Page 361 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Core thermal hydraulics 327

6.2.3.4.1 Blockages due to particles

A local blockage, either at the inlet or within a fuel assembly, can lead to severe dam-
age and failure of the fuel. Therefore, the thermal-hydraulic behavior of a partially or
completely blocked fuel assembly is of crucial importance for the safety analysis.
A blockage in a fast reactor fuel assembly may have serious effects on the safety
of the plant leading to fuel assembly damage or melt. The main goal of the safety stud-
ies with respect to blockages is to demonstrate that local effects do not propagate
toward neighboring assemblies. The external or internal blockage of a fuel assembly
may impair correct cooling of the fuel pins, be the root cause of anomalous heating of
the cladding and of the wrapper, and potentially impact also fuel pins not directly
located around the blocked area. One of the root causes of a fuel assembly blockage
is the aggregation of solid matter (oxides), dislodged from its intended location or gen-
erated in the coolant and transported inside and along with the coolant’s flow or orig-
inating from a cladding failure, releasing fuel particles in the bundle. This matter could
stop inside the fuel assembly with its narrow spaces and interfere with the coolant. The
main consequence of such a blockage is a reduction of the coolant flow rate through
the fuel assembly. A blockage can be instantaneous (when a large enough piece of
material obstructs a portion of the subchannels) or time dependent (when the aggre-
gation of solid matter piles up in the subchannels). For grid-spaced fuel assemblies, an
internal blockage is generally located in the first grid, and it has a flat-like shape
(Schultheiss, 1987), while for a wire-spaced fuel assembly, the shape of the internal
blockage is elongated following the wire.
The experimental study of the effect of blockages in LMFRs is difficult, expensive,
and time-consuming. Traditionally, numerical analysis of fuel assembly blockage was
carried out by system thermal-hydraulic codes. Nevertheless, for internal blockages,
local phenomena are dominant, and CFD is strictly necessary to capture temperature
peaks. The use of CFD allows to gain detailed insight in the effects prior to the exper-
iments, even with more details than can be extracted from the experiments. The exper-
iments on the other hand will allow to validate the CFD approaches.
Di Piazza et al. (2014) studied the impact of internal central blockages (up to
61 subchannels) at the entrance of the heated zone in an ALFRED (grid-spaced) fuel
assembly as shown in Fig. 6.2.3.14. They conclude that there is a local effect increas-
ing the temperature of pins and cladding in the recirculation zone behind the blockage
that is dominant for large blockages. On the other hand, a global effect caused by a
decrease in mass flow rate and therefore an increase in local temperature at the
end of the heated zone is observed to be dominant for small blockages. Furthermore,
they conclude that blockages occupying more than 15% of the flow-through area can
be detected by proper temperature measurements in the outlet of the fuel assembly.
Marinari et al. (2016) describe the design of an experimental campaign in the
NACIE-UP facility to check these conclusions. In this design phase, they perform pre-
test CFD simulations for side blockages in a 19-pin fuel assembly mock-up. From the
simulation, they see the same effects in this small-scale mock-up as in the full-scale
ALFRED fuel assembly. Fig. 6.2.3.15 shows the recirculation region (right) and the
temperature distribution at different planes for one-sector blockage case. The pretest