1. Integrity testing of the TVSA-ALFA fuel elements by gamma spectrometry of the gas plenum
© Sagalov S.S., Zhitelev V.A., Shevlyakov G.V.,
Strozhuk A.V., Teplov V.G., Ilyina E.G., 2019
XI Conference on Reactor Materials Science,
in commemoration of 55 th anniversary of material science department of JSC “SSC RIAR”
Dimitrovgrad, May 27-31, 2019

2. Introduction
In 2017, JSC SSC RIAR from unit 1 of the Kalinin NPP delivered the TVSA-ALPHA No. ED 3296, which, by results of the tightness control of fuel rods claddings was identified at the NPP as leaking. According to the results of primary research at RIAR, leaky fuel rods in the fuel assembly were absent.
Taking into account the contradictory conclusions made by the specialists of JSC SSC RIAR and Kalinin NPP, an additional check was carried out on the tightness of all fuel rods (FR) of the fuel assembly using the gamma spectrometry method based on the content of the 85Kr in the gas plenum region.

3. Object of study Bav. = 32,9 MWd/kgU a b
306 FR, enrich. 235U – 4,4%
6 FR with Gd2O3 (8%), enrich. 235U – 3,6%
18 guide channels
28,2 MWd/kgU
35,2 MWd/kgU a b
Fig. 1 – The location of the fuel rods (a) and average fuel burnup in TVSA-ALPHA (b)

4. Gamma spectrometry equipment
Fig. 2 – Installation scheme: 1 – fuel rod, 2 – shielding container, 3 – HPGE-detector, 4 – detector shield, 5 – Dewar vessel, 6 – lifting device, 7 – container for fuel rods, 8 – collimator, 9 – gamma spectrometer Ortec Dspec jr 2.0, 10 – computer for collecting and processing data , 11 – hot cell

5. Gamma spectrometry method
The “live” time exposure - 10 minutes, the collimator - 20x10 mm.
The total study time of a single fuel rod (taking into account transport operations) - about 0.5 hours.
Fig. 3 – Spectra of a hermetic fuel rod 149 and a punctured fuel rod 148 in the area of the 85Kr gamma peak

6. Results of gamma spectrometry examination
The fuel rods of the outer row, especially those located on the side of faces 1, 2 and 6, have elevated values of the 85Kr peak area compared to the fuel rods of the inner rows. Therefore, the fuel rods of the outer and inner rows were analyzed separately.
Fig. 4 – 85Kr peak area for fuel rods (FR) of the outer row along faces 1–6 and inner rows

7. Results of gamma spectrometry examination
Fig. 5 – The relationship between the average fuel burnup and the 85Kr peak area in the fuel rods of the diagonal 1–312

8. Results of gamma spectrometry examination
Fig. 6 – The peak area of 85Kr at fuel rods of internal rows
Fig. 7 – Deviation of the peak area of 85Kr from the approximating function
Approximation function:
Coefficient ао а1 а2 а3 a4 b
Value
0,65
0,0568
0,0135
0,0049
0,0001
28,5

9. Results of destructive examination
1. Laser puncture of the fuel rod cladding and the determination of pressure and gas volume.
2. Determination of the composition of gases by mass spectrometry (MAT-253 Thermo Fisher Scientific).
Gas parameters under the fuel rod cladding
Fuel
rod
(твэл)
Gas volume (объём газа), cm3
Gas pressure (давление газа), MPa
Volume fraction (объёмная доля), %
Partial pressure (парциальное давление) 85Kr, kPa
Yield of gas fission products
(выход ГПД), %
Peak area (площадь пика)
85Kr, keV·cps He Kr Xe
148
419,0
2,46
97,28
0,280
2,380
0,37
0,87 ― 37
416,1
2,51
97,52
0,235
2,200
0,31
0,72
5,926
158
410,4
2,44
98,86
0,100
0,995
0,13
0,34
2,357
266
400,0
2,42
99,61
0,034
0,312
0,04
0,12
0,463

10. Results of destructive examination
Fig. 8 – Dependence between the peak area and 85Kr partial pressure in fuel rods 37, 158 and 266

11. Conclusion
To determine the integrity of the TVSA-ALPHA fuel rods, a gamma-spectrometric method was used to monitor the content of 85Kr under the cladding of the fuel rod placed in a protective container. The use of the container has significantly reduced the duration of measurements. Selective comparison of gamma-spectrometry data with the results of destructive measurements of krypton pressure under the fuel cladding showed the presence of a linear relationship between them and confirmed the validity of the gamma-spectrometric method for monitoring the tightness of fuel elements.
A well-defined dependence of the peak area of 85Kr on the average fuel burnout was obtained. The fuel elements of the outer row have increased activity values of 85Kr in the gas plenum compared with the fuel rods of the inner rows. The yield of fission gas products from the fuel did not exceed 1%, which corresponds to the normal operation of TVSA-ALPHA. The conducted studies did not reveal fuel rods with abnormally low concentration of 85Kr, which confirmed the conclusion about the tightness of fuel assembly, made on the basis of primary studies in JSC "SSC NIIAR". 11

12. Thanks for attention!