Nuclear Science and Engineering / Volume 140 / Number 1 / January 2002 / Pages 70-85
Pressure vessel surveillance and benchmark dosimetry measurements are used to determine the effects of the plant-specific as-built core/internals/vessel materials and geometry on the vessel fluence. In several recent applications, uncertainties in these measurements and their interpretation have prevented the use of the dosimetry measurements in the benchmarking of the vessel fluence calculations. In this analysis, the uncertainties having a significant effect on the measurement-to-calculation comparisons used in the benchmarking are identified and evaluated, and the effect of these uncertainties on the >1-MeV vessel fluence derived from the measurements is determined.
The vessel >1-MeV fluence is determined by a weighted sum of the response from a set of 63Cu, 46Ti, 58Ni, 54Fe, 238U, and 237Np fast neutron dosimeters located on the outer wall of the thermal shield, vessel inner wall and/or in the cavity outside the vessel. The uncertainty estimates assume a well-maintained and calibrated measurement system and the use of state-of-the-art methods for interpreting the measurements. In the case where the effects of the individual uncertainties on the fluence are correlated, the specific correlation is calculated and properly included in the fluence uncertainty estimate.
The uncertainty in the >1-MeV fluence inferred from dosimeters located on the outer wall of the thermal shield or on the inner wall of the vessel ranges from 11 to 15% (1) depending on the specific type of fast neutron dosimeter. The uncertainty in the >1-MeV fluence inferred from dosimeters located in the cavity is significantly higher, due to the uncertainty in the iron cross section and the resulting uncertainty in the extrapolation to the vessel inner wall, and ranges from 19 to 23% depending on the type of dosimeter. These vessel fluence uncertainties are substantially larger than the uncertainty in the measured dosimeter reaction rates of 6 to 8% from which the fluence was derived.