Page 200 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

System thermal hydraulics for liquid metals 171

Theoverallheight,measuredbetweentheaxesoftheupperandlowerhorizontalpipes,
is 7.5m, and the width is 1m. The maximum inventory of LBE is in the order of 1000kg,
and the loop is designed to withstand to temperatures and pressures up to 550°Cand
10bar, respectively. The facility can work both in natural and forced circulation condi-
tions, and the transition from forced to natural circulation can be investigated as well.
Concerning the operation under natural circulation regime, the difference between
the thermal center elevation of the heat source (FPS) and the heat sink (heat
exchanger, HX), H, is about 5.7m. This difference is able to provide the pressure head
(Δp DF ¼gβΔTH) required to guarantee a suitable LBE mass flow rate even under nat-
ural circulation conditions. Under forced circulation conditions, a gas-lift technique is
adopted to promote LBE mass flow rate along the loop. A pipe with an inner diameter
of 10mm is housed inside the riser connected through the expansion gas top flange to
the argon feeding circuit, while at the pipe lower section, a nozzle is installed to inject
argon into the riser promoting enhanced circulation inside the loop. The gas injection
system is able to supply argon flow rate in the range 1–20NL/min with a maximum
injection pressure of 5.5bar. The argon gas flows into the riser and is separated, in the
gas expansion vessel, from the coolant flowing upward to the cover gas, while the
LBE flows back into the heat exchanger through the upper horizontal branch.
According to the described configuration, the maximum LBE mass flow rate is around
20kg/s in gas-lift forced circulation and 5kg/s in natural circulation conditions.
Fig. 4.7 shows the NACIE loop as installed in the HLM experimental-hall laboratory
at the ENEA Brasimone Research Centre.
Under forced circulation isothermal conditions, the density difference between the
LBE in the “downcomer” and the LBE/argon mixture in the riser can be expressed as

h i
Δρ ¼ ρ ρ ¼ ρ ρ 1 αð Þ + αρ g ¼ αρ ρ g
l
m
l
l
l
where ρ l is the density of the LBE in the downcomer, while ρ is the average density of
m
the two-phase mixture in the riser, defined by the void fraction α, and ρ represents the
g
average gas density in the riser.
The gas density strongly depends from pressure, which is mainly given by the LBE
column mass above. Then, the pressure changes as the gas rises up in the riser. Since the
gas injection line runs all along inside the riser, driving the gas to its bottom part, it is
reasonable to assume the rising gas in thermal equilibrium with the liquid metal. In order
to simplify the model, it is useful to define the average gas density ρ as the density
g
calculated at the average pressure in the riser, which means the pressure at z¼H R /2.
Since the void fraction is relatively small, the average pressure can be estimated as
H R
p ¼ p 0 + ρ g
l
2
where p 0 is the cover gas pressure. Then, it is possible to write the pressure difference as

Δp DF ¼ αρ ρ g gH R
l