BACKGROUND
Proper control of heavy loads is critical in any industrial application as faulty equipment or practices can have severe consequences. The lifting technique, equipment, and operator qualifications must all meet or exceed applicable standards to ensure industrial safety. The significance of heavy lifts at commercial nuclear facilities is, perhaps, even greater. In addition to the consequences of an adverse event that are common to any industry (bodily injury or human fatality, equipment damage, etc.), the nuclear industry adds additional challenges. Such an adverse event in the nuclear industry can also affect (depending on the specific lift) fuel geometry / criticality, system shutdown capability, damage to safety systems, etc. One example of a critical lift in nuclear power facilities is the reactor vessel head / reactor internals lift. The requirement to inspect the heavy lifting equipment for structural integrity is prescribed in NUREG-0612, Control of Heavy Loads At Nuclear Power Plants, as enforced by NRC Generic Letter 81-07. The aforementioned NUREG document describes specific requirements for special lifting devices. The requirements prescribed include:
- Special lifting devices are subject to 1.5X rates load followed by visual
inspection, or - Dimensional testing and nondestructive examination (NDE) of the load bearing welds
In the case of the former requirement, it can be difficult or even dangerous to test these lift rigs, which are designed to carry over 150 tons, at a factor of 1.5x. In the case of the latter requirement, employing the more traditional NDE techniques of MT, PT, and UT to inspect the lift rigs can be costly (both in terms of labor and radiological dose) and time consuming, in terms of impact to outage critical path, depending on when the inspection is performed. In PWRs or BWRs, inspections are performed in the reactor containment, or radiation-controlled area, and are typically only performed during the outage.
Ultimately, the NRC requires licensees to determine how they will comply with the NUREG requirements. One method that has been adopted (primarily by PWR plants) is Acoustic Emission (AE) testing. AE testing is a non-destructive testing process that uses high-frequency sensors to detect structure-borne sound emissions from the material or structure when under load. The process detects these acoustic emission events and, based on sensor locations and the known sound velocity and attenuation, can identify the approximate location of the sources or areas of concern. If such areas are identified, based on analysis of the data captured under load, those areas must be further investigated to characterize the indication. Such additional techniques may include surface examination (MT or PT), or volumetric UT to precisely locate, characterize, and size any indications.
