USE OF THE AXIAL BURNUP PROFILE AT THE
NUCLEAR SAFETY ANALYSIS OF THE VVER-1000
SPENT FUEL STORAGE FACILITY IN UKRAINE
Olena Dudka, Yevgen Bilodid, Iurii Kovbasenko, Vladimir
Khalimonchuk
State Scientific and Technical Centre on Nuclear and Radiation Safety
(SSTC N&RS)
35-37 Stusa St., 03142 Kyiv, Ukraine
[email protected]
17th SYMPOSIUM of AER
on VVER Reactor Physics and Reactor Safety
September 24-29, 2007, Yalta, Crimea, Ukraine
State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Nuclear safety of fresh and spent fuel is assessed in
compliance with current technical regulations, among
which the following documents should be singled out
«Safety Rules for Storage and Transportation of
Nuclear Fuel at Nuclear Power Facilities, PNAEG-14029-91».
«Basic Rules for Spent Nuclear Fuel Intermediate
Dry Storage Facilities Safety Evaluation, NP
306.2.105-2004».
According to this documents, the effective neutron
multiplication factor Keff must remain below 0.95 in
normal operation and design-basis accidents.
September 24 - 29, 2007
17th Symposium of AER, Yalta, Crimea, Ukraine
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Burnup, MWd/kgU
45
40
average burnup of the end parts of FA
distribution of burnup over FA length
35
30
1
2
3
4
5
6
7
8
9
10
layer number along FA height
Fig. 1 – burnup profile over FA length
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
BASIC DATA FOR GENERALIZED COEFFICIENTS
OF CONSERVATIVE AXIAL BURNUP PROFILE
Table 1. Fuel assembly number in ZNPP storage pools
till 11.29.06
Power unit #
Fuel assembly
number in
storage pools
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1
2
3
4
5
6
271 318 269 270 299 353
17th Symposium of AER, Yalta, Crimea, Ukraine
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Fuel assembly burnup was calculated by
simulating a fuel campaign accounting the
following experimental data
 power unit load curve
 control rod positions
 core coolant temperature at the core input
 core coolant rate
 boric acid concentration
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
GENERALIZED COEFFICIENTS OBTAINING FOR
CONSERVATIVE AXIAL BURNUP PROFILE
Burnup irregularity coefficient for each fuel assembly
layer was calculated by the following equation:
~( z ) 
P
B(z)
B 
B( z )
B
(1)
burnup at point z from the core bottom through
the fuel assembly
H R .C .
 B( z )dz
0
fuel burnup average value through the fuel
assembly
H R.C .
September 24 - 29, 2007
17th Symposium of AER, Yalta, Crimea, Ukraine
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Burnup, relative units
1.1
E58.84.93 B=46.10 MWd/kgU
E8355.96 B=41.95 MWd/kgU
E7822, B=40.70 MWd/kgU
E6484, B=39.88 MWd/kgU
E8877, B=38.59 MWd/kgU
ЕД8137, B=41.86 MWd/kgU
ЕД6815, B=37.73 MWd/kgU
ЕД5791, B=34.00 MWd/kgU
В 2608, B=33.88 MWd/kgU
ЕД6191, B=41.67 MWd/kgU
В7687, B=33.88 MWd/kgU
В 8127, B=26.05 MWd/kgU
0.9
0.7
0.5
1
2
3
4
5
6
7
8
9
10
layer number along FA height
Fig. 2 – Relative burnup profiles for arbitrary FA
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
During profile calculation the burnup in each layer of
fuel assembly is normalized to the average burnup
value over the fuel assembly. Sum of values obtained
through 10 layers for each fuel assembly in this case is
equal to 10.
Figure 1 demonstrates, fuel assembly burnup profiles
have only a weak dependence on a type of fuel
assembly, of initial enrichment and average fuel burnup
value, what allow their generalization to all types of the
spent fuel assembly independently on enrichment and
burnup of FA.
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Then minimal burnup irregularity coefficients for each of
10 layers through all the spent fuel assemblies in
storage pools were selected.
Conservative axial profile for burnup distribution for the
all studied fuel assemblies was obtained on the base of
the selected minimal burnup irregularity coefficients for
each of 10 layers through all the spent fuel assemblies
in ZNPP storage pools.
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Burnup, relative units
1.2
1.0
0.8
axial conservative burnup profile
0.6
0.4
1
2
3
4
5
6
7
8
9
10
layer number along FA height
Fig. 3 Axial conservative burnup profile
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Such formation of the conservative axial profile makes it
different from the obtained one by the equation (1) and
sum of its values is less 10 through ten layers.
Table 2
~conservativ
P
i
conservative axial burnup profile coefficients of
10 layers SFA
1
2
3
4
5
6
7
8
9
10
Layer #
Burnup,
relative 0.65 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.73 0.42
units
The sum
8.8
on layers
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
The sum of irregularity coefficients for burnup values
for conservative axial burnup profile through the ten
layers makes 8.8. So, transfer from the actual
distribution burnup profile to a conservative one results
in underrating of the fuel assembly burnup average
value to 12% as to its real value (for comparison,
average burnup value underrating makes 50-60% at the
uniform burnup profile).
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
APPLICATION OF GENERALIZED CONSERVATIVE AXIAL
BURNUP PROFILE COEFFICIENTS
At performance of estimation of nuclear safety the absolute
conservative fuel burnup value in each considered fuel assembly
layer should be calculated in the following way:
conservativ
Bi
where
B
September 24 - 29, 2007
conservativ
~
 Pi
B
(2)
is an average fuel burnup value in a
fuel assembly
17th Symposium of AER, Yalta, Crimea, Ukraine
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Burnup, MWd/kgU
State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
40
30
conservative axial burnup profile of FA
average burnup of the end parts of FA
distribution of burnup over FA length
20
1
2
3
4
5
6
7
8
9
10
layer number along FA height
 Fig. 4 – burnup profiles over FA length
September 24 - 29, 2007
17th Symposium of AER, Yalta, Crimea, Ukraine
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
As the result of equation (2) application to the average
burnup of any spent fuel assembly, the most
conservative burnup distribution profile should be made
for ten layers. Such distribution burnup profile results in
the maximum neutron multiplication factor Keff for all the
fuel assemblies' types and all the points of burnup.
Taking into account the axial fuel burnup distribution
according to the given methodology the reserve of 12%
compensates possible errors in determination of
burnup, which according to the software specifications
for NPPs make 7-10%.
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Accounting the fuel burnup for ventilated storage casks
of the dry storage facility for VVER-1000 spent fuel at
ZNPP critically calculations only 5 fissionable isotopes
(U-235, U-238, Pu-239, Pu-240, Pu-241) are taken into
account. This introduces additional conservatism to the
calculation results which makes 14% in magnitude Keff.
Summing up these data the conservative reserve, which
assumed for spent nuclear fuel storage safety, makes
up not less than 26% in magnitude Кэфф in connection
with possible burnup calculation errors and U and Pu
isotope concentrations.
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
CONCLUSIONS
Present-day approach to estimation of SFA burnup for ZNPP
Interim Dry Storage System for Spent Nuclear Fuel, where each fuel
assembly burnup is assumed uniform over assembly length and
equal to average burnup of the end parts is conservative. It results in
1.5-2.5 times decrease of fuel assembly burnup value comparing to
the average value as far as the fuel assembly is burned more
significantly near the center as to its ends. This in its turn increases
the number of the spent control rods loaded into containers required
for maintenance nuclear safety.
The results of the analysis of the spent fuel assembly energy-
producing placed in the units’ storage pools, which are presented in
the report, allow reducing soundly of conservatism to the accepted
level. To avoid excessive conservatism an axial conservative burnup
profile determined with coefficients, shown in Table 2, should be
used for analysis of nuclear safety of spent fuel dry storage system.
September 24 - 29, 2007
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State Scientific and Technical Centre on Nuclear and Radiation Safety of Ukraine
Application of conservative profile provides underrating of fuel
assembly burnup average value to 12% as to its real value,
additional conservatism in the result of only fissionable isotope
accounting will cause the design factor Keff increase higher than
26%.
Calculations based on the examples of two casks of ZNPP Interim
Dry Storage System prove that fuel burnup axial profile which has
been taken into account for substantiation of spent fuel dry storage
nuclear safety, allows to reduce the number of the control rods
loaded into casks at least on two without violation of the
Requirements for Nuclear Safety..
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