Mechanical design codes are used to evaluate the mechanical behaviour of the fuel assembly. They often also have the functionality to be used for thermal analyses if the temperatures of the assembly structural components (i. e. everything excluding the fuel pins) are of interest. The standard technique employed is the finite element method, where each component of the assembly is discretised into a number of volume elements. Given suitable material properties, and any (potentially time dependent) external loading, the (potentially time dependent) stresses, strains and displacements applicable to each finite element are then determined by solution of the underlying matrix equations for the force balances, stress-strain relations and strain-displacement relations.

The primary application of mechanical design codes is to calculate the stresses imposed by the loads applied to the various assembly components (during normal operation, anticipated operational occurrences and accidents). Other uses include vibrational mode and harmonic response analysis, and buckling assessments.

Mechanical design calculations have historically been performed using in-house codes; more recently, ‘off the shelf’ commercial finite element software packages have tended to be employed.

418 Nuclear fuel cycle science and engineering