 INES 2 |
Inoperability of the main steam lines isolation valves (MSIV) |
Classification according to the International Nuclear Event Scale (INES) of the incident reported by Cofrentes NPP due to the inoperability of the main steam line isolation for a period longer than permitted by Technical Specifications (TS)
This report describes the event that was notified on May 1, 2026, after discovering the inoperability of several Main Steam Isolation Valves (MSIVs), when Cofrentes NPP was in cold shutdown.
Cofrentes Nuclear Power Plant is a GE-designed BWR-6 light-water reactor with a MARK-III containment and a thermal power output of 3,237 MWt
Technical Specifications
Technical Specifications (TS) require all isolation valves to be operable in modes 1, 2 and 3. If one MSIV is inoperable, the affected steam line must be isolated within 8 hours; if two valves of the same line are inoperable, isolation must be performed within 1 hour. Otherwise, the reactor must be placed in mode 3 within 12 hours and in mode 4 within 36 hours.
System Description
The plant has four main steam lines (MSL) connecting the reactor vessel to the turbine. Each line includes two MSIVs, one inside containment (B21F022A/22B/22C/22D) and one outside containment (B21F028A/28B/28C/28D). These valves automatically close upon a safety signal within 3 to 5 seconds.
The valves are pneumatically operated and kept open by compressed air supplied through two solenoid valves connected in series. The design is fail-safe, since loss of power or air causes the valves to close, triggered by an actuator spring.
Events
On May 1, 2026, during plant shutdown process and after reaching mode 4 (cold shutdown), the MSIVs were slowly closed in accordance with the unit’s standard shutdown procedure, and, once the closing was complete, the MSIVs’ switches were set to CLOSE. At this point it was observed that, once closed, valves B21F022C (inner) and B21F028A/B/C/D (all outer) reopened automatically.
After identifying the fault, these valves were “quickly closed” by setting the switch directly to CLOSE, but three of the isolation valves (B21F022C and B21F028B/D) remained in the open position.
The cause was identified as sticking of at least one solenoid valve stem in each failed MSIV, preventing proper venting and MSIV closure.
The issue was originated after the replacement of 16 solenoid valves during the October 2025 outage, in accordance with its qualification programme. Fourteen of these valves belonged to the same manufacturing batch (2024) and showed lack of lubrication and internal wear, making them susceptible to a common cause failure.
INES Classification
The event was discovered in mode 4, where the MSIVs are not required; however, the condition had existed since October 24, 2025 (after the replacement of its solenoids), despite the fact that the MSIVs were successfully tested for closure according to Technical Specifications.
The classification follows INES guidance for events without an actual initiating event. Five MSIVs failed to perform their safety function on the first attempt and three of them also failed on the second attempt, meaning operability was below the minimum required by Technical Specifications. It should be noted that the failure of a single MSIV under the same conditions as this incident would have already resulted in an INES 1 classification, as it would have been outside the LCO; because there are only 8 hours available to restore operability or to isolate the line with the inoperative MSIV. If these actions cannot be carried out within the 8-hour period, a cold shutdown is required. Analysis of the incident shows that the failure occurred in a batch of valves manufactured in 2024 (14 of the 16 pilot valves installed during the October 2025 refueling) that exhibited signs of erosion and wear most likely attributable to a lack of lubrication on the actuator shaft, defects in the material, or a combination of both. The 14 solenoid valves were distributed such that all MSIVs were affected, since at least one solenoid valve from this 2024 batch was installed in each MSIV. Therefore, in accordance with section 5.1.2 of the INES Manual, it must be determined whether the operability of the safety function was “adequate” or “inadequate”.
Since alternative means existed to isolate the steam lines using the shut-off valves (N11-F098A/B/C/D - non safety-related valves) downstream of the MSIVs with guaranteed tightness, and the possibility of venting the supply of compressed air to the MSIVs to cause their closure —albeit not within the times required by the ETF— it can be concluded that the safety function, although degraded, is “adequate,” since, should any of the postulated initiating events occur, their consequences would be limited by the actions of plant personnel.
The initiators considered (LOCA, steam line break, fuel handling accident) are classified as “unlikely”, leading to a basic classification of INES Level 1.
Additional Factors
No procedural deficiencies or safety culture issues were identified.
With regard to common-cause failures, the failure occurred in a batch of valves manufactured in 2024 (14 of the 16 pilot valves installed during the October 2025 refueling) that showed clear signs of erosion and friction when disassembled, most likely attributable to a lack of lubrication on the actuator shaft, defects in the material, or a combination of both; and although the exact cause of the failure has not yet been determined by the supplier, it can be concluded that there is clearly a common cause for the valve failure, as all the failed valves were from the same year of manufacture and exhibited similar indications on the valve internals that were not present in valves from other batches that were also examined and did not fail. The 14 solenoid valves were distributed such that all MSIVs were affected, as at least one of the solenoid valves from this 2024 batch was installed in each one.
In this case, evidence identifies that a specific cause has resulted in failures of five solenoid valves.
Since all the MSIVs were affected, the probability of a loss of safety function increases; therefore, this additional factor must be taken into account in the INES classification, raising it by one level above the basic classification.
The classification reduction based on the time of inoperability cannot be applied, since the valves that failed were installed during the 2025 refueling, and the failure was not detected at that time through the operability tests performed.
Final Classification
Considering the common cause failure, the event classification is increased from INES Level 1 to INES Level 2.
Although there were no actual consequences, the event revealed a significant degradation of a key safety system. The presence of common cause failure increased the severity, resulting in a final classification of INES Level 2 (incident).
Event date: Fri, 01-05-2026
