Although a brief summary of the experimental technique is given below, a more detailed discussion can be found in:

F. Gavelli and K. T. Kiger, High-resolution boron dilution measurements using laser induced fluorescence (LIF), (2000) Nuclear Engineering and Design.195, pp.13--25.

Laser Induced Fluorescence (LIF) is a common technique used in experimental fluid mechanics to measure the concentration of a scalar species within a fluid. The fundamental principle of quantitative LIF is that the fluorescent intensity given off by an illuminated volume of dyed fluid will be proportional to the number of dye molecules contained within that volume. With that being said, the difficulty in obtaining quantitative measurements is usually in ensuring that the illuminating intensity is constant (i.e. there is no significant absorption of the incident light). This is usually done by controlling both the maximum concentration of the dye and the path length over which the illuminating beam has to travel before reaching the observation volume.

When applied to the Boron dilution event in the downcomer of the PWR, we needed to develop a method to illuminate and view the downcomer volume with a high spatial resolution, while maintaining adequate temporal resolution and minimizing the illumination beam path within the dye. Since the downcomer is largely an thin annular volume, it made the most sense to have the illumination source pojected radially outward from the centerline, and then scanned azimutally with a sufficient rotation speed to satisfy our temporal resolution requirements.

Illumination was provided by a 5 W Ar+ continous laser, which gave a narrow wavelength, highly collimated light source that could be used to excite many commonly available fluorescent dyes. The PWR was modified from its sister integral facility by removing the core internals, which redirected the water that would normally occupy this volume through an exit in the bottom of the tank (after the flow passed through the first lower plenum distributor plate). This allowed the optical instrumentation to be placed in the region normally occupied by the core.

The light sheet in the radial plane was created with a high-speed octogonal mirror which rotated at 6000 rpm and sheet was scanned azimuthally at a rate of 2 Hz by mounting the entire optical platform on a heavy-duty crane bearing. The fluorescene intensity was monitored by placing a half-silvered mirror along the illuminating beam path, which allowed for a continuously aligned view of any fluid that intersected the instantaneous beam trajectory. Thus the intensity measurements represent concentrations which are inherently radially averaged due to the optical set-up. The light collected from the half-silvered mirror was long-pass filtered to block any light produced by specular reflections and then measured by a sensitive photomultiplier tube which was sampled at a rate of 500 MHz. This gave an effective spatial resolution of approximately 3 mm in the axial and 4 mm in the azimutal directions.

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