David Davidson of EDF Energy explains the importance of pure water even for planned outages
EDF Energy owns and operates 15 nuclear plants at eight nuclear power stations in the UK. These stations have a combined capacity of just under 8,800MW - about 10% of the UK's total installed electricity generation capacity from all energy sources, which comes to about 85,000MW. One of the stations, Sizewell B, is a pressurised water reactor whilst the other seven - Dungeness B, Hinkley Point B, Hunterston B, Hartlepool, Heysham 1, Heysham 2 and Torness - all use Advanced Gas cooled Reactor (AGR) technology. Hunterston B power station at West Kilbride on the west coast of Scotland started generation in 1976, and its two advanced gas cooled reactors are capable of generating 890MW of electricity, sufficient for 1.7 million homes.
In an AGR, the nuclear core consists of enriched uranium-clad stainless steel cans with graphite moderator, and reaches a temperature of over 600°C. This heat is transferred from the core by pressurised carbon dioxide coolant. This hot gas gives up its heat to water in the boilers, forming superheated steam at high pressure. This steam is then piped away to drive the turbines, which in turn drive the electricity generating sets, and is then condensed back into water by sea water cooled condensers. A condensate polishing ion exchange plant removes impurities from condenser in-leakage and corrosion residues from the condensed steam prior to its return to the boilers.
At such high operating steam pressures, control of boiler water chemistry is critical to minimise corrosion and deposition. Like all nuclear power stations, Hunterston B has a sophisticated ion exchange plant to produce high purity boiler make-up water with conductivity less than 0.1µS/cm and silica concentration less than 10µg/l SiO2. Clearly an uninterrupted supply of high purity water is vital to the continuous operation of the power station, and its loss would result in a station shutdown - no simple matter in the case of nuclear reactors which take a considerable time to shut down and re-start. The loss of revenue, should this occur, is estimated at £1m per day. But water of the same quality is also required for reactor cooling prior to planned maintenance outages. The demand in this situation is higher than the ion exchange plant can deliver and, although the station has reserve feedwater storage to cover 48 hours of normal operation, the financial implications of water loss are so high that EDF Energy decided that additional security was required.
The solution that EDF adopted was to set up a service contract with Veolia Water Technologies' Mobile Water Services team (formerly known as AQUAMOVE) under which one of Veolia's fleet of mobile deionised water trailers would be on site within 24 hours of a phone call. The trailer houses two streams of ion exchange units (cation-anion-mixed bed) which can be run in sequence or in parallel, and are capable of producing 150m3/h of 0.1µS/cm high purity water. All of the equipment is installed in a standard 40' trailer which is insulated and supplied with heating and lighting so that, inside the container, the environment is just like most plant rooms. The trailers are also kitted out with safety equipment as necessary. Once exhausted, the complete trailer is returned to the central regeneration facility, which means zero discharge on site and no problems of handling or disposal of regenerant chemicals. That means that the on-site chemical systems can be completely isolated whilst maintenance or repair work is carried out.
The advantage of a contract of this type is that all the important details - where the deionised water trailer can be parked, the location and type of connections for raw and treated water - are on record so there are no delays when the trailer arrives. Further, Veolia's technicians have been inducted in site safety and method statements have been agreed in advance, so installation of the trailer can commence immediately.
Hunterston B has used a temporary trailer over thirty times to cover routine water treatment plant maintenance, to supplement the ion exchange plant during planned outages and for the rare ion exchange plant failure or potential problem such as a resin change.
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