Page 61 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

36 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Lead-water interaction
l
Challenge
LFR designs are pool-type reactors, which have the steam generators (or the heat exchangers)
inside the reactor vessel. This implies that the interaction between the secondary side coolant
and the liquid metal may occur. Thus, the primary to secondary leak (e.g., steam generator tube
rupture) shall be considered as a safety issue not only in the design but also in the preliminary
safety analysis, of these reactor types. Two are the topics of investigation in case of steam
generator tube rupture (SGTR) postulated event: firstly, to understand the phenomena involved
in the accident scenario and, secondly, to study how to prevent or mitigate the consequences of
the event, reducing the primary system pressurization. (Del Nevo et al., 2016).
State of the art
The interaction of lead alloys with water has extensively been studied in collaborative
European projects. Water was injected in liquid-metal pools at various temperatures, crack
configurations, flow rates, and pressures. CFD and SIMMER analyses (Pesetti et al., 2016a,
b, 2017) were applied to numerically model the experiments.
Development needs
Further studies on this topic are scheduled within the Italian national program up till 2020
(Tarantino, 2017). These studies basically will focus on enlarging the range of parameters
especially with respect to the crack configuration and with that the validation basis. More-
over, other areas of investigation are related to pressure wave propagation across the primary
system, sloshing, steam transport in primary system, steam entrainment into the core, and
lead-water interface phenomena. Rupture/leakage detection systems and tube rupture miti-
gation countermeasure are being investigated experimentally.
Severe accidents:
Containment thermal hydraulics
l
Challenge
Containment thermal hydraulics for LMFRs is different from containment thermal hydrau-
lics for light-water reactors in which the formation, distribution, and combustion of hydro-
gen play an important role, which is less important for LMFRs. Containment thermal
hydraulics for LMFRs will include temperature and pressure loads on the containment
and must be addressed in any safety evaluation. Herranz et al. (2018) explain that in
sodium-cooled reactors, also the effects of a sodium fire in the reactor containment need
to be taken into account. A core disruption by supercriticality involves energetic destruction
of fuel assemblies. The interaction between hot fuel and liquid sodium can lead to a vapor
explosion that could create a breach in the primary system, and contaminated liquid sodium
at high temperature would be ejected into the containment. In contact with oxygen, the
sodium burns and forms sodium oxide particles. The aerosol formed and its chemical trans-
formations would be responsible to a great extent for the radiological and chemical impact.
State of the art
Perez and Gomez (2013) showed that the exposure to the public is probably far below the
limits for currently operating LWRs and for currently designed power plants. Nevertheless,
it is foreseen to include it in international collaboration in the near future. Herranz et al.
(2018) show that models to include the effects of sodium fire in the reactor containment
are being developed and are partly validated. Even though they are already valuable, further
development will be needed as explained below.