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

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.

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)

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

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

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

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

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

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

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

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

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