Steam Generator Tube Rupture Diagnosis and Cooldown

The most likely event is a steam generator tube rupture in SG B. The primary-to-secondary leak creates the radiation signature in SG B blowdown and in the condenser off-gas because non-condensable radioactive gases reach the condenser. The primary side is losing inventory: primary pressure is slowly decreasing, pressurizer level is decreasing at about 2% per minute, and charging flow of 25 m3/h exceeds letdown of 15 m3/h, showing that the charging system is making up a primary inventory loss. SG B level is rising despite normal feedwater control, which is consistent with primary coolant entering the secondary side. SG A is unaffected, which localizes the leak. Indications that could argue against SGTR include a steam-line leak or feedwater problem, but those would not normally produce the SG B radiation signal with a simultaneous primary inventory loss. A primary-side leak inside containment would also lower pressurizer level, but it would not raise SG B blowdown radiation and SG B level. The combination of secondary radiation, rising SG B level, and primary inventory loss is the key diagnostic triad.

The crew should trip the reactor and confirm the trip, then isolate the affected SG B on the secondary side to stop the radiological release, while preserving the ability to remove heat through the intact SG A. Feedwater to SG B should be controlled to avoid overfilling, and SG B steam isolation should be established as required by the emergency procedure. Primary pressure and pressurizer level should be maintained with charging, and the crew should cool down the primary to reduce the break flow. At 6.5 MPa, the saturation temperature is approximately 280 C. To stop primary-to-secondary leakage, Tavg should be reduced below that value with margin, for example to about 275 C. The required cooldown is 296 - 275 = 21 C. At the 55 C/h limit, the minimum cooldown time is 21/55 = 0.38 h, or about 23 minutes. That is a minimum time at the maximum allowed rate, not a target to rush; the actual cooldown should respect thermal stress and procedure limits. Cooldown should be done using the intact SG, and SG B pressure must be kept below its safety valve setpoint to avoid an uncontrolled release.

Cooling the primary too fast creates thermal stress on thick-walled components and can cause a rapid primary contraction that challenges pressurizer level control. It can also increase the break flow transiently or cause a primary-side bubble if the cooldown is not coordinated with charging and letdown. Overfilling SG B is hazardous because it can lead to water hammer, moisture carryover, and a higher probability that the rupture path is submerged or that the safety valve lifts with contaminated steam. It also reduces the steam space available for pressure control and can worsen the radiological release. Two specific limits to respect are the plant cooldown limit of 55 C/h and the SG B level program, which must not be exceeded; the SG B safety valve setpoint of 7.0 MPa is a third hard limit that must not be approached. The crew should maintain adequate subcooling, keep pressurizer level on program, and use the intact SG for controlled heat removal rather than aggressively cooling both SGs.