Mechanical interaction between fuel and cladding

Abstract Because UO 2 expands more than the zircaloy cladding tube, is hotter, cracks and swells, the UO 2 will interact with the cladding. How much, is dependent upon the gap, length of pellet, whether it has flat ends or not, the depth of the dish, the width of the shoulder, heat load, rate of power increase and cladding properties. The stress-strain pattern can be very complicated. It usually has a changing multiaxiality with consequent varying strains to fracture, and the strains may be concentrated over small lengths of the cladding tube. All this adds up to a threat to the integrity of the zircaloy, particularly since the properties of this material are so adversely affected by irradiation. To avoid difficulties and failure, the pellet shape, dimensions and density and cladding properties should be chosen with due regard to reactor system conditions, the way the reactor is loaded and operated. The ideal cladding should have good ductility and high strength at the same time. Such a combination seems impossible, unfortunately. Many of the processes described we know only qualitatively. We have, therefore, a definite need for many more precisely defined and executed experiments, preferably in-pile. Precisely dimensioned, shaped and formed hardware has but little use, if flux shape along a rod and heat production cannot be determined with commensurable accuracy. So, while on this end of the fuel design spectrum we toil to sharpen out tools, may I express the hope that our colleagues on the other end of the spectrum, the physicist and the thermohydraulicist, will make equal progress.