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Experimental neutron characterization (or dosimetry) of surveillance capsule irradiation is a main step in the surveillance program of irradiation effects on pressurized water reactor (PWR) vessels. Recently in France, results were obtained with 50 capsules removed from 29 900-MW (electric) reactors and analyzed using a standard method implementing, among others, the TRIPOLI Code to determine the perturbations caused by structural parts (stiffeners) and the neutron spectra in the capsules with their variance-covariance matrices.
The method is briefly explained, emphasizing some choices that were made concerning the uncertainty evaluations. Then the results of neutron fluxes and fluences are discussed, and the mean measured value for each reactor series is compared with the computed one.
In France, PWRs in operation are highly standardized. There are only 2 models of 900- MW reactors, called “CPO” and “CPY.” The 50 capsules examined were taken from these 2 series of reactors and can be arranged into 6 classes only. Some classes have as many as 18 elements, as, for example the “U” capsule class, irradiated at a 20° position in the “CPY” series. In each class, the observed standard deviation and estimated uncertainty are discussed. Because some classes have a strong statistical weight, the averaged results are highly significant.
These reactors are constructed and operated very close to a standard model in power, flux, and dosimetry measurements. Therefore, results are weakly scattered. They confirm the calculations and indicate that the uncertainty we find for capsule fluence is reasonable. The best is only 6.1% (2 σ). Knowledge of the vessel fluence (and of its uncertainty, which is being evaluated) is an essential factor for plant life anticipation.
light water reactors, reactor dosimetry, surveillance dosimetry
Commissariat à l'Energie Atomique, Centre d'Etudes Nucléaires de Grenoble, Grenoble,