1. XI Конференция по реакторному материаловедению
г. Димитровград
Development and application of techniques for evaluation of coolant activity during operation of WWER power units
P. М. Kalinichev, I.А. Evdokimov, V.V. Likhanskii, Е.U. Afanaseva, А.G. Khromov

2. Some tasks of coolant activity monitoring during reactor operation
Task 1. Detection of fuel failures
Risk, followed by financial looses:
Escalation of primary coolant activity
High dose rates for personel
Increase in the amount of liquid radioactive waste
Search for and replace FA with leaking rods
Fuel washout
Defect in cladding
(Postirradiation examination SRC NIIAR)
Fuel failures cannot be always clearly identified by conventional techniques
Task 2. Detection of fuel washout
High background activity in the following campaigns (planning of corrective actions)
Specific in handling of leaking FA
WWER regulatory documents not include techniques for detection of fuel washout

3. Task 1. Detection of fuel failures
Iodine activity may be governed by release from tramp uranium even if there is a small failure
Surface-active iodine may adsorb inside leaking fuel rod in contrast to inert xenon
Относительное изменение активности
133Xe
131I
– small defect in cladding
– higher burnup
– fuel with solid pellets
“Defect size”
133Xe release rate, s-1 (reciprocal time of fission product release)
Xenon activity may be more sensitive to fuel failure than iodine activity

4. Detection of fuel failures by Xe activities in WWERs
Ratio of 133Xe to 135Xe activity
Detection of fuel failures by Xe activities in WWERs
Calculation for coarse & porous fuel chips in deposits
Calculation for small fuel particles in deposits
Failure in the core
Uncertainty
No leaking fuel in the core
The cleaning rate of coolant from Xe, s–1
Small sensitivity to loss of gas during taking samples
Small sensitivity to fuel burnup

5. Benchmarking for several cycles
MAX threshold
Cycles with fuel failures
MIN threshold
Cycles with no failures
Ratio of 133Xe to135Xe activity
Fuel cycle
The criterion on the basis of Xe activities is able to unambiguously distinguish between ‘clean’ cycles and cycles with leaking fuel

6. Problems in detection of fuel failures
No indications of fuel failure during cycle H-1 according to analysis by guideline methods, no iodine spiking during power shutdown
In such cases, according to guidelines, it is believed that there are no failures:
It is possible not to perform fuel leakage tests during outage
Cycle Н-1 at WWER-1000 power unit
Reactor power, MW
Iodine activity spiking was recorded after pressure drop in primary circuit
Activity of 131-iodine, Ci/kg
Time, days
Prompt detection of leaking fuel assemblies (FAs)
Avoiding risks of secondary degradation, activity escalation and fuel washout during the next cycle
Chances to mitigate the failure during the current cycle

7. Higher sensitivity of failure detection by Xe activities
Ratio of iodine normalized release rate to that of 134I
Cycle Н-1 at WWER-1000 power unit
Отношение активностей 133Xe/135Xe
Criterion treashold
Fuel failure
Failure is reliably detected by 133Xe/135Xe activity ratio
Ratio of iodine normalized release rates 131I/134I does not show any problem

8. Technique application (2)
Fuel cycle I-1, 2018 г.
131I activity, Ku/kg
Reactor power, МWt
Normalized activity ratio 131I/134I
time, days
According to iodine activity, there was no leaking fuel rods in first two monthes

9. Technique application (2)
Cycle I-1, 2018 г.
Activity, Ku/kg
Fuel failure
133Xe
135Xe
Reactor power, МWt
133Xe/135Xe ratio
Max threashold
time, days
133Xe /135Xe ratio exceed the threashold after ~160 day

10. Technique application (2)
Cycle I-1, 2018 г.
Fuel failure is detected by Xe
Activity 131I, Ки/кг
Reactor power, МWt
Ratio of normalized release rates131I/134I
threashold
time, days
After ~ 45 days fuel failure was confirmed by increase of ratio of iodines normalized release rate

11. Technique application (3)
кампания G-2, 2018 г.
Iodine normalized release rates ratio
Reactor power, МWt
133I/134I
135I/134I
132I/134I
time, days
At the beginning of fuel cycle, 131I activity was lower then MDA *
Normilized release rates ratio for 132I, 133I, и 135I – similar to cycles without fuel failures
*MDA – minimum detectable activity

12. Technique application (3)
Ratio 133Xe/135Xe
Technique application (3)
Cycle G-2, 2018 г.
Reactor power, МWt
Threashold for this NPP
Fuel failure
time, days
Ratio 133Xe /135Xe exceed threashold at the start of fuel cycle

13. Technique application (3)
Cycle G-2, 2018.
131I activity in steady state power mode was lower then MDA*
Activity 131I and 133I
Spike-effect
Reactor power, МWt
МДА* для 131I
time, days
Spike-effect by 131I and 133I confirm the fuel failure in one month
*MDA – minimum detectable activity

14. Technique application (4)
Cycle C-3, 2018
There was no any symptoms of fuel failure by iodines
131I activity
Spike-effect
Reactor power, МWt
time, days
In two months spike-effect of 131I was observed: from this moment there is a leaking fuel rod in core

15. Technique application (4)
Cycle C-3, 2018.
131I/134I normalized release rates ratio
Reactor power, МWt
time, days
According to iodine normalized release rates, there was no leaking fuel rods in core at the beginning of the cycle

16. Technique application (4)
Cycle C-3, 2018.
Ratio133Xe/135Xe
Fuel failure
Reactor power, МWt
Max threashold
time, days
133Xe/135Xe activities ratio show, that fuel failure occur at the beginning of the cycle

17. Summary for the task 1
A technique, which makes it possible to detect fuel failures by analysis of 133Xe and 135Xe activities ratio is developed
It is not sensitive to gas losses during sampling of primary coolant and to nuclide composition (burnup) of fuel deposits
There is no need in any additional measurements besides those that are routinely performed at power units
Given examples show that
Technique allow to draw a line between “clear” cycles and cycles with fuel failure in the core
In some cases it is able to detect a failure, even if detection is not possible by iodine activities

18. Task 2. Detection of fuel washout
Conventional way of detection of fuel washout
Indicator for amount of fuel deposits in WWER reactors is 134I activity
Low change in case of fuel failure
131I
133I
134I
power
Reactor power, МWt
Activity, Ku/kg
time, days.
The growing up of 134I activity may be associated with fuel washout

19. 134I activity may increase even if there is no leaking fuel in reactor
power
Reactor power, МWt
Activity, Ku/kg
I-134
time, days
If mass of fuel deposits is constant, why activity increase?
When an increase of 134I activity is associated with fuel washout?

20. Fissions dole in fuel pellets
Possible reason of increase of activity: Evolution of nuclide composition in fuel
Activity A
Fission 235U
Fission 239Pu +
Fission rate
Because neutron capture in 238U, 239Pu is produced
Amount of fissionable nucleus is change 1
0.8
0.6
0.4
0.2
Fissions dole in fuel pellets
Burnup МWt*day/кgU
In fuel deposits Pu production differs from it’s in fuel pellets

21. Production of 239Pu 238U, concentration n 239Pu, concentration n9 Ф
Cross-section σ
238U, concentration n
239Pu, concentration n9
Neutron flux Ф
Fission or transformation into 240Pu
Neutron resonance capture by 238U
«burning» 239Pu
Concentration of 239Pu depend on

22. Average plutonium breeding in fuel pellets several-fold lower then periferal
Dole of mass of Pu*
Number of neutron captures by 238U due to fuel depth Ф
Radius, rel.
Radius, rel.
in fuel deposits is higher then in pellets
value
Density of 238U is high
Neutron flux is changing in fuel
An increase of activity even without leaking fuel rods
Force production of Pu
An increase of fission rate in fuel deposits
*Ф.Н. Крюков Электронно-зондовый рентгеноспектральный микроанализ топливных композиций и оболочек тепловыделяющих элементов ядерных реакторов, диссертация на соискание ученой степени доктора ф-м наук, ОАО «ГНЦ НИИАР», Дмитровград 2006

23. A way of identification of fuel washout
Developed physical model provide an upper estimate for increase rate of activity in case of constant mass of fuel deposits
Simulated result, provided by qualified neutronic code SVL:
Relative activity increase
Enrichment 2.4%
If increase rate of 134I activity during the fuel cycle is higher then calculated threashold
Enrichment 3.6%
Fuel washout
Burnup of FA, МWt day/кgU

24. Example of practical application for different NPPs
Cycles without fuel failures
Cycles with fuel failures
Without fuel washout
With fuel washout
Ratio of relative increase rate of activity and threashold
Fuel washout
Fuel cycles

25. Summary for the task 2
A technique for identification of fuel washout during reactor operation is proposed
As exemplified by several WWER-1000 fuel cycles (with/without fuel failures), criterion allow to reveal cases when activity increase because of fuel washout
Criterion applied for exploitation monitoring of fuel, produced by AO TVEL

26. Conclusion 2 techniques are developed: Identification of fuel failure
Identification of fuel washout
Developed techniques expand oportunities for fission product analysis during reactor operation

27. Thank for your attention!