Isıl-Nötronik Etkileşimlerin Ve Nükleer Veri Kütüphanelerindeki Belirsizliklerin Nötronik Parametrelere Etkisi
Özet
Recently, investigation of effects of simplifications on nuclear models and used nuclear data libraries on calculation results have gained importance. Predicting uncertainties caused by these effects will provide more accurate approaches in calculations and more efficient results in nuclear reactor safety.
In first stage, the aim of this study is the investigation of the effect of using radial temperature distribution on the neutronic parameters in unit cell calculations. Modelling of nuclear fuel unit cell is a frequently used method to simplify nuclear reactor calculations and constant temperature in radial direction is used by defining fuel as one region during investigation. In this study, the fuel was divided into several regions having equals volumes and investigation was carried out by determining temperature distribution in radial direction in fuel with using thermal conductivity correlations. In order to perform this investigation, the communication between neutron transport code OpenMC and nuclear data processing code NJOY and thermal module which was developed with Python was provided. Different types of light water reactors were examined and the results were compared with the one region approach. This investigation was performed with ENDF/B-VII.1, TENDL-2014 and TENDL-2015 nuclear data libraries and thus, it is aimed to observe the effects of arising from the usage of different nuclear data libraries.
In second stage, uncertainties based on nuclear data libraries are examined by using TENDL random nuclear data libraries. TENDL-2014 random nuclear data libraries were used for two fundamental isotopes 235U and 238U in UO2 nuclear fuels. Again, the investigation was carried out with reactor types in the first stage. Different multiplication factor value was obtained due to usage of different nuclear data library for each of the simulation of the same isotope with same parameters. A distribution was obtained from multiplication factor value results of numerous simulations by using different random nuclear data libraries. The uncertainty values, which were caused by the nuclear data library of used isotopes in examined reactor types, were determined with using obtained distributions.
In the results, it was found that an underestimation of up to about 100 pcm in multiplication factors of different type of light water reactor types due to using constant temperature instead of using temperature gradient in radial direction in nuclear fuel unit cell. There is an underestimation of up to about 8% in calculations of Doppler coefficient due to not using temperature gradient. There are about 100 pcm difference for multiplication factors obtained with used ENDF/B-VII.1 and TENDL nuclear data libraries. Uncertainties caused by used TENDL 235U and 238U nuclear data libraries were observed differently in different reactor types and the highest values are 594 and 470 pcm for 235U and 238U isotopes, respectively.
