J. Sanza,b, R. Falquinaa, J.F. Latkowskic, S. Reyesc
a Dept. of Power Engineering Universidad Nacional de Educación a Distancia,
Ciudad Universitaria s/n, 28040, Madrid. Spain.
b
Instituto de Fusión Nuclear (UPM)
c
Lawrence Livermore National Laboratory, Livermore, CA, USA
The need to estimate the effect of activation cross section uncertainties in the accuracy of isotopic inventory calculations is an issue that is drawing more and more attention. Concerning this problem, cross section uncertainty files have been made available, such as FENDL UN/A-2.0, and some calculational procedures have been proposed. We developed a method based on the first order Taylor series, which was found practical for providing the uncertainty indices associated to each of the reaction cross sections in a continuous irradiation scenario. One of the drawbacks of the method is that its application to pulsed scenarios is difficult, and the other and most important is that it is impractical to deal with the synergetic/global effect of the uncertainties of the complete set of cross sections. To overcome these limitations, we have developed a Monte Carlo procedure based on simultaneous random sampling of all the cross sections involved in a problem, and it has been implemented in the activation code ACAB. The main characteristics of the method are presented here.
The procedure is applied to the analysis of the uncertainty in the radionuclide inventory and contact dose rate at the National Ignition Facility (NIF) gunite chamber shielding under a reference pulsing operation schedule, using the recently measured elemental composition. The parameters defining the shape of the cross sections distributions, assuming they are log-normal, are based on the data available in the FENDL libraries. Random sampling is used to produce different sets of cross sections values that are input to the ACAB code to obtain the probability distribution function for the random variables contact dose rate (D) and relative error of the contact dose rate (E), where E = (D — Do)/Do and Do is the contact dose rate obtained in the standard activation calculations, i.e. using the best estimate cross sections and not considering uncertainties data. Preliminary results show that the 95 percentile of the distribution of E (E95) can take values up to 1.2. Several cross sections are identified as contributing significantly to the contact dose rate uncertainty. E95 can be reduced down to 0.2 for the cooling times of interest by reducing a few critical cross sections uncertainties to 1/3 of the current value. Following the approach described above we can evaluate the required reduction in the uncertainties of the cross sections in order to obtain a required uncertainty in the contact dose rate.
The suitability of the method to deal efficiently with the global effect of the uncertainties of the complete set of cross sections on pulsed activation problems in NIF-type experimental facilities is proved. The present work also suggests promising expectations for the success of the model when applied to the uncertainty analysis of activation in IFE power plants, by using a continuous-pulsed model to represent the IFE real pulsed irradiation scenario. It is shown that this methodology together with that developed based on the first order Taylor series, allow fully comprehensive uncertainty estimation on activation calculations.
