An Analytic Model for the Prediction of In-Pile Behavior of Oxide Fuel Rods

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Abstract
The prediction and analysis of fuel rod performance in the CYGRO-3 model is an extended version of the earlier models CYGRO-1 and CYGRO-2 arising from the LWBR development program. The model assumes that circumferential and axial variations in conditions are small compared with radial variations. Fuel and clad are considered as a set of interacting concentric ring elements. Time-dependent values of temperatures, stresses, and deformations (elastic and creep effects) are calculated as the response to a history of coolant water conditions and of rod power and neutron flux. Provision is made to calculate effects of swelling due to pore growth and of thermally induced pore migration in the fuel. An approximation to fuel property changes as a result of cracking are introduced via changes in the elastic relationships. Frictional interaction between fuel and clad when the latter is in the collapsed condition is provided. Forces introduced by fuel rod supports are included. A first-order calculation of straightening moments introduced by circumferential variation in power can also be performed.

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