State Scientific Center– Research Institute of Atomic Reactors Comparative analysis of the fuel rod state with E110 and E110 opt. claddings irradiated up to burnup of 49,6–63,7 MW·day/kgU © A.V. Strozhuk, V.A. Zhitelev, I.N. Volkova, Yu.D. Goncharenko, A.S. Khrenov, G.V. Shevlyakov, A.A. Bokov, 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

Zr angle of diagram Zr-Fe-O 1 INTRODUCTION One of the ways to improve the performance of alloy E110 is to increase the content of oxygen and iron in it. Since 2007, the optimized E110 opt. alloyed with oxygen (600-990ppm) and iron (400-500ppm) has been used as a cladding material at both Russian and foreign VVER-1000. RIAR conducted PIEs of experimental fuel rods from two FAs TVSA-ALPHA with claddings made from spongy and electrolytic E110 opt. after pilot operation at Unit 1 of Kalinin NPP. Purpose: Comparative analysis of the experimental fuel rods with E110 opt. and standard E110 claddings. Zr angle of diagram Zr-Fe-O 1 1 Optimization of alloy E110 for VVER-1000 claddings. V.A. Markelov, V.V. Novikov, M.M. Peregud, V.F. Konkov, V.N. Shishov, A.A. Balashov. Presentation at the 5th International Scientific&Technical Conference “Provision of NPP with VVER-1000 Safety” May 29 –June 1, 2007 , Podolsk, http://www.myshared.ru/slide/666504

KEY FUEL ROD CHARACTERISTICS Thinned cladding 0,57mm. Fuel pellet with an increased OD Ø7,8 mm without central hole. Grain size of 25-27 µm. Experimental fuel rods with claddings from E110 opt. and standard E110 were compared by: Deformation (diameter, length); Corrosion state (oxide film thickness, morphology of hydrides, hydrogen content); Mechanical properties.

Cladding diameter change lengthwise fuel rods FUEL ROD DEFORMATION Deformation mechanisms: Anisotropic radiation and thermal creep; Irradiation-induced growth; Fuel-to-cladding mechanical interaction. Two stages in VVER-1000 fuel rod deformation under operation: From the beginning of operation and till the fuel- to-cladding gap close, the fuel rod diameter decreases under excess coolant pressure. After the gap is closed and fuel come to a tight contact with cladding the fuel rod is operated with the cladding loaded by the swelling fuel. The cladding transverse deformation changes its sign into the opposite one and fuel rod diameter increases. Cladding diameter change lengthwise fuel rods

DEFORMATION. PROFILOGRAM PATTERNS CREEP STRAIN OF CLADDING IN CONTACT WITH FUEL CLADDING SHRINKAGE S – area under diameter distribution curve; L – length of cladding creep strain area after contact with fuel davg – average diameter over the fuel rod ; dpl – diameter in the gas plenum area Changes in shrinkage with increasing fuel burnup for claddings with high (1) and low (2) creep rate Fuel rod diameter change lengthwise At the first stage, with an increasing burnup, shrinkage increases first. At the second stage, after the fuel-to-cladding contact, with an increasing burnup, and, accordingly, fuel swelling, the shrinkage will decrease.         At high burnups, with increasing burnup, the cladding shrinkage will decrease for a fuel rod with both high and low creep rates. At the same time, the shrinkage value for a fuel rod with a low creep rate will be higher.

DEFORMATION Fuel rod creep strain Cladding shrinkage Fuel rod elongation Creep strain increases and shrinkage decreases with burnup . Creep strain of the experimental fuel rods with E110 opt. claddings is less by about 10-12µm and shrinkage is more by 12-15µm at the same burnup. Elongation of experimental fuel rods with E110 opt. claddings is about 3mm more than the one of standard fuel rods.

Oxide film on the cladding outer surface Cladding microstructure CORROSION STATE  = 5 m  = 10 m  = 10 m Oxide film on the cladding outer surface E110 opt. sponge E110 opt. electrolytic E110 Cladding microstructure Hydrogenation of E110 opt. and E110 claddings is not significantly different. The cladding microstructure in the area of maximum oxidation is characterized by lamellar hydride precipitates of a predominantly tangential orientation.

H content in the cladding lengthwise fuel rods CORROSION STATE H content in the cladding lengthwise fuel rods No significant difference is observed in the H content in the E110 and E110 opt. claddings. The H mass fraction in the claddings increases with the elevation. At the fuel rod bottom it is in the range 0,0029–0,0048%, at the top – 0,0056–0,0084 % .

MECHANICAL PROPERTIES OF CLADDING RING SAMPLES 20оС 380оС Yield strength E110: 545–600 MPa E110 опт.: 590–680 MPa E110: 330-385 MPa E110 опт.: 355-435 MPa 20оС 380оС Total relative elongation

CONCLUSION Comparative analysis of PIE results obtained for E110opt. and E110 claddings revealed the following differences in their state: Creep rate of E110 opt. claddings Is less than of standard E110 ones. At the same burnups, after the contact with fuel, E110 opt. claddings have creep strain about 10-12µm less, while the shrinkage is 12–15µm higher. The oxide film thickness on the outer surface of the experimental fuel rods with E110 opt. cladding is somewhat higher than that of fuel rods with standard E110 claddings. The maximum oxide thickness for E110 opt. is in the range of 9–13µm, for E110 it is 4–7µm. The strength of E110 opt. claddings is higher than that of standard E110 claddings. For E110 claddings, the yield strength at 20оС is in the range 545–600MPa and at 380оС it is 330–385MPa. For E110 opt. claddings, it is 590–680MPa and 355–435MPa, respectively. 10

11