Assessing the mechanical performance of a fuel bundle

The computer code BEAM helps assess the mechanical integrity of a fuel bundle. This report describes the features and the validation of the code. The focus of the code is to provide a fast and simple tool for calculating the following parameters of a fuel element and of the adjacent end plate: axial and lateral stiffness of the fuel element; spring constants of the end plate; stresses in the end plate and in the end cap/end plate weld; frequency of lateral vibrations (radial and tangential); collapse pressure; and buckling load. On-power effects such as pellet expansion, hourglassing, densification, creep and fission product swelling are especially important in these calculations. The code is based largely on analytical closed-form equations. A small segment of the code contains numeric integrations. The rigidity enhancement factor is derived from on-power measurements of frequencies. The calculations of the code show reasonable agreement with axial and flexural rigidities measured in out-reactor tests using many combinations of sheath and pellet dimensions and using various degrees of pellet/sheath interferences. In addition, the calculations also compare well with measurements of lateral frequencies in out-reactor and in-reactor tests. The BEAM code is fast. To simulate a typical irradiation history representing a residence period of about one year (136 time steps) the code requires 12 CP seconds on the Chalk River CYBER 990 computer. The turnaround time is about 20 seconds. The BEAM code was used in an investigation of end plate cracking in the Darlington reactor. The calculations suggested that the axial stiffness of the fuel elements is influenced significantly by element power, pellet densification, and sheath creep. Another application of BEAM involved the assessment of end plate integrity due to differential axial expansion of fuel elements in a low void reactivity bundle. (Author) (22 figs, tab., 16 refs.)