1. Ignalina NPP, Nuclear Safety Department
Transferring and reuse of the spent nuclear fuel, unloaded from the 1 unit reactor,
at the 2 unit reactor
Andrius Jurkevičius
Ignalina NPP, Nuclear Safety Department
Visaginas, Lithuania

2. Introduction
There are 2 units with the graphite moderated boiling water RBMK 1500 reactors at the Ignalina NPP.
The INPP 1 unit was in operation for the 21 year from the end of 1983 year till the end of The reactor of the 1 unit was shut down in the for the decommission due to political pressure.
The 2 unit of the Ignalina NPP is in operation since the August It is planed that the 2 unit will be operated until the end of 2009 year.
The RBMK-1500 reactors are reactors with the on-line nuclear fuel refueling. So, the significant amount of the relatively fresh, non-burned spent nuclear fuel was remaining in the reactor core after the 1 unit reactor final shut down.
Visaginas, Lithuania

3. Introduction
It was economically unprofitable to sent such non-burned nuclear fuel to the spent nuclear fuel storages. The significant increase of the spent fuel amount should take the place in this case also.
Due to this economical and ecological disadvantages, an alternative courageous decision to transfer nuclear fuel, unloaded from the 1 unit reactor, to the 2 unit for the reuse and further burnup in the 2 unit reactor was made.
The results of the estimation of the optimal 1 unit spent nuclear fuel loading sequences at the 2 unit reactor will be presented. The influence of the 1 unit spent fuel reuse at the 2 unit reactor to the reactor neutron-physical parameters and economical effect will be discussed.
Visaginas, Lithuania

4. The object of the project
The object of the project – transferring and reuse of non-burned nuclear fuel, unloaded from the 1 unit reactor, at the 2 unit, economy of the fresh nuclear fuel.
The aims of the investigation – the choice of the optimal conditions for the nuclear fuel reuse, justification of nuclear safety for the chosen conditions.
The task for the optimization – the minimization of the fresh fuel usage.
Limitations:
Available time interval;
The refueling frequency – depends on refueling machine capacity;
The reactivity effects and factors, other limiting parameters.
Visaginas, Lithuania

5. The problem need to solve
What amount of spent fuel transferred from the 1 unit it is possible to reuse and burnup in the 2 unit reactor?
In what sequence fuel transferred from the 1 unit must be loaded in the 2 unit reactor depending on transferred spent fuel burnup?
How should be nuclear fuel loaded in the 2 unit reactor - only transferred spent nuclear fuel or spent fuel together with fresh nuclear fuel? In last case in what sequence and proportion the spent and fresh nuclear fuel must be loaded?
Visaginas, Lithuania

6. The problem need to solve
How much fresh nuclear fuel can be saved depending on chosen spent nuclear fuel refueling sequence? What is an economy effect?
How fuel burnup, refueling frequency, reactivity factors depends on chosen spent nuclear fuel refueling sequence?
What is optimal proportion of spent fuel loading at the center and at the periphery of the reactor core?
Visaginas, Lithuania

7. Refueling schemes investigation
4 different spent nuclear fuel reuse and 2 unit reactor core loading schemes were investigated:
beginning from the nuclear fuel with minimal burnup 
beginning from the nuclear fuel with maximal burnup 
nuclear fuel with minimal and maximal burnup combined loading sequence 
nuclear fuel with minimal and maximal burnup combined loading sequence beginning from middle burnup levels  
As well as fresh and spent nuclear fuel combined refueling schemes and sequences
Visaginas, Lithuania

8. Refueling schemes investigation
Fig.1 Refueling frequency (only spent fuel are loaded at the beginning)
Visaginas, Lithuania

9. Refueling schemes investigation
Fig. 2 Refueling frequency (fresh and spent fuel are loaded at the proportion 1:1)
Visaginas, Lithuania

10. Refueling schemes investigation
Fig. 3 Average fuel burnup in the reactor core
(fresh and spent fuel are loaded in proportion 1:1)
Visaginas, Lithuania

11. Refueling schemes investigation
Table 1. Comparison of FA refueling schemes
Visaginas, Lithuania

12. Optimal refueling scheme
Main parameters of the optimal refueling scheme
The INPP 2 unit reactor should be refueled alternately with transferred from the 1 unit spent fuel with minimal and maximal burnup. The difference of the loaded spent fuel burnup levels should gradually decrease (refueling scheme ).
The refueling of the spent nuclear fuel, transferred from the 1 unit, interleaves with the refueling of the fresh fuel at the equal proportion 1 : 1.
The amount of the spent fuel for the reuse is selected in such a way that the last spent fuel assemblies will be loaded in the reactor exactly before final shut down of the 2 unit for the decommission at the end of 2009 year.
Visaginas, Lithuania

13. Optimal refueling scheme
The spent fuel with the higher burnup levels should be loaded at the periphery of the reactor core in order to get maximal economy of nuclear fuel.
The performed calculations and investigations of the optimal 2 unit reactor refueling scheme showed that it is economically efficient to transrerr from the 1 unit and reuse 1000 spent nuclear fuel assemblies.
Table 2. Number and maximal burnup of spent fuel assemblies chosen for reuse at the 2 unit reactor
Fuel enrichment by 235U 2%
2.4%
2.6%
Number of assemblies for the reuse 36 57
907
Part in the total amount of the considered enrichment spent fuel
0.43
0.10
0.90
Maximal burnup MWd/FA
1287
1549
1750
Visaginas, Lithuania

14. Nuclear safety justification
The evaluation calculation of reactor core neutron-physical parameters were performed in order to justify 2 unit reactor nuclear safety during spent ant fresh nuclear fuel loading process at the 2 unit reactor from the middle 2006 year till the end of 2009 year.
It was shown that all 2 unit reactor neutron-physical parameters can be kept within the required passport limits if spent and fresh nuclear fuel is loading at proportion 1 : 1.
The INPP 2 unit reactor nuclear safety is fully ensured.
Visaginas, Lithuania

15. Nuclear safety justification
Table 3. Reactor core neutron-physical parameters during spent ant fresh FA loading.
Time, days
204
405
701
900
1061
Refueling number ()
390
770
1330
1700
2000
Number of loaded spent fuel assemblies
195
385
665
850
1000
Number of 2.8% 235U enriched FA
200
348
596
668
805
Average FA burnup in the reactor core E, MWd/FA
1401
1456
1504
1542
1566
1585
Graphite temperature reactivity factor c, 10-3 /0С
2.57
2.83
3.06
3.03
3.00
3.12
Fuel temperature reactivity factor
т, 10-3/0С
-2.51
-2.56
-2.57
-2.59
Power reactivity factor w, 10-4 /МВт
-2.50
-2.48
-2.55
-2.54
Void reactivity factor , 
0.73
0.70
0.74
0.68
Subcriticality , %
2.70
2.73
2.87
2.84
2.91
2.59
Visaginas, Lithuania

16. Spent fuel reuse and further burnup
A lot of existing technical systems and equipment modifications, as well as implementation of the quite new systems and equipment, including spent fuel transferring cask for reused assemblies and transportation machine, were done in order to make possible safe transfer of the spent nuclear fuel from the INPP 1 unit to 2 unit.
The first batch of the spent nuclear fuel was transferred from the INPP 1 unit to the 2 unit in the middle of the 2006 year.
Reuse of the transferred spent nuclear fuel in the 2 unit reactor core begins at the end of the 2006 year.
Visaginas, Lithuania

17. Spent fuel reuse and further burnup
From the 2006 year until the middle of the 2008 year:
More then 550 spent nuclear fuel assemblies were unloaded from the 1 unit reactor core;
More then 500 spent nuclear fuel assemblies were transferred from the 1 unit to the 2 unit;
More then 450 spent nuclear fuel assemblies were loaded in the 2 unit reactor core.
The amount of spent nuclear fuel assemblies damaged during transportation does not exceeds 4 % and are far less designed level of 10 %.
Visaginas, Lithuania

18. Spent fuel reuse and further burnup
Fig. 4 Number of reused spent nuclear fuel assemblies
Visaginas, Lithuania

19. Spent fuel reuse and further burnup
Average burnup of FA at the reactor core increased by 7 % from 1425 MWd/FA to 1525 MWd/FA during process of spent FA reuse.
Average burnup of unloaded spent FA decreased from 2700 MWd/FA to 2550 MWd/FA during the first 150 effective days and then stabilized on this level.
Fig. 5 FA burnup at the reactor core, and burnup of the unloaded spent nuclear fuel
Visaginas, Lithuania

20. Spent fuel reuse and further burnup
Fig. 6 The results of measured void reactivity factor
Visaginas, Lithuania

21. Spent fuel reuse and further burnup
There is set preventive limits of the measured void reactivity factor 0,450,75 ef for spent and fresh fuel loading in order to ensure the 2 unit reactor nuclear safety.
The proportion of loaded spent and fresh fuel should be corrected if measured void reactivity factor exceeds this limits.
Beside this, 6 additional cluster absorbers were loaded in to the reactor core in order to keep required reactor subcriticality level during shut down maintenance, as well as for the decreasing the void reactivity factor.
There should be mentioned, that the spent nuclear fuel loading is also very useful measure to keep or at least to slower the reactor subcriticality decreasing due to fresh 2.8 % 235U enriched nuclear fuel loading.
Visaginas, Lithuania

22. Economic effect
It was estimated that reuse of one spent nuclear fuel assembly is approximately equal to the 0.400.42 of the fresh 2.8 % 235U enriched fuel assembly.
So, there could be saved ~370 fresh fuel assemblies till the end of 2009 year, or in monetary equivalent ~70 mln. Euro. Even excluding the cost of the spent fuel transferring complex implementation the savings will be still more then 20 mln. Euro.
Beside this, the decreasing of spent fuel waste amount by the same 370 fuel assemblies will allow save further expenses for the 10 additional spent fuel storage casks buying and this casks storage.
Visaginas, Lithuania

23. Conclusions
1. There is possible to transfer and reuse 1000 spent nuclear fuel assemblies, unloaded from the 1 unit reactor, at the 2 unit reactor.
2. The refueling of the 2 unit reactor core should be performed by the spent and fresh nuclear fuel at the proportion 1 : 1. The spent fuel with minimal and maximal burnup should be loaded in such a way that the difference of the loaded spent fuel burnup levels gradually decreased.
3. The optimal spent and fresh nuclear fuel refueling sequence allows to keep stable the refueling frequency (2.0 FA/day), and main reactor neutron- physical parameters, including void reactivity factor.
4. The economy of fresh nuclear fuel is ~370 FA or ~ 70 mln. Euro.
5. The INPP 2 unit reactor neutron-physical parameters are kept within the required reactor passport and operational limits. The INPP 2 unit reactor nuclear safety is fully ensured.
Visaginas, Lithuania

24. Thank You For Your Attention
Visaginas, Lithuania