Page 158 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

130 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

3.4.2 LBE melting and first time filling

LBE is usually delivered in solid ingots, which have to be melted in order to be intro-
duced into the facility. For large facilities requiring significant volumes of LBE, a ded-
icated melting vessel is recommended with a possible batchwise filling procedure.
Batchwise filling would typically require a repeated procedure of melting some of
the ingots in the melting vessel, performing mechanical removal of oxides from
the free surface when necessary, and draining the melt from the bottom of the melting
vessel into the experimental facility. Draining from the bottom of the melting vessel
reduces the possibility of introducing buoyant solid oxides into the facility. This pro-
cedure therefore requires a valve on the filling line between the melting tank and the
experimental facility, and the procedure is repeated until the facility is full. For precise
feedback of the LBE volume transferred into the facility, the melting vessel could be
mounted onto a scale, to record the mass of LBE transferred per batch. The volume is
easily calculated if the temperature of the melt is known and the density is therefore
also known. Alternatively, the volume of transferred LBE can be estimated from the
LBE level inside the drain tank using preinstalled level sensors. Continuous level mea-
surement is possible with laser or acoustic distance meters or float level sensors. Dis-
crete level measurements are possible with electric contact probes that are easy and
cheap to manufacture.
If the melting vessel is an open vessel, workers should take the necessary safety
precautions against splashing of liquid metal, such as wearing protective full-face
shields, suitable long-cuffed gloves, and suitable aprons. A removable swing-arm
lid is a useful safety addition to the melting vessel. The melting procedure should
be performed at temperatures as low as possible, without risking localized freezing,
to reduce oxidation. For LBE, this could be 150–180°C and 380–400°C for lead.
A heavy inert cover gas such as argon could be used to reduce oxidation at the free
surface, but this is not mandatory (take note that argon gas is an asphyxiant and is
heavier than air). During and after melting, inherent impurities in the supplied ingots
and resulting oxides will float to the free surface where they can be mechanically
removed, for example, with a metallic mesh scoop. To avoid redistribution of any
oxides, the removal of oxides should be performed with minimal disturbance to the
melt mixture.
The filling line from the melting vessel and the sections of the facility being filled,
usually the drain tank and connected piping, should be preheated before making the
first transfer of liquid metal from the melting vessel. Ideally, the facility preheat tem-
perature should not be too different from the temperature of the melt being added. This
is to reduce thermal shocks to the facility piping and components. Careful selection of
the melting temperature and facility preheat temperature should consider any risks of
localized freezing during filling. It is also recommended that the filling line be con-
nected to the facility drain tank or attached piping, where minimal instrumentation or
sensitive components are installed. This is not only to avoid any possible thermal
shocks in the event of temperature differences between components and the melt
but also to prevent possible free-falling HLM impinging on delicate instrumentation