Nuclear Engineering and Design 235 (2005) 1635–1649 Development of SIRIUS-N facility with simulated void-reactivity feedback to investigate regional and core-wide stability of natural circulation BWRs M. Furuya a,b, ∗ , F. Inada a , T.H.J.J. Van der Hagen b a Nuclear Energy System Department, Central Research Institute of Electric Power Industry, 2-11-1 Iwado-kita, Komae, Tokyo 201-8511, Japan b Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received 21 July 2004; received in revised form 26 November 2004; accepted 18 January 2005 Abstract The SIRIUS-N facility was designed and constructed for highly accurate simulation of core-wide and regional instabilities of a natural circulation BWR. A real-time simulation was performed in the digital controller for modal point kinetics of reactor neutronics and fuel-rod conduction on the basis of measured void fractions in reactor core sections of the thermal-hydraulic loop. Stability experiments were conducted for a wide range of thermal-hydraulic conditions, power distributions, and fuel rod time constants, including the nominal operating conditions of a typical natural circulation BWR. The results show that there is a sufficiently wide stability margin under nominal operating conditions, even when void-reactivity feedback is taken into account. The stability experiments were extended to include a hypothetical parameter range (double-void reactivity coefficient and inlet core subcooling increased by a factor of 3.6) in order to identify instability phenomena. The regional instability was clearly demonstrated with the SIRIUS-N facility, when the fuel rod time constant matches the oscillation period of density wave oscillations. © 2004 Elsevier B.V. All rights reserved. 1. Introduction Regional oscillations have been observed (D’Auria et al., 1997) in several forced circulation BWRs under close-to-natural circulation conditions where the sta- bility margin is lower in the power/flow map. The term ∗ Corresponding author. Tel.: +81 3 3480 2111; fax: +81 3 3480 2493. E-mail address: furuya@criepi.denken.or.jp (M. Furuya). ‘regional oscillations’ refers to oscillations of neutron flux and flow in different regions of the core, which are out of phase with each other. By nature, the regional os- cillation observed was in the first azimuthal mode and thus out of phase between symmetry regions separated by a diagonal line of the core. The stability margin to re- gional oscillation becomes narrower when (1) nuclear coupling between regions in the core becomes weaker, (2) channels are thermal-hydraulically less stable, and (3) core diameter is larger (D’Auria et al., 1997). 0029-5493/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2005.01.007