Page 163 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Operational aspects of experimental liquid metal facilities 135

allowing the pressure resistance in the system to build up gradually to the required
operating head.
With this said, most liquid-metal facilities are likely to be closed circuit facilities
that are completely filled prior to start-up. While each facility will be different, some
degree of soft starting (gradual increase in pump speed) is recommended for HLM
facilities with high-density fluids. Although primarily used to improve process con-
trol, a variable frequency drive could be used to reduce the starting current, providing
the lowest possible current at the highest possible torque. It provides a performance
similar to a soft starter at the start-up sequence.
It should be noted that certain types of pump, for example, induction pumps on
rotating permanent magnets, do not set strict requirements on pump start-up proce-
dure. However, magnetic pumps might be sensitive to overheating.

3.4.7 Cooling

Similar to heating procedures, the effects of LBE cooling should be carefully consid-
ered when establishing cooling procedures. Avoid the possibility of creating excessive
thermal gradients in components. Depending on the heat tracing control layout and the
thermal inertia of components, setting rapid cooling rates could result in some sections
cooling down faster than others resulting in low-temperature warnings, automated
draining, and even localized solidification.

3.4.8 Draining

Draining can be initiated by two typical but different draining sequences: a manual
user initiated controlled draining sequence or an automated, emergency draining
sequence triggered by one or more criteria. Actuated drain valves are therefore rec-
ommended for an experimental facility to facilitate the need for automated facility
draining. For some experimental facilities, actuated valves may in fact be a safety
requirement.
The manual, controlled draining sequence is initiated by opening the automated
drain valves to the drain tank, allowing the LBE to drain passively under the influence
of gravity. Precautions should be taken to reduce the driving pressure differential
between the loop and the drain tank, in order to reduce the LBE velocity flowing from
the loop into the drain tank. These actions could include the following: ensure the LBE
pump is not running (drain sequence to be interlocked), reduce cover gas overpressure
in the loop, and increase the drain tank cover gas pressure (not too high to prevent
draining). In general, the following prerequisites/interlocks could be defined in order
to initialize a drain:
l Stop all pumps (forced circulation)
l Deactivate all active coolers to avoid solidification in a heat exchanger
l Opening of all valves (except the main drain valve/s) that otherwise may prevent complete
draining of the facility