Shutdown Margin Erosion After a Rod Worth Measurement
You are the shift supervisor on a small PWR at the end of a fuel cycle. The core is at hot zero power, all control rods fully withdrawn except the regulating bank, which sits at 220 steps. Tech specs require a shutdown margin of at least 1.3% Δk/k with the most reactive rod fully withdrawn. During a routine rod worth measurement, the reactor operator pulls the highest-worth rod (worth 1.8% Δk/k) to 200 steps, then to 250 steps, and the operator reports the rod position indicator 'sticking' between 250 and 300 steps. The digital rod position indication (DRPI) shows 250 steps, but the pulse-count-derived position suggests 285 steps. The operator wants to continue the test to complete the curve. Boron concentration is 1,150 ppm, and the predicted critical boron concentration at this condition is 1,140 ppm.
1. What immediate action do you direct, and why?
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Direct the operator to stop the rod motion and drive the rod back to a known, verifiable position (or insert fully if position cannot be confirmed). The core is at HZP with only 10 ppm of boron margin to critical, so the reactor is very close to critical and any unplanned positive reactivity insertion matters. The DRPI and pulse-count disagreement means the actual rod position is unknown; you cannot assume 250 steps. Because the rod is the highest-worth rod (1.8% Δk/k), the difference between 250 and 285 steps could correspond to a nontrivial fraction of that worth. Continuing the test would insert positive reactivity with an unverified position indication. The conservative action is to stop, restore a known configuration, and treat the DRPI discrepancy as an equipment problem to be evaluated before further testing.
2. The DRPI is confirmed failed. How do you assess whether shutdown margin is still met, and what do you do if it is not demonstrable?
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Shutdown margin must be demonstrated, not assumed. With the DRPI failed, you cannot credit the indicated position of the affected rod for the SDM calculation. Recompute SDM assuming the worst credible position of that rod consistent with the evidence (e.g., fully withdrawn), using the measured rod worth and the current boron concentration and temperature. If the recomputed SDM is below 1.3% Δk/k, the reactor is outside the tech spec limit. Options: borate to restore margin (calculate the ppm needed using the moderator temperature and boron worth), or take the reactor to a condition where SDM is demonstrable (e.g., insert rods and verify by alternate means). Do not simply declare the rod 'at 250 steps' because the DRPI says so. Document the calculation, the assumption, and the compensatory action, and initiate a condition report for the failed indication.
3. What is the regulatory and human-performance lesson from the operator's initial proposal to 'finish the curve'?
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The proposal prioritizes completing a test over maintaining a verified safe configuration-a classic goal-conflict error. The operator focused on the task (finish the curve) and treated an anomalous indication as an obstacle rather than a signal. The supervisor's role is to break that fixation: stop the evolution, restore a known state, and re-plan. From a regulatory standpoint, continuing a reactivity evolution with an unverified rod position and only 10 ppm boron margin could constitute operating outside the analyzed condition and would be difficult to defend in an inspection. The lesson: when instrumentation disagrees, the conservative interpretation governs until proven otherwise, and test completion never outranks configuration control.