Cooling collapse stress

A large natural-draft cooling tower collapsed in a 70-mph (110-km/hr) v/ind, probably due to imperfections in the shape of the tower, which led to stresses greater than those it was designed to take and caused bending collapse [10]. 

Most blown film operations extrude the resin in an upward direction. However, blown polypropylene film is generally extruded downwards and water or mandrel quenched. The extruded tube is then reheated, to a point still below its melt temperature, before it is blown. The collapsed bubble can be fed over a series of heated rollers to reheat it and relieve thermal stresses if a heat-stabilized film is wanted or it can be heated and reinflated in what is known as the double bubble process, which will be discussed in Section 7.3.7. In either case, the film is restrained until cooling is complete, to keep it from shrinking. 

In the reactor, the clad is subjected to a very complex set of phenomena, particularly when the power is cycled. The most important parameters alfecting clad performance are thermal stress cycling and stress concentrations caused by nonuniform axial and nonaxisymmetric fuel behavior. Both these factors are expected to be less damaging in sodium-cooled reactors than in gas- or water-cooled systems, because high coolant pressures force the clad to collapse onto the fuel and follow its movements as the power varies. They can, however, still significantly affect fuel pin life. 

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