COMPARISON OF RELAP5-3D/ATHENA AND ANSYS/CFX THERMAL-HYDRAULIC RESULTS
Juan J. Carbajo, Joel McDuffee, and A. Louis Qualls
Reactor and Nuclear Systems Division
Oak Ridge National Laboratory
P.O. Box 2008 MS-6167
Oak Ridge, TN 37831-6167
A heat exchanger (HX) for a space reactor has temperatures inside the shell fluid are shown in Fig. 3.
been modeled by the codes RELAP5-3D/ATHENA (Ref. Every tube is modeled. This model has been used for
1) and ANSYS/CFX (Ref. 2), and some thermal- component design to calculate material stresses.
hydraulic results have been compared. The parameters The results from both codes (RELAP5 and
that have been compared are the total heat transferred ANSYS) for different input conditions are shown in
within the component and the pressure drop across the Table 1. The primary NaK flow is 1.1 kg/s, the
component. secondary NaK flow is 1.75 kg/s. Two different hot inlet
The HX modeled is a shell and tube HX made temperatures were investigated: 875 K and 880 K; and
of stainless steel with 61 tubes, 9.65 mm OD and 0.55 m two different cold inlet temperatures: 814 K and 818 K.
long. The overall dimensions of the HX are 0.16 m in The manufacturer’s design value of the HX is 48.2 kW
diameter and 0.8 m long. Both the primary and the when a hot temperature of 875 K and a cold temperature
secondary fluids are liquid NaK, which is a eutectic of 818.6 K are used.
mixture of 22% sodium and 78% potassium. Different The maximum heat transferred in the HX is
hot and cold fluid inlet temperatures were investigated. calculated by RELAP5 for the case with the maximum
The transient analysis 3-dimensional (3-D) hot temperature (880 K) and the minimum cold
computer code RELAP5-3D/ATHENA was one of the temperature (814 K) with a value of 55.4 kW. The
codes used in this modeling. This code can employ a minimum heat transferred is calculated for the lowest hot
variety of coolants in addition to water, the original temperature of 875 K and the highest cold temperature of
coolant employed in early versions of the code. Liquid 818 K with a value of 48 kW. The difference between
metals (sodium, potassium, NaK, lithium) and cryogenic the maximum and the minimum heat transferred is 15%.
fluids (hydrogen, helium, nitrogen) are some of the Small changes in the inlet temperatures (~5 K) result in
available coolants. The thermo-physical properties for significant changes in the heat transferred (15%). The
NaK in RELAP5 have been modified to correct some RELAP5 calculated value of 48 kW is very close to the
observed property errors. The code can also use 3-D design value of 48.2 kW using about the same inlet
volumes and 3-D junctions, thus allowing for more temperatures. ANSYS/CFX calculated 51.6 kW which is
realistic representation of complex geometries. above the design value of 48.2 kW and above the
This HX has been modeled using simple 1-D RELAP5 calculated value.
volumes with 14 axial nodes for the volumes inside the Pressure drops calculated by RELAP5 and
shell and headers and 10 axial nodes for the volume ANSYS/CFX for the primary side were in good
inside the tubes. The primary flows inside the 61 tubes agreement (0.6 kPa for both). For the secondary side,
and the secondary flows inside the shell, outside the ANSYS predicted 2.4 kPa while RELAP5 predicted only
tubes. The tube walls are modeled as a heat structure that 0.2 kPa. Other calculations showed that RELAP5 does
separates the primary and the secondary volumes but not predict accurate pressure drops in bundles with cross-
transmits heat between both volumes. The primary and flows since it uses correlations for flow in parallel with
the secondary fluids flow counter-currently. Fig. 1 shows the bundles.
this model with the NaK temperature distribution inside In conclusion, both RELAP5-3D/ATHENA and
the tubes. This model has been used as part of the system ANSYS/CFX results appear to be in reasonably good
design to calculate flows, temperatures, and pressure agreement. RELAP5 does not calculate correctly
drops. pressure drops for cross-flow in bundles.
ANSYS is a multipurpose finite-element
code that can perform a variety of calculations, including REFERENCES
stress analysis, temperature distributions, and thermal
expansions in solid materials. CFX is one of the 1. INEEL, RELAP5-3D Code Manual, INEEL-EXT-
computational fluid dynamics packages of the code and 98-00834, Rev. 2.4, Idaho National Laboratory
can perform thermal-hydraulic fluid calculations. The (2006).
ANSYS/CFX representation of this HX with the 2. ANSYS Multiphysics 8.1 Code, ANSYS Inc., 275
temperatures inside the tubes is shown in Fig. 2. The Technology Drive, Canonsburg, PA 1531.
Table 1. Comparison of RELAP5-3D/ATHENA and ANSYS/CFX results
Tprimary Tsecondary Tprimary Tsecondary
CODE IN (K) IN (K) OUT (K) OUT (K) Q (kW)
RELAP5 875 818 825 849.5 48
RELAP5 875 814 821 848 51.2
RELAP5 880 818 825.8 852 52
RELAP5 880 814 822 850.5 55.4
ANSYS/CFX 875 818 821.4 851.7 51.6
Design 875 818.5 825 850 48.2
Fig. 1. Heat exchanger as modeled by RELAP5-3D Fig. 2. ANSYS/CFX model of the heat exchanger
showing the temperature of NaK inside the tubes and showing the temperature of NaK inside the tubes like
headers. Hot NaK at 875 K enters at the top left into in the RELAP5 model of Fig. 1
Fig. 3. ANSYS/CFX model of the heat exchanger
showing the temperature of NaK circulating inside
the shell (cold NaK enters at the bottom right).