Thermal expansion of a compressed Li2TiO3 pebble bed

Transcription

1 CBBI-14 6 September 2006 Petten, The Netherlands Thermal expansion of a compressed Li2TiO3 pebble bed H. Tanigawa, T. Hatano, M. Enoeda and M. Akiba Blanket Technology Group, JAEA

2 Thermo-Mechanical properties of a pebble bed Neutron flux Neutron Multiplier Tritium breeder Temperature distribution Thermal conductivity Thermal expansion Interaction Packing state Deformation, Stress

3 Purpose of the study Difference in thermal expansion between pebble bed and structural materials Thermal stress and deformation There is few data of thermal expansion for packed pebble beds Thermal expansion of a Li2TiO3 pebble bed is investigated.

4 Series of studies for thermo-mechanical analysis of blanket module Empirical continuum model for the pebble bed Effective thermo-mechanical properties Stress-Strain property Thermal conductivity Thermal expansion reported in CBBI11, 12 reported in CBBI13, 14 Finite element calculation code (ABAQUS)

5 Test apparatus INSTRON quartz tube load cell upper load rod Thermal expansion measured at the actuator includes that of load rods and the alumina container. IR furnace pebble bed alumina container lower load rod actuator coolant

6 Thermal expansion of system is calibrated , h60mm pebble bed Thermal expansion / mm whole system load rods and containor copper column measured expansion calculated expansion Cu column , h: 50.4mm : 60.0, h: 60.0mm Temperature / K estimated expansion of system (mmk -1 ) T (K)

7 Experimental conditions Sample Li2TiO3 pebble; 2mm 81.1% of T.D. Dimensions of packed bed 75mm, h60mm; 265.1cm 3 Initial packing factor Atmosphere Temperature Load on the bed during heating 65.0 ~ 68.0% (hand tapping) He; 1atm purge rate; 30ccm R.T. ~ 973K 0.44kN - 0.1MPa (friction of O-rings; about 0.03kN)

8 Definition of words in the present study Stress load cross section of bed Packing factor (P.F.) defined at R.T. 0.1MPa dl L Vpebble (Mpebble/density) Vall Average coefficient of thermal expansion (CTE) dl L@R.T. 973-R.T. dl dl P.F.= a in heating in P.F.= a

9 Thermal expansion of Li2TiO3 pebble bed Height of pebble bed / mm Bed is thermally treated in 3times. 1st heating 1st cooling 2nd heating 2nd cooling 3rd heating 3rd cooling Temperature of pebble bed / K initial packing state heat treatment compaction thermal expansion in heating cooling heat treatments heating < ~ compaction cooling

10 Relationship between CTE and P.F. Average thermal expansion coefficient / K E E E E E-05 1st heating 1st cooling 2nd heating 2nd cooling 3rd heating 3rd cooling Packing factor / % After a few heat treatments, the expansion coefficients in both heating and cooling processes have close values, and an increase of the packing factor becomes small.

11 Thermal expansion of pre-loaded bed Height of pebble bed / mm st heating 1st cooling compression 2nd heating 2nd cooling Temperature of pebble bed / K initial packing state heat treatment heat treatments compaction loading up to 10MPa 5times, loading at 0.1MPa thermal expansion in heating cooling > relaxation

12 Relaxation of residual deformation Temperature of pebble bed / K for 4 beds with different P.F. relaxation ratio: relaxation compression 0.33 ~ st heating 1st cooling compression 2nd heating 2nd cooling The compressive deformation of the pebble bed and its relaxation are affected by the friction among a large number of pebbles. This effect is considered to work statistically. competitive process ratchet-like deformation relaxation heating excess stress

13 Correlation between CTE and P.F. (reported in CBBI-13) Average thermal expansion coefficient / K E E E E E E E E E-05 heating cooling Packing factor / % heat treatment higher P.F. equilibrium P.F. heat treatment lower P.F. thermal expansion large small CTE P.F. constant

14 In larger region of P.F., CTEs are constant Average thermal expansion coefficient / K E E E E E-05 heating cooling Packing factor / % in each series for fresh packing CTE P.F. for all data with different P.F. and loading histories CTE ~ const. 1.4± K -1

15 Thermal expansion is explained by considering progress of compaction and its relaxation Average thermal expansion coefficient / K E E E E E E-05 All data are classified into four Packing factor / % 1st heating: small 1st cooling: large 1st heating for loaded bed: large others: progress of compaction progress of compaction relaxation of residual deformation about K -1

16 Summary Thermal expansion behaviour of Li2TiO3 pebble bed has been studied. In the temperature range from R.T. to 973K, thermal expansion of the pebble beds is measured under compressive load of 0.1MPa. For the beds with different packing factors and loading histories, average thermal expansion coefficients are 1.4± K -1. A residual deformation in the bed caused by a previous loading can be annealed when the bed is heated without the load. The thermal expansion behaviour of pebble beds is explained by taking into account progress of compaction and its relaxation.