CRDM housing gets a needed inspection1 March 2000
When a leak was discovered in a housing of a part length control rod drive mechanism, a remote inspection approach was developed – and accepted by the USNRC – that could determine whether plants with similar components could avoid expensive parts replacements.
Early last year, a US plant shut down when a leak in the reactor vessel closure head began to increase. On investigation, it was determined that the leak was in the upper pressure housing for a part length control rod drive mechanism (PL-CRDM), as shown in the diagram. Initially, it was presumed that the leak was in the canopy seal weld, which has had occurrences of similar leaks in other plants in the past.
After a weld repair to the canopy seal, the leak unexpectedly continued and it was visually confirmed to originate above the canopy seal near a dissimilar metal weld joining sections of 304 stainless steel and 403 stainless steel. All four of the PL-CRDM assemblies were removed from the reactor closure head and replaced with welded caps.
ABB CE Nuclear Power was called in to perform a high-priority inspection of the leaking pressure housing to help identify the cause and source of the leak. The initial manual ultrasonic inspection revealed a crack-like indication 360° around the circumference and through-wall for approximately 20 mm. This result caused concern that similar flaws could lead to complete joint rupture and a small-break loss of coolant accident (LOCA). The US Nuclear Regulatory Commission requested all utilities with this design PL-CRDM to evaluate performing either an immediate inspection or a replacement of the suspect components to mitigate LOCA concerns.
Manual ultrasonic inspections of installed PL-CRDMs were not possible, because there was limited access and high radiation levels in the region of the suspect weld. Within a week ABB CE Nuclear Power developed and qualified a remote inspection capability based on ABB CE Nuclear Power’s IntraSpect™ ultrasonic imaging system and the companion AMAPS™ automated pipe scanning system. The initial system qualification was performed using the actual removed cracked housing during the destructive examination in a hot cell facility.
The IntraSpect inspection system used a modified pipe scanner with a custom self-centering and locking track device, to access the weld from above the reactor vessel head and control rod drive pressure housings. With the low profile of the AMAPS pipe scanner, a minimal amount of hardware had to be removed from the CRDM housings.
Following the emergency field deployments, a more detailed technique qualification was performed to demonstrate the detection capability of the system. Realistic PL-CRDM mockups were manufactured for a demonstration of the ultrasonic technique. ABB performed a blind test on these samples, which was monitored by the Electric Power Research Institute at its NDE Center. The results of these demonstrations were presented to the NRC, which accepted the approach as a basis for continued operation of other potentially-affected plants.
The destructive testing of the leaking housing revealed that the flaw was a fabrication welding hot tear at the fusion line between the 403 stainless base material, and that there was no active degradation mechanism. However, there was still a safety concern that other housings in service could contain similarly produced hot tears that could lead to leakage or joint failure.
In rapid sequence, a total of ten US plants used ABB’s automated ultrasonic inspection method. In combination, the total population of PL-CRDMs inspected was used as a basis for a statistical sampling argument to quantify the remaining risk for other plants. Ultimately, the USNRC accepted this sampling approach and further testing or repair at other plants was no longer required.
While some plants elected to replace the PL-CRDMs rather than perform the inspection, the economic benefits of the inspection option became obvious. Comparing the inspection and repair approaches, the inspections were performed with one tenth the labour hours required and 100 times less radiation exposure. Tens of millions of dollars were saved in avoiding the costs of inspection and replacement of CRDM housings.
ABB’s project team was rewarded for this work with the ABB Customer Focus Award for the Americas Region, an annual award presented for “an outstanding contribution that demonstrates attention to the customers’ needs”.