A regulatory investigation team has pointed at "faulty computer modelling" and "manufacturing issues" as contributing factors to the rapid deterioration of steam generator tubing at the San Onofre Nuclear Generating Station (SONGS).
Unit 3 at SONGS was shut down on 31 January after operator Southern California Edison (SCE) noticed reactor coolant in the secondary circuit of the pressurized water reactor. Inspection revealed that a steam generator tube had ruptured - an unexpected event given that both of the unit's steam generators had been replaced as recently as 2010. Unit 2, a twin to unit 3 with new steam generators as of 2009, was in outage at the time but remained shut down as SCE and the Nuclear Regulatory Commission (NRC) became concerned that the cause of failure might be common to both units.
Subsequent eddy current testing revealed that this was indeed the case and a significant number of tubes in each of the four steam generators were plugged and retired from service. In total some 386 tubes had thinned by more than 35% from their original state - a level that required mandatory plugging - while hundreds more were plugged as a precaution. Both units remain shut down with no clear timetable for their return to service.
Last week SCE identified vibration and tube-to-tube fretting as the major cause of the unusual wear. It found the cause of this to be higher-than-predicted thermal hydraulic conditions and wear found at the retainer bars. These retainer bars are unique to steam generators made by Mitsubishi.
The NRC Augmented Inspection Team report, issued yesterday, largely agreed with SCE's diagnosis. It noted that "the changes in manufacturing resulted in less contact forces between anti-vibration bars and tubes." The combination of these two causes allowed excessive vibration to occur. The team also found that SCE had provided the NRC with all the information required before it replaced the steam generators, ending speculation that the utility had failed in this regard.
It also identified ten follow-up areas which require further investigation and may result in disciplinary action. Among these are actions that relate to SCE's handling of the event and assessment of the adequacy of Mitsubishi's computer simulation software.
Each steam generator contains 9727 tubes and is designed to cope with the loss of some of these throughout its life, however the speed of degradation was surprising and presented a possible safety issue if a large failure were to suddenly occur prompting the need for further investigation and planning. The new steam generators are unique; built by MHI as a like-for-like replacement of the original Combustion Engineering model 3340 recirculating steam generators, but with improvements in design and the use of new materials. Many nuclear plants throughout the world have benefited from replacing their steam generators, and Mitsubishi is one of the major suppliers.
Researched and written
by World Nuclear News