Question 7 of 7: Nuclear Energy Worker Dose Limits and Airborne Exposure

Question 7: Nuclear Energy Worker Dose Limits and Airborne Exposure (7 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Facility-assigned dose ceiling 10 mSv over a 6-month temporary NEW contract; at the 3-month mark, external dose 2.5 mSv plus the tabulated airborne exposures above; DAC defined for a 2,000 h/y (40 h/week) reference work-year.

Find. (a) whether 10 mSv/6 months is within the Canadian regulatory NEW dose limit; (b) the worker's total (external + committed internal) effective dose at 3 months and whether it is within the prescribed limit, and whether the remaining 3 months should be radiation-free.

Approach. Compare the facility's 10 mSv/6-month ceiling against the CNSC regulatory NEW dose limit for part (a); for part (b), convert each nuclide's air concentration to a fraction of its DAC, weight by actual hours exposed (from the given exposure durations at the 40 h/week reference rate implied by 2,000 h/y), sum to total DAC-hours, convert to a committed effective dose via the ALI-to-20 mSv relationship, add the external dose, and compare the running total against the prescribed limit.

1. Part (a) — is 10 mSv/6 months within regulatory limits? Under the CNSC Radiation Protection Regulations, a nuclear energy worker's effective dose limit is 50 mSv in any one-year dosimetry period (and 100 mSv over any 5-year period). Pro-rating the annual limit linearly over a 6-month contract gives an indicative half-year allowance of $$50\text{ mSv/y}\times\tfrac{1}{2} = 25\text{ mSv}$$ $$\boxed{10\text{ mSv} < 25\text{ mSv (pro-rated)} < 50\text{ mSv (annual limit)} \implies \text{within regulatory limits}}$$ The facility's own 10 mSv administrative ceiling for this contract is in fact considerably more conservative than the regulatory limit itself.
2. Part (b) — total dose at 3 months. The reference work-year of 2,000 hours corresponds to $2{,}000/50=40$ hours per week (a standard work week), so each nuclide's actual DAC-hours of exposure is (concentration/DAC) $\times$ (weeks $\times$ 40 h): $$\begin{aligned} {}^{125}\text{I}:&\ \tfrac{0.4}{1.2}\times(2\times40)=0.333\times80=26.7\text{ DAC-h}\\ {}^{131}\text{I}:&\ \tfrac{0.4}{0.8}\times(2\times40)=0.500\times80=40.0\text{ DAC-h}\\ {}^{89}\text{Sr}:&\ \tfrac{8}{16}\times(1\times40)=0.500\times40=20.0\text{ DAC-h}\\ {}^{90}\text{Sr}:&\ \tfrac{0.08}{0.32}\times(1\times40)=0.250\times40=10.0\text{ DAC-h} \end{aligned}$$ $$\text{Total} = 26.7+40.0+20.0+10.0 = 96.7\text{ DAC-h}$$ One full working year at the DAC (2,000 DAC-h) corresponds to one Annual Limit on Intake, defined to deliver a 20 mSv committed effective dose, so the committed internal dose from this airborne exposure is $$\boxed{D_{\text{internal}} = \frac{96.7}{2{,}000}\times20 = 0.967\text{ mSv}}$$ Adding the measured external dose, $$\boxed{D_{\text{total,3mo}} = 2.5 + 0.967 = 3.47\text{ mSv}}$$ Against a straight-line half of the facility's 10 mSv contract ceiling for the elapsed 3 months (5.0 mSv), $3.47<5.0$ — the worker is within the prescribed limit at the 3-month checkpoint (and far inside the regulatory 25 mSv half-year pro-ration from part a).
3. Part (b), continued — should the remaining 3 months be radiation-free? The remaining allowance against the facility's own 10 mSv contract ceiling is $10-3.47=6.53$ mSv for the final 3 months. If the worker's exposure rate over the remaining period simply continues at the same average pace as the first 3 months ($\approx3.47$ mSv per quarter), the projected 6-month total would be $\approx2\times3.47=6.93$ mSv — still comfortably under both the facility's 10 mSv ceiling and the regulatory 25 mSv half-year pro-ration. $$\boxed{\text{Remaining allowance }6.53\text{ mSv} > \text{projected further accrual }\approx3.47\text{ mSv}}$$ No — the worker does not need to be reassigned to radiation-free work; there is ample margin under the prescribed limit to continue in radiation work for the remaining three months, subject to ALARA practice and continued dose monitoring.