Seal Replacement Under a Tight Dose Constraint
A radiation protection supervisor must plan a 6-hour window for replacing the mechanical seal on a primary coolant pump. A dose-rate survey gives 3.20 mSv/h at the seal flange, 0.80 mSv/h at 1 m from the pump and 0.15 mSv/h at the valve station 3 m away. The job needs two fitters at the flange for 40 minutes each, one fitter at 1 m for 2 hours, and one person at the valve station for 3 hours. The station applies an administrative dose constraint of 10 mSv per person per quarter above natural background. One of the four fitters has already accumulated 7.50 mSv this quarter. Only one shield can be installed: either a 50 mm lead sheet at the flange (dose reduction factor 2) or a blanket at the 1 m position (factor 1.5); space prevents both. During the pre-job survey a smear from the pump casing reads 12 Bq/cm² beta-gamma against a derived surface contamination limit of 4 Bq/cm², and no worker reports a breach of coveralls or gloves.
1. Calculate the individual and collective dose for the unshielded job and decide where the single shield should be placed. Justify the choice numerically.
Read the worked answer
Flange fitters: 3.20 mSv/h × 0.667 h = 2.13 mSv each, 4.27 person-mSv for the pair. 1 m position: 0.80 × 2 h = 1.60 mSv. Valve station: 0.15 × 3 h = 0.45 mSv. Collective dose without shielding = 4.27 + 1.60 + 0.45 = 6.32 person-mSv.
Shield options:
- Lead sheet at the flange: 2.13/2 = 1.07 mSv per fitter, saving 1.07 each, i.e. 2.13 person-mSv.
- Blanket at 1 m: 1.60/1.5 = 1.07 mSv, saving 0.53 person-mSv.
The flange shield saves about four times as much collective dose, so it goes there. The real ALARA lever is time, not shielding: rehearse the fit-up on a mock-up, pre-stage tools and the spare seal inside a shielded bag, and split the 40-minute stay into two shorter entries if the seal can be left part-fitted. This reduces the highest individual dose, which the shield alone does not do.
2. How should the fitter who has already accumulated 7.50 mSv this quarter be deployed, and what must be recorded?
Read the worked answer
Compare the three options against the 10 mSv quarterly constraint:
- Flange with shield: 7.50 + 1.07 = 8.57 mSv, leaving 1.43 mSv for the rest of the quarter.
- 1 m position: 7.50 + 1.60 = 9.10 mSv, leaving 0.90 mSv.
- Valve station: 7.50 + 0.45 = 7.95 mSv, leaving 2.05 mSv.
Assign him to the valve station. All options keep him below 10 mSv, but the constraint is not a target that may be fully consumed early in the quarter; he still has weeks of outage work ahead, including possible unplanned entries. Using 1.07 mSv for a task that another fitter can do at 2.13 mSv is a poor allocation when the other fitters start near zero.
Record the prior dose from his official dose record (not from memory), the basis of the assignment, the pre-job briefing, and the estimated versus actual dose after the job. If the seal job later needs a third body at the flange, request RPS review rather than quietly exceeding the constraint.
3. The smear result of 12 Bq/cm² is received after the dose plan is written. What should happen before any work starts?
Read the worked answer
12 Bq/cm² is three times the derived limit, so it cannot be filed as a curiosity. Treat the pump casing as a contaminated surface until a controlled re-survey says otherwise.
Actions before entry:
- Stop and rope off the area; post contamination signage and a step-off point.
- Re-smear with a documented 100 cm² wipe, recording fixed versus loose contamination and the exact location. Also take an air sample: a weeping seal can produce airborne activity, and that changes the respiratory protection decision.
- Survey the four fitters with hand and foot monitors and check their dosimeters and clothing; if loose contamination is found on them, they leave the controlled area by the step-off point and decontaminate.
- If the activity is loose, decontaminate with strippable coating or tape before the job, then re-survey. Fit the lead shield only after the surface is clean enough to handle.
- Raise coveralls to double gloves and add a change of outer boots; re-brief the team.
One smear is not proof of a contaminated room, but the conservative response is to assume it is real until a proper re-survey has been done and logged.