Thermal expansion behavior of a compressed Li2TiO3 pebble bed

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1 CBBI /12/1 Santa Barbara, USA Thermal expansion behavior of a compressed Li2TiO3 pebble bed H. Tanigawa, S. Suzuki, M. Enoeda and M. Akiba Blanket Technology Group, JAEA

2 Thermo-Mechanical properties of a pebble bed Neutron flux Neutron Multiplier (Be alloy) Tritium breeder (Li 2 TiO 3 ) Temperature distribution Thermal conductivity Thermal expansion Interaction Packing state Deformation, Stress 2

3 Purpose of the study Thermo-mechanical analysis of the blanket module with pebble beds using finite element calculation code Empirical continuum model of the pebble bed including following properties should be established. Effective thermo-mechanical properties of Li 2 TiO 3 pebble bed. Thermal expansion Stress-Strain property (Young s modulus) Thermal conductivity reported in CBBI-11, 12 3

4 Test apparatus coolant alumina container IR furnace pebble bed hot wire for measurement of effective thermal conductivity 4

5 load cell Load and deformation in apparatus Load on the bed is measured by the load cell. alumina container Actuator controls the lower loading rod, and compresses the bed in this test. The deformation of the bed is obtained by measuring displacement of the lower loading rod. actuator The measured deformation (nominal deformation) includes thermal expansion of the alumina container and the loading rods when the test section is heated. 5

6 Estimation of actual deformation of the bed A rod made of pure copper is heated instead of the pebble bed. Cu rod Thermal expansion of the whole system is measured. pebble bed A correlation formula is determined. The obtained data of deformation are calibrated and actual deformation of the pebble bed can be estimated. 6

7 Thermal expansion of the apparatus whole system Thermal expansion / mm apparatus copper rod y y = x apparatus = T 1.19 R 2 R2 = Temperature Temperature of test section, / K T / K thermal expansion of bed / mm =(nominal deformation) ΔΤ 7

8 Experimental conditions Sample Li2TiO3 pebble; φ2mm 81.1% of T.D. Dimensions of packed bed φ75mm, h60mm; 265.1cm 3 Initial packing factor Atmosphere Test temperature Holding load on the bed during heating 66.8% (hand tapping) He; 1atm purge rate; 30ccm R.T. ~ 973K 0.1MPa (=0.44kN) (friction of O-rings; about 0.03kN) 8

9 Definition of words in the present study Stress load cross section of bed Packing factor (P.F.) defined at R.T. Vpebble (Mpebble/density) Vall Thermal expansion coefficient P.F.= a 0.1MPa dl dl dlë l P.F.= a 9

10 Thermal expansion of bed without pre-loading 1.0 Thermal expansion / mm heating cooling After thermal transient (RT-->973K-->RT), degree of compaction increased. 0.0 Small holding load of 0.1MPa was present Temperature / K 10

11 Thermal expansion of pre-loaded bed at 10 MPa After the measurement of the bed without pre-loading, the bed was compressed at 10 MPa in 5 times. Then, thermal transient (RT --> 973 K --> RT) was loaded. Thermal expansion / mm heating cooling After thermal transient, degree of compaction decreased. Stress and deformation caused by the pre-loading of 10 MPa were relaxed Temperature / K 11

12 Change in thermal expansion coefficient and P.F. Thermal expansion / mm compression 3 heating, noload cooling, nolaod heating, preloaded cooling, preloaded Temperature / K P.F. / % thermal expansion coefficient / K K compression of 10 MPa x 5 times K

13 Correlation between thermal expansion coefficient and packing factor Thermal expansion coefficitnt / K E E E E E E E E E-05 heating cooling equilibrium Packing factor / % thermal transient higher P.F. equilibrium P.F. thermal transient lower P.F. large thermal expansion constant thermal expansion small thermal expansion 13

14 Summary Thermal expansion behaviour of Li2TiO3 pebble bed was studied. After (several) thermal transient, packing state of the pebble bed reaches equilibrium. When the bed with higher P.F. than equilibrium one is heated, thermal expansion is large so that P.F. becomes small. In the case of smaller P.F., thermal expansion is small. These results suggest that residual stress in the bed caused by a compressive load can be annealed when the bed is heated with or without small load. 14

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17 Future works Analysis of controlling factors for equilibrium packing state (load on the bed, surface state of the pebbles, atmosphere, etc.) Determination of correlation between P.F. and thermal expansion coefficient 17

18 Thermal expansion coefficient of bulk Li2TiO3 Thermal expansion coefficient / K E E E E E E E E %T.D. by Oarai Temperature / K 81.5%T.D. by CEA present results 18

19 最後の 2 枚は IEA subtask 用 19

20 Current state for subtask in JA Preliminary results of effective thermal conductivity of compressed pebble beds were obtained for JA Li2TiO3. The measurement of thermal conductivity for EU material/osi is in progress. Measurement for height of the heated bed was improved. Thermal expansion coefficient of the bed was estimated. Correlation between thermal conductivity and packing state is being analyzed. 20

21 Future works for subtask Following issues will be checked and correlation between effective thermal conductivity and packing state will be determined so that it can be used in the thermomechanical analysis. Effects of cyclic loads or thermal creep Effects of volume or shape of the packed bed Difference in materials for the container 21