Dose Constraint Conflict During a Steam Generator Tube Inspection
A two-unit PWR has one unit in a refueling outage. During eddy-current inspection of steam generator tubes, a through-wall indication is found on a tube in the hot leg. The utility's radiation protection program sets an annual dose constraint of 15 mSv (1.5 rem) per worker, below the regulatory limit of 50 mSv (5 rem). The repair requires a welder to enter the channel head. Estimated dose for the repair is 18 mSv (1.8 rem) for the welder and 6 mSv (0.6 rem) for the support crew. The welder has already accumulated 11 mSv (1.1 rem) this year. The outage schedule has 9 days remaining; deferring the repair to the next outage would require power derating on the affected unit. The plant manager asks you, as the RP lead, to 'make the dose constraint work' so the repair proceeds on schedule.
1. Can the repair proceed as planned under the dose constraint? Show the reasoning.
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No, not as planned. The welder would receive 11 + 18 = 29 mSv (2.9 rem) this year, which exceeds both the utility's 15 mSv constraint and is within but close to the 50 mSv limit. The constraint is a management commitment, not a legal limit, but it exists to keep doses ALARA; exceeding it requires a formal, documented justification and senior management authorization, not a verbal workaround. The support crew at 6 mSv is within the constraint if they have low year-to-date dose, but that must be verified individually. So the answer is: the repair cannot proceed as scoped without either reducing the welder's dose below the constraint or invoking the constraint exception process with justification.
2. What ALARA and engineering options would you evaluate to reduce the welder's dose?
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Evaluate, in order: (1) dose reduction by shielding-install temporary lead shielding around the channel head and use a remote or semi-remote welding technique; (2) source reduction-flush and decontaminate the channel head, verify dose rates with survey before entry, and consider chemical decontamination if justified; (3) time reduction-pre-fabricate and rehearse the weld, use a mockup, and minimize in-field fit-up time; (4) distance-use extended tooling so the welder spends less time close to the source; (5) worker selection-assign a welder with lower year-to-date dose if qualified, which is permissible if not done to evade limits and if all workers remain under the constraint; (6) rescheduling-split the task or defer non-critical portions. Each option should be costed in dose saved versus schedule and cost, and the selected combination documented in the ALARA review.
3. The plant manager suggests simply raising the dose constraint to 20 mSv for the outage. How do you respond?
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A dose constraint is not a knob to turn when convenient. Raising it mid-outage to accommodate a specific task inverts the ALARA logic-the constraint should drive the work, not the reverse. If the constraint is genuinely too low for the plant's work mix, that is a program-level decision made in advance with documented rationale, regulatory awareness, and worker involvement, not a one-off adjustment to fit a schedule. I would refuse the ad hoc change, document the refusal, and offer the legitimate paths: reduce dose through the ALARA options, or invoke the formal exception process with senior management authorization, a written justification, and confirmation that the individual remains well below the regulatory limit. I would also flag the schedule pressure as a potential undue-pressure issue under the code of conduct.