In Pressurized Water Reactor (PWR) nuclear power plants, boric acid is added to the cooling water to act as a passive neutron absorber which controls the nuclear fission reaction rate within the core. Therefore, a major concern for nuclear reactor safety is the possiblity that a volume of boron dilute water may accumulate in a region of the primary system and, subsequently, flow through the reactor core: in this case, the neutron absorption effect would be reduced, causing a potentially serious power surge. Several scenarios have been identified, by which a slug of boron free water can accumulate in portions of the primary cooling loop: for example, through a fresh water leak from the secondary cooling system, or through the boiling and condensing of primary coolant following a Small Break Loss Of Coolant Accident (SB-LOCA) (Hyvarinen, 1994). In either scenario, the actuation of one (or more) of the primary coolant pumps will put the slug in motion towards the reactor core. The turbulent flow along the path to the core will cause the dilute slug to mix with the surrounding borated water. The impact of the slug flow through the core is clearly dependent upon the degree of mixing it encounters before entering the fuel region. Thus the important question is: How much mixing will take place as the slug moves into the reactor core?
 

FIG 1: View of RPV annulus, cold	FIG 2: Close-up cross-section of
leg inlet pipes and pump.	RPV annulus.

Although much of turbulence research has been dedicated to trying and understand the turbulent mixing of a passive scalar, most work has been confined to studies of prototypical flows within pipes, jets, wakes, or boundary layers. The current flow configuration is a complex amalgam of all these geometries, further complicated by the fact that the injection process is unsteady (i.e. the flow is accelerating during the injection event). Hence, the previous work does not directly extrapolate to the current problem. Previous experimental work performed at the University of Maryland provided an analytical tool to quantify the global effect of the mixing phenomena occurring in a PWR downcomer. However, to gain a better understanding of mixing within this complex geometry, a transparent replica of the inlet pipes and the annular coolant jacket surrounding the reactor pressure vessel (RPV) has been constructed in order to enable direct visualization and quantitative measurement of the mixing process (see FIG 1 and FIG 2 above). 

In this study we have used a combination of Laser Induced Fluoresence (LIF) and Laser Doppler Velocimetry (LDV) to measure the concentration and velocity of flow as the slug is transported to reactor core. Details are available on the following pages about the experimental procedure and results .
 

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