Auto-correlation and variance-to-mean measurements in a subcritical core obeying multiple alpha-modes

[EN] Neutron noise measurements based on the Rossi-α and Feynman-α methodologies have been performed in a heterogeneous subcritical system. It is shown that the traditional single alpha-mode formulations of the Rossi-α and Feynman-α methods are not applicable due to the presence of higher alpha-modes. Formalisms taking into account multiple alpha-modes are applied resulting in satisfactory results. Three alpha-modes could be identified using the Rossi-α method, whereas only two could be obtained using the Feynman-α method. In the Feynman-α case, the possibility to obtain the fastest decaying alpha-mode was diminished due to detector dead time effects. It was found that the slowest decaying alpha-mode does not exactly correspond to the prompt decay found in pulsed neutron source measurements, which confirms the results of previous studies. Strengths and weaknesses of the multiple alpha-mode Rossi-α and Feynman-α methods observed in this study are pointed out.This work was supported by IP-EUROTRANS contract no. FI6 W-CT2005-516520, the ENRESA-CIEMAT agreement for the Transmutacion Aplicada a los Residuos Radiactivos de Alta Actividad, Svensk Karnbranslehantering AB (SKB, the Swedish Nuclear Fuel and Waste Management Co) and the Swedish Institute through the Visby program.Berglöf, C.; Fernández Ordóñez, M.; Villamarín, D.; Bécares, V.; González Romero, EM.; Bournos, V.; Muñoz-Cobo González, JL. (2011). Auto-correlation and variance-to-mean measurements in a subcritical core obeying multiple alpha-modes. Annals of Nuclear Energy. 38(2-3):194-202. doi:10.1016/j.anucene.2010.11.009S194202382-