Corresponding author: Evgeny A. Kinev ( kinev_ea@irmatom.ru ) Academic editor: Yury Korovin
© 2020 Evgeny A. Kinev.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Kinev EA (2020) 16Cr-19Ni steel swelling at dose rates from 1×10-8 to 1.6×10-6 dpa/s. Nuclear Energy and Technology 6(4): 249-252. https://doi.org/10.3897/nucet.6.60371
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The article presents the first data on EK-164ID steel swelling after operational irradiation in a fast nuclear reactor in the temperature range of 370–630 °C and maximum damaging doses of 66–77 dpa. The dose accumulation rate along the cladding tubes made of this material was 1×10-8–1.6×10-6 dpa/s. The swelling was determined by the hydrostatic weighing method with an error of no more than 0.5%. The results obtained were analyzed depending on the irradiation parameters and in comparison with the 16Cr-15Ni grade material. The objectives of the study were to estimate the characteristic values of the maximum swelling temperature and dose as well as to calculate the average material swelling rate at the working temperature of irradiation, the incubation period for the onset of swelling, and the stationary swelling rate. It was found that the tube samples, characterized with austenite grain sizes of 9–12 µm before irradiation, have an average swelling rate of 0.035–0.05 %/dpa after reaching the maximum damaging doses of 66–77 dpa (at a rate of (1–1.5)×10-6 dpa/s) and not more than 0.035%/dpa at doses less than 20 dpa (at a rate of 5×10-7 dpa/s). The characteristic maximum swelling temperature of the studied material is in the range of 430–500 °C. The characteristic maximum swelling dose is in the range of 61–72.5 dpa or 70–80% of the maximum accumulated dose. The incubation stationary swelling period for the material is 30 dpa. The stationary swelling rate is 0.1% /dpa. The radiation resistance characteristics of the studied material have an advantage over those for 16Cr-15Ni grade cladding materials under similar irradiation conditions and a similar structural state, which inherits grain sizes of 9–14 μm during the tube processing.
16Cr-19Ni grade steel, temperature, dose rate, average radiation-induced swelling rate
A promising material for manufacturing fuel element claddings for fast neutron reactors is EK-164ID steel with increased chromium contents of up to 19% and a complex set of alloying elements used to enhance resistance to radiation swelling (
The first data on the ratio of these parameters were obtained in 2009 on EK-164ID steel of single heat, from which a batch of cold-worked tubes was made for separate irradiation as fuel claddings in the low enrichment zone (LEZ) and high enrichment zone (HEZ) of a fast nuclear reactor to the maximum damaging doses from 66 to 77 dpa and the temperature range of 370–630 °C. The minimum damaging dose of less than 1 dpa during irradiation was achieved at 370 °C whereas the maximum dose values were achieved at 510–550 °C. The dose accumulation rate along the length of the tubes was 1×10-8–1.6×10-6 dpa/s (Fig.
The microstructure of individual tubes of the batch at the initial stage of mastering the tube technology after finishing heat treatment at 1050 °C (
The steel radiation swelling S was calculated based on the generally accepted ratio of hydrostatic density ρ of each swollen sample with that ρ0 in areas without swelling at a damaging dose of less than 1 dpa within one pipe:
The error in determining the swelling was no more than 0.5%.
The average metal swelling dose rate S/D (hereinafter referred to as ‘swelling rate’) at each irradiation temperature was calculated relative to the corresponding damaging dose D, including the incubation swelling period. The incubation dose D0 was determined by approximating the values of the characteristic maximum swelling doses Dch at the corresponding characteristic swelling temperature.
The average swelling rate of the EK-164ID steel irradiated in the LEZ of reactor, depending on the temperature and radiation dose, has a non-monotonic character (Fig.
The maximum swelling rates of the steel under study in the LEZ are realized at a dose rate of about 1.3×10-6 dpa/s (Fig.
The dependences of the swelling rate of the batch tubes irradiated under the conditions of the HEZ are shown in Fig.
One tube sample with a maximum burnup of 72.5 dpa demonstrated an increased swelling rate as 0.075%/dpa at Тch = 500 °C, D/Dmax = 1 (Fig. 3а) and a dose accumulation rate of 1.47×10-6 dpa/s (Fig.
The incubation dose for the maximum swelling of the considered batch of tubes made of EK-164ID steel was 30 dpa (Fig.
Comparison of the swelling characteristics of the 16Cr-19Ni and 16Cr-15Ni grade samples (
Steel grade | Dmax, dpa | Dch, dpa | D0, dpa | Тch, °C | S/D, %/dpa | Dose accumulation rate, dpa/s |
16Cr-19Ni | 66–75 | 61–72.5 | 30 | 430–500 | 0.035–0.075 | (1–1.47)×10–6 |
16Cr-15Ni | 69–76 | 60–75 | 55 | 460–480 | 0.06–0.1 | (1–1.55)×10–6 |
The first experience of reactor irradiation of EK-164ID steel as a material for manufacturing fuel element claddings, despite the above scatter of the investigated swelling characteristics, shows the prospect of increasing the radiation resistance of the 16Cr-19Ni grade steel as compared to the 16Cr-15Ni grade steel. This conclusion is based on a comparison of the data presented in this work with the previously obtained results (
The table shows that the average swelling rate of the 16Cr-19Ni grade steel for the entire irradiation period is noticeably lower than that of the reference object. This is due to the fact that, despite a decrease in the incubation period for the onset of intense swelling, the rate of stationary swelling decreases six times as compared to that for steels with lower nickel contents (
In addition, it should be noted that, in isolated cases of accelerated (S/D ≈ 0.075 dpa/s) swelling of the EK-164ID steel in the LEZ and HEZ, there were no obvious metallographic differences (grain size, micro-hardness) or operational factors (dose, temperature), highlighting specimens of tubes with abnormally high swelling from the main group with an average swelling rate of less than 0.05 dpa/s.
A similar situation was repeatedly observed for the 16Cr-15Ni grade steels due to the inhomogeneity of the tube processing technology (
Taking into account the criterion of permissible shape change and maximum swelling of 15% (
The article investigated the radiation swelling of EK-164ID steel samples (16Cr-19Ni grade) at the initial stage of tube production after neutron irradiation in a fast nuclear reactor at temperatures of 370–630 °C.
It was found that the tube samples with grain sizes of 6–12 µm have average swelling rates of 0.035–0.05%/dpa after reaching the maximum damaging doses of 66–77 dpa (at a rate of (1–1.5)×10-6 dpa/s) and not more than 0.035%/dpa at doses less than 20 dpa (at a rate of 5×10-7 dpa/s).
The characteristic temperature of maximum swelling of the studied material is in the range of 430–500 °C.
The characteristic dose of maximal swelling is in the range of 61–72.5 dpa or 70–80% of the maximum dose.
The incubation period for stationary swelling is 30 dpa.
The stationary swelling rate is 0.1%/dpa.
The radiation resistance characteristics of the studied material have an advantage over those for Cr16Ni15 grade cladding materials under similar irradiation conditions and a similar structural state, which inherits a grain size of 9–12 μm during the tube processing.