EXTENSION OF VVER-440 FUEL CYCLES USING IMPROVED FA DESIGN
Pavel Mikoláš
e-mail: [email protected]
Jiří Švarný
e-mail: [email protected]
ŠKODA JS a.s.
Orlík 266, 31600 Plzeň
Czech Republic
ABSTRACT
Practically full five years cycle has been achieved at NPP Dukovany in the last
years. There are two principal means how it could be achieved. First, it is necessary to use
fuel assemblies with higher fuel enrichment and second, to use fuel loading with very low
leakage. Both these conditions are fulfilled at NPP Dukovany at this time.
However, the efficiency of fuel cycle can be improved by increasing the fuel
residence time in the core up to six years. There are at least two ways how this goal could
be achieved. The simplest way is to increase enrichment in fuel. There exists a limit, which
is 5.0 w% of U235. Taking into account some uncertainty, the calculation maximum is 4.95
w% of U235. The second way is to change fuel assembly design. There are several
possibilities, which seem to be suitable from the neutron – physical point of view. The first
one is higher mass content of uranium in a fuel assembly. The next possibility is to enlarge
pin pitch. The last possibility is to “omit” FA shroud. This is practically unrealistic;
anyway, some other structural parts must be introduced.
The basic characteristics of these cycles for up-rated power are presented showing
that the possibilities of fuel assemblies with this improved design in enlargement of fuel
cycles are very promising.
1. INTRODUCTION
As stated in Abstract, the efficiency of fuel cycle can be improved if six years cycles
would be applied. There are at least two ways how this goal could be achieved. The simplest
way is to increase enrichment in fuel. There exists a limit, which is 5.0 w% of U235. Taking
into account some uncertainty, calculation maximum is 4.95 w% of U235. The second
possibility is to change fuel assembly design.
In this paper, both possibilities are checked in different range individually.
2. FUEL ASSEMBLY WITH HIGHER ENRICHMENT
As it has been stated in introduction, we will suppose enrichment of 4.95 w% of U235
in some of fuel pins in a fuel assembly. The other characteristics (it means excluding
enrichment) are the same as for fuel assembly of Gd2 or Gd2M for up-rated power. It seems
to be clear that this maximal enrichment can not be applied in all fuel pins because in this case
pin power non-uniformity would be very high (more than 1.15), which would create a
problem from the point of view of pin power factor in the core. Therefore, enrichment is
lower in some pins and one from possible solutions is shown in Fig. VIIa, where the
maximum (1.072) has been found in pin No: 17 according to Fig. II. at FA burn-up of 10000
[MWd/tU]. Kinf values of some similar designs are shown in Graph 1K (also with zoom for
the beginning of burn-up process) and values of maximum of Fdh (Kr) in Graph 1P (three
digits in the name identify lower enrichment in FA). Fuel assemblies (according to Fig. VIIa)
have been loaded into core in transient end equilibrium cycles of Dukovany NPP for up-rated
power (cycles 27 to 34 of Unit III; without change of existing loadings – only with a change
of overall cycle length; the same is valid in chapter 3) and basic characteristics of these cycles
are shown in Tables 1-4 (where in column 1 is cycle number, in column 2 are values for base
calculation (WFA of enrichment 4.38 w% of U235 and CFA of 4.25 w% of U235)), in column 3
for the variant with WFAs according to Fig. VIIa and CAs acc. to Fig. III, in column 4 for the
variant with WFAs according to Fig. VIIa and CAs according to Fig. VI and in column 5 for
the variant with WFAs according to Fig. VIIa (but with higher Gd2O3 content [5.0 w%] and
CAs according to Fig. III. It is seen that fuel cycles prolongation has been achieved, but this
prolongation may not be sufficient for six-year cycle.
3. NEW FUEL ASSEMBLY DESIGN
The other possibility how to extend the fuel cycle (and how potentially achieve six
year cycle) is the application of fuel assembly with an improved design. There are several
possibilities, which seem to be suitable from the neutron – physical point of view. The first is
higher mass content of uranium in a fuel assembly. This can be achieved by two ways: first to
remove central hole in fuel pellet and second, to enlarge fuel pellet diameter. In our checking,
both conditions were applied together. Other characteristics are the same as for FA for uprated power (Gd2M).
The next possibility is to enlarge pin pitch. It has been also applied. The last
possibility is to “omit” FA shroud. This is practically not realistic, in any way, some other
structural parts must be introduced; in our simplified model, thickness of FA shroud was
lowered in two steps, namely from 1.5 mm on 1.0 mm and then on 0.5 mm. Kinf values for 5
different FAs are shown in Graph 7 (also with zoom for the beginning of burn-up process),
(where G- means FA according to Fig. VII, Gbezd – FA of the some type as in Fig. VI, but
fuel pellet is without central hole and with higher diameter, G124 - as previous, but fuel pin
pitch is 12.4 mm (instead of 12.3 mm), G12410 – as previous, but FA shroud thickness is
only 1.0 mm and G12405 – as previous, but FA shroud thickness is only 0.5 mm. The basic
characteristics of these cycles for up-rated power are shown in Tables 5-7 showing that the
possibilities of fuel assemblies with this improved design in enlargement of fuel cycles are
very promising.
All FAs features were calculated by WIMS8 code [1] and core calculations by
MOBY-DICK code [2].
4. FUEL CYCLES WITH 4.75W% ENRICHMENT FUEL (QO3)
Application of the higher fuel enrichment (WFA enriched on 4.75w%, see FA type
QO3 with 3.35w% Gd2O3 and CFA enriched on 4.38w%) was realized for power up-rated
design (1444 MWt) of NPP Dukovany. Optimization of low leakage fuel cycle sequences was
provided by program OPAL_B [3] on the 3D level core burn up modeled by macrocode
MOBY-DICK. Nearly 5.5 year cycle (see Fig. VIII) was reached by optimization, which is in
Table 8 compared with original 5 years fuel cycle for up-rated power of NPP Dukovany
(WFA enriched on 4.38w%, see FA type QS3 or Gd2M with 3.35w% Gd2O3 and CFA
enriched on 4.25w%). From Table 8 is seen that average burn up of six years FA will be
lower than 60 000 MWd/tU.
During 5 years cycle we have loaded 12 fresh FAs in average in each cycle and during
6 years cycle we have loaded 10 fresh FA. From this it follows that each 6 years FA should
(approximately) brought excess of reactivity by 20% higher compared to FA of 5 years cycle.
That represents excess by 65 FPD in 326 FPD length of 5 years cycle. According Table 2 we
have for QO5 FAs excess 27 FPD, which represents 8.3% increase in fuel cycle length. This
agrees with decreasing number of FA by 9%.
Additional excess cycle length (or reactivity) can be achieved for example by loading
of FA of new design QN1.
Combination of improvement QO5 + QN1 potentially can assure pure 6 years fuel
cycle.
5. DISCUSSION
The values shown in Tables 1 - 4 cannot be supposed to be quite real because pin
power non-uniformity is too high. Different loadings have been found as it is shown in Table
8 and Fig.VIII. Pin power non-uniformity is still relatively big; it could be lowered by
optimization, because we have still excess of reactivity (positive residual boric acid
concentration). Loading with lower number of fresh FA has been found instead of simple
enlargement of the fuel cycle. As stated above, although not yet proved, application of FAs
with higher enrichment and improved design (see Tables 5-8) could lead to full six years
cycle. So, it seems to be clear that FA have still potential in sense of the fuel cycle economy.
6. CONCLUSIONS
Although only preliminary analyses described in the paper have been performed,
it can be concluded that temporary design of VVER-440 FA is not optimal (as it is well
known) and it exists a great potential how to increase FA reactivity, which is a necessary
condition in achieving full six years loading strategy. (Potential in lowering core neutron
leakage is practically exhausted as is also the possibility in reducing neutron absorption
in construction parts of a FA.)
Design change (it means FA of “KARKAZ” type) seems to be more encouraging
than an attempt still to increase fuel enrichment.
Of course, both possibilities (or their combination) must be proved on very well
designed loading strategies.
REFERENCES
[1]
Coll.:
WIMS - A Modular Scheme for Neutronics Calculations, User Guide for Version
8, ANSWERS/WIMS(99)9, Winfrith, 1999
[2]
Krýsl, V.:
MOBY-DICK Users Manual, Report of ŠKODA JS a.s. No.: Ae10068/Dok, Rev. 3,
Plzeň 2005, (In Czech)
[3]
Švarný, J.:
OPAL_B The In Core Fuel Cycle Management System Development,
Proceedings of the 13th Symposium of AER, Dresden, Germany, September 2003
Figures and Graphs
Fig. I Calculation scheme of an „asymtotic“ fuel assembly
Fig.II Numeration of fuel rods
17
18
12
19
20
13
8
14
9
5
10
6
3
22
16
11
7
4
2
21
15
n.n % U235
central tube
Fig.III Fuel enrichment [w%U235] in fuel rods of Russian design of FA „Gd-2“
(average enrichment 4.247619 w%U235)
Gd
Gd
4.4 % U235
Gd
4.0 % U235
Gd
4.0 % U235
+ 3.35 % Gd2O3
3.6 % U235
Gd
Gd
central tube
Gd
Fig.VI Fuel enrichment [w%U235] in fuel rods of modified design of FA Gd-2
„Gd2M“ (average enrichment 4.380952 w%U235) for uprated power
Gd
Gd
4.6 % U235
Gd
4.0 % U235
Gd
4.0 % U235
+ 3.35 % Gd2O3
3.6 % U235
Gd
Gd
central tube
Gd
Fig.VIIa Fuel enrichment [w%U235] in fuel rods of modified design of FA Gd-2
„Gd2Max“ (average enrichment „4.757143“ w%U235) for six-year cycle
Gd
Gd
4.95 % U235
Gd
4.4 % U235
Gd
4.4 % U235
+ 3.35 % Gd2O3
4.2 % U235
Gd
Gd
central tube
Gd
Fig.VIII The odd and even equilibrium loadings of 5.5 years fuel cycle
(Numbers represent residence time [years] of FA in core)
Graph 1 K-inf FAs with higher enrichment and different profilation
[Gd2O3 content 3.35 w%]
1,22
Gd2M
G440415
1,16
G440420
G435420
G435415
1,1
k-inf
1,04
0,98
0,92
0,86
0,8
0
10000
20000
30000
burn-upí [MWd/tU]
40000
50000
60000
Graph 1K (zoom) K-inf FAs with higher enrichment and different profilation
[Gd2O3 content 3.35 w%]
1,21
Gd2M
G440415
G440420
1,2
G435420
G435415
k-inf
1,19
1,18
1,17
1,16
1,15
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
vyhoření [MWd/tU]
Graph 1P Maximal relative power in FPs of FAs with higher enrichment and different
profilation [Gd2O3 content 3.35 w%]
1,08
G440410
G440420
1,07
G435420
Relative power
G435415
1,06
1,05
1,04
1,03
0
10000
20000
30000
burn-up [MWd/tU]
40000
50000
60000
Graph 2K K-inf FAs with higher enrichment and different profilation
[Gd2O3 content 5.0 w%]
1,22
G440415
G440420
G435420
G435415
k-inf
1,12
1,02
0,92
0,82
0
10000
20000
30000
40000
50000
60000
burn-up [MWd/tU]
Graph 2K (zoom) K-inf FAs with higher enrichment and different profilation
[Gd2O3 content 5.0 w%]
1,2
G440415
1,19
G440420
G435420
G435415
k-inf
1,18
1,17
1,16
1,15
0
1000
2000
3000
4000
5000
burn-up [MWd/tU]
6000
7000
8000
9000
10000
Graph 2P Maximal relative power in FPs of FAs with higher enrichment and different
profilation [Gd2O3 content 5.0 w%]
1,08
G440415
G440420
G435420
1,07
Relative power
G435415
1,06
1,05
1,04
1,03
0
10000
20000
30000
40000
50000
60000
burn-up [MWd/tU]
Graph 7 K-inf of FAs with improved design
1,2
G
Gbezd
G124
1,1
G12410
k-inf
G12405
1
0,9
0,8
0
10000
20000
30000
burn-up [MWd/tU]
40000
50000
60000
Graph 7 (zoom) K-inf of FAs with improved design
1,2
G
Gbezd
1,195
G124
1,19
G12410
G12405
1,185
k-inf
1,18
1,175
1,17
1,165
1,16
1,155
1,15
1,145
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
burn-up [MWd/tU]
Tables
Table 1 Cycles length with FAs with higher enrichment
Cycle\FA
27
28
29
30
31
32
33
34
Base (QS3)
326
326
328
328
328
327
326
326
QO3 (Gd-2+)
341
346
357
357
348
353
353
349
QO3 (Gd-2M)
342
348
360
359
350
356
355
351
QO5 (Gd-2+)
336
344
356
355
346
352
351
346
Table 2 Difference (profit) in cycles length with FAs with higher fuel
enrichment in relation with base variant (base [Gd2M])
Cycle\FA
27
28
29
30
31
32
33
34
Base (QS3)
0
0
0
0
0
0
0
0
QO3 (Gd-2+)
15
20
29
29
20
26
27
23
QO3 (Gd-2M)
16
22
32
31
22
29
29
25
QO5 (Gd-2+)
10
18
28
27
18
25
25
20
Table 3 Residual boric acid concentration after power stretch-out at fuel cycle
length found for FAs with higher fuel enrichment
Cycle\FA
27
28
29
30
31
32
33
34
Base (QS3)
+0.026
-0.013
+0.061
+0.082
-0.041
-0.027
+0.008
-0.062
QO3 (Gd-2+)
+0.000
+0.000
-0.001
+0.002
-0.002
+0.007
+0.009
-0.011
QO3 (Gd-2M)
+0.010
-0.001
-0.007
-0.004
+0.002
-0.008
+0.006
-0.012
QO5 (Gd-2+)
+0.001
+0.001
-0.002
+0.001
+0.005
-0.011
-0.002
+0.001
Table 4 Maximum Fdh (Kr) in cycles with FAs with higher fuel enrichment
(value in bracket gives burn-up [in FPD] where this maximum occurs)
Cycle\FA
27
28
29
30
31
32
33
34
*
Base (QS3)
1.519 (2*)
1.511 (0)
1.518 (2)
1.497 (2)
1.514 (0)
1.500 (2)
1.511 (2*)
1.510 (0)
QO3 (Gd-2+)
1.557 (200*)
1.563 (180*)
1.548 (160)
1.539 (160*)
1.542 (180)
1.537 (180)
1.539 (180*)
1.542 (140*)
QO3 (Gd-2M)
1.556 (200)
1.561 (160*)
1.544 (140*)
1.537 (160*)
1.539 (180*)
1.535 (180*)
1.537 (180*)
1.540 (160*)
QO5 (Gd-2+)
1.541 (240*)
1.543 (240)
1.543 (0)
1.517 (240)
1.525 (8.260)
1.523 (4)
1.532 (4*)
1.528 (4*)
means that value shown has been found in more consecutive time steps [ in FPD];
the first from these steps is marked
Table 5 Cycles length with different FAs types
Cycle\FA
27
28
29
30
31
32
33
34
Base „G“
(QS3)
326
326
328
328
328
327
326
326
„Gbezd“
(QND)
329.91
333.82
339.81
343.74
347.68
346.62
345.56
345.56
„G124“
(QN4)
329.91
333.82
339.81
343.74
347.68
346.62
345.56
345.56
„G12410“
(QN1)
331.16
336.36
343.68
348.96
354.29
353.21
352.13
352.13
„G12405“
(QN0)
341.46
345.50
351.70
355.77
359.85
358.75
357.65
357.65
Table 6 Difference (profit) in cycles length with different FAs types in relation with base
variant (base [Gd2M])
Base „G“
(QS3)
Cycle\FA
27
28
29
30
31
32
33
34
0
0
0
0
0
0
0
0
„Gbezd“
(QND)
3.91
7.82
11.81
15.74
19.68
19.62
19.56
19.56
„G124“
(QN4)
3.91
7.82
11.81
15.74
19.68
19.62
19.56
19.56
„G12410“
(QN1)
5.16
10.36
15.68
20.96
26.29
26.21
26.13
26.13
„G12405“
(QN0)
15.46
19.50
23.70
27.77
31.85
31.75
31.65
31.65
Table 7 Maximum Fdh (Kr) in cycles wit different FAs types (value in bracket gives burn-up
[in FPD], where this maximum occurs)
Cycle\FA
27
28
29
30
31
32
33
34
*
Base „G“
(QS3)
1.519 (2*)
1.511 (0)
1.518 (2)
1.497 (2)
1.514 (0)
1.500 (2)
1.511 (2*)
1.510 (0)
„Gbezd“
(QND)
1.547 (4)
1.531 (200)
1.537 (2)
1.489 (0)
1.511 (180)
1.502 (2)
1.517 (2*)
1.508 (0)
„G124“
(QN4)
1.547 (2*)
1.525 (200*)
1.547 (2)
1.504 (0)
1.525 (0*)
1.513 (0)
1.531 (2*)
1.516 (0*)
„G12410“
(QN1)
1.545 (2*)
1.530 (200)
1.536 (0)
1.489 (0)
1.512 (0)
1.497 (0)
1.507 (2*)
1.515 (0)
„G12405“
(QN0)
1.571 (2)
1.564 (180)
1.562 (0)
1.545 (180)
1.542 (180)
1.539 (180)
1.550 (180*)
1.546 (140*)
means that value was found in more consecutive steps [in FPD]; the first step is marked
Table 8 Comparison of design fuel cycle for Gd2M FA (QS3) loadings with upgraded fuel
cycle with 4.75w% FA (QO3) loadings (both for up rated power 1444 MWt)
QS3 fuel loadings
5 year cycle
32
33
11 + 1
10 + 2
327
326
4.565
4.732
-0.027
0.008
1.500
1.511
Eq. cycles
WFA + CFA
Teff [FPD]
CBBOC [gH3BO3/kg]
CBEOC [gH3BO3/kg]
Fdh
Aver. burn-up
[MWd/tU]
Core
35161
Aver.burn-up
[MWd/tU]
Reloaded FA
50794
QO3 fuel loadings
(5.5 year cycle)
39
40
9 +2
10 + 1
326
330
5.009
5.203
0.028
0.084
1.558
1549
35374
38589
38556
51304
57391
56825
Aver.burn-up
[MWd/tU]
1 year
2 year
3 year
4 year
5 year
6 year
13990
26764
38366
47908
51598
13618
27081
39170
47877
51601
14200
27694
37219
51014
56034
59073
14515
27542
40058
47209
57496
58832
Table 8 (continuation) Comparison of design fuel cycle for Gd2M FA (QS3) loadings with
upgraded fuel cycle with 4.75w% FA (QO3) loadings (both for up rated power 1444 MWt)–
reactivity coefficients.
QS3 fuel loadings
5 year cycle
BOC
BOC
EOB
EOB
QO3 fuel loadings
(5.5 year cycle)
BOC
BOC
EOB
EOB
/Nti [1/%N]
Power coefficient
with constant input
temperature
-1.738E-4
-2.015E-4
-1.757E-4
-2.018E-4
-1.733E-4
-2.044E-4
-1.736E-4
-2.057E-4
-1.154E-4
-1.135E-4
-1.177E-4
-1.134E-4
-1.163E-4
-1.150E-4
-1.173E-4
-1.162E-4
-3.607E-4
-5.612E-4
-3.583E-4
-5.640E-4
-3.519E-4
-5.706E-4
-3.483E-4
-5.717E-4
2.056E-1
3.137E-1
2.040E-1
3.149E-1
1.993E-1
3.153E-1
1.967E-1
3.161E-1
-1.146E-2
-1.246E-2
-1.143E-2
-1.245E-2
-1.087E-2
-1.196E-2
-1.081E-2
-1.196E-2
/Nta [1/%N]
Power coefficient
with constant
average temperature
/tM [1/C]
Moderator
temperature
coefficient
/ [cm3/g]
Moderator density
coefficient
/CB [kg/g]
Boron coefficient