The bottom inspectors3 August 2003
During a scheduled refuelling and maintenance outage in April, engineers at the South Texas Project (STP) plant found a powdery material on the outside of two instrument guide tubes on the lower part of the unit 1 reactor vessel. STP immediately began the process of selecting a firm to complete the non-destructive examinations, root cause analysis and any repairs that might be needed.
While leaks caused by flaws in Alloy 600 material have been identified on a number of penetrations on the closure heads of some PWRs in the USA and Europe, the discovery on 12 April 2003 of a small amount of a substance (weighing 153mg) at the South Texas Project (STP) plant was the first indication anywhere in the world of the possibility of a similar issue with the lower reactor pressure vessel.
Laboratory tests were performed by STP chemists, and the results indicated that the crystallised mineral found in the residue was boric acid, a clear indication that the source was reactor coolant fluid. STP officials immediately began working closely with the US Nuclear Regulatory Commission (NRC), experts from other commercial nuclear power operators and the Electric Power Research Institute, based in Palo Alto, California, USA. Initial findings were quickly shared with the public and the nuclear power industry at an NRC meeting in Washington, DC on 16 April 2003.
The next step for STP was to offload the full inventory of fuel in the reactor core and perform exhaustive inspections on all 58 penetration tubes on the unit 1 reactor bottom.
Following non-destructive examination (NDE) demonstrations by a number of vendors, STP selected Framatome ANP. As part of the demonstration, Framatome ANP experts identified hidden metallurgical flaws in mock-ups similar in design and material to the actual penetration nozzles.
Framatome ANP has considerable in-core nozzle and Alloy 600 inspection and repair experience in Europe and the USA. The company began a programmatic campaign for Electricité de France in 1991 to examine all Alloy 600 sensitive areas of reactor pressure vessels, including the in-core nozzles. To date, Framatome ANP has performed 19 in-core reactor pressure vessel inspections worldwide, involving more than 500 penetrations. Framatome ANP has also been heavily involved in Alloy 600 issues related to reactor vessel closure heads. The company has performed more than 80 reactor vessel closure head inspections since the early 1990s, and it completed more than 1200 inspections in 2002.
Framatome ANP has developed a number of inspection and repair tools and techniques (see NEI May 2003, p14). To help Framatome ANP's technicians practise the inspection and repair techniques with these tools, STP had a local metal working shop fabricate a full-scale mock-up of the reactor vessel bottom complete with nozzles.
In addition, Framatome ANP acquired a small section of a reactor vessel bottom identical to STP's Westinghouse designed and fabricated vessel. Practice was an important step, since it would ensure that personnel dose rates would be minimised and the job would be completed safely and efficiently.
After preparing for the job at its training facility and US headquarters in Lynchburg, Virginia, Framatome ANP technicians on 16 May began a comprehensive NDE programme consisting of ultrasonic (UT), visual and eddy current examinations. UT probes were mounted to the in-core inspection delivery tool, a type of robot designed to function as an integrated part of a plant's existing refuelling equipment. The probes were inserted into the penetrations from inside the reactor core. The probes are designed to identify the existence and geometry of any axial and circumferential flaws or cracks, as well as detect any weld flaws where the penetration tube and vessel are joined together. The probes convert ultrasonic sound waves into an electronic signal that is analysed by computers and converted into a series of graphic images that are recorded digitally and monitored in real-time on a computer screen outside the containment.
Eddy current tests, similar to UT but with electric current, and bare metal visual examinations were also performed on the penetrations and the J-groove weld surfaces, inside and outside the vessel. Framatome ANP employed a specially-designed eddy current tool to examine the J-groove weld, and the visual inspection was accomplished with a high resolution camera.
The NDE was completed over the 24-26 May Memorial Day weekend, and it was revealed that there were axial flaws in the tube walls of the two suspect penetrations. No circumferential flaws were found. The J-groove welds showed no crack-line indications on the eight penetrations inspected with J-groove eddy current. And there was more good news for STP, when the other 56 penetrations showed no flaws. Framatome ANP is replacing sections of the two penetrations with newer, more crack resistant Alloy 690 tubes and welding material.
Additional NDE will be performed on the repaired penetrations before the 15-year-old plant is expected to be returned to service. Further examinations on removed samples of the affected material will be conducted and analysed to support conclusions regarding the root cause of the flaws. The NRC will be looking closely at this data and is considering requiring all nuclear units in the USA to conduct bare metal inspections of reactor vessel bottoms during the upcoming autumn outage season. The 1315 MWe unit 1 should be back on line by the end of this summer.
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