One of the largest and most complex heat treatment operations has been successfully completed as part of a 30 million-pound project which will ensure the future of BNFL's Sizewell power station well into the Millennium. The project involved removing and replacing a large circumferential weld on three of the four boilers associated with the second Magnox Reactor. Up to 350 specialist contractors took part and much of the time was spent on preparation work since the boilers had to be stripped down and taken to pieces. State-of-the art technology was used in the form of new checking methods to ensure that these units will operate safely and efficiently for many years to come.
The preheating and postweld heat treatment of each boiler was a critical part of the project and this was carried out by Didcot Heat Treatment Limited of Abingdon who allocated a total of 84 engineers for the operation. Preheating was applied to an extensive area surrounding the weld to eliminate undesirable stresses and drive off water and the post weld heat treatment was designed to reduce internal stresses caused by welding and to soften any tempered areas. To achieve the desired results a considerable amount of heating was required due to the construction of the boilers which are substantial thick walled steel vessels. The process could only be attempted once since any error may have rendered the boiler unsuitable for service. For this reason heat treatment experts worked closely together to specify the equipment and process to ensure that every precaution was taken. Managing Director of Didcot Heat Treatment, Andy Anderson said "this has been a very demanding job and is a remarkable achievement for all concerned." Large areas on each boiler were heated and every section was shown on detailed drawings and meticulously logged. Extensive preparation was required since electrical heaters and measuring thermocouples had to be installed and wired up before the whole area was covered with thermal insulation in order to retain the heat. Over 800 temperature-measuring thermocouples were fixed to each weld and a computerised monitoring system was used to show instantaneously the individual and average weld temperatures. Detailed computer printouts were produced and the temperature of every segment was adjusted to maintain the profile required. The operation needed an enormous amount of electrical power and ten transformers were required for the task, each rated at 11 KVA.
Most of the equipment used for the heat treatment process was provided by Mannings Thermal & Environmental Engineers of Southport who had to manufacture special heaters to meet the requirements of Didcot Heat Treatment and their consultants.
Over 12,000 ceramic pad heaters were supplied and these were arranged to suit the contours of the shell and to fit around obstructions on the surface, such as tubes and nozzles. The ceramic heaters consist of multi-strand nickel chrome element wire which is electrically insulated by interlocking sintered alumina beads to produce a flexible mat with a high heat transfer capacity. In addition, Mannings supplied over 100 heat treatment centres, as well as special recorders and other equipment, including over 200 miles of cable. Les Oliver, who is Manager of Industrial Heating at Mannings said "this is one of the most interesting jobs we have been involved in and probably one of the most technically demanding ever undertaken due to its size."
At the end of the contract BNFL as well as Mitsui Babcock Energy Ltd, who were main contractors on the project, commended Didcot Heat Treatment for their performance and for the reliability of the equipment used. All three boilers are now being reassembled, tested and recommissioned and BNFL will soon be in a position to seek permission from the independent watchdogs, the Nuclear Installations Inspectorate, to start operating Reactor Two once more.
For further information please contact Mr Les Oliver or Mr A Anderson, Didcot Heat Treatment Limited, The Quaternion Unit 1, Station Yard, Steventon, Abingdon, Oxon, OX13 6RX. Tele : 01235 862017. Fax : 01235 862018 Email: dhtinfo@aol.com
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