Page 51 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

26 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

excursion, all pathways are then characterized by a postaccident heat removal phase, where
the core debris, generating decay heat only, are released in the plena. In this phase, one must
guarantee that particle deposits do not become critical again and that they are cooled prop-
erly to avoid vessel wall degradation. The introduction of mitigating provision in the new
SFR designs will sensibly reduce the risk of having severe accidents with energetic excur-
sions. In LFRs, the risk is already reduced by some beneficial coolant properties: slightly
negative coolant temperature feedback; similar density with the fuel, which limits the fuel
compaction speed; and high boiling temperature.
State of the art
Computational tools are used to simulate energetic and nonenergetic severe accident
sequences for safety analysis purposes and for the design of mitigating provisions. These
tools have to capture the core material degradation and relocation in an adequate way for
reliable predictions. Mechanistic codes to assess the risk of CDA in SFR have been devel-
oped continuously since the 1970s. The SAS-SFR (SAS4A) and SIMMER codes have been
widely used to simulate CDA in SFR. SAS-SFR is devoted to the initiating phase of a severe
intraassembly scale, while SIMMER simulates the transition phase, when the damage
extends to the intersubassembly scale. A realistic assessment of the core degradation process
is only possible by coupling SAS-SFR and SIMMER. The validation database of these two
codes is quite large for SFR. SAS-SFR was validated against the CABRI experiments
(Perez-Martin et al., 2014), while the validation of the SIMMER code was carried out
through separate effect tests and integral tests during two assessment phases (Maschek
et al., 2008) at the end of the 1990s. Nowadays, the general tendency is having an integrated
framework for studying the whole severe accident sequence, from the initiating phase to the
postaccident heat removal phase. Therefore, there is a common effort from CEA, JAEA, and
KIT to develop the platform SEASON, coupling SIMMER with other neutronic,
thermomechanical, and thermohydraulic tools (Rouault et al., 2015).

Development needs
The code validation program is an ongoing effort. A continuous evaluation of the validity of
the mechanistic code models for the analysis of liquid-metal-cooled reactors needs to be per-
formed, and where necessary, new validation programs need to be developed. In this respect,
an important difference between SFRs and LFRs is the presence of lead (-alloys) coolant in
all the core degradation stages. Therefore, the mechanical degradation and melting and
freezing models involving structural materials and fuel must be reassessed, to take into
account the effect of the coolant on this phenomena and, generally, on the mobility and evac-
uation of the fuel from the core. A reassessment of the severe accident code validation is also
required for new SFR designs as well, because the presence of mitigating devices in the core,
which avoid the occurrence of energetic excursions, suppresses typical phenomena of CDA.
On the opposite, phenomena that were negligible in a CDA gain more relevance, for exam-
ple, because of the longer duration of the severe accident sequence.

2.4.3 Pool thermal hydraulics
The category of pool thermal hydraulics includes the following topics:
Normal operation:
l Pool modeling (see also Section 6.2.4)
Challenge
The study of pool thermal hydraulics integrates many of the thermal-hydraulic challenges
described in this chapter, including thermal stratification, thermal striping, jet interaction,