ML19250C134

Draft Residual Activity Limits for Decommissioning

ML19250C134
Person / Time
Issue date:	09/30/1979
From:	Conti E NRC OFFICE OF STANDARDS DEVELOPMENT
To:
References
NUREG-0613, NUREG-613, NUDOCS 7911080271
Download: ML19250C134 (9)

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h NUREG-0613 DRAFT Residual Activity Limits for Decommissioning Enrico F. Conti Office of Standards Development U.S. Nuclear Regulatory Comission September 1979 1274 187 (NOTE:

Any opinions or conclusions contained in this paper are those of the author and do not represent official NRC policy.)

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NUREG-0613 9

Introduction The Plan for the Reevaluation of NRC Policy on Decommissioning of Nuclear 2

Facilities, NUREG-0436, identified the need for specifying acceptable cri-teria for residual radioactivity following decommissioning for both suface and volumetric facility component contamination and for residual radioactivity in soil. The plan also identified the need for an NRC staff report on specific criteria for residual radioactivity levels following decomissioning.

This paper presents the approach and considerations of an NRC staff technical task group, with particular focus on reactor decommissioning. These considerations have not been completed and have not been reviewed by the NRC. A technical report on residual activity limits for decommissioning various facilities operated under NRC license as well as a Regulatory Guide on this subject are scheduled to be issued by calendar year 1980.

Background

Guidance on residual activity limits for surface contamination has been 1

available for reactors in Regulatory Guide 1.86 and for byproduct, source, or 1

special nuclear material licensees in an NRC Staff Technical Position.

Similar guidance for surface contamination has recently been issued in a draft ANSI standard 2 Analyses of exposures from recycle of metals with induced radioactivity and/or radioactive contamination were performed by F. R. O'Donnell3 Historically, the question of acceptable residual radioactivity levels in soil has arisen in cases which required some type of restoration to conditions

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approximating initial background. Specific cases inc1t h the Palomares acci-dent, Grand Junction mill tailings, and the Florida Phosphate tailings. The recent focus on survays of excess sites by ERDA (DOE) and NRC, as well as the need for criteria usaLle for making licensing decisions on facility decomis-sioning, necessitates the development of a practical method for specifying residual activity levels for purposes of regulation.

Residual contamination may be in or on structures, equipment, and soils.

The acceptable residual level of any fann of contamination is not a simply set, predetennined value similar to the allowable concentration values from the tables in 10 CFR Part 20.

It is a matter of assessing the radiological impact of the residual contamination and the cost and advantages of removing it.

Constructing and using models to determine this radiological impact can be difficult.

Models have been used for some time to simulate the release of radionuclides from operating facilities, the environmental transport of the radionuclides, and the exposure or ingestion by man. of these radionuclides, wnich lead to estimates of the radiological impact (presentud as a dose) to a hypothetical individual. This dose assessment methodology has been utilized in making regulatory decisions for some time in individual reactor licensing actions and was used for the generic analysis for the use of recycle plutonium in mixed oxide fuel (GESMO)". The methodology used by the NRC ' staff is des-cribed in guides developed to implement Appendix I of 10 CFR Part 50. These 1.109, 1.110, 1.111, 1.1128, and 1.1138 5

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are Regulatory Guides 1274 188 1

NUREG 4613 Studies conducted at Battelle (PNL)l0, 11, 12 on generic facility decomissioning have demonstrated the utility of using the predictive method-ology in a partial inverse manner, i.e., backcalculating the concentration and/or areal radiontclide levels equivalent to a unit radiation dose to a hypothetical individual. The exposure to a mixture of radionuclides with the involvement of different critical organs makes it very difficult to perform a summation of impacts equivalent to a whole body dose equivalent limit.

The lack of any authoritative definition of a "de minimis" dose (i.e., a dose-equivalent corresponding to a risk that is comparable to the risk from other activities that are generally accepted without special concern) also meant that a proposed acceptable level of residual radioactivity in soil was certain to be in jeopardy. The recommendation of the International Commission on Radiological Protection adopted on January 17, 1977 (ICRP 26)l3, provides the conceptual basis for constructing a methodology which can provide practical performance objectives for stipulating acceptable surface contamination and residual radioactivity levels in soil for purposes of regulation.

The conceptual approach in ICRP 26 makes it possible to sum impacts of groups of nuclides in terms of equivalent risk and to delineate surface contam-ination levels and soil cormntrations which are correlated with a specified risk to an individual from unrestricted use.

The decommissioning of a facility usually has as its goal the return of the site to the public for unrestricted use after cessation of operations.

Some of the site buildings could remain if they meet the regulatory standards for residual radioactivity and be used for other purposes, most likely of an industrial nature. The retrieved land is to be suitable for farming or any other unrestricted use.

There are many different types of nuclear fuel cycle facilities, each having different potential residual radionuclides and different exposure path-ways. This means that there should not be a single criterion based on residual activity since different radionuclides carry differing stochastic risks per unit of activity.

Each type of facility to be decommissioned may require separate consideration with a set of radioactivity levels specified both to protect the health and safety of the public and to permit unrestricted use of the facility and site. These criteria should be capable of implementation and amenable to verification by the Commission's inspection programs to ensure com-pliance. This paper focuses on the radionuclides and exposure conditions to be characteristic of Light Water Reactor facilities.

Approach to Problem The NRC technical staff effort on developing residual activity limits for decommissioing has been predicated on the assumption that the following objec-tives would be met:

residual activity limits should present a small risk from exposure, the limits should allow conducting an effective measurement pro-gram to demonstrate compliance, and they should be consistent with existing guidance.

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NUREG-0613 Existing Guidance and Recommendations _

The U.S. EPA has the statutory authority for establishing generally applicable environmental standards for radioactivity.

In the absence of such a standard for residual activity, the NRC staff has considered existing appli-cable guidance and recommendations in the literature as a benchmark of comparison for the residual activity limits under development.

Close coordina-tion with EPA will minimize the possibility that residual activity limits issued by the NRC would be in conflict with an EPA standard issued at a later date.

The standards and guidelines pertinent to the develogment of residual activity limits have been reviewed in reports by Dickson 1 and Schilling, et al. ".

In addition, it should be noted that the radiation & -e objective for the ambient waters of the Great Lakes Water Quality Agreeme.

is one mrem per year". The EPA standard for radioactivity in processed drinking water has been specified as 4 mrem per year". Adler and Weinbergie have recommended setting a de minimis level at one standard deviation of natural background (22 mrem per year).

Consideration of risk of exposure as a basis for setting residual activity limits suggests using an annualized risk to an individual of about one chance in a million of a health effect from the radiation exposure resultant from the planned activity. This level of risk was identified in the proposed EPA FRC guidance for Plutonium in Soil for accident situations" and was also identi-fled as ?.a acceptable risk in ICRP 26.

For purposes of this study, the NRC staff group has used the risk-dose conversion of 10-* per rem from the BEIR Committee."'

Methodology To place in perspective the residual radioactivity levels for the spectra of radionuclides associated with the operations of the various facilities, numerical estimates of radiation dose to man were developed. These estimates provide insight into a) what residual radioactivity levels would be related to a particular dose level, b) which of the various exposure pathways are signifi-cant, and c) the nuclides within the spectra of nuclides associated with the facility which are significant dose contributors. The important nuclides and pathways of exposure for reactor situations are identified below. As discussed previously, more detail on the methodology as well as application to other nuclear facilities will be included in the staff technical report to be issued at a later date.

Given the information on the particular radionuclides and their quantities, the computational model estimates:

a) the relative quantities of various spectra at various times following the decomissioning of the facility, and b) the quantities that could exist at the time the facility was relased for unrestricted use such that the dose at a future date does not exceed a particular numerical value.

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NUREG-0613 This modeling approach assumes that the residual radioactivity is associated with soil or contained on or within the structure of the facility.

The residual levels in these somewhat ' fixed' media serve as the model ' initial conditions' and the driving function for the various pathway models.

The mathematical modeling used drew heavily from past staff efforts, e.g.,

GESM0 and Regulatory Guide 1.109. However, considerable effort was devoted 5

toupdatingthemethodolog{8, in particular, adoption of the recently published ICRP-26 dosimetry methods.

The environmental exposure modes considered are irradiation from surface deposits and resuspended radionuclides, inhalation of resuspended radionu-clides, and the ingestion of food products contaminated by plant uptake of material in the soil and the foliar deposition of resuspended radionuclides.

For reactor sites, the predominant exposure pathway is direct irradiation from surface deposits.

Over the past decade, infonnation has become available to pennit a prudent quantification of the relationship between absorbed dose and the risk of bio-logical effects. This information pennitted the International Comission on Radiological Protection (ICRP)28 to consider two categories of radiation effects, namely, stochastic and non-stochastic effects. Stochastic effects are those disorders, e.g., cancer, for which the probability of an effect occurring, rather than its severity, is a function of the absorbed dose.

For stochastic effects, the ICRP recomended a dosimetric system that pennits the total stochastic risk incurred fro;a the irradiation of all body tissues to be calculated. This concept thus permits the suming of the absorbed doses to the various organs or tissues of the body. This methodology was used for the modeling done by the NRC technical staff.

The ICRP has not published, at the time of this writing, the report of -

Comittee 2 setting forth the detailed dosimetric and metabolic models and the results of calculations by its task group. However, the National Radiation 2

Protection Board has published a report employing the Comittee 2 methods in conjunction with the approved dosimetric and metabolic models of the ICRP Com-mittee. The dose conversica factors for ingestion and inhalation exposures used in this study were obtair.2d from this report. We note here, however, that:

a)

A quality factor of Q of 20 for alpha particles and recoil particles has been employed, b)

The modifying factor, n, has been taken as unity, c)

The dose is based on a fifty-year comitment period, d)

For inhalation calculations, particles of 1 pm AMAD-(Activity Median Aerodynamic Diameter) were assumed, e)

The lung model of the ICRP Task Group on s ung Dynamics was used and, 22 z

f)

The GI tract model of Eve was employed.

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NUREG-0613

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For external dosimetry, the work of Kocher24 was used.

In the case of external dosimetry, the body organs are exposed to much the same dose and thus the stochastic dose equivalent is equal to the total body dose.

fionitoring for Compliance The NRC has a contract with Oak Ridge National Laboratory (ORNL) to develop monitoring programs for decommissioned sites to insure compliance with residual radioactivity limits.

This monitoring program is being designed in a manner to assure that the survey of a site will have a high probability of detecting any areas of excess radioactivity or hot-spots.

Hot-spots which are detected will need to be removed or decontaminated to meet the residual activ-ity limits. Additional monitoring will be performed by an NRC inspector to verify compliance.

Reports on surveys of several excess DOE sites by ORNL haveindicatedtheneedforgostdecommissioningsurveystobebasedupon statistical considerations.2 In cases where the radionuclide spectrum is not sufficiently established onsite, it will be necessary to take samples of soil or other media from the site and perform laboratory analyses in order to determine the radionuclide composition of contaminants.

It is expected that the actual monitoring necessary in a termination survey will be conducted with portable instrumentation backed up by some lab-oratory analyses. Considerations of confidence and of cost and time will be involved in establishing the balance between surveying methods. The detection limits presented in Table I are derived from experience with the Department of Energy's Formerly Utilized Sites-Remedial Action Program.

The detection limits listed in this table are based on practical considerations.

Scanning speed has been considered as one of the limiting factors.2 For example, the sensi-tivity of an alpha survey meter may be an order of magnitude better if the meter could be held in a fixed position for a minute or more. This is imprac-tical in actual survey work since excessively long periods of time would be required to survey any large area or major piece of equipment.

For reactor situations, this emphasis on field surveys is expected to be appropriate.

Conclusion The staff analysis conducted to date indicates that residual activity levels which would be expected to result in exposures of 5 mrem per year to an individual from realistic exposure pathway conditions would both be consis-tent with existing guidance and result in activity levels which can be effectively monitored for enforcement.* The radionuclides which are of parti-cular importance to light water reactor decommissioning are Co-60, Cs-137, and Cs-134. The exposure pathways which are most significant for reactor sites are external irradiation from deposited radionuclides, with ingestion of contaminated foods and inhalation of resuspended activity of much smaller magnitude.

• An illustrative example of a typical radionuclide mix on a light water reactor site which is equivalent to 5 mrem / year is presented in Table II.

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NUREG-0613 Table I Detection Limits for Direct Surveys with Portable Instruments Radiation Detected Monitoring Technique Detection Limit a

Gamma NaI(T1) scintillator 2.5 - 5 pR/hr with count-rate meter Beta Thin window GM probe 2000 - 20,000 w/ count-rate meter cpm /100 cm b 2

2 Alpha ZnS scintillator with 200 dpm/100 cm count-rate meter aDependent upon background radiation levels which typically range from 5 to 10 pR/hr. This detection limit represents a F3% increase over the background.

For fallout radionuclides, an exgosure rate of 5 pR/hr corresponds to 0.1 to 20 pCi/m2 (e.g., 0.1 pCi/m2 for oCo and 0.5 pCi/m2 for 137Cs).

For a wide range of soil concentrations (e.g., 5 pR/hr corresponds to approximate 2.5 pCi/g of 22sRa in the soil).

bHighly dependent on beta energy; low energy betas (<l50 kev) may require special monitoring techniques.

The stated sensitivity may not be achieved where background exceeds 100 cpm.

Table II Residual Activity Levels Equivalent to 5 mrem / year for a Light Water Reactor Site Mn-54 5,000 pCi/m2 Co-60 310,000 pCi/m2 4.0 mrem Sr-90 70,000 pCi/m2 Cs-134 15,000 pCi/m2 0.2 mrem 2

Cs-137 370,000 pCi/m 0.6 mrem 5.0 mrem 1274 193 6

NUREG-0613 REFERENCES 1.

U.S. NRC, Plan for Reevaluation of NRC Policy On Decomissioning of Nuclear Facilities, NUREG-0436, Office of Standards Development, December 1978.

2.

Draft American National Standard N13.12, Control of Radioactive Surface Contamination on Materials, Equipment, anu Facilities to be Released for_

Uncontrolled Use, American National Standards Institute, August 1978.

3.

O'Donnell, F.R., et al., Potential Radiation Dose to Man from Recycle of Metals Reclaimed from a Decomissioned Nuclear Power Plant, NUREG/CR-0134, ORNL/NUREG/TM-215, December 1978.

4.

U.S. NRC, Final Generic Environmental S~atement on the Use of Recycled Plutonium in Mixed 0xide Fuel in Light Water Cooled Reactors, NUREG-0002,

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Volume 3, August 1976.

5.

U.S. NRC, Calculation of Annual Doses to Man from Routine Releases of Reactor Effi ents for the Purpose of Evaluating Compliance with 10 CFR Part 50, Appendix I, Regulatory Guide 1.109, October 1917.

6.

U.S. NRC, Cost-Benefit Analysis for Radwaste Systems for Light-Water-Cooled Nuclear Power Reactors, Regulatory Guide 1.110, March 1976.

7.

U.S. NRC, Methods for Estimating Atmospheric Transport and Dispersion of Gaseous Effluent in Routine Releases from Light-Water-Cooled Reactors, Regulatory Guide 1.111, July 1977.

8.

U.S. NRC, Calculation of Releases of Radioactive Materials in Gaseous and Liquid Effluents from Light-Water-Cooled Power Reactors, Regulatory Guide 1.112, April 1976.

9.

U.S. NRC, Estimating Aquatic Dispersion of Effluents from Accidental and '

Routine Reactor RC.ea:es for the Purpose of Implementing Appendix 1, Regulatory Guide 1.113, April 1977.

10.

K.J. Schr. eider and C.E. Jenkins, Technology, Safety, and Costs of Dacom-missioning a Reference Nucleur Fuel Reprocessing Plant, NUREG 0278, Vol. I, October 1977.

11. Smith, R.I., Konzek, G.J. and Kennedy, W. E., Jr., Technology, Safety and.

Costs of Decomissioning a Reference Pressurized Water Reactor Power Station, NUREG/CR-0130, Pacific Northwest Laboratory for U.S. Nuclear Regulatory Comission, June 1978.

12. Jenkins, C.E., Murphy, E.S., and Schneider, K.J., Technology, Safety and Costs of Decomissioning a Reference Smal_1 Mixed 0xide Fuel Fabrication Plant, NUREG/CE-0219 Pacific Northwest Laboratory for U.S. Nuclear Regulatory Comission, June 1978.

13. Recommendations of the International Comission on Radiological Protection, ICRP Publication 26, Annals of the ICRP, Vol.1, No. 3,1977.

7 1274 194

NUREG-0613 14.

H.W. Dickson, Standards and Guidelines Pertinent to the Development of Decommissioning Criteria for Sites Contaminated with Radioactive Material, ORNL/0 EPA-4, August 1978.

15 Schilling, A.H., Lippek, H.E., Tedger, P.D., Easterling, J.D.,

Decommissioning Commercial Nuclear Facilities: A Review and Analysis of Current Regulations, NUREG/CR-0671, August 1979.

16.

" Refined Radioactivity Objective for the Great Lakes Water Quality Agreement", Federal Register, 42 (65),18171, April 5,1977.

17. EPA Drinking Water Regulations for Radionuclides, Federal Register, 41 (133), 28402, July 9,1976.

18. Adler, Howard I., and Weinberg, Alvin M., "An Approach to Setting Radiation Standards", Health Physics _, Vol. 34, pp. 719-720, Pergamon Press, Ltd. Great Britain, June 1978.

19. Proposed Guidance on Dose Limits for Persons Exposed to Transuranium Elements in the General Environment, U.S. Environmental Protection Agency, September 1977.

20. National Academy of Sciences / National Research Council, The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, Report of the Advisory Committee on the Biological Effects of Ionizing Radiations, NAS/NRC, Washington, D.C., 1972.

21. Adamr., N.,

et al., ' Annual L iits of Intake of Radionuclides for Workers',

National RadToI gical Protection Board, NRP6-R82, October (1978).

22.

International Commitsion on Radiological Protection, Task Group on Lung Dynamics, ' Deposition and Retention Models for Internal Dosimetry of the Human Respiratory Tract', Health Physics, 12,173,(1966).

23.

Eve, I.S., ' A Review of the Physiology of the Gastro-Intestinal Tract in Relation to Radiation Doses from Radioactive Materials', Health Physics, 12, 131 (1966).

24.

D.C. Kocher, Nuclear Decay Data for Radlonuclides Occurring in Routine Release from Nuclear Fuel Cycle Facilities, ORNL/NUREG/TM-102, August 1977.

25. Leggett, R.W., Dickson, H.W., Haywood, F.F., "A Statistical Methodology for Radiological Surveying," Symposium on Advances in Radiation Protection Monitoring, Stockholm, Sweder, June 26 - 30, 1978.

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