At the service of steam generator maintenance30 May 2000
Improving the equipment available to service steam generators has helped operators achieve shorter outage times and reduced personnel exposure
To perform eddy current inspection, tube plugging, tube sleeving activities and other maintenance, workers have to enter a steam generator primary channel head through the primary manway. Improved equipment can increase reliability, shorten outages and reduce exposure.
New equipment can be the result of innovative design, or the foresight to transfer technology from another industrial application. Connecticut-based NES is one company whose work in steam generator servicing over many years has brought improvements in the US and more recently in Europe, South America and Asia.
Many utilities use nozzle dams inside their steam generators to isolate the primary channel head from the refuelling pool, allowing eddy current testing or repair of the tubes to be carried out simultaneously. NES nozzle dams are designed to fit into nozzles of diameter 30-42in (760-1070mm). The only access into the steam generator channel head is through a manway opening, typically 16in (406mm) wide and the NES dams are segmented to allow passage through this opening. There are high dose rates and contamination levels in the channel head so to meet ALARA objectives the dam must be rapidly installed, and sealed with zero leakage.
The most common type of installed retention ring in both Westinghouse and Framatome-designed PWRs is a flange ring with 20 threaded holes. The usual method for sealing the nozzle has been to use a folding bulkhead bolted against a compression seal applied to the top surface of this flange ring. NES’s Type-WR nozzle dam was developed to seal SG nozzles with these flange rings. The dam, which seals within the smooth inner diameter of the flange ring, consists of two rugged pneumatic seals and a third passive seal secured to eight of the 20 flange ring threaded holes. By using dissimilar materials, special thread configurations and other measures, the galling problems experienced with dams which are sealed at the top surface of the flange ring and use standard threaded connections have been eliminated.
NES has developed a nozzle dam specifically for replacement steam generators and those with no retention rings. It replaces the traditional flange ring configuration (with its threaded holes which are susceptible to galling, misalignment and breaking of bolts) with a single grooved retention ring that accommodates fast-acting lock pins resulting in short installation times.
Both types of nozzle dams also have:
•Redundant, pneumatic seals with sufficient throw and serrated face.
•Lightweight, self-aligning dam segments that can be assembled quickly.
•Manual installation time of one to two minutes.
•Remote installation capability.
•Eight connectors to transmit hydrostatic force from the dam to the flange ring.
•A design allowing failure of any one active component without breaching the dam.
•A one-piece diaphragm seal with integral pneumatic seals and a passive seal.
•Back-up passive emergency sealing activated by hydrostatic pressure.
•A leak detection system.
A segmented multi-stud tensioning system (SMTS) is a portable system designed to tension all studs on system closures. The system applies a predictable and accurate pre-load to all studs simultaneously, eliminating the problems – such as uneven gasket pre-loads resulting in closure leakage, galling and broken fasteners – associated with conventional torque-tightening techniques. As it speeds installation and removal of system closures in high radiation areas it helps meets ALARA objectives.
The NES SMTS consists primarily of several ring segments, an air-operated hydraulic control unit, and interconnecting high pressure hoses with quick couplings. Each segment contains hydraulic load cells, interconnected so they can be pressurised simultaneously from one port in each segment (using ethylene glycol or oil-based hydraulic fluids). The hydraulic load cells are fitted with self-energising lip seals that snap-fit into the piston housing to ease replacement. A handle on each segment means it can be lifted by one person.
The studs are tensioned via reaction nuts that are supplied with the system. To help achieve uniform initial tightness the system is also supplied with an in-line torque wrench and square torque adaptor bit, and round nut rotating sockets that incorporate magnets so they will be self-supporting when assembled over the closure nuts.
manway cover tool set
The NES manway cover handling tool set requires no eternal power source, avoiding failure-prone hydraulics or pneumatics. Operation relies on existing features of the manway, and the tool set can be used with studs or bolts. The set is hand-operated and is independent of the variables and interferences that normally make the operation and handling of floor-supported machines difficult. It handles primary manway covers weighing up to 1750lb (800kg).
The set comprises two guide rods, one jackscrew and two hoist bar studs and one chain hoist support bar and two chain hoists with cover support brackets. With additional tools, the tool set can also be used to handle a stud-tensioning ring.
The shield door reduces the radiation emitted from open SG channel head manways. The door can be locked to restrict entry of unauthorised personnel into the channel head and includes ventilation features.
The NES shield door consists of a support ring, an inner and outer shield (the door) and four support bolts (made of aluminium to prevent galling) which attach the support ring directly to the manway flange. The ring is split to allow door removal, while hoses and cables pass through the manway.
The lead shield and ventilation opening are integrated into the NES design. This is done by separating the radiation shield to create an annulus for airflow between the manway and the inner shield. A circular opening through the middle of the door allows quick movement of air between the hot and cold leg manways. There is a hinged, shielded opening for hoses and cables.
Threaded fasteners, studs, nuts and bolts from system closures must be cleaned each outage. Thread cleaning is usually performed manually with wire brushes in a low dose area away from the closure. The operation is time-consuming and could contaminate the area and the personnel.
The NES fastener cleaner rapidly removes lubricants, oxides or other chemical deposits, eliminating the spread of contaminants. The portable housing has a hinged cover that supports two rows of wire brushes, along with a vacuum suction outlet to direct all contaminants to a plant vacuum and HEPA filter system.