1. Ingeniería

CHF Enhancement of Advanced 37-element Fuel bundles
aKorea
Joo Hwan Parka , Jong Yoeb Junga and Eun Hyun Ryua
Atomic Energy Research Institute, 989-111 Daedukdaero, Yuseong-gu, Taejon, Korea
Abstract
A standard 37 fuel bundle (37S fuel bundle) has been used in commercial CANDU reactors for over
40 years as a reference fuel bundle. Most CHF of a 37S fuel bundle have occurred at the elements
arranged in the inner pitch circle for high flows, and at the elements arranged in the outer pitch circle
for low flows. It should be noted that a 37S fuel bundle has a relatively small flow area and high flow
resistance at the peripheral subchannels of its center element compared to the other subchannels. The
configuration of a fuel bundle is one of the important factors affecting the local CHF occurrence.
Considering the CHF characteristics of a 37S fuel bundle in terms of CHF enhancement, there can be
two approaches to enlarge the flow areas of the peripheral subchannels of a center element in order to
enhance CHF of a 37S fuel bundle. To increase the center subchannel areas, one approach is the
reduction of the diameter of a center element, and the other is an increase of the inner pitch circle. The
former can increase the total flow area of a fuel bundle and redistributes the power density of all fuel
elements as well as the CHF. On the other hand, the latter can reduce the gap between the elements
located in the middle and inner pitch circles owing to the increasing inner pitch circle. This can also
affect the enthalpy redistribution of the fuel bundle and finally enhance CHF or dryout power.
In this study, the above two approaches, which are proposed to enlarge the flow areas of the center
subchannels, were considered to investigate the impact of the flow area changes of the center
subchannels on the CHF enhancement as well as the thermal characteristics by applying a subchannel
analysis method.
Introduction
Wolsong unit 1, which is one of four CANDU units at the Wolsong site in Korea, was recently shut
down after reaching the end of its 30-year life time. It had been refurbished during the last several
years. One of the main refurbished components was the aged pressure tube, which was expanded
diametrically as well as axially owing to irradiation damage over a long period of time. It is known
that the diametrical expansion of the pressure tube can deteriorate the CHF performance and finally
reduce the thermal margin or operating power. There have been many studies on enhancing the CHF
and/or Critical Channel Power (CCP), which is determined by the dryout and hydraulic characteristic
curves of the primary heat transfer system of a CANDU reactor. In the late 1970s, a turbulent promoter
was invented to increase the turbulent intensity surrounding the fuel elements in a fuel channel [1].
The axial positions or number of bearing pad planes were changed, or the number of spacer pad planes
were increased to enhance the CHF by means of increasing the turbulent intensity or flow mixing
within a fuel passage [2]. These attempts provided a CHF increase, but an adverse effect on the CCP
existed to worsen the hydraulic characteristics of the primary heat transfer system when increasing
the pressure drop of the fuel channel [3].
Recently, one of the studies to enhance the CHF was the development of the CANFLEX fuel bundle,
which is composed of two element sizes and attaches the CHF enhancement buttons to 43 element
fuels [4]. It is known that the CANFLEX fuel bundle achieves a remarkable CHF enhancement by
attaching a special appendage called a CHF enhancement button on the fuel element [5]. However, it
has not been commercialized yet owing to a more complex design and higher fabrication cost
potentially than those of a 37S fuel bundle.
In particular, a 37S fuel bundle has 37 element fuel elements which are arranged circularly. It contains
four pitch circles, i.e., center, inner, middle, and outer pitch circles, to properly configure the circular
bundle structure. From the previous CHF experiments, most CHF of a 37S fuel bundle have occurred
at the elements arranged in an inner pitch circle at high flows [6], or under reactor conditions of which
the reference flow rate is 24kg/s [7]. It should be noted that a 37S fuel bundle has a relatively small
flow area and high flow resistance at the peripheral subchannels of the center element compared to
the other subchannels. The configuration of a fuel bundle is one of the important factors affecting the
local CHF occurrence. The diameter effect of the fuel elements arranged in the center, inner, middle,
and outer pitch circles of a 37S fuel bundle has recently been studied [8]. It was shown that the dryout
power of a fuel bundle has a tendency to increase as the element diameter decreases within the size
limitation.
Considering the CHF characteristics of a 37S fuel bundle in terms of CHF enhancement, there can be
two approaches to enlarge the flow areas of the peripheral subchannels of a center element in order to
enhance the CHF of a 37S fuel bundle. To increase of the center subchannel areas, one approach is
the reduction of the diameter of a center element and another is the increase of the inner pitch circle.
The former can increase the total flow area as well as the center subchannel area of a fuel bundle.
Additionally, the power densities of the other 36 fuel elements can be increased to compensate the
power density reduction of a small center element. It was noted that a 37S fuel bundle with a small
center element can enhance the CHF and finally improve the thermal margin of a CANDU fuel bundle.
However, it can redistribute the power density of the remaining 36 fuel elements and increase the
maximum fuel temperature of the hottest elements arranged in the outer pitch circle. Moreover, it may
deteriorate the safety margin related to the coolant void reactivity by increasing the coolant volume
in the fuel channel.
On the other hand, the latter can also increase the flow area of the peripheral subchannels of a center
element without any change in the total flow area or bundle configuration. However, it can reduce the
gap between the elements arranged in the middle and inner pitch circle owing to an increase in the
inner pitch circle. This can affect the enthalpy redistribution of the fuel bundle as well as the radial
CHF locations and dryout power. Finally, it can enhance the CHF without any impact on the safety
margin and fuel fabrication cost.
In this study, the above two approaches which were proposed to enlarge the flow areas of the center
subchannels were examined in terms of the CHF or dryout power enhancement by using the
subchannel analysis code, ASSERT [9]. In addition, the thermal characteristics of two kinds of
modifications were compared to those of a 37S fuel bundle.
1.
Subchannel Modelling
A 37 element fuel bundle is composed of 37 fuel elements and two types of appendages such as
spacers and bearing pads. Those appendages are welded onto the surface of the fuel elements to
maintain the gap among fuel elements and between the fuel bundle and pressure tube, respectively. In
addition, two end-plates were welded at both ends of 37 fuel elements to configure a bundle structure.
Twelve fuel bundles are loaded horizontally into a horizontal pressure tube. Figure 1 shows the cross-
sectional view of a 37S fuel bundle located in a pressure tube. When the coolant flows into the fuel
channel, the channel flow can be distributed into the open spaces among the fuel elements and pressure
tube depending on the flow resistance of the subchannels, which are divided by the hypothetical line
connected between the centers of the fuel elements, as shown in Figure 1. The number of total fuel
elements and subchannels are 37 and 60, respectively, as shown in Figure 2. When the subchannel
analysis is being performed, the minimum symmetric angle should be 180 degrees because a fuel
bundle can be laid down at the bottom inside of a horizontal pressure tube. However, the present
subchannel modeling of a 37-element fuel bundle selects a full bundle configuration, not considering
a symmetric angle.
Figure 1 Cross-sectional view of
a 37S fuel bundle
1.1
Figure 2 Rod and subchannel numbers of
a 37S fuel bundle
Area of center subchannels
A 37S fuel bundle is composed of four pitch circles, i.e., the center, inner, intermediate, and outer
circles. The center subchannels are composed of subchannel numbers 1 through 6, the inner
subchannels are composed of subchannel numbers 7 through 18, the middle subchannels are
composed of subchannel numbers 19 through 42, and the outer subchannels are composed of
subchannel numbers 43 through 60, as shown in Figure 2. To increase the flow area of the center
subchannels, there may be two approaches: one is the modification of the inner pitch circle and the
other is a modification of the diameter of the center element discussed in the previous section. When
an inner pitch circle is increased, the flow areas of the center subchannels can be increased while the
gap between the inner 6 elements and middle 12 elements should be reduced, as shown in Figures 2
and 3.
Figure 3 Schematic views of increasing the flow area of the center subchannels
In the case of the diameter decrease of a center element, however, the total flow area and element
power densities of the remaining 36 elements should be increased, if even slightly. This may affect
the fuel safety, such as on the void reactivity and the maximum fuel temperature, as well as fuel
management costs. These modifications can affect the subchannel flow and enthalpy distributions and
finally change the CHF location and dryout power. The geometries of both types of modifications are
summarized in Table 1.
Table 1 Geometries of the modifications of standard 37-element fuel
Pitch circle radius, mm
Element diameter
Pitch circle
No. of
identification
elements
37S fuel
Mod. Pitch
37S fuel
Mod. diam.
Center
0
0
13.08
11.50
1
Inner
14.88
14.88 ~ 15.38
13.08
13.08
6
Middle
28.75
28.75
13.08
13.08
12
Outer
43.33
43.33
13.08
13.08
18
The lengths of the inner pitch circle can be increased up to 15.38 mm, considering the minimum gap
interference between the inner 6 elements and middle 12 elements, as studied in reference [10]. On
the other hand, for the element diameter modification, the diameter of a center element was selected
as 11.5 mm, which was already used for the development of a 37M fuel bundle [11]. The flow area
variation of the center subchannels and total subchannels for the inner pitch modification were plotted
and compared to that of the modification of a center element, as shown in Figure 4.
For the present calculation, the length of the inner pitch for the pitch circle modification was selected
as 15.28 mm, which gave almost the same flow area of the center subchannel as that of a center
element diameter of 11.5 mm, as shown in Figure 4. The ratios of the increased flow area of a center
element modification to the center subchannel area and total flow area of a 37S fuel bundle were
found as 18 % and 0.9%, respectively.
1
Ratio to center subchannel area
Ratio to total flow area
0.8
15
0.6
Center pin
modification
Inner pitch
modification
10
0.4
5
ratio to total flow area, %
Ratio to center subchannel area, %
0.33782
25
0.33782
0.33782
0.33782
0.33782
20
0.33782
0.2
37S fuel bundle
0
0
14.8
14.9
15
15.1
15.2
Length of inner pitch circle, mm
15.3
15.4
Figure 4 Variation of the inner subchannel area according to increasing inner pitch circles
1.2
Modelling of AFD and RFD
A CANDU-6 core is composed of 380 fuel channels, and each fuel channel accommodates 12 fuel
bundles resting horizontally. Hence, the CHF of a fuel bundle can be affected by the radial power
profile (RFD) of a fuel bundle, as well as the axial power profile (AFD) in a fuel channel. Figure 5
shows the typical AFD of a 37S fuel bundle in a fuel channel [7]. For a subchannel analysis of the
37S fuel bundle and its pitch circle modification, the same AFD can be used because the change of
the inner pitch radius does not affect the AFD. However, the power density of the 37 elements by
increasing the radius of the inner pitch circle may be affected due to changes in the radial position of
the 6 inner rods. It was examined using the WIMS code [12] for different inner pitch circles, as
summarized in Table 2. It is shown that a small change of the inner pitch from 14.88 to 15.38 mm
affected RFD negligibly. Hence, the same RFD was used for the present calculation of the inner pitch
circle modification.
Table 2 Length effect of inner pitch circle on the relative power density of the fuel elements
Parameters
Relative power density of each pitch circle (% change)
Inner pitch circle, Reference 14.98
15.08
15.18
15.28
15.38
mm
(14.88)
0.8230 0.8243
0.8258
0.8271
0.8285
Center pitch
0.8218
(0.14)
(0.31)
(0.49)
(0.65)
(0.81)
0.8547
0.8552
0.8556
0.8565
0.8567
Inner pitch
0.8538
(0.1)
(0.16)
(0.21)
(0.31)
(0.34)
0.9305 0.9303
0.9302
0.9299
0.9298
Middle pitch
0.9308
(-0.03) (-0.05)
(-0.07)
(-0.1)
(-0.11)
1.1042
1.1046 1.1045
1.1044
1.1043
Outer pitch
1.1048
(-0.01) (-0.02)
(-0.03)
(-0.04)
(-0.05)
On the other hand, the radial power density of the 37 elements for decreasing a center element size
can be changed due to small uranium materials of the center element. The normalized RFD for a
center element size of 11.5 mm instead of the original size of 13.08 mm can be calculated by the
averaged volume, and compared to those of a 37S fuel bundle, as shown in Figure 6.
1.8
Standard 37 element fuel bundle
Center element modification (11.5mm)
1.4
Relative power density
Normalized Axial Flux Distribution
1.6
1.2
1
0.8
0.6
0.4
0.2
Outer Middle
0
0
1
2
3
Axial Location, m
4
5
6
Inner
Center
Inner
Middle Outer
Pitch circle
Figure 5 Normalized axial flux distribution
Figure 6 Relative power densities (RFD) of
(AFD) of a 37S fuel bundle
a 37S fuel bundle and center element modification
2.
Subchannel Analysis
When the twelve fuel bundles are loaded in the horizontal pressure tube, the upper section of the fuel
bundle has a larger flow area than the lower section owing to gravitational force. Even if the symmetric
angle of the cross-section is 180 degrees, as shown in Figure 2, this study considered a full bundle
geometry for the subchannel analysis, and the total numbers of fuel rods and subchannels were 37 and
60 respectively, as shown in Figure 2. The subchannels are composed of three types, i.e., triangular,
square, and wall subchannels. The minimum gap between the elements of a 37S fuel bundle was
designed as 1.8 mm. When increasing the inner pitch circle of 6 rods, the gap between the inner 6
elements and middle 12 elements can be reduced up to 1.3 mm in the case of the maximum outward
movement, 0.5mm. The 1.3 mm minimum gap could be accepted by the Fuel Design Manual [7].
For the sensitivity studies of the effect of the inner pitch circle and small center element size on the
CHF or dryout power of a fuel bundle, the subchannel analysis was performed using the ASSERT
code [9], which was transferred from AECL to KAERI under a Technology Transfer Arrangement
(TCA) between KAERI/AECL. The ASSERT code is originated from the COBRA-IV computer
program [13, 14]. It has been developed to meet the specific requirements for the thermalhydraulic
analysis of two-phase flow in horizontally oriented CANDU fuel bundles. Especially, it is
distinguished from COBRA-IV in terms of following features;
- The lateral momentum equation is also considered with the gravity term in order to allow gravity
driven lateral recirculation.
- The five-equation model was applied to the two-phase flow model in consideration of the thermal
non-equilibrium and the relative velocity of the liquid and vapour phases. Thermal nonequilibrium is calculated from the two-fluid energy equations for the liquid and vapour. Relative
velocity is obtained from semi-empirical models.
- The relative velocity model accounts for the different velocities of the liquid and vapour phases
in both axial and lateral directions. As well, the lateral direction modelling contains features that
consider: (a) gravity driven phase separation or buoyancy drift in horizontal flow, (b) void
diffusion turbulent mixing, and (c) void drift (void diffusion to a preferred distribution).
To find the subchannel and axial locations of the first CHF occurrence in a fuel channel, the
calculation will continue until the convergence tolerance is reached at the specified criteria,
‘ODVTOL’ in the ASSERT code. Once the first CHF for the given mass flow and inlet temperature
has occurred at any subchannel and axial location during iteration, the calculation is stopped and all
flow parameters are printed out. Onset-of-dryout iteration for the first CHF occurrence can be found
as follows:
MCHFLO £ MCHFR £ MCHFUP
(1)
where ‘MCHFLO’ and ‘MCHFUP’ are the lower and upper bounds, respectively, for the target
minimum CHFR (MCHFR), and ‘MCHFR’ is the minimum CHF ratio and is defined as

  = min 


(2)
where ′ is the CHF, and ′ is the zonal heat flux. ‘ODVTOL’ is the relative convergence
tolerance on the iteration parameter, which is defined as follows:

Y Y
 £
Y
(3)
where Y is the iteration parameter and n is the iteration number. Y and n are given as 1.00004 and 20,
respectively, for the present calculation.
3.
Results and Discussion
Subchannel analyses were performed for the modifications of the inner pitch circle and center element
size of a 37S fuel bundle using the ASSERT code with a CHF lookup table [15]. To examine the
dryout power enhancement for the present calculation, the inlet temperatures were selected as 256℃,
262℃, and 268℃, and the inlet mass flow as 20 kg/s, 24 kg/s and 30 kg/s. The inner pitch circle of a
37S fuel bundle is selected as 15.28 mm, which gave almost the same flow area of the center
subchannel as that of a center element diameter of 11.5 mm, as shown in Figure 4.
3.1
Axial enthalpy and void fraction
The bundle averaged enthalpies for the 37S fuel bundle and those modifications were calculated at
each axial location from the channel entrance. As shown in Figure 9(a), it is shown that the bundle
averaged enthalpies are increasing as the coolant flows downstream. The axial enthalpy distributions
for a 37S fuel bundle and those modifications have similar trends but the enthalpy rise for the
modifications is higher than those of a 37S fuel bundle owing to increasing dryout power of the
modifications. The axial void fraction distribution has the same trend as the enthalpy distribution, as
shown in Figure 9(b), and the void formations for three types of calculations are starting almost at the
same axial location or the middle of the fuel channel.
6.7329
6.7415
6.7482
6.7533
6.7574
6.758
6.7623
6.7677
6.7753
6.786
6.8014
6.818
6.8309
6.8408
6.8487
6.8556
6.856
6.8617
6.8696
6.8807
6.8963
6.9187
6.9424
6.9607
6.9746
0
6.9854
6.9942
34.2216
34.2216
34.2216
34.2216
34.221613.08
34.2216
15.28
34.2216
34.221611.50
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
34.2216
100
34.2216
34.2216
127.88
614.13
264.05
315.03
867.43
134.84
611.34
264.68
315.01
866.27
139.98
609.27
265.16
314.99
865.38
143.77
607.57
265.52
314.98
864.69
146.58
606.43
265.8
314.98
864.14
Reference
148.65
605.53
265.8
314.98
864.07
Inner 150.72
Pitch 590.48 266.08 314.85 863.49
Center153.53
Pin 589.12 266.46 314.85 862.76
157.32
587.65
267
314.84
861.75
162.46
585.63
267.76
314.83
860.33
169.42
582.87
268.83
314.81
858.27
177.43
577.44
269.97
314.13
856.09
Inlet
temp.:
184.39
574.59
270.84
314.75
854.39
268°C
189.53
572.47
271.5
314.74
853.09
193.32
570.73
272.01
314.73
852.06
256°C
196.13
569.51
272.39
314.73
851.16
198.2
554.21
272.38
314.6
851.11
200.27
553.32
272.77
314.6
850.36
203.08
552.03
273.29
314.6
849.33
206.87
550.52
274.02
314.59
847.89
212.01
548.44
275.03
314.57
845.87
218.97
545.59
276.45
314.55
843
226.98
540.03
277.92
314.51
839.97
233.94
537.1
279.04
314.49
837.64
239.08
534.92
835.88500
200
300 279.89 314.48
400
242.87
533.14
280.52
834.52
Axial
Distance
from
Inlet, 314.47
mm
245.68
531.93
281
314.46
833.42
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
131.880.71113.43
137.79 1116.37
142.16 1118.63
145.390.61120.36
147.77 1121.66
149.53 1121.66
151.91 1122.99
0.5
155.14 1124.82
159.51 1127.38
165.42 1130.98
173.430.4 1136.1
181.43 1141.51
187.34
1145.7
191.710.3 1148.9
194.94 1151.31
197.32 1153.13
199.080.21153.13
201.46 1154.98
204.69 1157.52
209.06 1161.05
0.1
214.97 1165.96
222.98 1172.87
230.98 1180.08
236.89 0 1185.59
241.26 1189.77
0
244.49
1192.9
246.87 1195.25
Void Fraction
Enthalpy, kJ/kg
5.8443
1600
5.8443
5.8443
5.8443
1500
5.8443
5.8443
5.8443
5.8443
1400
5.8443
5.8443
5.8443
5.8443
1300
5.8443
5.8443
5.8443
5.8443
1200
5.8443
5.8443
5.8443
1100
5.8443
5.8443
5.8443
5.8443
1000
5.8443
5.8443
5.8443
5.8443
600
0 -0.22999 20.00013
5.8443
6.7375 34.2216
0 -0.22729 20.00013
5.8443
6.7465 34.2216
0 -0.22521 20.00013
5.8443
6.7534 34.2216
0 -0.22363 20.00013
5.8443
6.7588 34.2216
Reference
013.08
-0.22246
20.00013
5.8443
6.7631 34.2216
015.28
-0.22249
20.00013
Inner
Pitch 5.8443 6.7637 34.2216
0 -0.22049 20.00013
5.8443
6.7682 34.2216
11.50 Center Pin
0 -0.21887 20.00013
5.8443
6.7739 34.2216
0 -0.21658 20.00013
5.8443
6.7819 34.2216
0 -0.21335 20.00013
5.8443
6.7931 34.2216
0 -0.20875 20.00013
5.8443
6.8094 34.2216
0 -0.20378 20.00013
5.8443
6.8268 34.2216
0 -0.19999 20.00013
5.8443
6.8403 34.2216
0 -0.19711 20.00013
5.8443
6.8507 34.2216
0 -0.19493 20.00013
5.8443
6.859 34.2216
0 -0.19334 20.00013
5.8443
6.8663 34.2216
0 -0.19254 20.00013
5.8443
6.8667 34.2216
0 -0.19093 20.00013
5.8443
6.8727 34.2216
0 -0.18867 20.00013
5.8443
6.8811 34.2216
0 -0.18553 20.00013
5.8443
6.8928 34.2216
0 -0.18116 20.00013
5.8443
6.9092 34.2216
0
-0.175 20.00013
5.8443
6.9328 34.2216
0 -0.16848 20.00013
5.8443
6.9578 34.2216
0 -0.16355 20.00013
5.8443
6.9771 34.2216
0 -0.15981 20.00013
5.8443
100
200
300 6.9918 34.2216
400
0 -0.15701 20.00013
5.8443
7.0032 34.2216
Axial Distance
from
Inlet, mm
0 -0.15493 20.00013
5.8443
7.0124 34.2216
127.88
597.37
134.84
594.66
139.98
592.65
143.77
591
146.58
589.89
148.65
589.01
150.72
574.19
153.53
572.89
157.32
571.48
162.46
569.53
169.42
566.84
177.43
561.48
184.39
558.72
189.53
556.66
193.32
554.97
Inlet
temp.:
196.13
553.78
268°C
198.2
538.71
256°C
200.27
537.87
203.08
536.63
206.87
535.17
212.01
533.16
218.97
530.39
226.98
524.9
233.94
522.05
500 239.08 519.94
600
242.87
518.2
245.68
517.03
(a) Enthalpy
(b) Void fraction
Figure 7 Enthalpy and void fraction distributions of the 37S fuel bundle,
inner pitch, and center element modifications
3.2
Subchannel enthalpy
Figure 8 shows a comparison of the subchannel enthalpy distributions of a 37S fuel bundle and its
modifications for an inlet temperature of 256℃. Since the enthalpies of the center subchannel numbers
1 through 6 for a 37S fuel bundle are much higher than those of other subchannels for the inlet
temperature of 256℃, as expected, the first CHF occurrence of a 37S fuel bundle was found at
subchannel number 1, as shown in Figure 8(a), while those of the inner pitch and center element
modifications were found at subchannel numbers 10 and 32, respectively, as shown in Figures 8(b)
and 8(c). It was noted that the large flow area of the center subchannels makes the subchannel enthalpy
distribution more uniform than that of a 37S fuel bundle. In addition, the first CHF for the inner pitch
and center element modifications was moved from the center subchannel to the inner and outer
subchannels, respectively. When compared with the subchannel enthalpies of two modifications, the
center subchannel enthalpies for a small center element are lower than those of the large inner pitch
because the small center element has a low power density, as shown in Figure 6. Additionally, the
inner subchannel enthalpies of the large inner pitch were higher than those of the small center element,
as shown in Figures 8(b) and 8(c). This caused the flow area of the inner subchannels to be reduced
by enlarging the inner pitch, even when the flow areas of the center subchannels of two modifications
were the same.
1408
1407
1387
1408
1408
1500
1491
1410
1616
1557
1423
1623
1493
1457
1490
1423
1572
1528
1576
1451
1529
1527
1523
1530
1484
1510
1476
1503
Enthalpy (56t20g14.88)
1507
1563
1452
1443
1522
1518
1516
1513
1524
1477
1458
1486
1515
1522
1495
Enthalpy (56t20g Inner Pitch)
1449
1510
1480
CHF subchannel loc.(O): 10
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.37888
1503
1543
1502
1504
1511
1482
1499
1556
1545
1538
1564
1507
1513
1466
1506
1498
1431
1575
1509
1499
CHF subchannel loc.(O): 1
CHF axial loc.(cm): 516.27
CHF (MW/m2): 0.95172
1440
1434
1510
1574
1553
1495
1554
1417
1496
1560
1572
1575
1443
1520
1527
1502
1497
1506
1575
1495
1501
1570
1460
1506
1415
1494
1494
1516
1473
1559
1563
1548
1410
1488
1571
1507
1559
1510
1527
1520
1596
1554
1474
1578
1594
1477
1398
1550
1555
1596
1487
1628
1601
1554
1520
1408
1510
1490
1398
1574
1576
1580
1501
1439
1426
1503
1578
1482
1484
1490
1478
1469
1670
1666
1508
1566
1470
1557
1700
1502
1471
1393
1574
1488
1396
1482
1488
1393
1596
1393
1396
1476
1484
1489
1472
1410
1489
1393
1390
1476
1510
1596
1387
1390
1500
1514
1510
1492
1389
1386
1407
1469
1492
1498
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 481.69
CHF (MW/m2): 1.58558
Enthalpy (56t20g Center Pin)
(a) 37S fuel bundle
(b) Inner pitch modification (c) Center element modification
Figure 8 Enthalpy distributions of the standard 37 element fuel and its modifications for 256℃
On the other hand, Figure 9 shows a comparison of the subchannel enthalpy distributions of a 37S
fuel bundle and its modifications for an inlet temperature of 268℃. The subchannel enthalpy
distributions of the modifications as shown in Figure 9 were similar to those for the inlet temperature
of 256℃, except that the first CHF location of the large inner pitch was moved from inner subchannel
#11 for the inlet temperature of 256℃ to outer subchannel #32 for the inlet temperature of 268℃. It
was noted that the subchannel enthalpy distribution and the first CHF occurrences for the case of the
high inlet temperature of 268℃ can be affected by mixing the characteristics among the subchannels
as well as the inlet temperature.
1414
1414
1414
1416
1416
1500
1493
1501
1417
1418
1599
1545
1431
1651
1506
1617
1543
1435
1508
1517
1592
1508
1461
1552
1509
1513
1450
1514
1570
1522
1485
1524
1527
1522
1519
CHF subchannel loc.(O): 1
CHF axial loc.(cm): 490.62
CHF (MW/m2): 1.02476
1525
1479
1504
Enthalpy (68t20g14.88)
1533
1534
1536
1491
1467
1512
1451
1526
1517
1523
1510
Enthalpy (68t20g Inner Pitch)
1574
1462
1536
1484
1520
1471
1529
1505
1468
1555
1517
1575
1534
1515
1566
1556
1551
1598
1532
1507
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.50815
1582
1564
1518
1576
1437
1514
1572
1581
1596
1461
1535
1499
1501
1513
1519
1587
1513
1571
1469
1520
1568
1490
1519
1435
1519
1524
1600
1456
1525
1437
1516
1518
1569
1464
1419
1562
1491
1542
1585
1570
1497
1419
1600
1615
1621
1505
1568
1613
1502
1501
1506
1598
1617
1515
1598
1603
1547
1433
1505
1581
1497
1494
1654
1579
1503
1445
1589
1495
1545
1686
1505
1500
1420
1599
1418
1502
1513
1420
1489
1415
1418
1506
1510
1513
1584
1490
1415
1419
1506
1507
1584
1417
1419
1500
1510
1508
1494
1411
1417
1414
1532
1537
1493
1530
1526
1509
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.49609
1535
1529
1486
1507
1512
Enthalpy (68t20g Center Pin)
(a) 37S fuel bundle
(b) Inner pitch modification (c) Center element modification
Figure 9 Enthalpy distributions of the standard 37 element fuel and its modifications for 268℃
3.3
Subchannel void fraction
Figures 10 and 11 show the void fraction distributions of a 37S fuel bundle and those modifications
at the location of the first CHF occurrence for 256℃ and 268℃ inlet temperatures, respectively. The
void fraction distributions of the modifications are more uniform than those of a 37S fuel bundle, as
shown in Figure 10. The location of the first CHF occurrence of a 37S fuel bundle was moved from
subchannel #1 to #10 and #33 for the inner pitch and center element modifications, respectively. It
was noted that the enthalpies or void fractions of the inner subchannels are more prone to have the
CHF occur firstly as they are closer by increasing the inner pitch. The axial CHF location of a 37S
fuel bundle was 508.48 cm, or at the axial 10th bundle from the channel inlet, while that of the inner
pitch and center element modifications were 478.82 cm and 481.69 cm, respectively, or at the 9th
axial bundle.
0.207
0.129
0.201
0.201
0.199
0.198
0.482
0.499
0.458
0.456
0.385
0.193
0.193
0.686
0.6
0.244
0.746
0.491
0.7
0.478
0.299
0.132
0.508
0.183
0.434
0.365
0.428
0.23
0.626
0.543
0.621
0.345
0.536
0.528
0.544
0.456
0.54
0.506
0.432
0.479
0.458
0.463
0.478
0.472
0.598
0.336
0.304
0.51
0.356
0.499
0.494
0.486
0.514
0.42
0.427
0.495
0.51
0.471
Void fraction (56t20g Inner Pitch)
0.326
0.557
0.46
0.443
CHF subchannel loc.(O): 10
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.37888
0.453
0.586
0.565
0.548
0.466
0.481
0.433
0.517
0.452
0.261
0.6
0.47
0.485
0.618
0.618
0.475
0.381
Void fraction (56t20g14.88)
0.293
0.213
0.45
0.595
0.615
0.582
0.44
0.577
0.267
0.481
0.499
CHF subchannel loc.(O): 1
CHF axial loc.(cm): 516.27
CHF (MW/m2): 0.95172
0.448
0.474
0.62
0.448
0.617
0.3
0.524
0.475
0.206
0.439
0.454
0.624
0.541
0.584
0.381
0.593
0.436
0.377
0.189
0.428
0.597
0.567
0.512
0.147
0.572
0.383
0.507
0.651
0.4
0.147
0.623
0.611
0.492
0.602
0.429
0.585
0.649
0.418
0.42
0.43
0.618
0.649
0.426
0.497
0.705
0.497
0.539
0.6
0.62
0.672
0.593
0.251
0.486
0.64
0.402
0.369
0.749
0.636
0.383
0.372
0.601
0.775
0.484
0.427
0.619
0.153
0.418
0.132
0.444
0.15
0.154
0.402
0.42
0.428
0.664
0.446
0.15
0.135
0.402
0.498
0.664
0.133
0.136
0.482
0.514
0.138
0.133
0.395
0.465
0.479
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 481.69
CHF (MW/m2): 1.58558
Void fraction (56t20g Center Pin)
(a) 37S fuel bundle
(b) Inner pitch modification
(c) Center element modification
Figure 10 Void fraction distributions of the standard 37 element fuel and its modifications at 256℃
0.205
0.2
0.206
0.206
0.207
0.206
0.466
0.49
0.445
0.457
0.203
0.202
0.657
0.724
0.473
0.685
0.467
0.306
0.206
0.556
0.263
0.477
0.495
0.362
0.493
0.616
0.516
0.447
0.525
0.512
0.505
0.491
CHF subchannel loc.(O): 1
CHF axial loc.(cm): 490.62
CHF (MW/m2): 1.02476
0.521
0.426
0.5
Void fraction (68t20g14.88)
0.532
0.532
0.538
0.455
0.375
0.484
0.322
0.514
0.495
0.508
0.506
Void fraction (68t20g Inner Pitch)
0.616
0.364
0.539
0.434
0.504
0.391
0.523
0.493
0.381
0.579
0.494
0.618
0.534
0.489
0.605
0.585
0.571
0.653
0.527
0.498
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.50815
0.63
0.601
0.496
0.614
0.273
0.491
0.615
0.628
0.649
0.355
0.536
0.486
0.485
0.503
0.637
0.49
0.498
0.384
0.51
0.607
0.424
0.505
0.267
0.509
0.656
0.349
0.517
0.205
0.593
0.613
0.495
0.614
0.375
0.205
0.27
0.495
0.632
0.606
0.447
0.426
0.555
0.676
0.485
0.315
0.466
0.655
0.644
0.479
0.58
0.459
0.608
0.674
0.465
0.467
0.655
0.678
0.494
0.69
0.48
0.523
0.636
0.653
0.667
0.571
0.259
0.477
0.633
0.449
0.435
0.727
0.63
0.455
0.438
0.57
0.759
0.475
0.485
0.655
0.211
0.465
0.207
0.43
0.569
0.253
0.481
0.207
0.212
0.475
0.485
0.638
0.433
0.207
0.211
0.475
0.48
0.638
0.209
0.212
0.466
0.481
0.447
0.19
0.209
0.529
0.54
0.463
0.525
0.515
0.506
CHF subchannel loc.(O): 32
CHF axial loc.(cm): 474.73
CHF (MW/m2): 1.49609
0.536
0.526
0.443
0.501
0.513
Void fraction (68t20g Center Pin)
(a) 37S fuel bundle
(b) Inner pitch modification
(c) Center element modification
Figure 11 Void fraction distributions of the standard 37 element fuel and its modifications at 268℃
3.4
Dryout power enhancement
Dryout powers of the inner pitch and center element modifications under each mass flow and inlet
temperature were investigated and compared to those of a 37S fuel bundle. To investigate the
enhancement of dryout power of its modifications in terms of a 37S fuel bundle, the dryout power
enhancement ratio is defined as the ratio of dryout power of its modification to that of a 37S fuel
bundle. Figure 12 shows the dryout power enhancement of the modifications for each mass flow and
inlet temperature conditions. It was shown that the dryout powers of the modifications are always
higher than those of a 37S fuel bundle for all mass and inlet temperature conditions. These results
agree well with references 8 and 10. In addition, most dryout powers of the small center element are
higher than those of the large inner pitch except for the low mass flow condition of 20 kg/s. It was
noted that the turbulent intensity and mixing characteristics among the subchannels of the small center
element may be different from those of the large inner pitch, and can be affected by the low power
density and small heated area of the center element, as shown in Figure 6.
While considering the dryout power enhancement for a mass flow of 20 kg/s, that of the large inner
pitch is higher compared to that of the small center element and this trend becomes significant as the
inlet temperature increases. However, for high mass flows, the dryout power enhancement of the small
center element is higher than that of the large inner pitch. It is noted that the higher dryout power
enhancement of the center element modification should be caused by the power density reduction of
a center element, as well as the large flow area of the center subchannels.
Even if the small center element resulted in a higher dryout power enhancement and might be more
effective in terms of the dryout power enhancement, the adverse impact on the safety and fuel
management cost for its modification should be considered.
1.1400
1.1400
t56g20
1.0600
1.0400
1.0200
1.1000
1.0800
1.0600
1.0400
1.0200
1.0000
Inn Pitch
Center Pin
Types of modification
1.1000
1.0800
1.0600
1.0400
1.0200
1.0000
Ref
t68g30
1.1200
Enhancement ratio of dryout power
1.0800
t68g24
t62g30
1.1200
Enhancement ratio of dryout power
Enhancement ratio of dryout power
1.1000
t68g20
t62g24
t56g30
1.1200
1.1400
t62g20
t56g24
1.0000
Ref
Inn Pitch
Center Pin
Middle ring radius, mm
Ref
Inn Pitch
Center Pin
Middle ring radius, mm
(a) Inlet temperature, 256℃
(b) Inlet temperature, 262℃
(c) Inlet temperature, 268℃
Figure 12 Comparison of dryout enhancements of inner pitch and center element modifications
in terms of a 37S fuel bundle
3.5
Void fraction imbalance factor
In this section, the void fraction distribution or non-uniformity of the subchannel void fraction was
selected to investigate its correlation with the dryout power enhancement ratio. Also, the comparison
of the correlation between the non-uniformity of the subchannel void fraction at the CHF locations
and dryout power enhancement ratios for the fuel bundle modifications were made under the same
inlet flow conditions because the dryout power enhancement ratio is not the local properties such as
CHF and quality.
To investigate the correlation between the local void fraction at the first CHF location and the dryout
power enhancement ratio, the dryout power enhancement ratio and the local void fraction at the axial
and subchannel locations corresponding to the first CHF occurrence were plotted together for each
flow condition, as shown in Figure 13. The void fraction imbalance factor is defined as a local
subchannel void fraction divided by a section averaged void fraction at the location of the first CHF
occurrence. This shows that the dryout power enhancement ratio of the center element modification
is lower than that of the inner pitch modification (see ‘t262g20’ and ‘t268g20’ in Figure 13) if the
void fraction imbalance factor of the center element modification is higher than that of the inner pitch
modification. It was noted that the dryout power of a fuel bundle would be higher when the local void
fraction imbalance factor at the location of the first CHF occurrence is lower.
3
1.20
VF imbalance factor
1.15
2
1.10
1.5
1.05
1
1.00
t256g20 t256g24 t256g30 t262g20 t262g24 t262g30 t268g20 t268g24 t268g30
0.5
0
Flow conditions [t: inlet temperature(°C), g: mass flow(kg/s)]
0.95
Dryout power enhancement ratio
Void faction imbalance factor
DP enhancement ratio
2.5
0.90
Figure 13 Comparison of void fraction imbalance factors and dryout power enhancement ratio
according to the flow conditions
4.
Conclusion
The subchannel analyses were performed to investigate the enthalpy distribution, void fraction
distribution, and dryout power enhancement of the fuel bundle modifications of a 37S fuel bundle.
Two approaches such as the increase of the inner pitch circle and the decrease of the center element
diameter to enlarge the flow area of the center subchannels were considered to enhance the dryout
power of an existing CANDU fuel bundle. The uncertainty of the dryout power could be existed for
a 37S fuel bundle and its modifications, but it was not considered because of the sensitivity studies
for a 37S fuel bundle and its modifications. From the present study, the following were concluded:
The subchannel enthalpy and void fraction of a 37S fuel bundle modification were redistributed as
expected when the flow area of the center subchannels are enlarged by increasing the inner pitch circle
or reducing the center element diameter. In addition, the subchannel locations of the first CHF
occurrence of those modifications were moved to the other subchannels such as the middle subchannel
or outer subchannel. In addition, the axial locations of the first CHF occurrence were moved from the
10th fuel bundle to the 9th fuel bundle according to the modifications. Finally, the dryout powers of
the two types of modification of a 37S fuel bundle could be enhanced. In addition, two approaches to
enlarging the center subchannel area were revealed as very effective ways to increase the dryout power
of a CANDU fuel bundle or to overcome the power deration owing to an aging CANDU power plant.
While considering the dryout power enhancement for a mass flow of 20 kg/s, that of the large inner
pitch is higher compared to that of the small center element and this trend became significant as the
inlet temperature increases. However, for high mass flows, dryout power enhancements of the small
center element are higher than those of the large inner pitch. It was noted that the enthalpy and void
fraction distribution of the large inner pitch are different from those of the small center element and
should be caused by the power density reduction of a center element, as well as the large flow area of
the center subchannels.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the
Korea government (Ministry of Science, ICT, and Future Planning) (No. NRF2012M2A8A4025960).
5.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
References
D.C. Groeneveld and K.C. Goel, “A method of increasing Critical Heat Flux in nuclear fuel
bundles”, CRNL-1763 (1978).
A.G. McDonald and S.C. Sutradhar, “CANFLEX bundle thermalhydraulic experiments: Part
4, Freon CHF tests on the 37E-hybrid bundle, equipped with two space planes and four
bearing pad planes”, HPBP-32/ARD-TD-124 (1988).
D.C. Groeneveld, “On the definition of Critical Heat Flux margin”, Nuclear Engineering and
Design 163, 245-247 (1996).
G.C. Dimmick, W.W.R. Inch, J.S. Jun, H.C. Suk, G.I. Hadaller, R.A. Fortman, and R.C.
Hayes, “Full scale water CHF testing of the CANFLEX bundle”, Proceeding of the 6th
International Conference on CANDU fuel, Niagara Falls, Ontario, Canada, 103-113 (1999).
L.K.H. Leung, J.S. Jun, G.R. Dimmick, D.E. Bullock, W.W.R. Inch, and H.C. Suk, “Dryout
power of a CANFLEX bundle string with raised bearing pads”, Proceeding of the 7th
International Conference on CANDU Fuel, Kingston, Ontario, Canada, 27-40 (2001).
L.K.H. Leung, and F.C. Diamayuga, “Measurements of critical heat flux in CANDU 37element bundle with a steep variation in radial power profile”, Nuclear Engineering and
Design 240, 290-298 (2010).
“Fuel Design Manual for CANDU-6 reactors”, DM-XX-37000-001. AECL (1989).
J.H. Bae and J.H. Park, “The effect of a CANDU fuel bundle geometry variation on
thermalhydraulic performance”, Annals of Nuclear Energy 38, 1897-1899 (2011).
M.B. Carver, J.C. Kiteley, R.Q.N. Zou, S.V. Junop, and D.S. Rowe, “Validation of the
ASSERT subchannel code; prediction of critical heat flux in standard and nonstandard
CANDU bundle geometries”, Nuclear Technology 112, 299-314 (1995).
J.H. Park and Y.M. Song, “The effect of inner ring modification of standard 37-element fuel
on CHF enhancement”, in publishing in Annuls of Nuclear Energy (2014).
A. Tahir, Y. Parlatan, M. Kwee, W. Liauw, G. Hadaller, and R. Fortman, “Modified 37element bundle dryout”, NURETH-14, Hilton Toronto Hotel, Toronto, Ontario, Canada
(2011).
S.R. Douglas, “WIMS-AECL Release 2-5d Users Manual”, COG-94-52(Rev. 4), FFC-RRP299, AECL, July (2000).
Wheeler, C.L., et al., “COBRA-IV-I: an interim version of COBRA for thermal-hydraulic
analysis of rod-bundle nuclear fuel elements and cores”, Battelle Pacific Northwest
Laboratories Report, BNWL-1962 (1976).
Stewart, C.W., et al., “COBRA-IV: The model and the method”, Battelle Pacific Northwest
Laboratories Report, BNWL-2214 (1977).
D.C. Groeneveld, L.K.H. Leung, P.L. Kirillov, V.P. Bobkov, I.P. Smogalev, V.N.
Vinogradov, X.C. Huang and E. Royer, “The 1995 look-up table for critical heat flux in
tubes”, Nuclear Engineering and Design, 163, 1-23 (1995).