry data sources expands updated HC-3 TQ listing to 1,252 radionuclides. Whereas a limiting exposure pathway comparative analysis shows minimal change in the number of radionuclides assigned to a particular limiting exposure pathway, the relative change in the HC-3 TQs is broadly noted. In fact, the HC-3 TQs change by more than +10% for 571 radionuclides. Table 1. Comparison of Current HC-3 TQs and Updated HC-3 TQs Current Isotopes Only All Isotopes Current Updated > +10% change All Updated Limiting Exposure Pathway HC-3 TQs HC-3 TQs HC-3 TQ HC-3 TQs Inhalation % Ingestion (food) % Ingestion (water) % -- 1 Direct Exposure & Noble Gas Immersion % TOTALS Of specific interest to many DOE facilities are the relative change in the HC-3 TQs for actinide radionuclides. As observed in Figure 1, many of the actinide radionuclides typically found in the radiological inventory of DOE facilities will incur a higher HC-3 TQ value (e.g., 234 U, 235 U, 238 U, 238 Pu, 239 Pu, 240 Pu, 241 Pu, and 242 Pu). Figure 1. Updated HC-3 TQs for Actinide Radionuclides A broader look at the relative changes in the HC-3 TQs (Table 2) reveals that approximately two-thirds of the HC-3 TQs for the current set of radionuclides have a lower value. Whereas Page 4 of 12

5 Inhalation Ingestion (food) Ingestion (water) Direct Exposure & Noble Gas Immersion TOTAL % of Current HC-3 TQs Hazard Categorization Modern Dosimetry Threshold Quantities the inhalation exposure pathway is evenly distributed between updated HC-3 TQs which are higher and lower than the current values, the ingestion (food) exposure pathway actually has a general trend of towards higher HC-3 TQs values. In contrast, the direct exposure / air-immersion exposure pathway has a distinct trend towards lower HC-3 TQs. Table 2. Relative Change In The Updated HC-3 TQs (Exposure Pathway Analysis) The updated HC-3 TQ is than the current HC-3 TQ. higher % the same as % lower % TOTALS This downward trend for the direct exposure pathway TQ values was not anticipated. The inhalation and ingestion HC-3 TQs were expected to be broadly affected because of the use of updated DCs. After all, the primary reason for publishing updated HC-3 TQs was based on the acknowledged use of updated inhalation and ingestion DCs. But, inhalation and ingestion DCs are not utilized in the direct exposure pathway calculation. It was assumed from the start of the TQ update effort that continued use of the nuclear transformation data of Publication 38 would yield updated direct exposure TQ values similar to the current TQ values. Given that the methodology utilized to calculate the updated direct exposure pathway TQ was the exactly same as was utilized for calculating the current direct exposure pathway TQs, then the use of the Publication 38 nuclear transformation data was expected to yield an almost identical value. And yet, this approach resulted in the updated HC-3 TQs having a lower value for the majority (366 out of 446) of the radionuclides assigned to the direct exposure pathway. It s important to note that the current HC-3 TQs for the 757 radionuclides from Publication 30 were not directly calculated by the DOE. Instead, the HC-3 TQs were determined from a U. S. Environmental Protection Agency (EPA) report (16) which published exposure pathway release values (equivalent to 0.5 rem exposure). The activity value for the radionuclide specific limiting pathway (i.e., the pathway with the smallest activity value) was selected from the EPA report and then multiplied by 20 to obtain the HC-3 TQ value (which is based on a 10-rem exposure). An understanding of why the updated HC-3 TQs for the direct exposure pathway changed requires a review of the EPA report. The methodology discussion for the direct exposure pathway in the EPA report does not specify the source of the nuclear transformation data used in the calculation (i.e., half-life values, and photon emission intensity /energy data). Page 5 of 12

6 The EPA report does a list table in the report with examples of photon intensity / energy data, citing the source of the data from the National Council on Radiation Protection and Measurements (NCRP) report No. 58. However a review of NCRP report 58 notes the published radionuclide database is only a subset of the 757 radionuclides evaluated by the EPA report. As such, it does not seem probable that NCRP report No. 58 was the source used by EPA to calculate the direct exposure pathway values. The methodology section of the EPA report also lacks specific detail regarding the scope of the photon intensity / energy data used in the direct exposure calculation. The updated TQs utilize the entire photon intensity / energy data from Publication 38 (and Publication 107). As described by the EPA, a default value was selected for the linear energy absorption coefficient for gamma rays in air. The EPA selected a default value within +15% of the actual coefficient values for gamma rays with energies between approximately 0.07 MeV and approximately 2 MeV. On its face, this statement seems to be limited to solely a justification of the selection of a default value for the linear energy absorption coefficient. Although nothing more is said about the photon intensity / energy data in the report (besides the previously noted data example from NCRP report No. 58), a footnote on the last page of the EPA report explains that radionuclides without a published value for the direct exposure pathway are due to: No gamma rays are emitted or the gamma rays which are emitted have gamma ray energies of less than 0.07 MeV and are strongly attenuated in air. No release value for the direct exposure pathway was calculated. The exact meaning of the note is vague. Does the note mean only photon intensity / energy data was used for emissions that were > 0.07 MeV? Or were all discrete energy levels considered, provided the total photon emission energy was > 0.07 MeV? Were only gamma photons evaluated, or were X-rays included in the total photon energy calculation? And still unknown, what data set was used for the photon intensity / energy data? Finally, the EPA report noted that annihilation gammas were considered in establishing the total photon energy value used in the direct exposure pathway calculation. Yet several examples have been noted where the direct exposure pathway value in the EPA report have not accounted for the annihilation gamma; specifically 18 F, 45 Ti, 64 Cu, 68 Ga, and 69 As. There were noted examples where radionuclides in the EPA report appeared to account for the annihilation gamma rays (or the lack of), but seemed to have not properly accounted for the intensity / energy of normal gamma rays and x-rays which were > 0.07 MeV; specifically 130 Cs and 235 Pu. The combination of; (i) noted errors in the direct exposure data from the EPA report, and (ii) the decision to use all photon intensity / energy data for calculating updated TQs, seems to account for the general trend in lower TQs for the direct exposure pathway. Without further clarification of the EPA report specific to the scope of photon intensity / energy data used for the calculations, the utilization of the entire photon intensity / energy data is conservative since it yields a lower updated TQ value than would be calculated if a truncated data set were used. Page 6 of 12

7 Comparative Analysis: HC-2 TQs Again, following the hierarchy of input dosimetry data as previously described, updated HC-2 TQs have been calculated for 1,261 radionuclides. Table 3 summarizes a comparison between the current HC-2 TQs and the updated HC-2 TQs. Table 3. Relative Change In The Updated HC-2 TQs The updated HC-2 TQ is than the current HC-2 TQ. TOTAL higher 264 the same as 1 lower 492 TOTALS 757 As similarly noted in the HC-3 TQ comparative analysis, many of the updated TQs differ by more than +10% (666 radionuclides). This result was reasonably expected since the input data (inhalation DCs and air-immersion DCs) used for calculating updated HC-2 TQs are completely different than original DCs used for calculating the current HC-2 TQs. It should be noted that the HC-2 TQs do not utilize photon intensity / energy data in the calculation. Therefore, there are no issues associated with nuclear transformation data as was previously discussed for the HC-3 direct exposure pathway calculations. Inventory Examples In many instances, radiological material is comprised of multiple radionuclide constituents. Only items such as specialty medical sources or sealed sources tend to be limited to single radionuclide composition. Table 4 provides a summary of the relative change in the sum of fraction (SOF) calculations for various uranium mixtures. Table 4. Relative Change in the SOF for Uranium Mixtures SOF Current /SOF Updated Uranium Mixture Examples Cat 2 Cat 3 FEM Limited? Depleted Uranium Natural Uranium LEU (5 wt%) YES HEU (20 wt%) YES HEU (50 wt%) YES HEU (90 wt%) YES HEU (95 wt%) YES There are more radionuclides in the updated HC-2 TQ data set (1,261 radionuclides) than the updated HC-3 TQ data set (1,252 radionuclides). The nine extra radionuclides are associated with the input data source used for the HC-2 TQ calculations which as previously described differ from the input data set used for the updated HC-3 TQ calculations. Page 7 of 12

8 In general, the SOF values will decrease for uranium mixtures using the updated TQs. For depleted and natural uranium mixtures, almost 11 times more material can be accommodated in a HC-3 facility (for hazard categorization purposes) and at least 3 times more material can be accommodated in a below HC-3 facility. Although Table 4 indicates a similar trend for enriched grades of uranium mixtures, the limiting inventory factor is now the 235 U fissionable material content. It should be noted that the updated TQs do not change the fissionable material inventory limits currently specified in DOE-STD A review of plutonium mixtures is captured in Table 5. Similar to the uranium mixtures, both HC-3 and below HC-3 facilities will realize in increase in the allowable amount of material in the respective facilities. Also similar to the uranium analysis, several grades of plutonium (high 239 Pu weight percent content) will be limited by the fissionable mass content. Pu Mixture Examples Table 5. Relative Change in the SOF for Plutonium Mixtures SOF Current /SOF Updated New Mass Limit (g) Cat 2 Cat 3 FEM Limited? Cat 2 Cat 3 MT42 84% E E+00 MT42 90% E E+01 MT42 95% E E+01 MT CAT 2 ONLY 4.62E E+01 MT CAT 2 ONLY 4.75E E+01 MT CAT 2 ONLY 4.87E E+01 MT CAT 2 ONLY 5.01E E+01 MT CAT 2 ONLY 5.18E E+01 MT CAT 2 ONLY 5.28E E+01 MT CAT 2 ONLY 5.54E E+01 MT 83 83% E E-01 MT 83 89% E E-01 The in-growth of radiological progeny can also impact the SOF calculations associated with facility inventories. Table 6 notes HC-3 and below HC-3 facilities will be able to accommodate more unrefined uranium ore, thorium and aged 241 Pu (which contains 241 Am). However, a lesser amount of 50-year old 226 Ra material will be permitted in a HC-3 facility than is currently allowed. This an example where the updated TQs could adversely impact inventory allowances with respect to current inventory limits. Page 8 of 12

9 Table 6. Relative Change in the SOF for Progeny In-growth Examples In-growth Examples SOF Current /SOF Updated Cat 2 Cat 3 Unrefined Uranium Ore Ra-226 Source (50-yr old) Thorium Ore Pu-241 (30-yr old) Finally, Table 7 provides a comparison of the relative change in the SOF for example medical isotopes. Here we encounter an assortment of instances where some radionuclide allowances increase or decrease for both HC-3 and below HC-3 facilities. The extreme example in Table 7 is the marked decrease in inventory allowance for 201 Tl in below HC-3 facilities. Table 7. Relative Change in the SOF for Medical Isotope Examples Medical Isotope Examples SOF Current /SOF Updated Cat 2 Cat 3 Mn Co Co Ga Y Tc-99m I Ba Tl Potential for Future Revision to TQs As can be gathered from information presented herein, dosimetry data available for calculating updated TQs are in a state of transition. The original TQs were calculated based on the dosimetry recommendations by the Commission in Publication 26. The current inhalation and ingestion DCs were developed based on the Commission s recommendations in Publication 60, which have since been superseded by recommendations in Publication 103. In Publication 103, the Commission recommended the revision of inhalation and ingestion DCs using updated nuclear transformation data in Publication 107. Upon the future fulfillment of this recommendation, Publications 68 and 72 are expected to be superseded with revised DCs. Additionally, the air-immersion DCs in DOE-STD-1196, which are based on Publication 107 data, are expected to supersede the air-immersion DCs in the FGR-12 report (which is based on Publication 38 data). Page 9 of 12

10 When future inhalation and ingestion DCs are published by the Commission, an effort can then be initiated to publish revised TQs. A paper published by Manabe (et. al.) detailed a comparative analysis of the impact of Publication 107 nuclear transformation data on the calculation of inhalation dose coefficients. The analysis reported that the inhalation DCs would increase 10% (or more) for 98 radionuclides, and would decrease 10% (or more) for 54 radionuclides. The comparative analysis by Manabe attributed changes in the inhalation DCs primarily to updated data associated with radiation energy emitted per nuclear transformation. Secondary contributors to the change in inhalation DCs were associated with revised half-life values and updated decay modes. The comparative analysis was limited in scope in that it only assessed the impact of using revised nuclear transformation data and did not assess other revised dosimetry recommendations from Publication 103. Based on the analysis of Manabe, it can be surmised that changes to the future TQs will not be trivial. A significant subset of radionuclides would incur significant changes in their inhalation DCs (approximately 160 radionuclides), whereas the rest will only change slightly (the relative change in the inhalation DC is expected to be less than +10%). Finally, as previously discussed, a revisitation of the direct exposure pathway calculation is encouraged to clarify the ambiguous scope of photon intensity / energy data, as described in the EPA report. Such a clarification could allow for a recalculation of HC-3 TQs to potentially higher values for the direct exposure pathway. Acknowledgements The author gratefully acknowledges the following: Pat Cahalane (NNSA), Ivan Trujillo (NNSA), and Robert Murphy (NNSA) for the regulatory guidance they provided, particularly in establishing a hierarchy of dosimetry input data; Lisa Pansoy-Hjelvik (LANL), Tammy Wheeler (Nuclear Safety Assoc.), Terry Foppe (Hukari), and Vern Peterson (Hukari), who are the team responsible for calculating updated HC-2 and HC-3 TQs; and Dr. Keith Eckerman (ORNL-retired) for providing the team with ICRP dosimetry data. Page 10 of 12

11 References 1. U. S. Department of Energy. Hazard Categorization and Accident Analysis Techniques for compliance with DOE Order , Nuclear Safety Analysis Reports. September DOE-STD , CN Los Alamos National Laboratory. Table of DOE-STD Hazard Category 3 Threshold Quantities for the ICRP-30 List of 757 Radionuclides - LANL Fact Sheet. October Revision 1. LA MS. 3. National Nuclear Security Administration. Guidance on Using Release Fraction and Modern Dosimetric Information Consistently With DOE STD , Hazard Categorization And Accident Analysis Techniques For Compliance With DOE Order , Nuclear Safety Analysis Reports, Change Notice No NNSA SD 1027 (Admin Change 1). 4. International Commission on Radiological Protection. Limits for the Intakes of Radionuclides by Workers ICRP U. S. Department of Energy. External Dose-Rate Conversion Factors for Calculation of Dose to the Public DOE/EH U. S. Department of Energy. Internal Dose Conversion Factors for Calculation of Dose to the Public DOE/EH International Commission on Radiological Protection. The 2007 Recommendations of the International Commission on Radiological Protection Publication International Commission on Radiological Protection. Dose Coefficients for Intakes of Radionuclides by Workers Publication International Commission on Radiological Protection. Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 5 Compliation of Ingestion and Inhalation Dose Coefficients Publication International Commission on Radiological Protection. Radionuclide Transformations: Energy and Intensity of Emissions Publication International Commission on Radiological Protection. The 2007 Recommendation of the International Commission on Radiological Protection Publication International Commission on Radiological Protection. Nuclear Decay Data for Dosimetric Calculations Publication Environmental Protection Agency. External Exposure to Radionuclides in Air, Soil, and Water EPA-402-R U. S. Department of Energy. Derived Concentration Technical Standard DOE-STD Japan Atomic Energy Research Institute. Dose Coefficients for Radionuclides Produced in High-Energy Proton Accelerators: Coefficients for Radionuclides Not Listed in ICRP Publications JAERI-Data/Code Page 11 of 12

12 16. ICF Incorporated and C-E Environmental. Technical Background Document to Support Final Rulemaking Pursuant to Section 102 of the Comprehensive Environmental Response, Compensation, and Liability Act: Radionuclides. s.l. : U. S. Environmental Protection Agency, RQ-RN Page 12 of 12