DP-THOT - A Calculational Tool for Bundle-Specific Decay Power Based on Actual Irradiation History

Abstract

A tool has been created for calculating the decay power of an individual fuel bundle to take account of its actual irradiation history, as tracked by the fuel management code SORO. The DP-THOT tool was developed in two phases: first as a standalone executable code for decay power calculation, which could accept as input an entirely arbitrary irradiation history; then as a module integrated with SORO auxiliary codes, which directly accesses SORO history files to retrieve the operating power history of the bundle since it first entered the core. The methodology implemented in the standalone code is based on the ANSI/ANS-5.1-1994 formulation, which has been specifically adapted for calculating decay power in irradiated CANDU reactor fuel, by making use of fuel type specific parameters derived from WIMS lattice cell simulations for both 37 element and 28 element CANDU fuel bundle types. The approach also yields estimates of uncertainty in the calculated decay power quantities, based on the evaluated error in the decay heat correlations built-in for each fissile isotope, in combination with the estimated uncertainty in user-supplied inputs. The method was first implemented in the form of a spreadsheet, and following successful testing against decay powers estimated using the code ORIGEN-S, the algorithm was coded in FORTRAN to create an executable program. The resulting standalone code, DP-THOT, accepts an arbitrary irradiation history and provides the calculated decay power and estimated uncertainty over any user-specified range of cooling times, for either 37 element or 28 element fuel bundles. The overall objective was to produce an integrated tool which could be used to find the decay power associated with any identified fuel bundle or channel in the core, taking into account the actual operating history of the bundles involved. The benefit is that the tool would allow a more realistic calculation of bundle and channel decay powers for outage heat sink planning. Information on the operating state of the core is calculated and recorded at regular intervals using the SORO fuel management code. To retrieve the complete history of an individual bundle requires tracking the bundle back from its current location, through all previously occupied locations in the core, to the fuelling operation at which the bundle first entered the core. Taking account of all intervening core states, including periods of shutdown or reduced reactor power, a histogram can then be constructed representing the entire power history of the bundle up to any subsequent point during its residence within the core. Existing capability within the SORO auxiliary suite of codes was built upon to facilitate the process of retrieving the complete irradiation history of a bundle specified in terms of its current location in the core. An interfacing routine was written to automate the process of obtaining input from the user, retrieving the bundle power history from SORO files, providing the history in the format needed for input to the decay power calculation module, executing the decay power program, and directing output back to the user. Information supplied by the user comprises identification of the reactor unit and bundles or channels which are of interest, the time and date of shutdown subsequent to which decay powers are to be calculated, as well as the associated decay time intervals. Where the decay power for complete channels is requested, the calculation is completed on a bundle by bundle basis, and then the resulting bundle decay powers are summed to yield the overall decay power for the channel. When a core scan is requested, the calculation is undertaken for every channel and bundle in the core. The tool has been extensively tested and results from its application are presented.