Sub-channel analysis of rod bundle thermal hydraulics: Effect of eccentricity and blockage

A comprehensive study of rod bundle thermal-hydraulics in a nuclear reactor, enables us with safe and reliable margins for its operation. Although computational fluid dynamics (CFD) is an ideal choice, the number of degrees of freedom and the detail increases exponentially with increase in bundle size. Performing a large number of simulations, over the entire range of operating conditions and other parametric combinations makes CFD prohibitively expensive. Hence, it is preferable to pursue simple and effective reduced order models. To this end, a wide range of sub-channel analysis codes have been devised in the literature. One such model is the popular deformation and intermixing analysis in nuclear assemblies (DIANA) algorithm. In the present study, this algorithm is numerically implemented and validated for different rod bundle configurations. The effect of eccentricity and blockage and their combined effect on the thermal hydraulics of a 19 rod bundle is systematically analyzed. The thermal-hydraulics simulations are performed in the eccentricity (e) range of 0.0 ≤ e ≤ 0.7. It was observed that, eccentricity leads to flow maldistribution in some sub-channels, which in turn effects the coolant temperature distribution at the outlet. Using the present approach, a number of what-if type scenarios such as, blockage, bundle deformation and their combinations are also numerically investigated. It was noticed that, the presence of blockage in a sub-channel results in a reduction in the local coolant mass flux and hence the local clad temperature shoots up. In the narrowest sub-channel, for a typical case of eccentricity e = 0.5 and blockage ratio (b) = 0.4, maximum decrease in mass flux was observed to be 85% and increase in the coolant and clad temperatures was found to be 150% and 304% respectively. To emphasize the utility of the present study, the simulations are extended for a 217 sub-assembly with multiple blockages.