October 11, 2001
This off-the-beaten-path article defines a remote GTAW process that allows weld operators to minimize their exposure to radiation when welding spent fuel canisters used to contain nuclear fuel.
Among many tasks that Nuclear Utilities perform is to ensure that radiation workers are exposed to the lowest possible amount of radiation. The industry has to follow strict exposure limits, and all nuclear energy workers must ensure the radiation exposure is as low as reasonably achievable (ALARA).
A particular area of concern is when spent nuclear fuel is sealed in storage canisters. Typically, workers weld the canisters shut using track-mounted pipe welding equipment, and the operators are relatively close to the radiation source. Recent innovations in canister welding allow operators to seal these canisters remotely, thus lowering their exposure risk while still being able to seal them efficiently.
A dedicated, automated, remote gas tungsten arc welding (GTAW) system can perform canister welding. The welding manipulator uses a multiaxis arm to position the welding torch at the proper location to accomplish the desired weld. The manipulator is mounted with a flange assembly to the radiation shield plate.
A video camera system with the necessary viewing angles for observation monitors the welding process. The video cameras are designed and installed so they do not impede the welding process and allow complete video monitoring. Under field conditions, the control consoles, power supplies, and high-efficiency particulate air (HEPA) systems are located up to 50 feet away for ALARA compliance.
The manipulator has three main components:
1. Central Mast. The bottom of the central mast has a mounting flange for attaching the manipulator to the intermediate radiological shield plate.
2. Rotating Arm. Above the flange is a bearing surface that supports the rotating arm. The top of the mast has a series of slip rings and rotating unions used to transfer services to the rotating arm from the stationary mast. The slip rings and rotating unions transfer data, control and video signals, welding current, welding shield gas flow, and recirculating torch coolant.
3. Torch Assembly. The torch assembly holds the tungsten electrode that provides the welding arc. It is liquid-cooled and has an integral water jacket. The torch assembly recirculates coolant through an external chiller located near the welding power supply.
The torch body is made from tellurium copper, a conductor of heat and electrical current. The front of the torch is fitted with a ceramic cup and gas diffuser similar to that used for narrow-groove welding. This provides the least amount of physical interference and allows for the maximum amount of temporary radiological shielding.
The assembly consists of modular components, each easily removed for repair or replacement. The wire feed nozzle steering mechanisms are mounted on either side of the torch body. The video camera assemblies attach to the torch body-mounting bracket.
The torch assembly has twin high-resolution, close-coupled discharge (CCD) color video cameras that provide the necessary viewing angles for monitoring the welding process and examining the dye penetrant.
The system can be programmed with a variety of weld sequences. The primary sequence performs the perimeter welds on the inner and outer closure lids. This sequence presets positions of the following:
Workers weld the closure lids using GTAW. The weld sequence includes an initial root pass, intermediate groove-filling passes, and a final cover pass. The inner lid is welded to the canister body shell around its circumference.
Drain or vent ports used to dry and immobilize the canister protrude through a hole in the inner lid at two locations. Depending on the canister system design, port covers or closure rings are placed into position and welded.
A work platform or temporary scaffolding erected around the cask at a suitable elevation provides access to the top of the canister. Technicians must stand on the work platform during the installation and removal of the welding manipulator and closure lids and while applying and removing the dye penetrant chemicals. In addition, any grinding required for weld repairs is done manually from the work platform.
All other operations are completely remote, with no support required on the work platform. Temporary radiological shielding can be provided as needed around the top of the canister to minimize the exposure to workers.
The canister welding system is designed to operate in harsh environments. These can include elevated temperatures, high radiation levels, and surfaces contaminated with radioactive materials and boric acid residue from contact with spent fuel pool water.