Analysis of Intermittent Buoyancy Induced Flow in Asymmetrically Obstructed CANDU Nuclear Reactor Fuel Channels

Abstract

<p>CANDU reactors have a large number of active and passive systems which help to mitigate events such as a loss of forced cooling or a loss of electrical power. During these events, decay heat must be removed using either emergency power driven active systems or through natural circulation phenomena. Depending on the configuration of the core at the time of the event, single or two-phase natural circulation is available to transport energy from the core to the steam generators which act as a heat sink. However, in some configurations (such as during a maintenance outage) natural circulation to the boilers may be precluded due to either the boilers or heat transport system being partially drained. Under these conditions flow may stagnate in the horizontal fuel channels which will initiated a mode of circulation known as Intermittent Buoyancy Induced Flow (IBIF).</p> <p>During IBIF, void generated in the center of the core will periodically reach a feeder and vent from the channel due to buoyancy induced forces, with liquid entering the channel from the opposite feeder, cooling the fuel. Analysis is typically performed to demonstrate that during the period of time from stagnation to venting fuel temperatures remain within their acceptance criteria. This mode of emergency heat removal can remove sufficient heat to preclude fuel failures until such time as forced flow can be re-established.</p> <p>It is assumed in most analysis that completely symmetrical configurations represent the worst possible configuration since most asymmetries (i.e., in channel axial heat flux, channel creep etc.), would lead to earlier venting and hence lower fuel temperatures. However, asymmetry in the form of a partial obstruction in the fuel channel during IBIF may occur due to maintenance or inspection activities- a scenario that has not been included in previous experimental and computational work on IBIF. This paper presents the analysis of the impact of partial flow obstruction on IBIF scenarios. A model is constructed using the code GOTHIC, and is benchmarked against experimental data from the Cold Water Injection Test (CWIT) facility for both the symmetrical and asymmetrical configurations. The work demonstrates that for single-phase IBIF, the impact of partial obstructions is not prohibitive. Computational demands precluded the completion of two-phase simulations in a reasonable timeframe; however useful insights and observations are drawn from the early stages that are successfully simulated.</p>