Page 215 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

186 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

θ relative angle position to gap
ε blockage ratio
ε M turbulent edge viscosity (Pas)
λ thermal conductivity (W/mK)
3
ρ density (kg/m )
ζ local pressure-loss coefficient
2
τ shear stress (N/m )

Subscript

B bulk
H energy/enthalpy
m mass
M momentum
i, j ID number of subchannels
k ID number if axial nodes
W wall

Superscript

0 transversal exchange due to turbulent fluctuation
* transversal exchange due to directed cross flow

5.1 Introduction

Since many decades, the subchannel thermal-hydraulic (SCTH) method is the most
widely applied numerical approach to analyze the thermal-hydraulic behavior in
nuclear reactor cores or fuel assemblies. It is also expected that the SCTH method
remains its importance for the next decades.

5.1.1 Structure of LMR fuel assemblies
The SCTH method is focused on the reactor core or even on one fuel assembly. Dif-
ferent types of reactors have different requirements on the SCTH method due to the
variety of fuel assembly structures, coolant properties, and thermal-hydraulic condi-
tions. Most fuel assemblies in liquid-metal-cooled reactors (LMRs) have a hexagonal
arrangement, as shown in Fig. 5.1.
In LMRs, the fuel assembly is usually hydraulically closed by a box. There is no
transversal exchange of mass and momentum between fuel assemblies. The fuel
assemblies are connected at their inlet and outlet via the lower plenum and the upper
plenum, respectively. According to the geometric structure, three different kinds of
subchannels are contained in one fuel assembly, that is, interior subchannel, edge sub-
channel, and corner subchannel as indicated in Fig. 5.1. For optimal performance in a