Page 342 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

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Core thermal hydraulics 6.2.3

†
F. Roelofs*, I. Di Piazza , E. Merzari ‡
*Nuclear Research & Consultancy Group (NRG), Petten, The Netherlands,
† ‡
ENEA, Kista, Stockholm, Sweden, Argonne National Laboratory, Lemont, IL,
United States

6.2.3.1 Introduction

This chapter focuses on one of the most studied and important topics of nuclear ther-
mal hydraulics, that is, core thermal hydraulics. The nuclear chain reaction, which is
the source of nuclear fission energy production, takes place in the core of a nuclear
reactor. Within this core, heat is produced in nuclear fuel and transported to a coolant.
Depending on the function and type, nuclear reactor cores consist of about ten up to a
few hundred fuel assemblies. In turn, not only most fuel assemblies consist of a large
number of fuel rods, but also other geometric configuration can be found, like straight
or curved plates and spherical fuel. Heat transfer plays an important role in the
thermal-hydraulic evaluation of the reactor core. In the past, design was based on
experimental research resulting in empirical correlations combined with analytic
modeling and subchannel code evaluations. As experimental data are often hard to
obtain, especially velocity profiles in opaque liquid metals, and very costly, nuclear
reactor designers rely more and more on simulation techniques like CFD. Apart from
that, CFD is also used to evaluate possible local effects that cannot be derived from
one-dimensional system thermal-hydraulic code simulations. Especially, the spacer
design of wires wrapped around the fuel pins employed by most LMFRs (IAEA,
2012) requires a thorough knowledge of the local effects. Such a wire-wrapped fuel
assembly design is also employed in MYRRHA, the Multipurpose hYbrid Research
Reactor for High-tech Applications (Abderrahim et al., 2010). Therefore, simulations
of the heat transport in the core are essential for the design and safety analyses of
LMFRs. The main challenges related to core thermal hydraulics are the occurrence
of hot spots, the thermomechanical loads, effects of fuel assembly deformations,
effects of partial and complete blockages, and influence of the interwrapper flow.
However, one should never forget that CFD approaches first need to be validated
against experimental data.
Apart from this, the interaction between flow and heat transport on one side and the
rod and spacer structures on the other side is considered an important topic. Espe-
cially, the occurence of (turbulence) flow fluctuations may lead to vibrations. In order
to simulate this fluid-structure interaction, accurate CFD is needed in combination
with good models for structural assessment. As flow-induced vibrations are the topic
of another lecture, the application to fuel assemblies will only briefly be shown in this
lecture.

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