Calculation of the importance-weighted neutron generation time using MCNIC method S.A.H. Feghhi a, * , M. Shahriari b , H. Afarideh a a Department of Nuclear Engineering and Physics, Amirkabir University of Technology, Tehran, Iran b Department of Nuclear Engineering, Shahid Beheshti University, Tehran, Iran Received 28 August 2007; received in revised form 23 January 2008; accepted 25 January 2008 Available online 10 March 2008 Abstract In advanced nuclear power systems, such as ADS, the need for reliable kinetics parameters is of considerable importance because of the lower value for b eff due to the large amount of transuranic elements loaded in the core of those systems. All reactor kinetic parameters are weighted quantities. In other words each neutron with a given position and energy is weighted with its importance. Neutron gener- ation time as an important kinetic parameter, in all nuclear power systems has a significant role in the analysis of fast transients. The difference between non-weighted neutron generation time; K; standard in most Monte Carlo codes; and the weighted one K y can be quite significant depending on the type of the system. In previous work, based on the physical concept of neutron importance, a new method; MCNIC; using the MCNP code has been introduced for the calculation of neutron importance in fissionable assemblies for all criticality states. In the present work the applicability of MCNIC method has been extended for the calculation of the importance-weighted neu- tron generation time. The influence of reflector thickness on importance-weighted neutron generation time has been investigated by the development of an auxiliary code, IWLA, for a hypothetic assembly. The results of these calculations were compared with the non- weighted neutron generation times calculated using the Monte Carlo code MCNP. The difference between the importance-weighted and non-weighted quantity is more significant in a reflected system and increases with reflector thickness. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The issue of determining the accurate value of kinetic parameters such as the neutron generation time has been demonstrated to be of major importance in designing reac- tor safety features. In advanced nuclear power systems, such as ADS, the need for reliable kinetics parameters is of considerable importance because of the lower value for b eff due to the large amount of transuranic elements loaded in the core of those systems. The calculation of these parameters is usually performed with deterministic codes. There are three general available methods in this regard (Verboomen et al., 2006): (i) the direct integration method (Keepin, 1965); (ii) the perturbation method (Henry, 1958); and (iii) the iterated fission probability method (Bohl and Margolis, 1964). For advanced nuclear power systems such as ADS, the Monte Carlo method is the preferred calculation tool since it has the ability to handle nuclear data not only in its most basic but also most complex form: continuous energy cross-sections, complex interaction laws, detailed energy- angle correlations, multi-particle physics, S(a, b) tables for thermal neutron scattering by molecules and crystalline solids, unresolved resonance probability tables. It also can handle very complex 3D geometries. As a result, normal critical systems as well as sub-critical systems with an exter- nal source can all be calculated with a single code, practi- cally without making any approximation. Due to this growing use of Monte Carlo codes one now also desires the calculation of the kinetic parameters by such codes. Among various time interval definitions characterizing the life of a neutron through a given system, the neutron 0306-4549/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.anucene.2008.01.018 * Corresponding author. Tel.: +98 21 64542591, +98 91 22109461; fax: +98 21 66495519. E-mail address: A.feghhi@aut.ac.ir (S.A.H. Feghhi). www.elsevier.com/locate/anucene Available online at www.sciencedirect.com Annals of Nuclear Energy 35 (2008) 1397–1402 annals of NUCLEAR ENERGY