Cook's pressure vessel upgrade

Thursday, 20 September 2007 Major maintenance and upgrade work has started at Cook 2. The nuclear power unit will get a new vessel head and new steam turbine generator control software.

The pressurized water reactor, built by Westinghouse in the early 1970s, set a number of records before the refuelling outage, during which the maintenance is being carried out. It generated power continuously for 497 days - a best for the unit. Generating 12.9 million kWh of electricity in that time, it output 100.6% of its design capacity.

Work began on 15 September to replace the reactor vessel head, the top portion of the reactor pressure vessel made removable for refuelling. It is through this enormous component that the control rod drive mechanisms operate to regulate the reactor power. These crucial components operate in conditions of high temperature and pressure and require regular inspection to guard against possible corrosion and cracking.

The new vessel head for Cook 2, measuring 5.1 metres in diameter and weighing 90 tonnes, comes complete with new control rod drive systems. It was forged in one piece in Japan and fabricated by Areva in France over three years. More than 1500 extra workers will be on-site during the works, in addition to the 1400 that normally work there.

The old vessel head will be housed on-site temporarily before transport for burial as low-level radioactive waste.

Another major upgrade to the reactor is the addition of inline temperature sensors, which will make it possible to remove 500 ft of piping and 40 valves previously used to monitor reactor coolant temperature. When this change was made at Cook 1, which underwent the same maintenance program a year ahead, it resulted in a 50% reduction in the radiation workers were exposed to during some jobs. It could reduce radiation dose to workers by 25% overall.

Cook 2 will also get a new digital turbine control system, meant to improve reliability and ease of operation. Owners American Electric Power spent over $70 million on the work carried out at both Cook units. With Nuclear Regulatory Commission approval for licence extensions already secured, unit 1 could operate until 2034, and unit 2 until 2037.

Further information

American Electric Power