Following a large loss of coolant accident in a PWR, cooling is performed by
superheated vapour with entrained droplets, which bounce from the hot metal without
wetting it. This thesis describes experimental and modelling studies aimed at the
evaluation of the direct cooling by these droplets. Droplet diameters are less than
2 mm, they spend ~15 ms near the surface, extract ~1/5 J, cooling the metal by ~50 oC
with heat fluxes of the order of MW/m2. An interface-tracking CFD code was used to
model the droplet approach, the generation of vapour from its underside and its
rebound or break-up, and to compute the transient cooling of the hot metal below the
droplet. Validation of this model requires measurements of the heat transfer. A novel
method to measure the transient surface temperature beneath the droplet is reported,
using transient high resolution infra-red spectroscopy. Spatial and temporal
resolutions of ~100.μm and ~4ms respectively are achieved, observing an opaque
metallic layer from beneath through an infrared-transparent substrate. Post-processing
via transient finite elements permits all thermal quantities (heat flux, energy, etc) to be
determined. Associated simultaneous high speed optical recording of the droplet
motion and deformation provided data for validation of the hydrodynamic aspect of
the prediction. It is estimated that these methods allow the heat extracted by (for
example) a 1.5 mm droplet during the 10 ms it spends in the vicinity of the hot surface
to be obtained with an uncertainty of 15%. This heat extracted is approximately
0.19 J, associated with a transient temperature reduction of ~47 oC, and is removed by
a heat flux peaking at 3.5 MW/m2. Encouraging agreement was obtained between
these measurements and the computational simulations. For this same case, the CFD
analyses predict 0.12 J and a peak heat flux of 5 MW/m2.