Pumping liquid CO2 in nuclear power stations
13 May 2010
Reactor cooling medium requires low temperature and high pressure, while its inherent compressibility and poor lubricating properties also place special demands on pump design
Within the nuclear power industry, liquid carbon dioxide (CO2) is the main form of temperature cooling during the reaction process in the UK’s advanced gas-cooled reactors. The CO2 to top up the reactors is stored at two pressures: at 20 bar to receive supplies from road tankers; and at 50 bar in order to be able to top up the reactors and refuelling plant.
To overcome the problems with the original rotary pumps used for CO2 pumping at its Heysham and Torness nuclear power stations, British Energy, part of EDF Energy, is currently installing four purpose-designed triplex positive displacement pumpsets at each site. The equipment was supplied by Cat Pumps, which partnered with Star Refrigeration on the contract.
“The main problem experienced at both power stations when transferring liquid CO2 was the very short operating life of the existing rotary pumps,” reports Cat Pumps UK general manager Brian Hubbard. “These pumps required frequent maintenance and eventual replacement.”
Each nuclear reactor is cooled using CO2 which is held in a high pressure tank at 50 bar. This is replenished by pumping liquid CO2 from low pressure tanks where it is stored at 20 bar. The eight Cat 6861K Series pumps are specially adapted to handle liquid CO2 and configured to handle a flow rate of 6 tonnes/hour.
For reactor topping-up duties only one of the four pump systems is in service, with the other three being on stand-by. During refuelling the CO2 storage system is segregated into two parts. In this mode there is a pair of pumps in each part - one duty and one on auto-standby.
To maintain the CO2 in its liquid phase, it is sub-cooled using Star Refrigeration R404A air-cooled refrigeration plant. The sub-cooler lowers the temperature of the CO2 from the low pressure bulk storage tanks from -16ºC down to -18ºC prior to the inlet to the pumps.
Mounted in purpose-designed skid frames, the 6861K triplex plunger pumps feature a forged-billet fully machined 316 stainless steel pump-head for optimum liquid and process compatibility, strength and corrosion resistance. The plunger seals are lubricated only by the liquid CO2 and run on concentric, high density polished alumina ceramic plungers.
The pump seal arrangement is configured to predict future seal life and allow planned routine servicing. Three high pressure seals are connected to a leak detection system for CO2 monitoring and the three low pressure back-up seals are installed in a way that minimises the risk of moisture build-up inside the seal spacers, which could otherwise result in ice formation.
The high pressure CO2 pump-head is completely oil-free with a spacer section and extended plunger rod and nitrile elastomers/PTFE seals between the CO2 pump end and the oil lubricated crankcase end, preventing cross-contamination. The pump is gear-driven via a 12kW TEFC motor built to the demanding BE electrical and mechanical specifications.
Reports indicate that the first two pumpsets now installed at Torness are performing well and are a vast improvement on the previous rotary pumps. Two more sets have been installed at Heysham and Torness is installing its third unit.
