Updating Dosimetry for Emergency Response Dose Projections

Abstract

In 2013, the U.S. Environmental Protection Agency (EPA) proposed an update to the 1992 Protective Action Guides (PAG) Manual. The PAG Manual provides guidance to state and local officials planning for radiological emergencies. EPA requested public comment on the proposed revisions, while making them available for interim use by officials faced with an emergency situation. Developed with interagency partners, EPA’s proposal incorporates newer dosimetric methods, identifies tools and guidelines developed since the current document was issued, and extends the scope of the PAGs to all significant radiological incidents, including radiological dispersal devices or improvised nuclear devices. In order to best serve the emergency management community, scientific policy direction had to be set on how to use International Commission on Radiological Protection Publication 60 age groups in dose assessment when implementing emergency guidelines. Certain guidelines that lend themselves to different PAGs for different subpopulations are the PAGs for potassium iodide (KI), food, and water. These guidelines provide age-specific recommendations because of the radiosensitivity of the thyroid and young children with respect to ingestion and inhalation doses in particular. Taking protective actions like using KI, avoiding certain foods or using alternative sources of drinking water can be relatively simple to implement by the parents of young children. Clear public messages can convey which age groups should take which action, unlike how an evacuation or relocation order should apply to entire households or neighborhoods. New in the PAG Manual is planning guidance for the late phase of an incident, after the situation is stabilized and efforts turn toward recovery. Because the late phase can take years to complete, decision makers are faced with managing public exposures in areas not fully remediated. The proposal includes quick-reference operational guidelines to inform re-entry to the contaminated zone. Broad guidance on approaches to wide-area cleanup and cleanup goals is also provided. EPA adapted the cleanup process from the 2008 U.S. Department of Homeland Security (DHS) Planning Guidance for Protection and Recovery Following Radiological Dispersal Device (RDD) and Improvised Nuclear Device (IND) Incidents, and the final PAG Manual will supersede that DHS guidance. Waste management guidance is also provided. Recognizing that an incident could result in radioactive waste volumes that severely strain or exceed available resources and capacity, officials may consider alternatives for disposal of waste that is relatively lightly contaminated. Waste management, including treatment, staging, and interim and long-term storage, must be an integral part of recovery.

INTRODUCTION

RADIOLOGICAL DOSIMETRY is being updated in the United States according to international recommendations. In order to set policy for implementation of new dosimetry for emergency response, the U.S. Environmental Protection Agency (EPA) facilitated a symposium among interagency partners in radiation protection to delineate their uses of updated dosimetry. Federal guidance currently in use for radiation emergency response across federal, state, local, territorial, and tribal governments is based on a 1992 guidance manual (^{USEPA 1992}) that provides calculated trigger levels based on the International Commission on Radiological Protection (ICRP) Publication 26 series recommendations (^{ICRP 1977}). At the EPA facilitated symposium, sensitivity analyses were performed to explore the practical implementation impacts of moving to the ICRP Publication 60 series recommendations (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}) for emergency response; the resulting policy decisions are discussed in this paper.

The symposium in March 2014 enabled interagency federal partners to share current practices for updating radiation dosimetry in the United States. The purpose was to address the question, “How should updated ICRP Publication 60 dosimetry models (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}), including age-specific factors, be used for United States emergency response dose assessment, particularly early (4 d) and intermediate phase (1 y) projections?” Participants from EPA, several U.S. Department of Energy (DOE) programs and national laboratories, the U.S. Nuclear Regulatory Commission (NRC), the U.S. Food and Drug Administration (FDA), the Federal Emergency Management Agency (FEMA), and the U.S. Department of Agriculture (USDA) discussed how each organization is implementing updated dosimetry in their respective programs. While these various radiation programs have different mission areas to protect public health and the environment, the discussion provided a better understanding of dosimetry approaches and treatment of age groups in different applications.

In the United States, radiation emergency actions to protect the public are guided by the EPA’s Protective Action Guides (PAG) Manual (^{USEPA 1992, 2013}) developed by interagency federal experts in radiological and nuclear consequence management. Early or emergency phase protective actions, which will be discussed in further detail later in this paper, include sheltering or evacuation and the administration of prophylactic potassium iodide (KI). This paper also discusses intermediate phase protective actions: relocation of the public, and food and water restrictions.

1992 PAG MANUAL BASIS

Focusing first on the PAGs currently in use, the 1992 PAG Manual dose calculations were based on the adult receptor or Reference Man, as defined in ICRP Publication 23 (^{ICRP 1975}). When the 1992 PAG Manual was drafted, DCs for only the adult receptor were available. Interagency experts setting the PAGs did include consideration of higher risks for the fetus and those more difficult to evacuate (e.g., hospital patients or prison inmates), so conservatism was built in through the risk-benefit balancing process used to derive the PAGs. Additionally, assumptions made to generate default dose parameters and derived response levels ensure adequate protection for all members of the public, including sensitive subpopulations such as young children.

Making age group-specific dose calculations would add complexity to managing the early phase of a radiation response that may not be warranted. According to ICRP Publication 63, Principles for Intervention for Protection of the Public in a Radiological Emergency (^{ICRP 1992}), being too conservative can result in the population being exposed to more risk from protective actions (e.g., evacuation) than the risk from radiation exposure or avoiding less dose than intended: “The derived levels should be calculated as far as possible using realistic rather than excessively conservative parameters and assumptions in the models. Otherwise the protective action will avert less dose than was intended.” That guidance document focuses on how to set PAG levels, but it does not specify how to use age group factors in such decisions. Doses for hypothetical radiation emergency scenarios were projected for each of the six ICRP Publication 60 age groups for the early phase. The early phase includes dose from the airborne plume (usually the largest contributor to dose) and from material deposited on the ground. The most sensitive child whole body doses are typically no more than 15% higher than adult whole body doses.

The U.S. radiation emergency response community is not ignoring the fact that more current international recommendations are available in ICRP Publication 103 (^{ICRP 2007}). However, a complete set of DCs based on the ICRP Publication 103 recommendations have not yet been developed. Therefore, this analysis is geared toward implementing ICRP Publication 60 series recommendations first, and then another evaluation will be needed when all the DCs are available and the U.S. radiation protection community is ready to adopt ICRP Publication 103. A quote from ICRP Publication 109 (^{ICRP 2009}), though, paints a similar picture: “In accordance with the Commission’s advice on the representative person, it is important that the dose estimates reflect those likely to be received by the groups at greatest risk, but that they are not grossly pessimistic.”

NRC’S USE OF AGE GROUPS

NRC is in the process of updating their regulations. When those are revised, their Standards for Protection Against Radiation, 10 CFR 20 (^{USNRC 1991}), may use Reference Person (age and gender weighted) DCs for chronic exposures. This option was offered for public comment in an Advance Notice of Proposed Rulemaking published in July 2014 (^{USNRC 2014}). 10 CFR Part 50, Appendix I, Numerical Guides for Design Objectives and Limiting Conditions for Operation to Meet the Criterion "As Low as is Reasonably Achievable" for Radioactive Material in Light-Water-Cooled Nuclear Power Reactor Effluents (^{USNRC 2007}), has used four age groups since 1976 and may go to the six ICRP age groups. Using age groups makes sense for many radiation protection standards, particularly for assessing lifetime exposure.

During the March 2014 symposium, the NRC emergency preparedness staff described their primary emergency assessment tool, the Radiological Assessment System for Consequence AnaLysis (RASCAL) code (^{USNRC 2015}). RASCAL has been updated with adult and 1-y old thyroid DCs for the early phase to enable quick decision making on the use of KI. No further updates on age groups will be made in RASCAL until a final revised PAG Manual is published. States and localities have flexibility in how they implement federal guidelines within NRC’s and FEMA’s emergency preparedness regulatory requirements around nuclear power plants (^{USNRC 1980; FEMA 2015}). NRC and utilities provide recommendations for protective actions during an incident, and the state or county has the authority to order the protective action. Some localities with nuclear power plants evacuate children at Site Area Emergency based on plant conditions and not on projected exposure, a very conservative measure aimed at sensitive subpopulations, over and above federal guidelines.

DOE ENVIRONMENT, HEALTH, SAFETY, AND SECURITY DOSIMETRY APPROACHES

At regulated facilities, DOE’s Environment, Health, Safety, and Security programs currently use the ICRP Publication 60 dosimetry models (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}). DOE is demonstrating compliance using an age- and gender-weighted Reference Person. Inhalation doses for most age groups exposed to most radionuclides are within a few percent of each other, the exceptions being a newborn exposed to ³H, ³²P, ⁹⁰Y, and ¹³¹I. Ingestion values may differ by up to an order of magnitude.

DOE’s age and gender-weighted Reference Person DCs were developed to enable dose assessments in demonstrating protection of the public from normal releases that may expose the public working or living near a DOE site and ensure that compliance with annual public dose limits and constraints are protective of the population over their lifetime. The Reference Person DCs are discussed in DOE Standard 1196, Derived Concentration Technical Standard (^{USDOE 2011}). DOE staff noted at the Symposium that age specific DCs may have merit for some radionuclides in an emergency scenario that are one-time, short-term exposures (less than a year or so). Making age group-specific calculations during the intermediate phase (where ingestion becomes more of a factor) should be considered as well. Certainly when incident-specific information is available and dose assessors have enough resources, more precise projections should be made that identify and evaluate the most sensitive age groups. However, it may not be appropriate to compare such dose estimates to annual dose constraints that are developed to protect the public from chronic long-term exposures.

EPA FEDERAL GUIDANCE IMPLEMENTING NEW DOSIMETRY

EPA staff provided an update at the March 2014 symposium on Federal Guidance Report (FGR) revisions that are currently underway. FGRs provide the building blocks for most radiation protection programs, where calculated values of detriment can be translated to operational values to limit emissions, protect workers, and inform environmental protection standards.

Currently the ICRP Publication 60 dosimetry models (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}) are being implemented as a step in the right direction until the updated ICRP Publication 103 coefficients are available. Another analysis will be required when the ICRP Publication 103 DCs become available to assess how the new recommendations would impact these decisions. Note that ICRP has shifted to the residual dose concept in their ICRP Publication 103 recommendations, which may necessitate a shift from projected dose and avoidable dose used now in the PAGs.

FGR revisions will continue before and after the ICRP 103 factors are finished, so at this time, FGR 11 for workers (^{USEPA 1988}) will include factors for adults only, and the revision to FGR 12 for external exposure factors (^{USEPA 1993}) will include age and gender groups. That revision will include 29 organs, six age groups, gender averaging, and updated tissue weighting factors and decay data from ICRP. The revision of FGR 12 is presently undergoing the last stages of editing and quality assurance as of the 2015 NCRP Annual Meeting.

ORNL generating dosimetry coefficients

Staff at the Oak Ridge National Laboratories (ORNL) Center for Radiation Protection Knowledge continued the federal guidance presentation with more details at the March 2014 symposium. The ORNL staff noted that ICRP Committees 2 and 4 and the Main Commission are looking at use of effective dose. For effective DCs, the age-averaged values versus the adult values are very similar due to 65% of the population being adults. According to ORNL staff, per capita (also known as Reference Person) DCs work well for chronic exposures, but for emergency response even age-averaged coefficients may underestimate some risks.

PAG: EARLY PHASE EVACUATION

Shifting to focus on the PAG Manual that guides radiological emergency response in the United States, EPA staff at the symposium described the 1992 EPA PAG Manual where early phase dose calculations used Reference Man, an adult default that was the only option available. Interagency experts setting the PAGs did include consideration of higher risks for the fetus and those more difficult to evacuate (e.g., hospital patients and prison inmates), so conservatism was built in through the risk-benefit balancing process used to derive them. Additionally, assumptions made to generate default dose parameters and derived response levels ensure protectiveness for all members of the public, including sensitive subpopulations such as young children.

FRMAC METHODS TO IMPLEMENT THE PAGS

Sandia National Laboratories staff provided more detail at the March 2014 symposium on the Federal Radiological Monitoring and Assessment Center (FRMAC) methods that are used to implement the PAGs (^{USDOE 2010}). Early phase derived levels are based on the assumption that a person is outdoors 24 h a day for 4 d being exposed to the plume. Intermediate phase derived levels also conservatively do not account for shielding provided by being indoors part of each day of the projection year. People are assumed to remain in the contaminated area during the entire time (not going to work or school in an uncontaminated area, for instance). Another conservatism built in is that it is assumed radionuclides are in the chemical and physical form that yields the highest dose, and that the particle size is 1 μm mean aerodynamic diameter. These conservative assumptions allow dose assessors to project whole body doses to an adult male for simplicity, and then decision makers can make protective action decisions that apply to entire communities including children, adults, and the elderly. FRMAC is also developing the capability to account for partial occupancy and building protection to make more accurate dose projections to support improved public protection decisions during the intermediate phase.

PAG: KI THYROID BLOCKING AS AN EXAMPLE OF AGE SPECIFICITY

At the March 2014 symposium, FDA explained that their KI (potassium iodide) guidance (^{USFDA 2001}) has seven age groups, three thresholds and four dosages. While that is medically appropriate, FDA staff acknowledges that it may be difficult to implement in an emergency. A simplified approach is to provide KI to the whole community if doses of 0.05 Sv are projected to the 1-y old thyroid. That is a conservative way to cover those most sensitive and ensure that communicating the protective action to the affected community is as simple as possible. Not all communities use KI, however, and, since it has limiting implementation challenges, it is a supplemental protective action to evacuation.

PAGS: FOOD AND WATER

The FDA food PAG guidance (^{USFDA 1998}) has six age groups and is based on diets for 10 age groups. Diets are based on EPA intake factors from 1984 and USDA statistics from 1977 and 1978 (^{USEPA 1992}). FDA has no immediate plans to update their food PAG guidance, but in a real incident, several case-specific factors should be considered in place of default assumptions. Because of the globalization of the food supply, intake periods of <1 y should be examined as well as allowing for radionuclide decay. For ¹³¹I, allowing decay over 1 y results in 1% of the original calculated activity, so a shorter intake period should be considered. Using ICRP Publication 60 (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}) factors does not lead to large differences in derived intervention levels, but when FDA does update their food guidance, they will use updated dosimetry.

Currently, there is not a specific emergency protective action guidance for drinking water in the United States, but it is being explored with sensitive subpopulations in mind. As in Japan in 2011 after the releases from Fukushima, authorities would have to pay special attention to limiting dose to infants whose formula is mixed with tap water should water systems become contaminated from an incident. If a new PAG is proposed, it will likely have the ICRP Publication 60 age groups considered specifically (^{ICRP 1991, 1992, 1994a, 1994b, 1995, 1996}).

PAG: INTERMEDIATE PHASE RELOCATION

For relocation in the intermediate phase, some implementation concerns are similar to those for early phase evacuation: The protective action, relocation, is best taken by entire households or neighborhoods, not subgroups based on age. Children and families need to stay together, especially in a stressful situation that takes them out of their daily routine. Relocation may require families to be out of their homes for months to years and for schools and businesses to close until decontamination and remediation can be accomplished.

CONCLUSION

After weighing these considerations, most experts participating in the March 2014 symposium endorse projecting doses to the adult with built-in conservatism for evacuation and relocation for simplicity. For ingestion pathways involving food or water and KI, the majority was of the opinion that age-specific calculations should be made. Age group specificity is already in place for KI and food PAGs. There are many updates being made within the federal agencies, which may result in all agencies using the same set of DCs in the near future. Evaluating worst-case projections, or bounding cases, enabled the interagency authors of the PAG Manual revision to decide on a practical usage of age groups, ensuring protectiveness of all subpopulations while keeping implementation as simple as possible.

To convey these conclusions to emergency preparedness programs that implement the PAGs in local and state emergency response plans, explanatory text will be added to the final PAG Manual revision, which is expected in early 2016.