TPT March 2017

T E CHNOLOG Y

Straightening of zirconium nuclear fuel clad tubing

NUCLEAR electrical power generation reactors are powered by uranium fuel. To separate the uranium and prevent contamination of the reactor coolant, the uranium fuel must be contained in a robust environment that can withstand the high temperatures and the intense neutron radiation as nuclear fission takes place. The material used to contain the uranium must have important characteristics such as high thermal conductivity, high strength, high resistance against corrosion and high dimensional stability. The containment of the nuclear fuel is normally a tube made from various zirconium alloys. These tubes are referred to as ‘fuel clad tubing’ or ‘fuel cladding’. Each zirconium fuel clad tube is filled with natural or enriched uranium pellets and sealed. A complete nuclear fuel cell assembly consists of a number of tubes contained in a rigid assembly. Extreme care has to be taken during the manufacture of zirconium fuel clad tubing to prevent any excessive cold working of the tube. Cold working causes the material to be susceptible to hydride orientation within the high temperature water environment in which they operate. During manufacture the tubes have to be formed, finished and, at the same time, not be subjected to excessive cold working. The desired accuracy of tube straightness is of particular importance and, therefore, straightening the tube without excessive working of the material is challenging. It is in this difficult nuclear arena that Turner has succeeded in becoming a foremost supplier of zirconium fuel clad tube straightening machinery.

Nuclear fuel clad tubing production

pressure applied by the rolls to the tube during straightening. Analytical software within the computer aided setting and management (CASAM) system monitors the load applied by each roll on the tube and provides control of the machine. The load cell signal conditioning and amplification is carried out by highly accurate, custom designed and built electronic circuits. The Turner CASAM system – now in its third decade of operation around the globe – has additional operational modes and calculating algorithms for use on this specific straightening application. A typical Turner ten-roll CASAM machine for straightening zirconium nuclear fuel clad tubing guarantees performance on all the important characteristics required in the final product. Because of the need to prevent even the slightest marking or contamination of the tube during straightening, the majority of Turner machines for the nuclear industry are supplied as complete straightening lines including fully integrated entry and exit tube handling equipment. Turner zirconium fuel clad tube straightening machines are in use throughout the world including China, Korea, Sweden, Canada and the USA. Turner Machine Company – USA Website: www.turnermachineco.com

Its history in serving the nuclear tubing industry stretches back more than four decades, during which its understanding of the problems and their solution have slowly evolved. For this challenging straightening ap- plication the most modern ten-roll com- puter driven machinery is now normal, replacing older six-roll machines. Turner has long been at the forefront in precision straightening machine innovation and specialised straightening applications and has developed important features applicable to straightening this particular product. These features prevent the possibility of resulting hydride orientation in the tube by ensuring that the amount of pressure applied to the tube during straightening is kept to a very low level. Some of these features include: special rolls which feature specific profiled regions that reduce the amount of friction applied to the tube as it is straightened; load cells positioned strategically within each roll pair, which measure the amount of roll pressure being applied to the tube; and analytical software, which receives the data from the load cells and ensures that the amount of cold working applied to the tube does not exceed set points, maintaining roll pressure at a very low, safe level. The special roll profiles ensure that the amount of friction generated between the roll and the tube is kept to the absolute minimum. The load cells monitor the direct

32

www.read-tpt.com

MARCH 2017