Preliminary Irradiation Effect on Corrosion Resistance of Zirconium Alloys

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18TH INTERNATIONAL SYMPOSIUM ON ZIRCONIUM IN

STATE ATOMIC ENERGY CORPORATION Preliminary

1 A.A. BOCHVAR HIGH-TECHNOLOGY RESEARCH INSTITUTE OF INORGANIC MATERIALS (SC «VNIINM») 18TH INTERNATIONAL SYMPOSIUM ON ZIRCONIUM IN THE NUCLEAR INDUSTRY MAY 15-19, 2016 «ROSATOM» STATE ATOMIC ENERGY CORPORATION Preliminary Irradiation Effect on Corrosion Resistance of Zirconium Alloys V. Markelov 1, V. Novikov 1, A. Shevyakov 1, A. Gusev 1, M. Peregud 1, V. Konkov 1, S. Eremin 2, A. Pokrovsky 2, A. Obukhov 2 (1) A. A. Bochvar High-Technology Research Institute of Inorganic Materials (SC VNIINM ) (2) Research Institute of Atomic Reactors (SC SSC RIAR )

2 Introduction Zr-Nb (E110, M5) and Zr-Nb-Sn-Fe (E635, ZIRLO) system alloys are used for fuel rod claddings and other fuel assembly elements in VVER and PWR reactors. Along with other properties, corrosion of Zr alloys depends on their chemical composition and structure-phase state. The initial structure and phase composition of alloys under neutron irradiation is subject to change. In order to forecast functional capability of the existing Zr alloys and to develop new ones, it is important to know whether changing of their structure-phase state under irradiation has an impact on corrosion and the extent of such impact. This paper covers autoclave corrosion tests performed on pre-irradiated and unirradiated fuel cladding samples made of Zr-Nb and Zr-Nb-Sn-Fe system alloys with different compositions. Structure-phase state of the alloys under examination before irradiation, after irradiation and after corrosion testing using TEM has been studied. The results allows to reveal factors of alloying and evolution of structure-phase state of the alloys, defining reduction or acceleration of their corrosion under irradiation. 2

Zr-Nb Industrial Zr-Nb-Sn-Fe Experimental Materials and Experiments System Alloy Nb Sn Fe O О2 1.02 0.41 0.33 0.09 О3 1.64 0.40 0.33 0.09 О4 2.33 0.40 0.32 0.09 О5 1.03 0.59 0.34 0.09 О6 1.02 0.58 0.

3 Zr-Nb Industrial Zr-Nb-Sn-Fe Experimental Materials and Experiments System Alloy Nb Sn Fe O О О О О О О О Cladding tubes Ø (7.93) mm with final annealing at 580 ºC-3 h for all alloys. Samples in the form of segments 6 mm wide and 70 mm long cut out of tubes Irradiation: BOR-60 research reactor; T irr = С; Fluence m -2 (E 0.1MeV); E E635M E E110opt E110M E Corrosion tests: an autoclave located in a hot cell of RIAR (Dimitrovgrad); Т = 350 С; Р = 16.3 MPa; Water with concentration of Li 10 ppm; Duration 240 days. 3

4 Materials and Experiments For TEM examinations of structure-phase state of irradiated and unirradiated samples, JEOL JEM-2000FXII electron microscope with the accelerating voltage up to 200 kv was used. Elemental analysis of the matrix and second phase precipitates (SPPs) was carried out using an EDAX Genesis XMS 60 X-ray microanalysis system. For the quantitative analysis of the SPPs, varnished-carbon replica was removed from each sample together with the particles. To determine the average size and number density of SPPs were measured from 400 to 700 particles for each alloy. 4

Structure-phase state of alloys before irradiation 1) The

state before irradiation is fully recrystallized with the

of SPPs): β-nb phase: 80 90 at. Nb % + 20 10 at.

% Zr + 35 at.% Nb + 30 at.% Fe (HCP: a = 0.534 nm, c = 0.

5 Structure-phase state of alloys before irradiation 1) The structure of tube samples of all the alloys in the initial state before irradiation is fully recrystallized with the average grain size around of 3 µm 2) Phase composition (3 type of SPPs): β-nb phase: at. Nb % at. Zr % (BCC: a = 0.33 nm ) Laves phase (L) Zr(Nb,Fe) 2 : 35 at.% Zr + 35 at.% Nb + 30 at.% Fe (HCP: a = nm, c = nm, c/a = 1.619) T-phase (Zr,Nb) 2 Fe: 60 at.% Zr + 10 at.% Nb + 30 at.% Fe (FCC: a = 1.22 nm) 5

Zr-Nb Zr-Nb-Sn-Fe Structure-phase state of studied alloys before irradiation System Alloy SPPs average size, nm SPPs number

35 T 0.05 L 3.75 T 0.05 L 3.45 T 0.05 L 4.45 T 0.05 E635M 95 3 L 3 О3 65 8.3 О4 62 11.9 О6 53 4 О7 56 12.2 β-nb 5.8 L 2.

6 Zr-Nb Zr-Nb-Sn-Fe Structure-phase state of studied alloys before irradiation System Alloy SPPs average size, nm SPPs number density, 10 19, m -3 О О О E SPP types and their calculated number density, 10 19, m -3 L 3.35 T 0.05 L 3.75 T 0.05 L 3.45 T 0.05 L 4.45 T 0.05 E635M 95 3 L 3 О О О О β-nb 5.8 L 2.5 β-nb 9.5 L 2.4 β-nb 2.4 L 1.6 β-nb 10.4 L 1.8 E β-nb 9 E110opt 45 8 E110M 50 7 E β-nb 7.8 L 0.2 β-nb 6.3 L 0.7 β-nb 15.8 L 0.2 Zr-Nb-Sn-Fe alloys O2, О5, О8 and E635: L-phase + Т-phase (too small) Nb E635M: L-phase Nb or Fe О3,О4,О7 and О6: β-nb phase + L-phase (smaller) Zr-Nb alloys E110: β-nb phase Fe E110opt, E110M and E125: β-nb phase + L-phase (too small) 6

subject to substantial changes (45-60 ат.

7 Structure-phase state of alloys after irradiation The microstructure of all alloys after irradiation also fully recrystallized О2: 1,02 % Nb О7: 2,37 % Nb β-nb precipitates were subject to substantial changes (45-60 ат.% Nb) The dependence of the elemental composition on particle sizes 7

8 Structure-phase state of alloys after irradiation L-phase L-phase precipitates changed their composition and transformed into β-nb (45-50 at.% Nb) Fe is almost completely removed from the precipitates into the matrix Т-phase practically the same 8

Structure-phase state of alloys after irradiation Radiation-induced precipitates (RIPs) in the form of nanometric elongated particles was detected in all alloys.

9 Structure-phase state of alloys after irradiation Radiation-induced precipitates (RIPs) in the form of nanometric elongated particles was detected in all alloys. The number density of such RIPs is higher in Zr-Nb system alloys and significantly lower in Zr-Nb-Sn-Fe system alloys especially with increase of Sn content in them. As it was noted earlier [9-14], formation of RIPs is associated with Nb exit from a matrix under irradiation and probably it is characteristic of all Nb containing alloys. 9

10 Zr-Nb Zr-Nb-Sn-Fe Structure-phase state of studied alloys after irradiation System Globular SPPs Nanometric RIPs Alloy D av, nm ρ, 10 19, m -3 L av, nm ρ, 10 21, m -3 О О О О О О О , E E635M E E110opt E110M E The average size of the transformed globular SPPs is increased The SPPs number density is reduced All globular precipitates were analyzed all together as particles of the same type Formation of RIPs 10

11 Zr-Nb Zr-Nb-Sn-Fe Corrosion System Alloy Unirradiated Weight gain, mg/dm 2 Irradiated Weight gain, mg/dm 2 K =WG ir /WG unir O O O O E E635M O O O E E110opt E110M Different irradiation effect on corrosion behavior of alloys Acceleration of corrosion : О2, О3, О5, О8, E635 and E635M No effect on corrosion: О4 and О6 Reduction of corrosion: E110, E125, E110M, E110opt and О7 E for TEM examination 11

and crystal structure of the precipitates return to its initial

12 Structure-phase state of alloys after corrosion tests Before corrosion tests β-nb After corrosion tests Elemental composition and crystal structure of the precipitates return to its initial state All alloys after corrosion tests had RIPs L-phase After corrosion tests 12

13 Discussion Influence of Nb content on corrosion : Zr-Nb ---- Zr-Nb-Sn-Fe (0.4 % Sn) Influence of Sn content on corrosion : ---- Zr-Nb-Sn-Fe (1 % Nb) 13

14 Discussion Unirradiated There is also no dependence of corrosion behavior on the size, number density or a volume fraction of all SPPs. The results show that neither β-nb phase, nor the Laves phase, nor T-phase with an insignificant volume fraction, have a noticeable effect on corrosion resistance of alloys. 14

15 Discussion Influence of Nb content on corrosion : Zr-Nb ---- Zr-Nb-Sn-Fe (0.4 % Sn) Influence of Sn content on corrosion : ---- Zr-Nb-Sn-Fe (1 % Nb) 15

16 Discussion Irradiated Transformed precipitates of the second phase which became "β-nb" particles have also no direct effect on corrosion. Figures shows that there is a very slight dependency of corrosion on the particles size and number density and the absence of such dependency on the volume fraction of particles, it can be assumed that other factors play an important role in this case. 16

17 Discussion The composition of solid solutions from the matrix of alloys has significantly changed under irradiation compared to the unirradiated state. Atoms of Fe from Laves phase particles pass into solid solution. A portion of Nb atoms in the solid solution is used to form nanometric RIPs. This process is significantly faster in Zr-Nb alloys. In Zr-Nb-Sn-Fe alloys the process of RIPs formation happens more slowly. It is evident that the presence of Sn in the initial solid solution of an alloy and transition of Fe atoms thereto under irradiation retain the output of Nb atoms from the matrix to form RIPs. It can be noticed that the greater the total content of Sn and Fe in a solid solution of the irradiated alloy is, the lower RIPs are formed. Thus, high density of nanometric RIPs containing Nb is structural indicator of rate reduction of the alloy corrosion under irradiation. On the contrary, smaller numbers of such precipitates in the structure of irradiated alloys indicate higher rate of corrosion under irradiation. 17

18 Discussion Based on the above, there must be a connection between the number of RIPs and the content of Nb, Sn and Fe in a solid solution of irradiated alloys, and the same connection between the above mentioned characteristics and corrosion of irradiated alloys. This interconnection allows us to reasonably combine corrosion of irradiated alloys of both systems with different alloying composition on the same diagram. Nb in a solid solution of alloys has a positive effect, as corrosion rate thereof is reduced under irradiation. The content Sn and Fe in a solid solution of irradiated alloys has a negative effect, as it accelerates their corrosion. In this paper it was impossible to determine the exact concentration of alloying elements in the alloy matrix using an analytical device. 18

19 Discussion Therefore, it was suggested to use the connection between the quantity of RIPs in the irradiated alloy and the ratio of contents of all alloying elements in its composition of Nb/(Sn+Fe). Where Nb is in the numerator, as it has a positive effect on corrosion, and Sn+Fe is in the denominator, as they have a negative effect on corrosion. 19

20 Discussion The volume fraction of RIPs and alloying parameter increase while the corrosion acceleration coefficient of the studied alloy systems is reduced. Volume fraction of RIPs > 0,03 % Alloying parameter Nb/(Sn+Fe) > 4 Corrosion acceleration coefficient < 1 20

21 Conclusion 1. In comparison with unirradiated state, irradiation reduces corrosion of Zr-Nb alloys containing from 1 to 2.5 % of Nb, and this effect becomes more apparent with increased content of Nb in the alloy. 2. For Zr-Nb-Sn-Fe alloys, irradiation leads to more severe corrosion due to the increased content of Sn. The increased content of Nb and reduced content of Fe in the alloys of this system contribute to a higher corrosion resistance. 3. A structural indicator of alloys corrosion reduction under irradiation is formation of nanometric niobium-rich RIPs with volume fraction of more than 0.03 % for the given experimental conditions. 4. Interconnection has been suggested between Nb/(Sn+Fe) alloying parameter, volume fraction of RIPs formed therein under irradiation and their corrosion performance. 21

22 Thank you for attention! 22

Study of Structure-Phase State of Oxide Films on E110 and E635 Alloys at Pre- and Post-Irradiation Stages

Study of Structure-Phase State of Oxide Films on E110 and E635 Alloys at Pre- and Post-Irradiation Stages A.A. BOCHVAR HIGH-TECHNOLOGY RESEARCH INSTITUTE OF INORGANIC MATERIALS (SC «VNIINM») 18TH INTERNATIONAL SYMPOSIUM ON ZIRCONIUM IN THE NUCLEAR INDUSTRY «ROSATOM» STATE ATOMIC ENERGY CORPORATION MAY 15-19,