Various reports of incomplete control rod cluster assembly insertion trig­gered investigations to identify causes of the sticking problem. The root cause was understood to be excessive deformation (bowing) of fuel assem­blies. When the bowing exceeds the limit, it increases friction between the control rod and the guide thimble and can result in the breaking of the con­trol rod cluster assembly.72 The mechanism of bow, though not clearly under­stood yet, is believed to be caused by creep affecting the overall assembly and guide thimble. If the fuel assembly experiences a flux gradient, the tubes at the lower flux side will grow less than those in high flux side causing the fuel rod to bow. Provision is made in the design, to accommodate an increase in length of the fuel rod (due to creep and growth) on either end and any restriction in the free movement leads to bowing of the rod. Rods under cold-worked stress-relieved (CWSR) conditions show more elonga­tion than those under fully recrystallized conditions.72