Page 48 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal-hydraulic challenges in liquid-metal-cooled reactors 23

for fuel assemblies employing wire wraps and fuel assemblies employing grid spacers.
From operating experience, it is well known that even under normal operation, deforma-
tions of the fuel assemblies will occur. Such deformations can be due to the effects of
manufacturing accuracy, tension of prestressed wires, contact pressure between cladding
and adjacent wires, thermal and irradiation creep of the cladding and wires, swelling,
fuel burnup, and eccentricity. Katsuyama et al. (2003) explain how a typical fast reactor
fuel pin will deform during irradiation being constrained by the wires and the housing
(wrapper).
State of the art
Pragmatic approaches for as-designed fuel assemblies have been established; however, as
explained by Roelofs et al. (2015b), they are not fully validated yet. A full validation for
the hydraulic field in wire-wrapped fuel assemblies is lacking. With respect to the assess-
ment of the effects of deformations, CFD techniques can be used. From the information
obtained, one may try to derive a hot channel deformation penalty factor that can, for exam-
ple, be applied in system codes and/or subchannel codes. Sosnovsky et al. (2015) provide
examples of both approaches.
Development needs
The lacking validation for the full hydraulic field should be addressed by provision of
matched index of refraction experimental and high-fidelity numerical data like the
European efforts described by Roelofs et al. (2016). A similar effort is ongoing in
the United States for a 61 pin bundle as described by Vaghetto et al. (2016) and
Obabko et al. (2016). With respect to fuel assemblies employing grid spacers, the sit-
uation is more complicated, as experiments and validation will probably be required for
each individual design, although generic validation approaches might provide a first
indication on the accuracy that may be expected from CFD simulations. The effects
of deformations will need a separate validation program including most importantly
experimental work to validate CFD methods that are currently being applied for
such cases.
Complete core modeling (see also Section 6.2.3)
l
Challenge
Natural convection regimes in liquid-metal-cooled reactors are associated with complex
flows inside and around the core. Indeed, in these regimes, recirculation loops can often
be observed both within each subassembly (between the warmer central region and the
cooler periphery), between subassemblies (with the coolant rising within warmer subassem-
blies and descending within cooler subassemblies), and inside the “interwrapper region”
between the hexagonal wrapper tubes. Each of these phenomena contributes to the overall
cooling of the core. However, modeling them successfully requires a 3-D description of the
flow inside each subassembly at the very least.
State of the art
Currently, complete cores are modeled in one-dimensional system codes or more in detail by
subchannel codes.
Development needs
It can be expected that with the growing use of CFD and the development of multiscale
thermal-hydraulic codes, such multidimensional codes may play a role. Model development
and especially validation for all these codes is required to obtain sufficient confidence in the
simulation results.