Browsing by Author "Becker, Björn"

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    On the influence of the resonance scattering treatment in Monte Carlo codes on high temperature reactor characteristics
    (2010) Becker, Björn; Lohnert, Günther (Prof. Ph.D.)
    The different interactions of neutrons in a nuclear reactor like absorption, production or scattering are generally introduced by solving the well known Boltzmann transport equation. Its solution scheme is based on the balance between losses and gain of neutrons. It turns out that all commonly practical ways to solve this basic equation exhibit an intrinsic inconsistency. The neutron scattering loss term includes explicitly the impact of the temperature and cross section shape on the interaction probability, namely the Doppler broadening of the integral scattering cross section. However, the calculations of the neutron gain based on the above mentioned scattering event assumes zero Kelvin temperature and ignores the shape of the cross section. For heavy nuclei with pronounced resonances like 238U this inconsistency leads to noticeable errors in the evaluation of core parameters as was shown by Ouisloumen and Sanchez, and Rothenstein and Dagan who developed the correct resonance dependent scattering kernel. In this work, a new approach, namely a stochastic method is presented for the calculation of the Doppler broadened integral cross sections as well as for the new developed resonance dependent scattering kernel. A stochastic methodology, based on an idea of Rothenstein, named as the “Doppler Broadening Rejection Correction” (DBRC) is implemented as a solver for the above mentioned missing resonance dependent kernel in the scattering treatment of Monte Carlo (MC) codes. The flexibility of using the unique form of a rejection method is confirmed, for the first time, by comparing it to the analytic S(alpha,beta) scattering tables approach. However, the DBRC scheme allows for a much wider and practical use of the improved scattering kernel theory. In addition, the DBRC method is validated by several experiments in particular by a dedicated 232Th scattering experiment done at the Gaerttner Institute of the Rensselaer Polytechnic Institute (RPI) in the USA. The influence of this new, stochastic resonance dependent scattering kernel on core parameters is extensively investigated for High Temperature Reactors (HTR). Both, pebble bed and block type HTR designs are considered. A significant impact is found in unit cell calculations as far as criticality, reaction rates and Doppler reactivity coefficients are concerned. The criticality decreases up to 1.20 % (depending on temperature and on the HTR design). The Doppler coefficient is found to be more negative by up to 10 %. In addition, the neutron flux shape in the vicinity of resonances and the fuel inventory during burn up change noticeably when the new resonance model is applied. Further on, for industrial relevant purposes, it is shown that for a full scale model of the Chinese HTR-PM reactor, the DBRC kernel leads to a reactivity decrease of about 0.20 % and renders the Doppler reactivity coefficient being more negative by about 6 % to 8 %.