Steam generator tube leak with a rising trend and grid pressure
At 100% power the condenser off-gas radiation monitor rises from 0.4 to 3.8 times background over 25 minutes. The 'B' main steam line monitor rises from 0.6 to 2.1 mSv/h while 'A' remains steady at 0.6 mSv/h. Condenser pressure has drifted from 5.1 to 5.6 kPa(a). A resample of the 'B' steam generator shows sodium 85 µg/kg (normal below 5) and cation conductivity 0.42 µS/cm; the 'A' steam generator is normal. The calculated primary-to-secondary leak rate from the first sample was 45 kg/h and has risen about 8 kg/h every 10 minutes on subsequent samples. The technical specification limit is 150 kg/h with a 4-hour action to reduce leakage, and 500 kg/h requires a shutdown. The grid has issued a capacity alert for the afternoon peak and the shift manager is asking whether the unit can stay on line until 20:00.
1. Confirm the leak and identify its location. Explain what the combination of off-gas, main steam line monitors and chemistry tells you, and what risk drives your timeline.
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The evidence is consistent and points to a primary-to-secondary leak in the 'B' steam generator.
- A single-loop main steam line monitor rising while the other stays flat localises the activity to that loop; gross primary activity reaching the steam line would affect both loops.
- Condenser off-gas rising reflects noble gases (xenon, krypton) carried with the leaking primary water through the turbine and into the condenser air-removal system, which is why off-gas responds before the steam line monitor at small leak rates.
- Sodium of 85 µg/kg with cation conductivity 0.42 µS/cm in 'B' only is the chemical fingerprint of primary water in the secondary; a condenser tube leak would raise sodium in the condensate and both steam generators.
- The slight condenser pressure rise is not itself diagnostic, but a growing leak adds non-condensable gas and moisture to the condenser.
Risk driving the timeline: the leak can grow non-linearly. A rapidly growing tube leak can lead to tube rupture, primary depressurisation into the secondary, steam generator overfill, and moisture carryover to the turbine - plus a release path through the safety valves. That is why you act on the trend, not on the 150 kg/h number alone.
2. Using the trend data, calculate when the technical specification limit would be reached and recommend what the shift manager should do about the grid request.
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Trend arithmetic: 45 kg/h, rising 8 kg/h every 10 minutes = 0.8 kg/h per minute. To reach 150 kg/h requires (150 − 45)/0.8 = about 131 minutes. To reach the 500 kg/h shutdown limit would take roughly 9.5 hours at this rate - but that assumes linearity, which is exactly the assumption you should not rely on.
Recommendation: begin an orderly shutdown now, not at the limit.
- About two hours of margin is not enough to reach 20:00. Waiting converts a planned shutdown into a scram taken under pressure while the leak is still growing.
- The 4-hour action in the technical specification is a limit for reducing leakage, not a licence to operate to the last minute; if the trend is upward, the corrective action (shut down) is already indicated.
- The grid capacity alert is a commercial consideration. It can be raised with dispatch, but it does not change the technical specification or the tube-integrity risk. It is legitimate to inform dispatch of the expected capacity loss and the reason.
- Actions in parallel: increase sampling frequency on both steam generators, monitor main steam line radiation and off-gas continuously, isolate or minimise the affected steam generator blowdown to limit activity spread, notify chemistry and health physics, and brief the crew that a trip may become necessary without notice.
3. Forty minutes into the power reduction the 'B' steam line monitor jumps to 8 mSv/h within one minute and the leak-rate estimate is 220 kg/h. What do you do?
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Treat this as a step change indicating tube degradation or rupture, not as a continuation of the trend.
Immediate actions, in order:
- Trip the reactor manually if it has not already tripped. A step increase of this size means the leak path can grow faster than any power reduction can compensate. Announce it clearly to the crew and the supervisor.
- Verify the reactor is shutdown and turbine tripped, and confirm the affected steam generator is isolated - stop blowdown, and be ready to isolate the steam line if the steam line radiation continues to climb or the safety valves lift.
- Monitor pressuriser pressure and level and be prepared for primary inventory loss through the leak; maintain charging and watch for a rapid depressurisation.
- Watch the 'B' steam generator level for overfill and the potential for water induction into the steam line; keep the unaffected steam generator available for heat removal during cooldown.
- Notify health physics and chemistry; sample the steam line and building radiation, and account for the release path through the steam safety valves.
Document the step time, monitor reading before and after, and the basis for the trip - an unexplained step change is more important than the absolute number in the decision.