10CFR20.1204 – Assessing dose used to determine compliance with occupational dose limits

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Practical Whole Body Counting and Internal Dosimetry
Tim Kirkham – Chesapeake Nuclear Services

 

(b) Unless respiratory protective equipment is used, as provided in § 20.1703, or the assessment of intake is based on bioassays, the licensee shall assume that an individual inhales radioactive material at the airborne concentration in which the individual is present.

c) When specific information on the physical and biochemical properties of the radionuclides taken into the body or the behavior or the material in an individual is known, the licensee may–

(1) Use that information to calculate the committed effective dose equivalent, and, if used, the licensee shall document that information in the individual’s record; and

(2) Upon prior approval of the Commission, adjust the DAC or ALI values to reflect the actual physical and chemical characteristics of airborne radioactive material (e.g., aerosol size distribution or density); and

(3) Separately assess the contribution of fractional intakes of Class D, W, or Y compounds of a given radionuclide (see appendix B to part 20) to the committed effective dose equivalent. (now known as F, M and S)

 

d) If the licensee chooses to assess intakes of Class Y material using the measurements given in § 20.1204(a)(2) or (3), the licensee may delay the recording and reporting of the assessments for periods up to 7 months, unless otherwise required by §§ 20.2202 or 20.2203, in order to permit the licensee to make additional measurements basic to the assessments.

(e) If the identity and concentration of each radionuclide in a mixture are known, the fraction of the DAC applicable to the mixture for use in calculating DAC-hours must be either–

(1) The sum of the ratios of the concentration to the appropriate DAC value (e.g., D, W, Y) from appendix B to part 20 for each radionuclide in the mixture; or

(2) The ratio of the total concentration for all radionuclides in the mixture to the most restrictive DAC value for any radionuclide in the mixture.

(f) If the identity of each radionuclide in a mixture is known, but the concentration of one or more of the radionuclides in the mixture is not known, the DAC for the mixture must be the most restrictive DAC of any radionuclide in the mixture.

(g) When a mixture of radionuclides in air exists, licensees may disregard certain radionuclides in the mixture if–

(1) The licensee uses the total activity of the mixture in demonstrating compliance with the dose limits in § 20.1201 and in complying with the monitoring requirements in § 20.1502(b), and

(2) The concentration of any radionuclide disregarded is less than 10 percent of its DAC, and

(3) The sum of these percentages for all of the radionuclides disregarded in the mixture does not exceed 30 percent.

(h)(1) In order to calculate the committed effective dose equivalent, the licensee may assume that the inhalation of one ALI, or an exposure of 2,000 DAC-hours, results in a committed effective dose equivalent of 5 rems (0.05 Sv) for radionuclides that have their ALIs or DACs based on the committed effective dose equivalent.

(2) When the ALI (and the associated DAC) is determined by the nonstochastic organ dose limit of 50 rems (0.5 Sv), the intake of radionuclides that would result in a committed effective dose equivalent of 5 rems (0.05 Sv) (the stochastic ALI) is listed in parentheses in table 1 of appendix B to part 20. In this case, the licensee may, as a simplifying assumption, use the stochastic ALIs to determine committed effective dose equivalent. However, if the licensee uses the stochastic ALIs, the licensee must also demonstrate that the limit in § 20.1201(a)(1)(ii) is met.

 

ICRP 30 – Limits for Intakes of Radionuclides by workers (7 volumes)

ICRP 54 – Individual Monitoring for Intakes of Radionuclides by workers: design and interpretation

Routine measurements –

baseline – conducted prior to work that involve exposure

periodic – determine a priori considering likelihood of exposure

considers likely exposure, measured levels of airborne, exposure time

can exclude half-life less than 2 hours

as a minimum – annually

Termination measurements

Estimating intakes

– evaluation levels generally are at 0.02 ALI

– investigation levels generally at 0.1 ALI

ICRP 66 – Human Respiratory Tract Model for Radiological Protection

ICRP 68 – Dose coefficients for Intakes of Radionuclides by workers

ICRP 78 – Individual monitoring for internal exposure of workers

Objective 2.0

A. Backscatter peak – Many gamma rays from the source/outer space, etc., do not interact in the detector but will interact with material surrounding the detector (shield material, walls, etc.,). These interactions will produce scattered gamma energies that are directed in reverse of the original gamma direction and thus strike the detector.  Since they have already lost some energy already they show up with energies less than the full energy peak.  A simple equation shows that based on Cs-137’s initial energy of 662 keV, the scattered gamma’s will have an energy of at least 180 keV.  The broad peak in the energy range from 200 keV to about 250 keV represents the back-scattered gamma rays.

B. Compton Continuum/Edge – If an incoming gamma undergoes a Compton scatter and the gamma then escapes the crystal, but all the energy is deposited, a compton continuum is observed.  The spectrum will show a continuum of events whose energies range from zero to a finite maximum value.  The compton edge occurs at an energy equal to the initial energy minus the minimum scattered energy.  In the case of Cs- 137: 662 keV – 180 = 482 keV.  The compton continuum = the compton background.  The counts that make up this region are not considered in the calculation of the peak area.  The amount of counts in this region do however contribute to the MDA in a given region of interest.

C. Results from the photoelectric interactions of a particular gamma ray.  We use these peaks to quantify and qualify the respective radionuclides.  Each radionuclide emits its own gamma peak(s).

•Performed 1 to 4 times per year
•Unknown quantities/isotopes tests both the machine and the man.  Have to interpret all the data to get the right answer.
•A very important  part of the program:
•The NRC and ANI like it
•Helps us to feel comfortable that we know what we are doing
•Many site require annually – no reason to (have TBD if desired)
•Energy calibration –

energy = offset + slope x channel + quad x channel2

•FWHM calibration – correlates energy with resolution (peak width)

mainly a characteristic of the detector

•Efficiency calibration – correlates energy with efficiency.  The calibration factors are basically a table of energy versus efficiency pairs.  A curve is then fitted to the data and an equation is developed
•Several different types of phantoms:
•BOMAB – BOttle Manikin ABsorption (BOMAB) phantom
•RMC
•Livermore
•Several organ phantoms (thyroid, wound)
•Human body has a specific gravity of 1.07 for males and 1.04 for females
•Some portions are greater than 1 (ribs are 2.0)

Objective 2.0

A – backscatter peak again

B – K-40 Peak

OE 5946 – Incident at Limerick station with a TIP.  Mo-99 nor Tc-99 were not identified on the gamma scan (why not?).  The NaI detector could not see them due to resolution and energy considerations.

 

Intake Date and Time – when performing the WBC of the individual, try to input the time of the intake.  The intake date & time can usually be found in the Dosimetric Assessment for the event.

This is an important piece of information for performing Internal Dose calculations.  The intake date and time is used as the starting point for all of the methods used to calculate the exposure.

Nuclides Identified – The whole body counter is very good at identifying nuclides, but sometimes it will not be able to identify or incorrectly identify a nuclide.

Give examples of this occurrence – misidentification of Zn when seen in the presence of Co.

There are also whole body counts that identify Mn-54 for Co-58 because it has the same energy peak.  Explain why we throw out peaks that have an error greater than 50%

Activity in each nuclide – this is pretty straight forward, but there could be problems due to doubling. For example, the peak the has the lower energy Co-60 and the Zn-65 made the Co-60 activity look lower that the actual activity.

Intake Date and Time – when performing the WBC of the individual, try to input the time of the intake.  The intake date & time can usually be found in the Dosimetric Assessment for the event.

This is an important piece of information for performing Internal Dose calculations.  The intake date and time is used as the starting point for all of the methods used to calculate the exposure.

Nuclides Identified – The whole body counter is very good at identifying nuclides, but sometimes it will not be able to identify or incorrectly identify a nuclide.  Give examples of this occurrence – misidentification of Zn when seen in the presence of Co.

There are also whole body counts that identify Mn-54 for Co-58 because it has the same energy peak.  Explain why we throw out peaks that have an error greater than 50%

Activity in each nuclide – this is pretty straight forward, but there could be problems due to doubling. For example, the peak the has the lower energy Co-60 and the Zn-65 made the Co-60 activity look lower that the actual activity.

These samples are usually used to develop scaling factors that is more appropriate than the scaling factors.

We only have scaling factors for major locations, i.e.  S/G, RFP.

These scaling factors can be used in other locations and can probably suffice, but to get the most accurate answer, appropriate scaling factors need to be used.

These samples can be sent off to a off-site contractor to analyze for the isotopes and their abundance.

This information is then used to produce scaling factors that are used for radionuclides that cannot be detected by the whole body counter.

Objective 3.0

Use an “easy-to-detect” (gamma emitter) nuclide that is easily detected by our detector as the nuclide to scale for scaling.  Co-60 and Cs-137 are two nuclides that make up a large portion of most radioactive profiles.

These isotopes also put out gamma radiation that is easily seen by the whole body counter.

We need to scale in hard to detects for a couple of reasons.

• The “hard to detects” (alpha, beta, low activity gamma) cannot be seen by the whole body counter because it can’t penetrate outside the body or it’s just too low activity that it’s washed out by background, other nuclides, or noise
• The “hard-to-detects” are the major dose contributor.  A small fraction of alpha emitters may give 50% of the exposure.

Objective 5.0

1.  How to identify nuclides and reasons for disregarding nuclides identified by the WBC

2.  Ways to verify if internal exposure is due to inhalation or ingestion.

3.  Reason for scaling in radionuclides

4.  Determining what parts of 4884 is used for the intake retention fraction (it is already determined, with the help of chemistry, what type{day, week, year} of nuclide characteristic is in our plant mix).

5.  Explain that organ dose comes from the largest exposure and is dependent on the type of radionuclide (most transuranics are bone seeker, while iodine will go to the thyroid).

6.  Give limit for reporting exposure – 10 mrem whole body or 100 mrem to any organ.

810 Bq = 4860 dpm

200 mSv = 2 rem

From the case examined above, it can be seen that even though the intake estimate rises using the new dosimetry models, the organ dose has fallen by approximately 50% and the whole body dose is lower by 63%, from 51 mSv to 19 mSv.

This reduction in assessed doses is as expected.  The new Respiratory Tract Model is now based on greatly improved understanding of the processes that occur in the airways, and the tissues that make up this organ.

The new model also takes account of differences in radiosensitivity of respiratory tract tissues.  In vivo and autopsy measurements, following accidental intakes, show that there is a degree of very long term retention in the lungs.

The long term component of the ICRP 66 model is 7000 days compared with 500 days for the ICRP 30 model, so that a percentage of material in the lungs is released much more slowly using the new model.

Apportionment factors have now been assigned to each region of the respiratory tract.  These factors relating to risks of cancer incidence weight certain tissues more than others according to their radiosensitivity.

This means that certain areas of the respiratory tract can receive much higher doses but with a very low risk of cancer.  For these reasons a higher intake does not necessarily lead to higher doses received by the lungs as a whole.

Objective 6.0

•Credibility is judged only 10% on what we say and 90% on how we say it.  Therefore, think about what and how we should say something before we say it.
•Remember who you are talking to.
•Example  – he already was upset but when a technician looked at the counts from the air sample and said something to the effect of “Wow”, we were had.  Also, when we talked to him regarding his intake, we told him it was the second highest we had ever seen – bad choice!
•Empathize – Put yourself in the subjects place.  How scared would you be?  We tell them in GOT that radiation is bad, and they think that internal radiation is even worse, especially when we make a big deal out of it.
•The proper way to interact with a subject is to answer their questions simply – “Yes, you have internal radioactivity but that this is only one indicator.”  If they want to see the spectrum, make sure you relate the peaks to the K-40 peak – it will let them see that they are radioactive already.
•Explain to them the process: WBC, other data, calculations according to standardized methods, etc.
•Offer a discussion with a HP.
•What else can you think of?  What interactions/questions have you encountered?

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