Page 160 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

132 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

3.4.4 Preheating

Preheating the facility is typically done with electric resistive heat tracing, installed on
the outside of the vessels, piping, and components. The preheating sequence is
required before liquid metal can be introduced into the facility. The preheating
sequence transitions the facility from the cold standby state to the hot standby state.
As a reminder, this preheating transition should be interlocked not to be active unless
the gas conditioning sequence has been completed.
As mentioned in Section 3.4.2, to avoid thermal shocks during filling, the facility
heat tracing set points should be set to a temperature not too different from the tem-
perature of the LBE entering from the drain tank. When increasing the temperature set
points, heating ramp rates should be limited for several reasons. Excessive heating
rates could result in nonuniform local temperatures and cause thermal stresses that
would be detrimental to some components, particularly ceramic components such
as oxygen sensors. LBE facilities at SCK CEN typically limit the heat tracing set-
l
point ramp rates to 1°C/min.
Precautions should also be taken to avoid excessive HLM heating rates to high tem-
peratures in a filled facility, particularly during the initial start-up or after insertion of
fresh specimens or test sections (OECD, 2015). The solubility of alloying elements in
contact with HLM increases rapidly with increasing temperature. HLM at high tem-
perature usually has oxygen concentrations far below solubility. Therefore, if no pre-
oxidation has been performed and the HLM temperature is raised too rapidly to high
temperatures (above 500–550° for 316L austenitic steels), rapid dissolution of the
steel alloying elements can occur. It is therefore recommended that heating be per-
formed at controlled rates or through stages, so that protective oxide layers can be
established (suitably with deliberate oxygen control).

3.4.5 LBE filling

A drain tank would typically be installed at the lowest part of the facility, to allow
complete draining under the influence of gravity. Considering this, Fig. 3.4.2 illus-
trates a typical drain tank design for an experimental liquid-metal facility. In this
case, the filling of the experimental facility from the drain tank is done by pres-
surizing the drain tank with an inert gas such as argon. This is possible by exten-
ding the supply/drain nozzle into the tank as shown in Fig. 3.4.2,the so-called
dip pipe.
This dip-pipe principle in the drain tank ensures that LBE can only enter the facility
from the bottom of the drain tank, when the drain tank gas pressure is increased. Float-
ing oxide precipitates and impurities are therefore kept floating in the drain tank dur-
ing the filling sequence. However, having a common pipe to both fill and drain the
loop has a disadvantage. During and after draining, any oxides floating at the free sur-
face of the loop are likely to remain in this common pipe, at or near the moving free
surface. This implies that the dip pipe could result in localized accumulation of