A comparison of constant power depletion algorithms

The depletion capability of the Common Monte Carlo Design Tool (CMCDT), under joint development at the Bettis and Knolls Atomic Power Laboratories, has been extended to include 'constant power' depletion calculations. CMCDT couples the Monte Carlo transport code MC21 and depletion routines that use the VODE variable-coefficient ordinary differential equation solver. Three predictor-corrector 'constant power' methods were implemented - the cell2 and monteburns algorithms, which rely on constant flux computations, and a linear rate method that allows selected components of the coefficient matrix to change linearly during the depletion interval. Comparisons are made of these three coarse step methods against a reference depletion calculation of a 17 x 17 PWR assembly containing Gd-UO2 fuel rods. The linear rate method gave consistently more accurate results, regardless of the size of the coarse time steps and the level of spatial detail within the Gd-UO2 fuel rods. All three methods have difficulty in modeling the fast rate of 157Gd depletion with the cell2 method having the most trouble. (authors)