Page 234 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Subchannel analysis for LMR 205

Fig. 5.12 Three-dimensional shape of oscillations in gap region. (A) Triangular array
(P/D ¼1.06); (B) four rod square array (P/D ¼1.12).

programs due to the deficiency in understanding the physical process and in the
reliable models.
It is agreed that large-scale oscillations can occur in tight lattices when no wire
wraps are applied, which can be the case in some LMRs. Fig. 5.12 shows example
results, that is, three-dimensional configuration of oscillation in triangular array with
P/D ¼1.06 and in four rod square array with P/D ¼1.12, obtained by CFD analysis
with a URANS approach (Yu and Cheng, 2012). Numerical results show that flow
oscillation develops gradually to a steady oscillation (i.e., the amplitude, shape,
and frequency of oscillation reaches a constant or quasi-constant value). A good
agreement between the numerical results and experimental data was achieved.
The future needs are (1) to develop models for the amplitude and frequency of the
oscillation in dependence of the geometric and thermal-hydraulic parameters; (2) to
couple the oscillation with the transversal source term of mass, momentum, and
energy exchange; and (3) to implement the source terms in SCTH programs.

5.2.2.4 Local wall temperature distribution

SCTH codes provide the average values of the thermal-hydraulic parameters in each
subchannel. Most SCTH codes solve also the thermal conduction model for calculat-
ing the temperature in the solid fuel pin. For this purpose, a third set of boundary
conditions are applied, that is, the fluid temperature in the subchannel and the
heat-transfer coefficient. SCTH codes typically cannot represent the variation of heat
transfer in the circumferential direction along the fuel pin surface, which may be sig-
nificant (Cheng and Tak, 2006). Usually in the gap region, heat transfer is less efficient
than in the region facing the center of subchannels. Fig. 5.13 presents the circumfer-
ential profile of the relative surface temperature in hexagonal lattice and square lattice,
respectively. In this example, LBE is used as coolant. The relative surface temperature
is defined as the ratio of the temperature difference between the rod surface temper-
ature and the fluid bulk temperature to its maximum value, that is, T W T B .
ð T W T B Þ max
In the tight triangular lattice (P/D ¼1.1), the heat-transfer coefficient in the gap is
only about 20% of that in the central region. This nonuniformity reduces rapidly with
increasing the pitch-to-diameter ratio. In a triangular rod bundle with a pitch-to-