Pressure tactics
A C Thompson and C M Hodge's efforts at
measuring pressure vessel flows have received a
warm response. Fluid Power understands why
The Department of Nuclear Science and
Technology (DNST) in UK had recently
undertaken a design study based on a
hypothetical two core pressurised water reactor. The
general layout of the plant established earlier utilised
Reactor Pressure Vessels (RPVs) with several inlet and
outlet nozzles. The initial work involved the
development of a Computational Fluid Dynamics
(CFD) model of a PWR whose objective was to
establish the flow patterns within the RPV of a PWR.
This was required so that any changes in flow could
be assessed when the number of inlet and outlet
nozzles was reduced to only one of each. If the flow
within the RPV remained satisfactory after such an
alteration it would allow the removal of the manifolds
between the primary loop and the inlets and outlets of
the RPV that are present in the NAC39 design. This
simplification would remove the associated shock and
frictional losses from the primary circuit reducing
head loss, thus improving flow. This alteration would
also simplify the manufacture of such a RPV due to
the reduction in transition pieces required. The
reduction in the number of welds required would also
lower through life costs and the dose burden due to
the reduced requirement for in service inspection.
The flow between the inlet to the RPV and the
core inlet has attracted a number of investigators
recently. The present work aims to examine the effect
of a single inlet on both the uniformity of core inlet
flow and the flow in the downcomer. These two
regions are thought to be the most likely to be
affected by changes to the inlet nozzle. The
importance of a well-distributed flow to the core inlet
is obvious; the downcomer contains thermal shields
that also require an even flow to promote cooling.
RPV flow model
Since the inlet flow patterns under steady state
conditions are the main object of study, steady,
incompressible flow with no heat transfer is assumed.
The fluid selected from the standard properties library
was water at 27 degree celsius. Water at ambient
temperature might well be used in a scale model test
to examine the same phenomena. Cylindrical polar coordinates
were adopted with 90x50x50 cells in the X
(azimuthal), Y (radial) and Z (axial) directions of the
fluid domain. A core barrel diameter of 1.0 m was
assumed in advance of the final design details being
fixed. This core diameter allowed for a square core of
25 modules of the type used in the Shipping port
Pressurised Water Reactor. The base of the RPV was
created using the 'Corebase' and 13 'Diffuser' blocks
to model the bowl shape of the lower head where the
flow is turned round from vertically downward to
vertically upward at inlet to the core. Each object has a
starting position and a size in the X, Y and Z
directions. The geometry 'polcube' of this first - 2 -
group of objects specifies a simple box in polar
coordinates, the type 'blockage' means the box is filled
by material given by 'attributes'. The next object is the
core barrel, a sleeve running from the top of the RPV
model to just above the base. Flow enters the annulus
between the core barrel and the RPV wall (called the
downcomer) and passes downward to the lower head
before flowing up into the core. Two further objects,
the thermal shields, sit in the lower half of the
....CONTD
