Page 20 - PR 2014 2016 09 Nuclear Reactors and Fuel Cycle

264 Nuclear Reactors and Fuel Cycle | Progress Report

with the need for a flow at high flow rates to collapse of the plates. The critical velocity was
ensure the cooling of the fuel element in op- reached with 14.5 m/s leading to the conse-
eration, may create problems of mechanical quent plastic deformation of the plates form-
failure of fuel plate due to the vibration in- ing the flow channel. The central channel had
duced by the flow in the channels. In the case a 3mm aperture in its center, causing a large
of critical velocity, it may cause collapse of the blockage of the flow in the lateral channels.
plates. Although there is no rupture of the fuel This behavior was observed visually during
plates during collapse, excessive permanent the disassembly of the test section. Blocking
deflections of the plates can cause blockage of of the channels was also observed by means
the flow channel in the reactor core and lead of graphs of pressure drop and graphs of the
to overheating in the plates. For this study, deformations of the entrance, center and exit
an experimental bench and a test section of the plates against the average speed of the
that simulates a plate-like fuel element with section of tests. It was observed a decrease of
three cooling channels were developed. The the hydraulic resistance of the section of tests
test section was constructed with aluminum due to the increase of the transversal section
and acrylic plates and was instrumented with of flow in the central channel and an expo-
strain gauge sensors, pressure sensors, acceler- nential increase of the deformations when
ometer and a tube of pitot. The dimensions of the critical velocity occurrence. Comparatively,
the test section were based on the dimensions the value obtained for critical velocity in the
of the designed fuel element of the Brazilian test section through the experiments was of
Multipurpose Reactor (RMB). The experiments the order of 85% of the value obtained by cal-
performed attained the objective of reaching culation with Miller’s theoretical expression
Miller’s critical velocity condition with the (Figures 27 to 29).

Figure 27. Loop and Test Section for the experiment.

Instituto de Pesquisas Energéticas e Nucleares