ML090760156
| ML090760156 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 03/10/2009 |
| From: | - No Known Affiliation |
| To: | NRC/RGN-III |
| References | |
| Download: ML090760156 (5) | |
Text
ENCLOSURE 1 POSITION PAPER 4 pages follow
TIs position paper relates to a shipping container sent from Prairie Island to Waltz Mill on October 29,2008. A spot on the container bottom was found to exceed 200 mR. TIs radiation field above the shipping limit was found to be quite small, indicating that the dose contrbution was dominated by a discrete radioactive paricle (DRP). Techncians strggled to locate the highest exposure rate, providing anecdotal evidence of the size of the radiation source. To fuer detal that aspect of the event, four independent survey instrents gave four higWy vared readings on the surace of the contaer:
Intrent GM Teletector GM Telepole GM E-600 Ion Chamber RO-2 Exposure Rate 2000 mR 1630 mR 1089 mR 800 mR
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Ths represented a quadar to what should be recorded as the contact exposure rate. It is importt to note that when using varous inents, especially in a scenaro of a discrete radioactive paricle, the surace exposure rate would be especially diffcult to defie. But strctly speakg, an ion chamber will always be the most appropriate instrent for the assessment of tre dose rate.
From Radiation Detection and Measurement, Thid Edition, Glenn F. Knoll, Copyright 2000, John Wiley & Sons, the following represents relevant inormation with respect to GM and Ion Chamber detectors.
Regarding Geiger-Mueller detectors:
"A common type of suey meter used in gam-ray monitoring consists of a portble Geiger tube, high-voltage supply, and pulse countig rate meter. The pulse rate is then taen as an indication of the intensity of the gama-ray exposure. The count rate meter scales are often calibrated in terms of exposure rate unts, but under some circumstaces these readigs can be in error by a factor of2 or 3 or more.
The diffculty arses from the fact that the count rate from a Geiger tube, in contrast to the measured curent from an ion chamber, bears no fudaental relation to the gama ray exposure rate."
Regarding Ion Chambers:
"One of the most importt applications of ion chambers is in the measurement of gama-ray exposure. An air-filled ion chamber is paricularly well-suited for ths application because exposure is defied in terms of the amount of ionization charge created in air. Under the proper conditions, a determination of the ionization charge in an air-filed ionition chaber can give an accurate measure of the exposure........"
GM detectors are used for intial shipping sureys for a variety of good reasons:
- They are rugged, inexpensive, lightweight, and easy to calibrate
- GM's have simpler electronics packages
. GM detectors are small, provide for auto-ranging, ca have speakers tht indicate changing fields, and can be on telescopic poles
. GM's have quick response times (as do most count rate instrents).
- They also have well-known over-responses to low-and high-energy photons. Yet because of their conservatisms, ths is very acceptable.
Although not tre indicators of actu dose rates, with speakers, small size, telescopic poles, and general over-responses, GM intrents provide the quickest, most effcient, and conservative detection of any radiation fields. With GM detectors, sureyor doses are lower. In addition, shipping surey measurements using GM's are tyically higher than the tre dose rates. Therefore, any subsequent measurements by other tys of instrents would be lower (GM's being conservative).
Tht presents the most suitable plan for initial shipping sureys; i.e., use conservative surey instentation ensurg that instrent differences would not be the cause of a shipment later exceeding a limt; i.e., GM detectors. With that plan in mind, the Conference of Radiation Control Program Directors (CRCPD) developed a set of generic procedures for shipping sureys. The CRCPD E-5 Committee on Radioactive Waste Management wrote very detaled model procedures for all of the agreement state shippers. In these, they stte the followig: Any observed radiation survey readings in excess of regulatory limits shall be confirmed using a portable ion chamber instrument.
The reasons are quite vald and logicaL. The GM detectors provide quick, easy to pedorm, conservative measurements. In an event where there would be a question about the tre dose rate, ion chabers should be brought in. They would be used only when assessments of the dose rates were necessar, to conf the tre dose rates. The CRCPD procedures recommend ths for surace measurements and also for personnel doses (e.g.,
cab dose rate). It is their goal to measure the tre dose rate (risk) when it is necessa.
Otherwse, the GM detectors generally provide a conservative set of surveys to manage the processes.
Regutory Guide 7.3 in its Anex Instrentation Secton describes obtaning the most appropriate intrent for accurte determations on shipments. For ths reguated shipping event, we propose tht the ion chamber meaarement of surace dose rate be used (Le., 800 mR). Of the measurements peda~ed, the ion chaber measurement was the most accurate assessment of surace dose rate, and the only one not measured with a count rate instrent. Furer, because the dose rate was dominated by a discrete radioactive paricle, we believe tht the tre risk of exposure to the bottom of the container would be better described by the effective dose equivalent (EDE) rate, which would be even lower than the ion chamber readng. Therefore, it is our contention that acceptace of the ion chamber reading as the surace measurement would allow for conservatism in the assessment of the tre risk of exposure to ths shipping container.
Perormed by: ~ L..~. GiL !, J 'li 0 "i Verfied by: I! cJ ~4ff, 1r,,1a7
Attachment to position on Ion Chamber VS. GM Tube Accuracy The following are a subset of references used while evaluating the accuracy of Ion Chambers vs.
GM Tubes for dose rate measurements.
Radiation Detection and Measurement - Third Edition, Glenn F. Knoll, Copyright 2000, John Wiley & Sons, Inc.
REGULATORY GUIDE 7.3, PROCEDURES FOR PICKING UP AND RECEIVING PACKAGES OF RADIOACTIVE MATERIAL, May, 1976 Radiation Instruments by Herman Cember, Copyright 2001 by the Health Physics Society, Medical Physics Pub Corp, Chapter 17, "Everything But The Counting Statistics: Practical Considerations In Instrumentation And Its Selection And Use", Ronald L. Kathren MARSSIM : NUREG-1575, Rev. 1, EPA402-R-97-016, Rev. 1, DOE/EH-0624, Rev. 1 ANSI N42.34-2006, American National Standard Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides