May 27-31, 2019, JSC “SSC RIAR”, Dimitrovgrad, Russia COMPARATIVE ANALYSIS OF POST-IRRADIATION EXAMINATION RESULTS OF MIXED NITRIDE FUEL IRRADIATED IN BOR-60 AND BN-600 REACTORS: INFLUENCE OF IRRADIATION PARAMETERS ON FUEL LIFE LIMITING FACTORS A.F.Grachev (1), L.M.Zabudko (1), F.N.Kryukov (2), S.I.Porollo (3), M.V.Skupov (4)  (1) ITCP "PRORYV", Moscow, Russia, (2) JSC "SSC RIAR", Dimitrovgrad, Russia, (3) JSC "SSC RF IPPE, Obninsk, Russia, (4) JSC "VNIINM", Moscow, Russia 11th Conference on Reactor Materials Science dedicated to the 55th Anniversary of the RIAR’s Reactor Materials Testing Complex. May 27-31, 2019, JSC “SSC RIAR”, Dimitrovgrad, Russia

INTRODUCTION Mixed nitride development is done in RF within the framework of the "Complex program of calculation and experimental substantiation of dense fuel for fast neutron reactors”: technology mastering, out-of-pile properties study, irradiation, post irradiation examinations (PIE), fuel codes improvement . In order to prove nitride fuel performance under operation conditions of BN-1200 and BREST-OD-300 reactors the experimental fuel assemblies (EFA) with fuel pins of various design and different cladding materials are tested in BOR-60 and BN-600 reactors. In BOR-60 reactor the irradiation of three EFAs with EP823 steel cladding is successfully continuing. The maximum burn-up of 5.4at% and damage dose of 81,2dpa are achieved (at the beginning of February 2019). At the end of 2018 the EFA with pin claddings made of advanced ferrite-martensitic steels EK181, CHS139 and the EFA with additions of Np and Am in mixed nitride have been loaded. In BN-600 reactor eighteen EFAs (more than 1000 fuel pins of different designs with pin claddings made of CHS68cw, EK164cw and EP823) have been loaded. The irradiation of ten EFAs is successfully completed. In combined EFA-7 the maximum fuel burn-up of 7.5at% and dose of 74dpa are achieved.

Parameters of helium-bonded mixed nitride pins of BOR-60 FAs , PIE of which have been completed 1st pin EFA-1 2nd pin EFA -1 1st pin EFA -2 Max fuel burn-up, at% 1,3 3,2 5,0 Max damage dose, dpa 18,7 46 74,7 Max linear rating, kW/m 36 33,2 Max cladding temperature, оС 610 615 Clad diameter x wall thickness, mm Cladding material 9,4 х 0,5 EP823

Post-irradiation examination (PIE) of BN-600 EFAs Completed PIE of: 20 mixed nitride pins and neigboring oxide pins of combined EFA-1, EFA-2, EFA-3, EFA-6, EFA-7 irradiated in BN-600 reactor 16 mixed nitride pins of EFA-4, EFA-5 of BN-600. EFA design Combined EFA-1,-6,-7 have standard BN-600 design pins: 4 pins with UPuN (2), 123 standard pins with UO2 : (1) neigboring oxide pins with PIE, (2)-other oxide pins Combined EFA-2 have БN-1200 type fuel pins; combined EFA-3 have BREAST type pins: 4 pins with UPuN per each EFA, 57 oxide pins in EFA-3 and 57 MOX fuel pins in EFA-2 EFA-4 has 61 BN-1200 type pins; EFA-5 has 61 BREST type pins.

Parameters of nitride pins of BN-600 FAs , PIE of which have been completed Параметр КЭТВС-1 КЭТВС-2 КЭТВС-3 ЭТВС-4 ЭТВС-5 КЭТВС-6 КЭТВС-7 Max fuel burn-up/ at% 5,5 5,0 4,5 3,8 3,9 7,5 Max damage dose, dpa 55 56,1 53 58 48 37,5 74 Max linear rating, kW/m 38,1 44 47 40 39,3 38 Max cladding temperature, оС 676 655 635 664 642 677 671 Clad diameter x wall thickness, mm Cladding material 6,9х0,4 CHS68cw 9,3х0,6 EК164cw 9,3х0,5 ЭEP823 EP823

Results of PIE and its analysis For BN-600 fuel pins with claddings made of ChS68 cw steel the max dose values varied from 37,5 dpa to 73,8 dpa, for BOR-60 and BN-600 pins with claddings made of EP823 steel from 18,6 dpa to 74,7 dpa. All fuel pins and EFAs are intact with no change of its initial geometry; no pins and FAS components defects, that may perform under irradiation or post irradiation fuel management As a result of the examination and analysis of the data obtained, the following potentially life limiting factors for mixed nitride pins were identified: pin deformation (cladding elongation and ovalization), higher level of cladding corrosion and short-term mechanical properties deterioration than in the neighboring oxide pins.

Potentially life limiting factors for mixed nitride pins – pins deformation (1) PIE showed that nitride pin cladding have higher level of cladding elongation and ovalization that neighboring oxide pins. Firstly it was received at PIE of CEFA-1. The similar results have been obtained for all other EFAs. The analysis of PIE profilometry data of all nitride pins of BN-600, BOR-60 EFAs, as well as BORA-BORA experiment (max burn-up 12,1at%) and nitride pins, irradiated in JOYO and PHENIX reactors, have shown, that cladding elongation and ovalization take place for all nitride pins as with austenitic and with ferrite-martensitic steels claddings Elongation of CEFA-7 fuel pins Ovalization of CEFA-7 pin cladding

Potentially life limiting factors for mixed nitride pins – pins deformation (2) As a result of the analysis it was found that the value of cladding ovalization, as well as an increase in the length, as a result of mechanical interaction with fuel pellets do not depend on the dose for austenitic cladding, and for ferrite-martensitic claddings. A possible reason may be that the cladding deformation occurs mainly under reactor transients power, when the fragmented pellet act on the cladding. Cladding made of ChS68 cw steel Cladding made of EP823 steel

Potentially life limiting factors for mixed nitride pins – claddings corrosion Cladding made of ChS68 cw steel Claddings corrosion can be divided into four main types: surface and subsurface ulcerous corrosion, increased edging of grain boundaries, nitriding and carburizing. The maximum depth of ulcerous corrosion was in the fuel pins of CEFA-1, minimum – in CEFA-1 -7. The depth of the zone of high edging was not so markedly different: 160мкм (CEFA-6), 200мкм (CEFA-7), 240 µm (CEFA-1). The depth of cladding nitriding - 10–20mkm. The content of impurities, density and other characteristics of nitride pellets of all combined EFA are similar. Possible cause of increased ulcerous corrosion in CEFA-1 is the higher fuel temperature, associated with increased value of fuel-cladding gap: for CEFA-1 ~100°C higher than CEFA-6 and -7. A possible reason for corrosion reducing could be the improvement of fuel manufacturing technology. Fabrication of CEFA-1 fuel pins is the first domestic experience of fabrication of full-scale nitride pins of the BN-600 reactor. Cladding made of EP823 steel For EP823 steel claddings the increased edging of grain boundaries is not observed. Not observed a noticeable effect of dose increasing from 18.6 to 74.7 dpa to all types of corrosion damage of EP823 steel claddings. Maximum depth of subsurface corrosion (170мкм) is recorded in the pin cladding of CEFA-3 (~50 dpa).

Potentially life limiting factors for mixed nitride pins – higher level of short-term mechanical properties deterioration than in neighboring oxide pins. Cladding made of ChS68 cw steel Ultimate tensile strength and total elongation of ring and longitudinal samples of CEFA‑7 fuel claddings as a function of irradiation/test temperatures: □ – pin № 64 (UPuN), ring samples; ■ – pin № 64 (UPuN), longitudinal samples ; ∆ - pin№ 71 (UO2), ring samples; ▲ - твэл № 71 (UO2), longitudinal samples Damage dose increase for CEFA-7 did not lead to noticeable change of the strength properties of cladding in comparison with CEFA-1 and CEFA-6. At the same time, the plastic characteristics of fuel cladding with the lowest dose were slightly higher.

Potentially life limiting factors for mixed nitride pins– higher level of short-term mechanical properties deterioration Cladding made of EP823 steel For ferrite-martensitic claddings the most important change in mechanical properties is associated with the phenomenon of low-temperature irradiation embrittlement (LTIE), which manifests itself already at a dose ≤10 dpa. The PIE results of BOR-60 fuel showed that dose increase to 74.7 dpa did not lead to the deterioration of short-term properties of EP823 steel, compared to the dose of 46 dpa. The experimental confirmation is obtained about the integrity of seven fuel mixed nitride pins of BOR-60 EFA-2 with max damage dose of ~81сна (to February 2018.) under irradiation conditions corresponding to EFA-11 conditions that is currently under irradiation in the BN-600 radial blanket. Irradiation conditions of nitride fuel in the BOR-60 are more strong compared to BN-600, from the point of view of the factors influencing the fuel behavior in case of LTIE cladding: 1) transients number is considerably greater because the duration of fuel cycle in BOR- 60 in ~2 times lower than in BN-600; 2) interim rearrangement (change of the wrapper) and replacement of EFA; 3) the minimum temperature of the EFA cladding (at core input) is ~30 ° C lower than in BN-600 . The experience of fuel management received today, confirmed that the integrity of nitride pins claddings made of EP823 steel is kept not only under irradiation, but also under fuel discharge - interim washing – change of wrapper– reloading-transportation - handling in hot cell. In this case, two types of washing were used: steam-water in BOR-60, liquid lead in BN-600 .

CONCLUSION The irradiation and post-irradiation examinations of mixed nitride fuel carried out within the framework of "Complex program of calculation and experimental substantiation of dense fuel for fast neutron reactors” allowed to identify the main factors potentially limiting the fuel performance: greater claddings elongation and ovalization, greater corrosion damage and more significant degradation of claddings short-term mechanical properties than in oxide fuel pins. With the achieved irradiation parameters, these factors did not lead to a fuel performance limitation. As the result of PIE data analysis of fuel pins with ChS68 and EP823 claddings it was shown that the cladding ovalization and elongation do not depend on the damage dose value. There is no obvious effect of dose increase on all types of claddings corrosion damage. Damage dose increase to ~75 dpa did not lead to a deterioration in claddings short-term mechanical properties. Comparison of PIE results of fuel pins irradiated in BOR-60 and BN-600 reactors shows that the specificity of irradiation in different reactors provides additional important data on the behavior of fuel and structural materials under irradiation. The choice of the two reactors for research within the Complex program is largely justified. The positive irradiation results of EFA-2 in BOR-60 and PIE allowed to prolong the irradiation in BN-600 of EFA-11 with the fuel pins of the BREST reactor type, which achieved a burn-up of 6.4at%, and dose of 77dpa.

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