XI Conference on Reactor Materials May 27 - 31, 2019 , Dimitrovgrad

1. SC «A.A. Bochvar High-technology Research Institute of Inorganic Materials»(SC «VNIINM») OPTIMIZATION OF STRUCTURAL-PHASE STATE OF FUEL PIN CLADDINGS MADE OF STEEL EK164 IN ORDER TO INCREASE THE RESOURCE EFFICIENCY OF FAST NEUTRON REACTORS FUEL ASSEMBLIES T. Churyumova, N. Mitrofanova XI Conference on Reactor Materials May 27 - 31, 2019, Dimitrovgrad

2. Introduction The achievement of fuel burn-up 15 % FIMA and the increase in the damage dose 112 dpa in the reactor BN-600, as well as 12 % FIMA, 116dpafor BN-800, are planned due to using a radiation-resistant austenitic steel EK164 (cold worked) as a cladding material. 16% Cr-19% Ni- based steel alloyed with carbide-forming elements (Ti, Nb, V), Si , B, P, Ce, with compared increased Ni content. During the development of the steel EK164, its chemical composition was optimized, metallurgical and pipe technologies were improved.

3. Stages of EK164 fuel pill claddings technology improvement The purpose of this work is to study the effect of the heat treatment parameters optimizing on structural-phase and dislocation stability during long-time aging at 700 °C, short-term and long-term mechanical properties (long-term strength and thermal creep) of cladding pipes made of steel EK164.

4. Structural studies Microstructure and microhardness after aging at 700 °С, 8000 hours - phase × 1000 АТОN × 1000 VSQ × 1000 PNTZ

5. Phase composition in the initial state Carbides of the MS type (where M is mainly Ti, Nb, as well as Mo and V) of various morphology. The ratio of the above elements varies as follows: 2Ti:2Nb:(1Mo), 3Ti:2Nb:(1Mo), 2Ti:2Mo:1Nb, 5Ti:4Nb:3Mo:(1V). TEM images of MC type carbides and spectra taken from them Particles of MC type carbides of various morphology pipes spructure (SEM)

6. Phase composition after thermal aging at 700 ° C Carbides М23С6. 5000 h 8000 h Phosphides М2Р needle shape. М2Р 8000 h 1000 h

7. Phase composition after thermal aging at 700 ° C 8000 часов Fine secondary MC carbides precipitated at dislocations. Laves’ phase Laves’ phase 3000 h 8000 h

8. Phase composition after thermal aging at 700 ° C SEM - images of the structure after aging at 700 ° C, 8000 hours Secondary МС VSQ d = (15 – 20) nm АТОN d = (5 – 10) nm

9. Dislocation structure in the initial state and after thermal aging at 700 ° C Using the TEM method, it was shown that a cellular dislocation structure is typical for samples of all technological designs. Initial state АТОN VSQ Aging 8 000 hours

10. Mechanical properties after aging at 700 °С

11. Long-term strength Long-term strength characteristics increased in 2 - 5 times, and the creep rate decreased by an order of value, compared to VSQ-made pipes, which, in turn, have similar advantages over pipes PNTZ PNTZ VSQ VSQ ATON PNTZ VSQ VSQ ATON Tension, MPa Tension, MPa Creep rate , % / h Time to rupture, h

12. Reactor tests and post-reactor research results • At present, reactor tests at BN-600 had been carried out on EK164 steel fuel pin claddings as part of 18 fuel assemblies, in which fuel element’s claddings were obtained with of various pipe technologies. The maximum damaging doses from 74 to ~ 100 dpa. have been achieved. • The results of BNPP and IRM non-destructive and destructive post-reactor studies confirmed the increasing radiation resistance of steel EK164 fuel pin claddingss with each stage of improvement of manufacturing technology. • It was shown that, the swelling value of VSQ cladding pipes (84 dpa) is lower than that of PNTZ pipes at lower damaging dose of 78 dpa. • The values of the fuel pins swelling in fuel assemblies irradiated up to ~ 100 dpa confirm the expected increase in the radiation resistance of ATON cladding pipes, which swelling is 1,7 times lower than that of VSQ claddings at a similar value of the damaging dose 96 dpa. • The results of BNPP and IRM studies (swelling, interaction with fuel and coolant, structure evolution under irradiation, mechanical properties of tubular samples, mechanical condition of fuel element claddings after operation to achieved doses and burnout of fuel) and predictive estimates allowed to justify the safe operation of ATON cladding pipes to damaging dose of at least 115 dpa. • Obtained results allow us to recommend use of EK164 steel as a fuel pin claddings material in order to extend service life of fast reactor fuel assemblies.

13. Conclusion 1. Comparative studies of the structure of steel EK164 made of various technologies after aging (8000 hours at 700 ° C) showed that the metal structure of the pipe, regardless of the technological performance, contains following phases: primary and secondary M (C, N) particles, Laves phase, M23C6 type carbides and needle phosphides. The transition from inertial (VSQ) to high-speed heating (ATON) results in increase in the structural and phase stability and the dislocation structure stability due to more rich in alloying elements solid solution and the formation of more resistant to growth M (C,N) and M2P particles. 2. The results of the study of short-term mechanical properties showed the advantage of pipes heat-treated at the ATON facility with respect to the strength characteristics in both the initial and aged (700 ºС for 1000-8000 h) states. 3. It is shown that ATON pipes have characteristics of long-term strength 2–5 times higher than that of VSQ pipes and an order of magnitude higher than those of pipes manufactured according to PNTZ technology; and the creep rate is 1 and 2 orders lower, compared to pipes manufactured using VSQ and PNTZ technology, respectively. 4. It is shown that the improvement of technology through the use of new heat treatment parameters, increases the service life of fuel claddings made of steel EK164, the swelling of the shells of VSQ fuel elements is lower (3,45 ± 0,3 % at 84 dpa) than that of the PNTZ shells (3,9 ± 0,6 %at 79 dpa). The swelling of ATON claddings irradiated up to 100 dpais 1.7 times lower than that of VSQ claddingsat a similar value of the damaging dose of 96 dpa.