Effective weight loss

A full assessment of core behaviour is extremely com­plex involving the many parameters already described. In some circumstances, such as coolant optimisation (see Section 10.3 of this chapter), a simpler parameter is required related to core integrity. A major change occurring during core life is the reduction in reserve factors caused by loss of strength due to radiolytic oxidation.

As described in Section Ю.4 of this chapter, changes in gas composition through the brick result in com­plex weight loss profiles through the brick, high carbon monoxide concentrations giving flatter profiles than low carbon monoxide concentrations. A series of ex­perimental strength measurements, together with two and three-dimensional finite element strength calcula­tions were carried out to determine the major para­meters determining brick strength.

The basic strength of a 25 mm brick slice was measured by applying a load via four keyways in a plane perpendicular to the brick axis. Relative loss of strength was measured on slices profiled to repre­sent various weight loss distributions, the reduction in thickness modelling the loss of modulus across the brick and the strength at the point of failure. All of the test slices failed by crack propagation from the root of a keyway generally propagating to the inner channel wall but in a few cases the failure was from key way to key way. The two — and three-dimensional calculations were carried out using the finite element program ‘BERSAFE’ with loadings as used in the brick slice tests. The two-dimensional mesh was suc­cessively refined to give a minimum radius at the keyway corner of l mm as this is the maximum notch radius which has a stress raising effect in graphite.

The two-dimensional mesh was ‘grown’ into the third dimension such that the thickness at any point was proportional to the modulus at that point. Both series of calculations showed that there was a stress concentration around the keyway root in agreement with the site of failure in the slice tests.

The relative failure loads for the three sets of data were plotted against various weight loss functions to determine any correlation with the strength loss re­lationship described in Section 10.4 of this chapter. It was found that the loss of strength can be deter­mined for all the weight loss profiles from the effective weight loss (EW) as given by: EW = 1/2 (mean weight loss — t — keyway root weight loss).