Department of Energy Office of Worker Protection Programs and Hazards Management Radiological Control Technical Position RCTP 99-02

Transcription

1 Issue: Title 10 Code of Federal Regulations, Part 835 (10 CFR 835), Occupational Radiation Protection, specifies occupational radiation protection requirements for Department of Energy (DOE) activities and includes provisions related to the monitoring and control of individual exposures to various airborne radionuclides. Appendix A to 10 CFR 835, Derived Air Concentrations (DAC) for Controlling Radiation Exposure to Workers at DOE Facilities, provides DAC values for numerous radionuclides that are used to control individual internal doses, identify the need for air monitoring, and identify the need for posting. The data in appendix A can also be used in determining individual internal dose. Tritiated particulate aerosols, often existing as small particles of metal incorporating tritium, have been used at a number of DOE facilities for many years. Also, tritiated particulate aerosols in the form of tritiated rust have been unintentionally formed through the contact of tritium with iron. Organically-bound tritium can also be found in the machinery oil used in tritium facilities. The decontamination and decommissioning of existing tritium facilities will involve a large number of individuals and the potential for poorly characterized radiological hazards resulting from tritiated particulate aerosols and organically-bound tritium. Appendix A of 10 CFR 835 specifies DAC values for elemental tritium and tritiated water vapor; however, it does not specify values for either tritiated particulate aerosols or organicallybound tritium. Introduction: Worker protection, monitoring, and dose assessment for tritium gas and tritium water vapor do not present a significant technological challenge. These materials can easily be detected using well defined techniques. Similar techniques can be used to detect tritiated particulate aerosols that disassociate fairly rapidly. However, the physical characteristics of tritiated particulate aerosols that disassociate more slowly (including materials commonly referred to as stable metal tritides) make effective and accurate workplace monitoring, bioassay, and dose assessment problematic.

2 This (RCTP) is limited to the assessment of dose and determination of air concentration values * for controlling exposures to tritiated particulate aerosols and organically-bound tritium. This RCTP does not discuss other radiological protection issues, such as bioassay techniques, surface contamination monitoring, or air monitoring. Individual sites are developing site-specific methodologies to address these areas. This RCTP may be used as interim guidance until such time that additional information concerning these issues is developed and appropriately reviewed, and DOE issues additional requirements and/or guidance. *For regulatory purposes, the air concentration values for tritiated particulate aerosols and organically-bound tritium discussed in this RCTP cannot be defined as DAC values. DAC values are defined in appendix A to 10 CFR 835. However, the values discussed in this RCTP may be used in a similar manner as DAC values. Discussion: Applicable Requirements (these requirements are based on the November 4, 1998, amendment to 10 CFR 835) 10 CFR (a) Definitions Airborne radioactivity area means any area, accessible to individuals, where: (1) The concentration of airborne radioactivity, above natural background, exceeds or is likely to exceed the derived air concentration values listed in appendix A or appendix C of this part; or (2) an individual present in the area without respiratory protection could receive an intake exceeding 12 DAC-hours in a week. 2

3 Derived air concentration (DAC) means, for the radionuclides listed in appendix A of this part, the airborne concentration that equals the ALI divided by the volume of air breathed by an average worker for a working year of 2000 hours (assuming a breathing volume of 2400 m 3 ) Concentrations of radioactive material in air. (a) The derived air concentration (DAC) values given in appendices A and C of this part shall be used in the control of occupational exposures to airborne radioactive material. (b) The estimation of internal dose shall be based on bioassay data rather than air concentration values unless bioassay data are: (1) unavailable; (2) inadequate; or (3) internal dose estimates based on air concentration values are demonstrated to be as or more accurate Air monitoring. (a) Monitoring of airborne radioactivity shall be performed: (1) Where an individual is likely to receive an exposure of 40 or more DAC-hours in a year; Workplace Controls. During routine operations, the combination of physical design features and administrative controls shall provide that: (a) The anticipated occupational dose to general employees shall not exceed the limits established at ; and (b) The as low as reasonably achievable (ALARA) process is utilized for personnel exposures to ionizing radiation. 3

4 Appendix A DERIVED AIR CONCENTRATIONS (DAC) FOR CONTROLLING RADIATION EXPOSURE TO WORKERS AT DOE FACILITIES The data presented in appendix A are to be used for controlling individual internal doses in accordance with , identifying the need for air monitoring in accordance with , and identifying and posting airborne radioactivity areas in accordance with (d). Radionuclide Inhaled Air-Lung Retention Class Inhaled Air-Lung Retention Class uci/ml Bq/m 3 Stochastic or Organ D W Y D W Y ( D/ W/ Y) H-3 (Water) 2.E-05 2.E-05 2.E-05 8.E+05 8.E+05 8.E+05 St/St/St H-3 (Elemental) 5.E-01 5.E-01 5.E-01 2.E+10 2.E+10 2.E+10 St/St/St Technical Position: In September 1995, the International Commission Radiological Protection (ICRP) adopted ICRP Publication 71, Age-Dependent Doses to Members of the Public from Intake of Radionuclides: Part 4, Inhalation Dose Coefficients. The report uses the revised kinetic and dosimetric models of the respiratory tract presented in ICRP Publication 66, Human Respiratory Tract Model for Radiological Protection, and gives the dose coefficients (doses per unit intake) for tritium gas, tritiated water vapor, organically-bound tritium, and particulate aerosols of various absorption types. ICRP Publication 66 recommends that material-specific rates of absorption be used in the model for compounds for which reliable human or experimental data exist. For other particulate compounds default parameters are recommended, depending on whether the absorption is considered to be fast (type F), moderate (type M), or slow (type S). Gaseous compounds, such as tritiated water vapor, are assigned to one of three classes of gas uptake patterns. 4

5 Particulate Aerosols ICRP Publication 71 includes dose conversion factors for 1 um activity median aerodynamic diameter (AMAD) size particles of types F, M, and S, with type S being the most restrictive. As an example, using the dose conversion factor from ICRP Publication 71 for type S tritium particulate aerosols would result in the following air concentration value: (0.05 Sv / 2.6E-10 Sv/Bq) / 2400 m 3 = 8.E+04 Bq/m 3 (2.E-06 uci/ml) The table provided in the summary of this technical position summarizes the air concentration values for type F, M, and S materials. The air concentration values provided are stochastic values which, for all material types of tritium particulate aerosols, are more limiting than organ (i.e., non-stochastic) air concentration values. Organically-bound tritium ICRP Publication 71 and supporting ICRP Publications 66, 67, and 68 also determine organically bound tritium is assigned to the same deposition class as tritiated water vapor (i.e., soluble and reactive gasses and vapors). Organically bound tritium differs from tritiated water vapor in: - a lower percentage skin absorption; and - the larger percentage of nuclear transformations within the stomach for organically bound tritium due to the assumed biokinetic gastrointestinal tract model. Using the dose conversion factor from ICRP Publication 71 would result in the air concentration value provided in the table in the summary of this technical position. The air concentration value provided is a stochastic value which is more limiting than the organ (i.e, non-stochastic) air concentration value. 5

6 Additional projects DOE is funding several projects to obtain additional material-specific data. These projects include the following: Self-absorption of tritium beta particles in the metal particle--icrp Publication 71 does not account for self-absorption, i.e., the model assumes that all of the activity is emitted. The values provided in ICRP Publication 71 reflect total activity. Preliminary data indicates that self-absorption for particle sizes typically encountered in the workplace may reduce the dose coefficient by up to a factor of 5; Bremsstrahlung production within particles---preliminary data indicates that bremsstrahlung production is not a significant factor; Dissolution rates--this data will support use of either type F, M, or S; Particle size distribution--studies are being conducted to determine actual particle size distributions in the work environment; and Biokinetic modeling--results may be used in determining appropriate dose conversion coefficient and DAC values. Summary: The adult dose conversion factors provided by ICRP Publication 71 for tritiated particulate aerosols and organically bound tritium provide an acceptable basis for determining air concentration values, which may be used for meeting the workplace controls (i.e., maintaining doses within the dose limits) and ALARA requirements in These air concentration values (summarized in the following table) are acceptable for use when air concentration values are used to estimate internal dose under (b). These values may also be used for identifying the need for air monitoring 6

7 and identifying the need for posting. The adult dose conversion factors for the most restrictive form of tritiated particulate aerosols (type S) should be used unless material-specific parameters are developed and appropriately reviewed. The following table summarizes resultant air concentration values for the tritiated particulate aerosol types of material and organically-bound tritium using the ICRP Publication 71 dose conversion factors (conservatively rounding the values) and provides comparison of these values to the DAC value for H-3 (water) given in appendix A to 10 CFR 835: AIR CONCENTRATION VALUES Tritium Form/Type uci/ml Bq/m 3 Ratio to H-3 (Water) DAC* H-3 (Water) 2.E-05** 8.E+05** 1 H-3 (Elemental) 5.E-01** 2.E+10** Type F Tritiated Particulate Aerosol Type M Tritiated Particulate Aerosol Type S Tritiated Particulate Aerosol 9.E-05 3.E E-05 4.E E-06 8.E Organically-Bound Tritium 1.E-05 4.E+05 2 * This column is a ratio comparing H-3 (water) to the tritium form/type i.e., H-3 (water) DAC / tritium form/type air concentration value. Numbers greater than 1 indicate a more restrictive ratio than H-3 (water). ** These are the appendix A 10 CFR 835 DAC values. 7

8 References: Title 10 CFR 835, Occupational Radiation Protection, U.S. Department of Energy, November 4, 1998 ICRP Publication 66, Human Respiratory Tract Model for Radiological Protection, International Commission on Radiological Protection, September 1995 ICRP Publication 67, Age-Dependent doses to Members of the Public from Intake of Radionuclides: Part 2 Ingestion Dose Coefficients, International Commission on Radiological Protection, April 1993 ICRP Publication 68, Dose Coefficients for Intakes of Radionuclides by Workers, International Commission on Radiological Protection, July 1994 ICRP Publication 71, Age-Dependent Doses to Members of the Public from Intake of Radionuclides: Part 4, Inhalation Dose Coefficients. International Commission on Radiological Protection, September