Outage Job Sequencing Under a Dose Budget
An outage has six jobs with the following durations, dose estimates, and precedence. A: radiation survey, 4 h, 2 mSv, no predecessor. B: valve rebuild, 8 h, 6 mSv, after A. C: pump seal replacement, 6 h, 5 mSv, after A. D: cable replacement, 4 h, 1 mSv, after B and C. E: weld NDT, 3 h, 3 mSv, after C. F: closeout, 2 h, 1 mSv, after D and E. Two work crews are available, each working one job at a time. The station's outage dose budget for this package is 15 mSv. The planner proposes to run B and C in parallel after A, then D, then E, then F. A health physicist objects that the total dose is already over budget and suggests deferring E until after D to simplify the schedule. The outage manager wants the shortest duration and a defensible dose plan.
1. Determine the critical path and minimum duration with two crews. Evaluate the planner's sequence.
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Paths: A-B-D-F = 4+8+4+2 = 18 h; A-C-D-F = 4+6+4+2 = 16 h; A-C-E-F = 4+6+3+2 = 15 h. The critical path is A-B-D-F, 18 h. With two crews the planner's parallel B and C is correct: A 0-4 h; B 4-12 h and C 4-10 h; E 10-13 h; D 12-16 h; F 16-18 h. This uses both crews without delaying the critical path. The proposal to defer E until after D would put E after D, so F could not start until E finished; that path becomes A-B-D-E-F = 4+8+4+3+2 = 21 h, which is longer. Deferring E does not improve the schedule.
2. Assess the dose budget and recommend ALARA options. Address the proposal to defer E.
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Total dose = 2+6+5+1+3+1 = 18 mSv, which is 3 mSv above the 15 mSv budget. The schedule is feasible but the dose plan is not. ALARA options: add shielding or use remote tools for the highest-dose job, B at 6 mSv, or C at 5 mSv; rehearse the job to reduce occupancy time; split the work so fewer people are in the field; use a temporary shield around the valve or pump; and track dose daily against the budget. Deferring E does not reduce total dose, and because F depends on E it extends the outage. If E cannot be eliminated, the manager should reduce dose on B or C rather than move E. If the budget cannot be met, the package should be re-scoped or escalated before work starts.