ML19343B560
| ML19343B560 | |
| Person / Time | |
|---|---|
| Issue date: | 12/22/1980 |
| From: | Slobodien M NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| To: | |
| References | |
| NUDOCS 8012240231 | |
| Download: ML19343B560 (22) | |
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THE U. S. NUCLEAR REGULATORY CC." MISSION'S NATIONWIDE PROGRAM FOR DIRECT RADIATION MONITORING AT LICENSED FACILITIES by Michael J. Slobodien Radiation Dosimetry Specialist United States Nuclear Regulatory Commission Office of Inspection and Enforcement Region I 631 Park Avenue King of Prussia, Pennsylvania 19406 Telephone (215) 337-5336 r9 o
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Abstract The United States Nuclear Regulatory Commission established a direct radiation monitoring program for all nuclear power reactors in August 1979.
The monitoring program, using thermoluminescent dos 1 meters (TLDs), was established as a result of the lessons learned from the Three Mile Island Nuclear Power Station accident in March 1979.
Major features of the program are automatic, computer based TLD system and a unified system of calibration and data reporting for the entire nation.
The program involves the coordination of federal and state programs.
As of July 1980, TLD monitoring programs have been established around all NRC-licensed nuclear power generating stations.
Major effon s are underway to fully characterize the response of the NRC TLD environmental monitor and to automate the data handling program.
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Introduction f
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During the famous accident at Three Mile Island (TMI) in the spring of
-1979 great pressures were felt by the NRC's Office of Inspection and Enforcement (IE) for an ongoing and rapid assessment of integrated doses resulting from releases of radioactive material into the environment.
Not only was current dose information required but past historical data were needed as well.
l The NRC staff was forced to rely on data supplied by the utility and its contractors.
Furthermore, the NRC found it necessary to contract for environ-mental radiation monitoring services from a vendor who was also used by the utility to provide quality assurance measurements for its environmental thermo-luminescent dosimetry (TLD) program.
Even though the utility's primary _ environ-l mental TLD program contractor was another vendor and both vendors utilized different types of TLDs and different from the NRC's contracted TLD service, the question of potential conflict of interest arose.
Within days of the initiation of the accident, a number of agencies also installed TLDs around i
i the TMI site, seemingly providing adequate, if not overlapping monitoring coverage.
However, none of the sources of data was completely sufficient for NRC needs.
In some cases data was not available in a timely fashion; in some the dosimeter calibration and quality control were inconsistent with existing l
standards (1, 2); -in some instances the dosimeters were insensitive to the I
beta radiation and low energy photons from krypton and xenon radioisotopes--
the principal radionuclides released to the environment, 4
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2 One of the lessons learned frcm the TMI accident (3, 4, 5) was that Congress, federal and state agencies, and the public expected and wanted an analytical capability on the part of NRC for such things as environmental radiation monitoring in the event of accidents and emergency situations.
The IE staff recognized that any analytical capability which is reserved for emergency situations is subject to misuse and failure because personnel are not familiar with its intricacies and because in most instances it would not be installed and available at the onset of an accident.
This would, therefore, severely restrict the value of such a system in evaluating the overall population exposures or potentiai exposures fo' lowing accidental releases.
Especially in the area of environmental monitoring data is required for long periods to establish baselines against which to compare current measurements.
Thus, IE established en ongoing TLD direct radiation monitoring program in August 1979.
Objectives The TLD Direct Radiation Monitoring Network has the following objectives:
l The establishment of pre-operational, historical baseline radiation dose l
levels whenever possible for each monitored facility.
i To provide ongoing radiation dosimetry data during routine operations.
1 To provide post-accident radiation dosimetry to aid in assessment of population exposures and subsequent government action.
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To allow for independent verification of the adequacy of NRC licensees' environmental radiation monitoring programs.
To provide uniform treatment of dosimeters with respect to handling, shipping, calibration, reading, and data processing for all monitored facilities in the United States'.
To report environmental radiation monitoring data annually in a consistent fashion to the Congress, federal and state agencies, monitored facilities, and the public by means of a written document.
The establishment of pre-operational baseline data for those facilities already in operation is obviously not possible.
However it may be possible to obtain what is essentially pre-op data during periods of extended outages.
As of July 1, 1950 TLDs have been'placed around all NRC licensed operating nuclear power reactors in the USA and several reactors which were authorized for low power testing.
Since July 1980 the NRC efforts have been directed at TLD environmental monitor placement around pre-operational power reactors.
For such sites it will be possible to obtain a minimum of three months pre-operational data and for many sites two or more years of such data will be obtained.
Ongoing radiation dosimetry data will be obtained by the collection and analysis of TL0s from the field on a quarterly frequency.
The period of a quarter for field residence was selected for reasons of dosimeter character-I istics and simple economics.
Durlng a one-month period it is expected 'that 'a
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4 typical TLD will be exposed to 5-10 cR from all sources.
This is not significantly different from the dose which may accrue during transportation to the field, interim field storag; prior to placecent, and return transportation to the NRC laboratory.
A three-month period overcomes that difficulty without creating the need for large fade correction factors for TLD fading wnich would be required if six-month or longer field periods were employed. From an economic standpoint the quarterly exchange frequency met the budgetary requirement for the program set by IE.
In the event that an accidental release of radioactive material does occur which would be of a magnitude detectable by the TLD system used by NRC, immediate retrieval from the field will be possible.
Furthermore, with a portable TLD reader NRC will be able to conduct field measurements of both environmental and personnel doses.
These will provide input to assit in rapid assessment of accrued population exposures by NRC and other federal and state agencies.
A major and growing component of the IE NRC inspection program is the ability to independently verify the analytical capabilities of licensees.
For several years the regional offices have operated mobile gamma spectroscopy l
i laboratories which intercompare data with the National Bureau of Standards (NBS) and licensees.
The IE Division of Safeguards has conducted a program of special nuclear material acccuntability by making measurements of unknown and i
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standard samples with fuel facility licensees.
In the same way the NRC TLD data will be compared at colocated monitoring stations with the environmental radiation monitoring data obtained by the licensee operated programs which are required by the license technical specifications.
Many systems for environmental radiation monitoring are used by NRC licensees. These include a variety of TLDs, film, pocket ionization chambers, and high pressure ionization chambers.
To compare data obtained from these is very difficult since they represent many types of radiation type and energy response characteristics.
The NRC program uses a common dosimeter, reader, and calibration for all locations thereby reducing.the variables which must be taken into account when making comparisons.
Lastly, an environmental monitoring program is worthwhile only if the data generated is available to interested parties.
Quarterly reports will be made to participating states and to the utilities being monitored.
An annual summary will be made available to all interested individuals probably as a NUREG document.
l Imolementation The general guidelines for the NRC TLD program have been made very flexible so as to meet the site specific characteristics of the-many licensees.
The basic requirements are that the monitoring is an offsite program consisting of-two concentric rings of monitoring stations at distances of approximately:2 l
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6 and 5 miles from the site airborne effluent release point, respectively.
The area around each site is divided into 16 sectors each of 22.5 degrees arc with one sector centered on true North (0 ).
Table 1 specifies these sectors.
In each sector a TLD station is placed at approximately 2 and 5 miles radial distance.
Internal guidance calls for TLD placement at 2 + 2 and 5 + 3 miles from the site release point (stack).
Care is taken to avoid topographical features such as ravines or back sides of steep hills which might shield the TLDs.
TLD stations must be accessible by automobile in all seasons of the year.
Monitoring stations are not placed over open water. For a site surrounded by land a total of 32 monitoring stations is possible in the two concentric rings.
Among these 32 stations a minimum of 5 are located adjacent to the monitoring stations of the NRC licensee.
These colocated stations are for the pu pose of verification of the licensee's environmental monitoring program.
Three TLD stations are placed in locations 15 miles or more in the most prevalent upwind direction from the facility airborne effluent release point.
These serve to establish the typical area radiation levels with minimal impact from plant operations.
Up to 14 TLD statians are available at the discretion of the environmental monitoring teams for placement at areas of high public interest and population centers (6).
One TLD station is designated as a control.
While the TLD badges are in the field, the control badge resides in a low background lead cask.
The cask has three inch walls designed to reduce background radiation typical of the Colorado Plateau by 99%.
In the planning stages personnel from the NRC regional offices use topo-graphical maps fiom the U.S. Geological Survey (USGS) in conjunction with local maps, meteorological data, plant siting plans, and plant environmental
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Table 1 Standard Sectors for Location of Environmental Monitoring Stations Each site is considered to be the center of mass of a circle.
The area of each circle is divided into sixteen standard windrose sectors each of 22.5*
arc.
The following is the sector identification table:
Sector Name Azimuth
- N 348.75 -11.25 NNE 11.25 -33.75 NE 33.75 -56.25 ENE 56.25 -78.75 E
78.75 -101.25 ESE 101.25 -123.75 SE 123.75 -146.25 SSE 145.25 -168.75 568.75-191.25 S
SSW 191.25 -213.75 SW 213.75 -236.25 WSW 236.25 -258.75 I
W 258.75 -281.25 WNW 281.25 -303.75 NW 303.75 -326.25 l
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- North - O and 360, is defined according to the map system.
Usually North will be either true north or grid north (TN or GN).
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8 monitoring plans to select approximate field locations for NRC TLD stations.
These selections are reviewed with officials from the states in the vicinity of the plants for concurrence and suggestions.
In most cases the NRC establishes a contract with the state in which the monitored facility is located for quarterly field placement.
TL Dosimeter and Field Station The TLD selected for the NRC program is the Panasonic Model UD801A.
The TLD badge, showq in Figure 1, incorporatas two lithium borate (Li B 0 :Cu) 2 47 and two calcium sulfate (CaSO :Tm) elements in one unit.
The TL phosphors are 4
powdered material bonded to a polyimide substrate and covered by a clear fluorocarbon bubble.
The lithium borate elements have windows of 14 mg/cm 2
and 300 mg/cm covering them respectively, while the two calcium sulfate 2
elements each have 700 mg/cm plastic and lead filters.
The badge includes two rows of holes which are scanned by an optical sensor in the TLD reader unit.
Coding of these holes allows the computer to associate field location and TL calibration data with each badge.
Field tests have demonstrated that the TLD badge is sensitive to moisture.
Small droplets have been observed under the fluorocarbon window.
To avoid this the entire badge is sealed in a protective bag of polyolefin / polyester.
2 The bag adds an additional 5.6 mg/cm filtration.
The bag also holds a color coded 3x5 inch stamped, addressed mailer.
On one side of the mailer there is a message to a finder giving a description of the device, a telephone informatio.7 number, and instructions for handling. These items are shown in Figure 2.
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9 In the field the plastic bag containing the TLD a.7d mailing label is pieced in a plastic mesh container.
The container serves to identify the monitor and protect the bag in very high winds and temperatures below 3 C at which it tends to be brittle.
The mesh container is shown in Figure 3.
The TLD badges are prepared for field use, deployed from, and analyzed in a central laboratory in the NRC's Region I Office in King of Prussia, Pennsylvania.
Field placement is performed on a calendar quarter frequency by state radiological and environmental protection departments or private firms under contract to NRC.
Dats Processing Hardware The NRC's data handling system for environmental radiation monitoring includes the following components:
A Panasonic Model 710A automatic TLD reader with 500 badge automatic sample changer, Texas Instruments 733 automatic send and receive recording terminal, Hewlett Packard 9845T desk top computer with 318 kbyte user core memory and 20 Mbyte hard disc mass storage device.
A block diagram of this system is shown in Figure 4.
The 710A TLD reader will process badges in groups of 50.
In the NRC system TLD badges are read twice.
The first reading establishes the gross thermoluminescence.while the second identifies any residual on the lithium borate elements.
There is generally no residual glow peak on the calcium sulfate phosphors.
Up to 500 badges can be in the reading cycle at any time.
The total time to read the 500 badges twice (4000 readings) is approximately 6.7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.
10 Data from the 710A reader is sent via RS-232 bus and switch to the Texas Instruments terminal where a hard copy is nade and data is stored on magnetic tape cassette.
Folicwing the completion of reading and eciting if necessary, the raw data is transferred frcm tape cri the terminal to the 9845T computer.
The large 20 Mbyte mass storage allows for carrying a unique calibration factor for each of the four TL elements on each badge.
The correct calibration factor is selected and applied to the raw data.
The use of individual calibration factors eliminates the need for a batch calibration value for the TLD badges.
This is one method to reduce the source of a potential + 12-15% uncertainty in the calculated dose.
Of course in order to perform this operation requires a very large mass storage device.
At the present time cver 24,000 active files store badge element calibration data.
The use of individual calibration factors also allows for rapid detection of a single damaged TL phosphor in a badge.
The 9845T calculates the doses for each location around each reactor site and prepares them in a tabular, report-ready form.
In addition, such studies j
as distribution of doses according to a normal distribution or the preparation of histograms are possible.
The unique code on each TLD badge enables the program to associate doses with a specific site around a specific reactor for a given time period.
The program prepares site dose histories upon request and can perform trend analysis by analysis of variance techniques.
Individual doses can be plotted on site maps stored in the mass storage memory or isodose l
curves can be prepared and superimposed on site maps.
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Quality Control
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Quality cos. trol is achieved by engineered features in the TLD reader and i
by independent testing of the entire dosimetry system.
The TLD reader is programmed to read critical parameters with each badge reading and compare f
these parameters with preselected range.
If any critical parameter is outside 1
its assi ned range, the reading is terminated with any accumulated data held C
j in non-volatile memory.
Two of the major engineered quality control features i
are the reading of the carbon-14 activated light source and the heating system i
power measurement. Reading the light source tests the entire optical system 4
I including the optical filters, photomultiplier tube assembly, and integrating l
picoammeters.
The system which, heats the TLD is a focused beam of infrared I
radiation from a standard tungsten filament projector lamp.
By measuring the voltage between the filament and ground and the current through the system, i
I lamp power is measured.
This is directly related to the heating of the TL element.
The TL phosphor temperature is not measured directly, i,
r In addition to the engineered quality control features identified, it is possible to connect a high speed recorder to the 710A TLD reader and obtain glow curves from each reading.
However to use this as a routine quality control feature would require either a manual or automatic'a method for curve shape analysis.
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A 12 There are features on the data' transmission components of the TLD reader which are related to quality control including the testing of the status of any external devices connected via the RS-232 bus.
The TLD reader sends a signal to confirm the " receive readiness" of the external terminal and following data transmission waits for a " data received" signal prior to starting the next reating cycle.
Failure to obtain the required " hand shake" signal terminates further TLD reading and sounds an alarm.
Complimenting the engineered quality control features is an independent i
testing program.
The NRC has contracted with the National Bureau of Standards (N35) to irradiate TLD bsdges to known doses with different types of radiation.
1 These are evaluated in blind tests of the TLD system.
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l As a routine part of the field measurement program a group of TLDs is irradiated with a cesium-137 source at the same time that field badges are prepared for deployment.
The irradiated badges and non-irradiated controls are stored in a heavy lead shield while indicator badges are in the field.
Upon reading of field badges once each quarter, the control and irradiated badges are read with each group.
This technique tests the TLD reader completely and provides information to correct for fading.
It is important to note that all badge irradistions are monitored using a seconaary standard ionization chamber which is traceable directly to the National Bureau of Standards.
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j To assess effect.s of fading under controlled and field conditions TLD 1
badges are irradiated at the end of the field period.
The recorded doses are r
compared to tnose of badges irradiated at the beginning of the field period.
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TLD badges irradiated at the beginnina of the field period are placed in the i
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field along with the indicator badges.
These are analyzed at the end of the i
field period along with the control badges and field indicator badges.
The total dose on the irradiated field bades is approximately ten times the anticipated i
dose which will accrue from transit and field residence.
This exposureallows for assessment of fading under actual field conditions without the confounding effects of field exposure.
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L System Calibration The response of the TLD badge and its packaging to various types of beta, gamma, and mixed field radiation is presently being studied for NRC by NBS.
f Once this study is complete a routine badge calibration will be performed by i
exposing badges to a cesium-137 source under reproducible geometry.
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The irradiated dosimeters are read on the TLD reader after a 24-hour.
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delay to allow for complete fading of all energy traps with short half times.
(This is primarily a characteristic of. lithium borate.) The ratio of delivered to reported dose is calculated for each individual element of each TLD badge.
These values are entered into historical files maintained in computer mass l
storage. A linear regression analysis of TLD correction factors for each element to confirm that the response has not significantly changed with time.
Major response changes result in removal of the badge from service.
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1 Transit Doses j
The TLD badges are sent to field locations via U.S. Mail.
They usually i
travel by air to those plants located further than 250 miles from the NRC T;h l
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In addition to the control badge wnich experiences all in transit det.es and cosmic radiation while badges are in the field, four special badges are used to assess the magnitude and lag of the journey in which an in-transit i
exposure occurred.
On the cutgoing trip from NRC lab to the field two of the i
special badges are sent unshielded and two are sent in a lead shield sufficient
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to provide 88% reduction of 150 kev electromagnetic radiation (the energy I
i typical of most radionuclides in transit in the U. S. Mail system).
Upon I
collection of field badges, all four special transit badges are sent back wi'.h those collected, however all of the special badges are unshielded.
Upon receipt of the shipment in the NRC laboratory, the special badges are read and.
the ratio of unshielded to shielded is taken.
From the ratia, the leg of the jo'urney in which the exposure occ'urred can be assessed.
This allows for recall of field TLDs if the exposure occurred in the outgoing portion of the
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References 1.
American National Sta,dards Performance, Testing, and Procedural Specif-ications for Thermo*uminescence Dosimetry (Environmental Applications)
A.Terican National Standards Institute N545, New York, 1975.
2.
dePlanque, Gail - Evaluation of Methods for the Determination of X-and Gamma-Ray Exposure Attributable to a Nuclear Facility Using Environmental TLD Measurecants, NUREG/CR-0711, USNRC, Washington, DC, June 1979.
3.
NRC /ctier Plan Developed as a Result of the TMI-2 Accident NUREG 0660, Nuclear Regulatory Commission, May 1980.
4.
Investigation into the March 28, 1979 Three Mile Island Accident By Office of Inspection and Enforcement, NUREG-0600, Washington, DC, August 1979.
5.
Three Mile Island, A Report to the Commission and to the Public, Nuclear Regulatory Commission Special Inquiry Group, National Technical Information Service, Springfield, Virginia, January 1980.
6.
Demographic Statistics Pertaining to Nuclear Power Reactor Sites, Office of Nuclear Reactor Regulation, U. S. Nuclear Regulatory Commission, NUREG 0348.
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FIGURE CAPTIONS FIGURE 1 TLD badge showing optical coding holes, filters, and thermoluminescent dosimeter elements.
The phosphor discs which are visible in the photograph are normally positioned behind the filters.
Scale is in inches.
FIGURE 2 TLD badges are return mailer sealed in the moisture proof plastic bag as prepare for field deployment.
FIGURE 3 Plastic mesh cartridge used for field deployment of TLD bag-mailer.
Scale is in inches.
FIGURE 4 Block diagram of data processing equipment.
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