RUSSIAN RESEARCH CENTER KURCHATOV INSTITUTE Development of annealing for VVER-1000 reactor vessels Experimental assessment of possibility for performance of restoration anealing Ya.I. Shtrombakh

The lifetime of VVER reactor vessels mainly depends on irradiation embrittlement of weld joint materials Generation 1Generation 2 VVER - 440/230VVER - 440/213VVER To 0,22% To 0,048% To 0,027% < 0,012% < 0,3% 1,2 - 1,9% To 0,22% < 0,3% 2 < 0,08% Elements impacting irradiation embrittlement

Performance of restoration annealing at VVER-440 reactor vessels 3 VVER-440 Annealing efficiency Annealing temperature

Rating of VVER-440 units by chemical composition of the weld joints By TecSpec for weld joints of Rovno NPP %Р Rovno NPP %Р 4

Cutting of templates for assessment of the real conditions in VVER-440/320 reactor vessels If T k 5  5  0°C: T k 10  10 = T k 5   (T k 5  5 ) 2, °C If T k 5  5  0°C: T k 10  10 = T k 5   (T k 5  5 ) 2, °C 5×55×5 10×10 5

VVER-1000 RO MKR raises significantly with increase of Ni concentration 6

Range of Ni concentration change in the weld joints Activities necessary for LTE Monitoring of radiation loads for ensuring of the designed life and ANNEALING for LTE Monitoring of radiation loads for ensuring of the designed life additional attestation for LTE The designed life is ensured Additional attestation for LTE 7

Mechanisms of irradiation embrittlement caused by nano-structure evolution Radiation-induced precipitates Brittle intergranular destruction Irradiation strengthening Development of intergranular segregations of impurities IRADIATION EMBRITTLEMENT Radiation defects 8

Brittle intergranular destruction The spectrum of OZ electrons from the intergranular surface of the reactor vessel destruction (F=6,5  м -2 ). The presence of phosphor intergranular segregations is visible 9 Kinetic energy, eV Intensity, s.u.

Changes of density of the radiation induced segregations in the weld joint of VVER-1000 Ф = 11,6×10 23 н/м 2 N= ×10 21 м -3 Ф = 6,5×10 23 н/м 2 N= ×10 21 м -3 Ф = 3,1×10 23 н/м 2 N=70-90×10 21 м -3 10

Changes of density in the dislocation loops of VVER-1000 weld joint Ф = 11,6×10 23 н/м 2 N= ×10 21 м -3 Ф = 6,5×10 23 н/м 2 N=10-20×10 21 м -3 Ф = 3,1×10 23 н/м 2 N=5-6×10 21 м -3 11

Radiation induced segregations in the weld joints of VVER-1000 made of the atoms of Ni, Mn, Si M.K. Miller, A. Chernobaeva, Y.I. Shtrombakh, K. F. Russell, R.K. Nanstad, D.Y. Erak, O.O. Zabusov., Evolution of the nanostructure of VVER-1000 RPV materials under neutron irradiation and post irradiation annealing., JNM, 2009 The higher the segregations density, The bigger displacement of Т К 12

Dependence of maximum temperature for temper brittleness from nickel concentration in the steel (duration 100 hrs) 13 Temperature-time diagram of isothermal embrittlement of Cr-Ni-Mo steels Displacement of embrittlement temperature Temperature

Experimental effectiveness justification for the annealing of VVER-1000 reactor vessel weld joints MaterialBalakovo NPP, unit 1Kalinin NPP, unit 1 Composition of elemets, %Ni=1,88 Mn=1,1 Ni=1,76 Mn=0,98 Fluency during primary receiving of the templates (flux 2-4 ×10 14 м -2 с -1 ), ×10 22 м Displacement of the embrittlement temperature after the primary irradiation of the templates, º С 9390 Annealing mode565 º С/ 100 hrs, cooling less than 20 º С/hr. to 100 º С; further on with disconnected heaters Displacement of the embrittlement temperature after the restoration annealing, º С 48 Fluency during the secondary irradiation, ×10 22 м Flux during the secondary irradiation, ×10 16 м -2 с -1 17,02,1 Displacement of the embrittlement temperature after the secondary accelerated irradiation, º С

Change of density in radiation-induced segregations under irradiation in the weld joint VVER-1000 SECONDARY SPEEDED- UP IRRADIATION Ф=5,0×10 23 н/м 2 PRIMARY IRRADIATION Ф=3,2×10 23 н/м 2 RESTORATION ANNEALING 15

The velocity of the secondary embrittlement of the VVER-1000 reactor steels after the restoration annealing is significantly slower than during the primary irradiation 16 Initial Loops Precipitates Primary irradiation Secondary irradiation Restoration annealing State, fluency

Dose dependences of the density in the radiation-induced structure elements Precipitates Radiation defects 17 Fluency

The share of brittle intergranular destruction in the Charpy tests of the main material thermal sets (Ni 1.6 %) 18 Weld joint, Kalinin 2 Main material, Kalinin 2 Main material, Rovno 3 Weld joint, Rovno 2 Duration of thermal treatment, eff. hrs. Initial The share of intergranular destruction, %

19 The share of brittle intergranular destruction, displacements of brittleness temperature and viscosity limits in various states for the weld joint of Balakovo NPP-1 The share of brittle intergranular destruction Displacement of brittle temperature Displacement of viscosity limit Initial IrradiationAnnealing The share of brittle intergranular destruction, % Annealing 565 C/100 hrs + secondary accelerated irradiation

Displacement of brittleness temperature in different states of Balakovo NPP-1 reactor vessel steels 20 Weld joint Main material Initial Irradiation + annealing + irradiation

Comparison of primary and secondary irradiation embrittlement in the materials of Balakovo NPP unit 1 reactor vessel Main metal Weld joint Consideration of temperature ageing effect 1. Consideration of flux effect 2. Consideration of temperature ageing effect

22 Comparison of the primary and secondary radiation embrittlement in the weld joint at Balakovo NPP, unit 1 VVER-1000 weld joint FluencyNeutron, m 2 Primary irradiation embrittlement Secondary irradiation embrittlement After annealing C/30 hrs After annealing C/100 hrs

Computed diagram of temperature distribution along the VVER reactor vessel in the process of the restoration annealing (stationary task) 23

24 The irradiation embrittlement of the vessel steels caused by radiation strengthening due to radiation induced changes of its nano-structure, as well as formation of intergranular and boundary-granular phosphor segregations. It is demonstrated that the velocity of radiation embrittlement in weld joints of VVER-1000 reactors is higher the one of main VVER-1000 materials. The maximum of temperature intervals for phenomenon of temper brittleness is raising along with increase of nickel concentration in the steel, which required to increase the restoration annealing temperature for weld joints of VVER-1000 with increased nickel concentration. The parameters of time and temperature for restoration annealing of VVER-1000 weld joints with increased nickel concentration are identified. The restoration of features and structure of steels in VVER-1000 reactor vessels after restoration annealing is demonstrated. It is identified that the velocity of the embrittlement after the annealing under the speeded-up irradiation is lower than during the primary irradiation. It is shown the presence of «flux effect» during the speeded-up irradiation after the restoration annealing. The flowchart of temperature distributions along the VVER reactor vessel within the scope of the stationary task is computed for the restoration annealing process. Conclusions