Page 198 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

System thermal hydraulics for liquid metals 169

In 2002, the version of RELAP5/Mod3.2β was used in a posttest validation process
of experiments conducted on CHEOPE, loop located at ENEA (Brasimone), designed to
study the LBE chemical and technological aspects of heavy-liquid-metal reactors. The
comparison between experimental and numerical results showed a reasonable agree-
ment and contributed to the assessment of the code capabilities in simulating LBE in
forced and natural circulation regime (Agostini et al., 2002). In 2006, an experimental
campaign was performed on an LBE pool-type facility, named CIRCE, located at
ENEA Brasimone, where gas-injection enhanced circulation tests were carried out.
In 2007, UniPi implemented the thermodynamic properties of lead and LBE in
RELAP5/Mod3.3 code version, adding the updated transport properties for viscosity,
thermal conductivity, and surface tension. Subsequently, this upgraded version of
RELAP5/Mod3.3 code was extensively employed in support of the experimental
activities performed at ENEA (Brasimone) in the field of HLM nuclear system. In
particular, two main LBE facilities, NACIE (loop-type) (Coccoluto et al., 2011)
and CIRCE (pool-type) (Tarantino et al., 2013), were reproduced in order to investi-
gate the related thermal-hydraulic phenomenology in both nominal conditions (gas
enhanced circulation) and transient conditions (e.g., transition to natural circulation
and ULOF).
In 2012, UniPi started a research activity on the development of a coupling tool
between the modified RELAP5/Mod3.3 code and the CFD Fluent code (Martelli
et al., 2017). In 2016, in order to perform this activity in a rational way, the last revised
liquid-metal properties available in the scientific literature were assigned to RELAP5/
Mod3.3, for sodium, lead, lead-bismuth eutectic, and lead-lithium eutectic (Angelucci
et al., 2017).
In particular, the equations needed to obtain temperature, pressure, specific volume,
specific internal energy, thermal expansion coefficient, isothermal compressibility, spe-
cific heat at constant pressure, and specific entropy both for single- and two-phase con-
ditions have been reviewed according to the OECD/NEA (2015), using the relationships
presented in the previous section. These relationships were used to generate the external
thermodynamic property file for the specific working fluid, while the upgraded transport
properties (thermal conductivity, dynamic viscosity, and surface tension) were
implemented directly inside the code. In addition, specific convective heat transfer cor-
relations for a liquid metal as working fluid flowing in a pipe or in a fuel bundle have
been also implemented according, respectively, to Seban and Shimazaki and Cheng and
Tak correlations and to Ushakov and Mikityuk correlations (see Section 4.1.2). In par-
ticular, when a liquid metal is adopted as working fluid, a specific convective boundary
condition can be set in the input file for the heat structures.

4.4.2 Application to NACIE

4.4.2.1 Facility description
The NACIE facility was commissioned to perform experiments in the field of thermal
hydraulics and fluid dynamics to investigate pressure drops and heat transfer correla-
tions in prototypical fuel bundle simulators (Coccoluto et al., 2011). The NACIE