USE OF THE AXIAL BURNUP PROFILE AT THE NUCLEAR SAFETY ANALYSIS OF THE VVER-1000 SPENT FUEL STORAGE...

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, [email protected]

17th SYMPOSIUM of AER on VVER Reactor Physics and Reactor Safety

September 24-29, 2007, Yalta, Crimea, Ukraine

September 24 - 29, 2007 17th Symposium of AER, Yalta, Crimea, Ukraine 2

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-14-029-91».

«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.



Fig. 1 – burnup profile over FA length

30

35

40

45

1 2 3 4 5 6 7 8 9 10

average burnup of the end parts of FAdistribution of burnup over FA length

layer number along FA height

Bu

rnu

p,

MW

d/k

gU



Table 1. Fuel assembly number in ZNPP storage pools till 11.29.06

Power unit # 1 2 3 4 5 6Fuel assemblynumber instorage pools

271 318 269 270 299 353

BASIC DATA FOR GENERALIZED COEFFICIENTS OF CONSERVATIVE AXIAL BURNUP PROFILE



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



Burnup irregularity coefficient for each fuel assembly layer was calculated by the following equation:

BzB

zP)(

)(~ (1)

GENERALIZED COEFFICIENTS OBTAINING FOR CONSERVATIVE AXIAL BURNUP PROFILE

)(zB burnup at point z from the core bottom through the fuel assembly

H

dzzBH

BCR

CR

..

)(..

0

fuel burnup average value through the fuel assembly



Fig. 2 – Relative burnup profiles for arbitrary FA

0.5

0.7

0.9

1.1

1 2 3 4 5 6 7 8 9 10

E58.84.93 B=46.10 MWd/kgUE8355.96 B=41.95 MWd/kgUE7822, B=40.70 MWd/kgUE6484, B=39.88 MWd/kgUE8877, B=38.59 MWd/kgUЕД8137, B=41.86 MWd/kgUЕД6815, B=37.73 MWd/kgUЕД5791, B=34.00 MWd/kgUВ 2608, B=33.88 MWd/kgUЕД6191, B=41.67 MWd/kgUВ7687, B=33.88 MWd/kgU В 8127, B=26.05 MWd/kgU


Bur

nup,

rel

ativ

e un

its



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.

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.



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.

Then minimal burnup irregularity coefficients for each of 10 layers through all the spent fuel assemblies in storage pools were selected.



Fig. 3 Axial conservative burnup profile

0.4

0.6

0.8

1.0

1.2

1 2 3 4 5 6 7 8 9 10

axial conservative burnup profile


Bu

rnu

p,

rela

tive

un

its



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.

Layer # 1 2 3 4 5 6 7 8 9 10

Burnup,relativeunits

0.65 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.73 0.42

The sumon layers 8.8

conservative axial burnup profile coefficients of 10 layers SFA

vconservatiiP~Table 2



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).



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:

BPBivconservati

ivconservati ~

is an average fuel burnup value in a fuel assembly

(2)

where B

APPLICATION OF GENERALIZED CONSERVATIVE AXIAL BURNUP PROFILE COEFFICIENTS



Fig. 4 – burnup profiles over FA length

20

30

40

1 2 3 4 5 6 7 8 9 10

conservative axial burnup profile of FAaverage burnup of the end parts of FAdistribution of burnup over FA length


Bur

nup

, M

Wd/

kgU



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%.



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.



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.



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..

USE OF THE AXIAL BURNUP PROFILE AT THE NUCLEAR SAFETY ANALYSIS OF THE VVER-1000 SPENT FUEL STORAGE...

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