Analysis of Sodium Void Reactivity Measured in FCA V-1 Assembly

Abstract

The analysis of sodium void reactivity measurements has been performed. The measurements were made in the FCA V-1, which is a fast critical assembly intended for physics mock-up of the experimental fast reactor “JOYO”. The voided zones were the central region of the core and channel drawers extending throughout the core height. The calculations were performed using the JAERI set with 70- group structure.

As a result it was found that the conventional two-dimensional calculations underestimate measurements by about 10–40% in the core region. However, if the axial streaming effects are considered, the ratio between calculations and experimental results become about 0.9 for almost all cases of channel void. The streaming effects are calculated with use of Benoist's formula of anisotropic diffusion coefficients.

The heterogeneity effects on the spatial neutron flux distribution are taken into consideration by the collision probability method. The effect is large in the central part of the core (about 20% negative reactivity). The elastic removal cross sections are precisely obtained for the predominant resonances of light elements and compared with the conventional set. The influence on sodium void reactivity is not so large in this assembly.

KEYWORDS:

• sodium void reactivity
• fast critical assembly
• slab lattice
• diffusion calculation
• neutron streaming effects
• heterogeneity effects
• leakage
• spectrum effects
• elastic removal cross section