Ali A. Simpkins
Westinghouse Savannah River Company
Aiken, SC 29808

Emergency response computer models at the Savannah River Site (SRS) are coupled with real-time meteorological data to estimate dose to individuals downwind of accidental radioactive releases. Currently, these models estimate doses for inhalation and shine pathways, but do not consider dose due to ingestion of contaminated food products. The Food and Drug Administration (FDA) has developed derived intervention levels (DIL) which refer to the radionuclide-specific concentration in food present throughout the relevant period of time, with no intervention, that could lead to an individual receiving a radiation dose equal to the protective action guide. In the event of an emergency, concentrations in various food types are compared with these levels to make interdictions decisions. Prior to monitoring results being available, concentrations in the environmental media (i.e. soil), called derived response levels (DRLs), can be estimated from the DILs and directly compared with computer output to provide preliminary guidance as to whether intervention is necessary. Site-specific derived response levels (DRLs) are developed for ingestion pathways pertinent to SRS: milk, meat, fish, grain, produce, and beverage. This provides decision-makers with an additional tool for use immediately following an accident prior to the acquisition of food monitoring data.

At the Savannah River Site (SRS), emergency response computer models are used to estimate dose following releases of radioactive materials to the environment. The current model used for atmospheric releases is PUFF-PLUME (Garrett and Murphy 1981), which uses real-time data to track either instantaneous (puff) or continuous (plume) releases. PUFF-PLUME calculates downwind air and ground concentrations and their associated doses from inhalation and ground shine pathways.

In 1982 the FDA (USEPA 1991) issued recommended dose levels to protect the public called Protective Action Guides (PAGs). PAGs were defined as "projected dose commitment values to individuals in the general population that warrant protective action following a release of radioactive material." This guidance is applicable to accidents where radiation dose could be received as a result of consumption of contaminated food (USFDA 1998). The recommended PAGs are 5 mSv committed effective dose equivalent, or 50 mSv committed dose equivalent to individual tissues and organs, whichever is more limiting.

Using the PAGs discussed above, a concentration in food can be back calculated using defined assumptions. This concentration, referred to as the derived intervention level (DIL), "corresponds to the concentration in food present throughout the relevant period of time that, in the absence of an intervention, could lead to an individual receiving a radiation dose equal to the PAG" (USFDA 1998).

The basic formula for the DIL is

where

Within USFDA (1998), DILs are reported for the following radionuclide groups: ⁹⁰Sr, ¹³¹I, ¹³⁴Cs+¹³⁷Cs, ²³⁸Pu+²³⁹Pu+²⁴¹Am, and ¹⁰³Ru+¹⁰⁶Ru. Also contained within the Appendices are DILs for several other isotopes. Using the methodologies contained within USFDA (1998) DILs were estimated for all radionuclides cited in the document. DILs for additional radionuclides contained within PUFF-PLUME could not be estimated because of the lack of age-specific dose conversion factors. DILs were calculated on an individual basis and if more than one radionuclide is released, the appropriate fraction of DILs could be summed to determine if intervention is necessary.

Using the DILs, derived response levels (DRLs) were estimated to represent the deposition concentration upon a given environmental media (i.e. soil)—or for the special case of tritium, the air concentration—that would lead to exceeding the DIL (USEPA 1991).

The equations for the DRLs vary based on type of food. The primary food classes referred to by the USFDA (1998) are produce, grain, dairy, eggs, meat, fish, and beverage (including water). At SRS the food classes that are expected to have the major impact on dose are milk, meat, fish, grain, produce, and beverage. Referring to Hamby (1991), eggs and poultry ingestion are excluded because chickens within the vicinity of SRS are typically housed in covered shelters and eat feed that is not produced locally. Pork is also excluded since hogs typically do not forage and are also fed commercial feeds. Contamination of drinking water is not expected to be a major pathway, due to mixing in the streams and rivers and deposition of particulates into the sediment, however, the pathway is included as a precaution.

The methodology for derivation of DRLs for tritium is different than for all other radionuclides. All calculations of DRL take no credit for hold-up time or reduction in concentration due to preparation of food. Measures such as these could be implemented to reduce dose.

When radionuclides are deposited on the ground, produce is affected in two ways. One is by direct uptake through the leaves of the plant and the other is via root uptake of the contamination within the soil. The DRL for produce contaminated externally through the leaves is calculated using the following equation for all radionuclides other than tritium:

(2)

where

The DRL for internal contamination of vegetables and grain is calculated using the following equation:

(3)

where

Tritium oxide (HTO) is assumed to exchange readily with the moisture within the plant. The DRL for tritium, therefore, refers to the tritium in the atmosphere and not the amount deposited on the ground. The DRL for HTO in vegetation is estimated using the following equation.

(4)

Milk and beef can become contaminated when cows ingest grass that has been contaminated with radionuclides. The equation for beef and milk is similar:

(5)

where

For this calculation, the density of milk is assumed to be the same as water (1g cm^-3).

For tritium, the DRL for milk and beef is estimated using the following equation:

(6)

Where all terms have been previously defined.

The DRL for ingestion of contaminated fish is estimated using the following equation:

(7)

where

	density of the water (1000 kg m-3)
d	depth of the water for farm ponds (1 meter assumed)
Bf	concentration factor for fish- see Table 2

This methodology is conservative in that it assumes that whatever deposits on the surface of the water is uniformly mixed throughout the water. This does not take into account deposition onto the sediments.

Contamination of fish from tritium following an atmospheric release is not expected to be an important pathway since tritium does not readily deposit on the surface of water unless it rains. Even if a rain event occurs during an accident the water would be highly diluted with uncontaminated water resulting in a low concentration. Also, the bioaccumulation factor for tritium in fish is low. Considering this, methods for estimating DRLs associated with ingestion of fish contaminated with tritium are not presented.

The DRL for the contamination of water is calculated using the following equation:

(8)

All terms have been defined above. This methodology is conservative for the same reason as stated above under the section on fish ingestion. Contamination of water from tritium is not expected to be a major contributor to dose for reasons discussed above.

Table 1 shows the DILs and DRLs for selected radionuclides. Some differences are seen when comparing the DILs published in USFDA (1998) and those presented here, which is primarily due to rounding. Calculations contained within the USFDA (1998) document contain rounding at several intermediate steps and the calculations performed here do not.

When multiple radionuclides are involved, summing the ratios of the environmental concentrations of each nuclide to its respective DRL is appropriate to verify that the sum is equal to or less than unity (USEPA 1991).

These DRLs were developed using conservative methodologies. In the event of an actual emergency, factors such as wind direction, population, and agricultural land type should be taken into account as to whether each of pathways is likely. This is especially true for the drinking water pathway since the origin of drinking water is only in one distinct location, which may not be the prevailing wind direction at the time of the accident. These DRL values can be used as a preliminary tool prior to the acquisition of field monitoring data.

Table 1. Derived Response Levels

	DIL Bq kg-1	Produce* External Bq m-2	Produce Internal Bq m-2	Beef Bq m-2	Milk Bq m-2	Fish Bq m-2	Beverage Bq m-2
3H	2.2x105	not calc	6.6x103	2.0x104†	2.3x104†	not calc	not calc
89Sr	1.4x103	1.4x104	2.0x107	8.0x105	5.6x105	4.8x104	1.4x106
90Sr	1.6x102	1.6x103	2.2x106	8.8x104	6.1x104	5.3x103	1.6x105
95Nb	1.2x104	1.2x105	3.1x108	1.5x104	1.5x106	4.1x102	1.2x107
129I	5.6x101	1.1x102	6.7x105	1.3x103	5.7x102	3.7x103	5.6x104
131I	1.7x102	3.3x102	2.0x106	3.8x103	1.7x103	1.1x104	1.7x105
133I	7.0x103	1.4x104	8.4x107	1.6x105	7.2x104	4.7x105	7.0x106
134Cs	9.3x102	9.3x103	2.2x107	7.7x104	2.4x104	4.6x102	9.3x105
137Cs	1.4x103	1.4x104	3.3x107	1.1x105	3.5x104	6.8x102	1.4x106
103Ru	7.4x103	7.4x104	3.6x107	6.2x103	2.3x109	7.4x105	7.4x106
106Ru	4.4x102	4.4x103	2.1x106	3.7x102	1.4x108	4.4x104	4.4x105
144Ce	4.8x102	4.8x103	4.6x107	1.3x105	1.5x106	4.8x105	4.8x105
237Np	4.1x100	4.1x101	3.9x105	1.1x103	2.5x105	4.1x102	4.1x103
239Np	3.2x104	3.2x105	3.1x109	5.3x107	2.0x109	3.2x106	3.2x107
238Pu	2.5x100	2.5x101	2.4x106	5.9x104	3.9x105	7.1x102	2.5x103
239Pu	2.2x100	2.2x101	2.1x106	5.3x104	3.4x105	6.3x102	2.2x103
241Pu	1.2x102	1.2x103	1.2x108	2.9x106	1.9x107	3.5x104	1.2x105
241Am	2.0x100	2.0x101	1.9x106	2.7x103	1.2x105	8.0x101	2.0x103
244Cm	1.6x100	1.6x101	1.5x105	2.1x102	9.9x104	6.4x101	1.6x103
Cs Group	1.1x103	1.1x104	2.8x107	9.6x104	3.0x104	5.7x102	1.1x106
Pu+Am Group	2.2x100	2.2x101	2.1x106	5.3x104	3.4x105	6.3x102	2.2x103

* Same used for grain
† Units for H-3 Bq m^-3

Methods have been developed for estimating derived response levels at the Savannah River Site following an accidental release of radioactive materials to the atmosphere. Methodologies such as these are gross estimates at best and should be treated as such. These methods provide decision-makers with an additional tool to use in the event of a radiological emergency.

1. Garrett AJ, Murphy Jr. CE. A puff-plume atmospheric deposition models for use at SRP in emergency response situations. Aiken, SC; Dupont Report; DP-1595; 1981.
2. Hamby DM. Average absolute humidity at the Savannah River Site, Aiken, SC: Westinghouse Savannah River Company Inter-Office Memorandum: SRL-ETS-900141, March 22, 1990.
3. Hamby DM. Land and water use characteristics in the vicinity of the Savannah River Site (U). Aiken, SC; Westinghouse Savannah River Company Report; WSRC-RP-91-17; 1991.
4. Hamby DM and Bauer LR. The vegetation-to-air concentration ratio in a specific activity atmospheric tritium model. Health Physics 66:339-342; 1992.
5. U.S. Environmental Protection Agency. Manual of protective action guides and protective actions for nuclear incidents. Washington, DC: U.S. Government Printing Office; EPA 400-R-92-001; 1991.
6. U.S. Food and Drug Administration. Accidental radioactive contamination of human food and animal feeds: recommendations for state and local agencies. Rockville, MD; U.S. Department of Health and Human Services; August 13, 1998.
7. U.S. Nuclear Regulatory Commission. Calculation of annual doses to man from routine releases of reactor effluents for the purpose of evaluating compliance with 10 CFR 20, Appendix I. Washington, DC: U.S. Government Printing Office; Regulatory Guide 1.109 (Rev 1.);1977.