1. RADIATION CREEP AND SWELLING
IN STEEL 08Х18Н10Т AT 350÷420°С
© E.I. Makarov, V.S. Neustroev, A.V. Obukhov, D.E. Markelov, 2019

2. Importance
Activities to justify the lifetime extension of VVER-440 and VVER-1000 reactor internals and the performance of new VVER reactor (VVER-1200, VVER-TOI) internals up to 60 years are quite pending.
Practically no data on creep and swelling of steel 08Х18Н10Т under neutron (E>0,1 MeV) high-dose and low-dose irradiation in special-purpose experiments;
Effect of tensile and, particularly, compression stress on properties and structure of austenitic steels (steel 08Х18Н10Т) used for VVER and PWR internals has not been practically studied either in Russia or abroad. 2

3. Goal and Tasks
To determine experimentally the mechanism of radiation creep and effect of stressed state on creep strain as well as microstructural characteristics of steel 08Х18Н10Т irradiated to different damage doses in BOR-60 at temperatures typical for VVER and PWR reactors.
Tasks to achieve the goal:
1. To develop and test pressurized samples of a new design under both tensile and compression stresses and to find a way how to test pressurized samples of new design in a nuclear reactor.
2. To plot dose dependences and stress strain dependences for pressurized samples as well as radiation creep modulus (at a stable creep stage) for steel 08Х18Н10Т at 90 dpa and 330 ÷ 350 and 400 ÷ 420°С.
3. To specify the behavior of irradiation-induced microstructure, in particular, porosity and dislocation loops, of steel 08Х18Н10Т irradiated in BOR-60 at 330 ÷ 350 and 400 ÷ 420°С under tensile and compression stress and with no stress. 3

4. Design of Steel 08Х18Н10Т Pressurized Samples and Irradiation Conditions
Experiment # (duration)
Sample design
Тirr., °С
Neutron fluence, ·1026 m−2 (Е>0,1 MeV)
Tangential stress at Тirr., MPa
Damage dose, dpa
# 1
(8 years)
Single tube and end components 23
192
up to 90
153
114 76 2
# 2
(more than 6 years)
Two co-axial tubes and end components 15
247; − 133
up to 60 (36)
185; − 99
124; − 67
82; − 44
# 3
(4 years) 9
206; − 111
up to 36 (17)
144; − 77
52; − 28 Не 4

5. Test Techniques
Sample geometry measurements (measurement error: diameter ±5m, length ±10 m);
TEM (relative measurement error: concentration (density) of microstructure elements: %, their size: 10%, volume fraction of phases and voids: 30-35%);
ICP measurement of alloying elements in steel Х18Н10Т.
Radiation creep modulus calculated by Von Mises formula:
H ,H – hoop strain and stress;
EQ, EQ – equivalent strain and stress;
Kt – damage dose (dpa).
where H = D/D/A, А~1,05. 5

6. New Design of a Pressurized Sample and its Testing in a Nuclear ReactorНе Не
Figure 1 ‒ New design of a pressurized sample:
1 – inner tubular element; 2 – outer tubular element;
3 – lower ring plug; 4 – upper ring plug; 5 – temporary plug
New-design pressurized samples will be tested by their irradiation in a reactor under the same temperature-dose conditions having quite different stressed-strained states of the structural material under testing. 6

7. Creep of Pressurized Samples (08Х18Н10Т) Irradiated at Тirr
Creep of Pressurized Samples (08Х18Н10Т) Irradiated at Тirr.= 330 ÷ 350°С up to 90 dpa
Figure 3 ‒ Diameter vs. length
Figure 2 ‒ Diameter vs. dose
Steady state creep
В=(2,3±0,4)∙10−6 (MPa∙dpa)−1
Figure 4 ‒ Diameter vs. stress
Figure 5 ‒ Creep modulus vs. dose 7

8. Creep of Pressurized Samples (08Х18Н10Т) Irradiated at Тirr
Creep of Pressurized Samples (08Х18Н10Т) Irradiated at Тirr. = 330 ÷ 350°С up to 60 dpa
Compressive
Figure 7 ‒ Diameter vs. stress
Figure 6 ‒ Diameter vs. dose
Steady state creep
В=(2,4±0,2)∙10−6 (MPa∙dpa)−1
Figure 8 ‒ Diameter vs. reduced dose
Figure 9 ‒ Creep modulus vs. dose 8

9. Figure 10 ‒ Diameter vs. dose
Creep of Pressurized Samples (08Х18Н10Т) Irradiated at Тirr. = 400 ÷ 420°С up to 36 dpa
S>0
S>0
Figure 10 ‒ Diameter vs. dose
Figure 11 ‒ Length vs. dose
В=(7,0±1,0)∙10−6 (MPa∙dpa)−1
B = Bo + D∙Ś,
where Ś – swelling rate (%/dpa),
B0 = 1∙10−6 (MPa∙dpa)−1,
D = (1,5÷3,0)∙10−3 (MPa)−1
Figure 12 ‒ Creep modulus vs. dose 9

10. Figure 13 ‒ Microstructure of an irradiated sample with no stress
Microstructure and Frank Loops in Steel 08Х18Н10Т Irradiated at Тirr. = 330 ÷ 350°С up to 36 dpa
Figure 13 ‒ Microstructure of an irradiated sample with no stress a) b)
Figure 14 ‒ Dark-field images of Frank loops: (a) σ=−123 MPa; (b) σ=+247 MPa 10

11. Dislocation Loops in Steel 08Х18Н10Т Irradiated at Тirr
Dislocation Loops in Steel 08Х18Н10Т Irradiated at Тirr. = 330 ÷ 350°С up to 36 dpa
Figure 15 ‒ Dislocation loops concentration vs. stress
Figure 16 ‒ Ave. dislocation loop size vs. stress
As tensile and compression stresses become higher, the loop concentration grows but the loop average size remains the same 11

12. Dislocation Structure of Steel 08Х18Н10Т Irradiated at Тirr
Dislocation Structure of Steel 08Х18Н10Т Irradiated at Тirr. = 400 ÷ 420°С up to 17 dpa
No stress
+ 206 MPa
− 111 MPa
Figure 17 ‒ Dislocation structure in the samples with different signs of pressure applied 12

13. Parameters of Loops and Voids in Steel 08Х18Н10Т irradiated at Тirr
Parameters of Loops and Voids in Steel 08Х18Н10Т irradiated at Тirr. = 400 ÷ 420°С up to 17 dpa
Figure 19 ‒ Size of voids and loops vs. stress
Figure 18 ‒ Concentration of voids and loops vs. stress
At the swelling incubation stage, the concentration of voids and loops becomes higher while their size remains the same under growing tensile and compression stress.
The obtained results can be used in theoretical models
of stress-to-swelling effect 13

14. Porosity of Steel 08Х18Н10Т Irradiated at Тirr
Porosity of Steel 08Х18Н10Т Irradiated at Тirr. = 400÷420°С up to 36 dpa
+ 206 MPa
− 111 MPa
Figure 20 – Porosity in irradiated steel samples with applied tensile and compression stress 14

15. Parameters of Voids in Steel 08Х18Н10Т Irradiated at Тirr
Parameters of Voids in Steel 08Х18Н10Т Irradiated at Тirr. = 400÷420°С up to 36 dpa
(1) S = S0∙(1+P∙σ)
Р=0,005±0,002 MPa‒1
(2) S = S0∙(1+P∙σeff),
where σeff =(1-η)∙σm+ η∙σeq, η = 0,15
σm = σθ / 2,
P=0,007±0,002 MPa‒1
Figure 21 ‒ Voids volume fraction vs. stress
Figure 22 ‒ Average void size vs. stress
Figure 23 ‒ Void concentration vs. stress 15

16. Conclusions
Three long-term experiments were performed in the BOR-60 reactor in a wide range of irradiation conditions to study radiation creep and microstructure of steel 08Х18Н10Т used for VVER-1000 reactors (VVER-TOI) internals.
The experimental dependences between strain and damage dose and stress in steel 08Х18Н10Т irradiated in the BOR-60 reactor in the temperature ranges 330÷350 and 400÷420°C to various damage doses are linearly increasing. The creep modulus of steel 08Х18Н10Т was determined to be (2.4 ± 0.4)10 −6 (MPa∙dpa)−1 for a damage dose up to 90 dpa in the irradiation temperature range 330 ÷ 350°С.
A methodical approach has been checked and applied to study the effect of compressive and tensile stresses on radiation creep strain and microstructure of steel 08X18H10T using proposed pressurized samples of new design and testing method in a nuclear reactor.
3. In the irradiation temperature range 330÷350°C at a damage dose of 36 dpa and 400÷420°C and 17 dpa, respectively, when swelling is close to zero (incubation period), the concentration of loops and voids increases, their size being constant, as absolute values of compressive and tensile stress become higher. 16

17. Conclusions (2)
4. In the irradiation temperature range 400÷420°С at a damage dose of 36 dpa, when swelling is more than 8%, the absolute value of both tensile and compressive stress causes an increase in swelling due to growing average size of voids, their concentration being constant.
5. Results of radiation creep studies (radiation creep modulus) of steel 08Х18Н10Т were used in strength calculations to extend the VVER-1000 internals lifetime and to justify the 60-year lifetime of a partitioning in new designs of VVER-1200 reactors (VVER-TOI). 17

18. THANK YOU FOR ATTENTION!
СПАСИБО ЗА ВНИМАНИЕ!