Page 58 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal-hydraulic challenges in liquid-metal-cooled reactors 33

transients and accident conditions. More recently, compact heat exchanger designs are being
developed, aiming at limiting the reactor vessel dimensions. A helical coil steam generator
design, like assessed by Yuan et al. (2017), is one of such. Another innovative concept is the
superheated steam double-wall bayonet tube type with leakage monitoring under consider-
ation for the ALFRED reactor (Frogheri et al., 2013). This concept allows the double phys-
ical separation between the primary lead in the pool and steam-water primary coolant that
flows in the tubes.
State of the art
Several efforts have been spent in Europe in improving primary system heat exchangers
design. Different solutions have been proposed, while prototypes have been realized and
tested in devoted experimental facilities. In Italy, a double-wall bayonet tube steam gener-
ator has been constructed and presently is under testing (Tarantino, 2017), while in France,
helical tube heat exchanger are considered.
Development needs
Innovative designs of heat exchangers need experimental and numerical validation. Numer-
ical models have to be validated considering innovative geometries (e.g., helical tubes, spiral
tubes, and double-wall bayonet tubes), coupling liquid metals on one side and water, steam,
or superheated steam on the other side. Modeling would address not only heat transfer behav-
ior but also mainly pressure losses both under forced and natural circulation, relevant for the
assessment of LMFRs primary system behavior under decay heat scenario. Experiments are
needed to both validate innovative design (simulating nominal and transient scenarios) and
support modeling validation.
l Pumps
Challenge
The main circulation pumps are placed in the primary pool of LMFRs. As consequence, the
component has to guarantee very high reliability and good performances (to reduce dimensions
in the primary system). For heavy liquid-metal fast reactors, the relatively high speed between
structural material and coolant implies that the pump impellers are subjected to severe
corrosion-erosion conditions that might not be sustained in the long term. The materials of
the pump impeller have to satisfy a couple of demanding requirements that deserve specific
experimental installation, such as capability to withstand to an exposure to high-temperature
heavy liquid metal (up to 480°C and higher for long-term perspective), capability to withstand
to corrosion/erosion effects due to high relative coolant velocity (10m/s and up to 20m/s), and
demonstration of reliability and performances of the pump for a long-term application.
State of the art
Currently,withinthe frameoftheItaliannationalprogram(Tarantino,2017),pumpsarebeing
tested and modeled. Three different mechanical pumps have been developed and are being
tested. One of those is a vertical pump as a prototype for a real application in a reactor.
Development needs
Experimental tests of innovative pumps are required to derive pump characteristics as input
for safety evaluations. But also experimental tests are needed to study effects of erosion,
corrosion, and cavitation of the coolant flow on the pump material.
l Start-up heating system
Challenge
Of peculiar interest is the start-up heating system. It is not needed for light-water reactors,
but for LMFRs for which the coolant at room temperature is not liquid, these systems and the
start-up procedure need attention and experimental validation.