A new method for calculating the design strength of fuel cans for liquid metal cooled fast reactors

Abstract

The method described shall provide a possibility for calculating the maximum design pressure and corresponding fission gas plenum length for cladding tubes of liquid metal cooled fast reactors with oxide fuel. The material properties — especially the creep behaviour — as well as temperature and power distributions must be given. The first part of the method deals with creep relaxation of a composite mechanical-thermal stress state on the basis of Norton's formula. Using integral relations for the strain rates the initial equations are transformed into a system of integro-differential equations. These, after substitution of the integrals by easily calculated constant mean values for only few and relatively long periods of time, change into ordinary differential equations for which an approximate analytic solution can be quoted. This solution is further simplified and finally leads to the definition of a “limiting stress” analogous to the yield strength of the material. Thus the problem is reduced to the well-known case of an elasticplastic tube under internal pressure and intermittent heat flux; its solution can be generalized to yield a criterion for the limitation of the tube loading caused by intolerable cyclic growth, the so-called “thermal ratchetting”. The method is applied to several examples with the aid of two small digital computer programs, and the results lead to conclusions for the appropriate design of fuel pin claddings.