Pellet-cladding interactions and length changes in SGHWR fuel pins

Fuel pins in the Steam Generating Heavy Water Reactor increase in length during irradiation, due largely to a mechanical interaction between the cladding and the fuel pellets. The length change produced by this process depends critically on the deformation behaviour of the fuel pellet under the interaction stresses. This paper presents a model of the pellet deformation which has been incorporated into a computer programme, SLIM, capable of predicting extensions from this and other causes in SGHWR fuel pins subjected to any power history. SLIM is shown to give good agreement with measured length changes for all variants of fuel rating, burn-up and pin type tested, and predicts that extension of the narrower pin design now in use should be reduced by a factor of 0.7 compared with similarly rated standard pins. (orig.)