Solutions Guide Meyerhof Elements Of Nuclear Physics
Nelson Suggs
Sections 4(a), 5(2)(a), and 13(1) of the Canadian Nuclear Safety Commission's (CNSC's) Radiation Protection Regulations, (SOR 2000/203) require licensees to implement a radiation protection program that keeps the amount of exposure to radon progeny and the effective dose and equivalent dose to persons as low as is reasonably achievable (ALARA). As part of the program, the licensee is obligated to directly measure and monitor the exposure to radon progeny, and adhere to prescribed effective dose limits for nuclear energy workers (NEWs), pregnant NEWs, and non-NEWs.
This regulatory document provides guidance to licensees regarding recommended elements of a thyroid screening program for workers handling volatile radioiodines, which may be required by licence conditions. Further to the licence conditions, this document includes recommendations for participation in the screening program, instrument selection, the screening method, monitoring periods, and validation procedures.
This regulatory document is intended to provide guidance for the CNSC's Radiation Protection Regulations (SOR 2000/203), and is partly based on the American National Standards Institute document entitled Design of Internal Dosimetry Programs.
Nothing contained in this document is to be construed as relieving any licensee from pertinent requirements. It is the licensee's responsibility to identify and comply with all applicable regulations and licence conditions.
This regulatory document describes the recommended elements of an effective thyroid screening program for volatile radioiodines. Thyroid screening of workers handling volatile radioiodines may be required by licence conditions. Further to the licence conditions, this document includes recommendations for selecting participants in the screening program, instrument selection, the screening method, monitoring periods, interpretation of results, validation procedures, and record keeping.
This document does not apply to the occupational health and safety of workers. The licensee should refer to applicable federal and provincial laws and regulations (see for example the Canada Labour Code ( R.S., 1985, c. L-2 ) and the Ontario Occupational Health and Safety Act, R.S.O. 1990, CHAPTER O.1).
Workers may be exposed to radionuclides in a variety of chemical forms that can be inhaled, ingested, or absorbed through intact skin or open wounds. Radiation protection programs include work area monitoring, such as surface contamination monitoring. Where intakes of radionuclides are possible, radiation protection programs should also include measurements to estimate the quantity of radioactivity deposited in the body or to establish a basis for judgment that significant intakes (in relation to applicable dose limits) of radionuclides have not occurred.
CNSC regulatory guide G-91, Ascertaining and Recording Radiation Doses to Individuals[2], provides guidance to licensees on approaches that may be used to ascertain and record radiation exposures and doses to workers. It includes guidance for ascertaining dose when one or more contributing component of a worker's overall effective dose is likely to exceed 1 millisievert (mSv) per year.
When workers are not likely to receive annual doses exceeding 1 mSv, increased resources for ascertaining dose may not always be justified. Hence other means of assessing possible exposure, such as screening, should be considered.
The purpose of a thyroid screening program is to monitor the intake of volatile radioiodines. Timely information produced by the program is used to assess any intake of volatile radioiodines, provide assurance that the radiation protection program is working, and demonstrate compliance with regulatory dose limits.
In the thyroid screening program, workers submit to an in-vivo count of the thyroid. Results are compared to a predetermined level without the need for dose assessment or intake estimation. Exceeding the predetermined level requires confirmation of intake and an investigation. When a worker frequently exceeds the predetermined level, that worker's need to participate in a routine bioassay program should be re-evaluated.
Although this regulatory document specifically addresses I-125 and I-131, the methods presented in this regulatory document may be applied when developing a screening program specific to other volatile radioiodines (e.g., I-123 or I-124).
Workers (NEWs and non-NEWs) who handle a total quantity of open-source volatile radioiodine in a 24-hour period that exceeds the amounts indicated in Table 2, should be screened for I-125, I-131, or both if necessary.
Workers should also be screened when they handle other amounts or types of open-source volatile radioiodine in ways other than those listed in Table 2. Appendix A to this document provides examples of volatile radioiodine compounds and illustrates actions that may generate such compounds.
The scintillation detector is currently the most common type of instrument used for measuring radioiodine in the thyroid. It typically consists of a probe (usually containing a sodium iodide (NaI) crystal) operated in conjunction with a counter, a scalar, and a rate meter, a channel analyzer, or a spectrum analyzer. Systems can be as simple as a portable unit that produces results in counts per unit time, or as sophisticated as a gamma spectroscopy system that generates the energy spectrum of the isotope and then quantifies the total activity.
When choosing an instrument, it is advisable to read the instrument specifications carefully or consult with the manufacturer to ensure that the probe and detector are capable of detecting the applicable radioiodine. For more information on selecting a detector for I-125 or I-131, consult the Canadian Medical Radiation Technology publication, Thyroid Monitoring Part VI: Choosing a Detector for Either I-125 and/or I-131.
Detection and measurement of I-125 requires only a thin crystal to efficiently detect low energy I-125 photons. Typically, NaI crystals approximately 1 mm thick are used to measure low-energy photon emitters such as I-125.
Another factor to consider is the diameter of the crystal. A large diameter results in greater overall counting efficiency. It also helps reduce error that may result from any variances such as neck-to-detector distances, misalignment of detector with thyroid, and size of thyroid. However, a larger detector diameter increases the background reading.
The window material of the probe is also a factor to be considered. The low energy I-125 photons require a window material, such as Mylar or beryllium, thin enough to allow the I-125 photons to penetrate the crystal.
As a best practice, verify controls annually by participating in a thyroid intercomparison program, such as the one provided by Health Canada and described in the Human Monitoring Laboratory technical report entitled, The Thyroid Intercomparison Program[1].
Thyroid screening for I-125 and I-131 on workers and other persons who meet the screening participation guidelines (see section 6.0) should be carried out between one and five days following the exposure.
For I-125 and I-131, a 10 kBq result is approximately equal to a dose of 1mSv. Under Section 16 of the Radiation Protection Regulations, the CNSC must be notified when a licensee becomes aware that a dose to a person may have exceeded an applicable dose limit (e.g., 1 mSv per year for a non-NEW). For NEWs, the steps listed below are recommended for consistency with the guidelines presented in CNSC regulatory guide G-91, Ascertaining and Recording Radiation Doses to Individuals[2] . In particular, G-91 recommends ascertaining the effective dose from each component of the dose that contributes more than 1 mSv per year.
Effective dose (E)
The sum of the products, in sievert, obtained by multiplying the equivalent dose of radiation received by and committed to each organ or tissue set out in column 1 of an item of Schedule 1 (Radiation Protection Regulations) by the tissue weighting factor set out in column 2 (Radiation Protection Regulations) of that item.
Effective half-life
The time required for a radionuclide deposited in the body to decrease to one-half of its initial quantity as a result of the combined action of radioactive decay and biological elimination.
Equivalent dose (HT)
The product, in sievert, obtained by multiplying the absorbed dose of radiation of the type set out in column 1 of an item of Schedule 2 (Radiation Protection Regulations) by the radiation weighting factor set out in column 2 (Radiation Protection Regulations) of that item.
NEW
A person who is required, in the course of the person's business or occupation in connection with a nuclear substance or nuclear facility, to perform duties in such circumstances that there is a reasonable probability that the person may receive a dose of radiation that is greater than the prescribed limit for the general public.
Radioiodine
A substance containing radioactive iodine in a chemical form which has a metabolic pathway similar to iodide, such as inorganic compounds and metabolic forms of organic iodine that are broken down in vivo; this includes the radionuclides Iodine-125 and Iodine-131.
Screening
The monitoring of workers for the purpose of detecting the presence of radioiodine deposited in the thyroid as an indication of radioiodine intake and is not intended for the purpose of quantitative dose assessment.
Standard source
A radioactive source characterized for the activity of radionuclides by the National Research Council of Canada, or another national standardizing laboratory for radioactivity measurements, and issued with a certificate that gives the results of the characterization.
Volatile radioiodine compounds include such compounds as sodium iodide (NaI) and radioiodines in a disassociated form. The volatility of radioiodine compounds may increase as a result of acidifying or freezing.
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