Mechanical strength testing of irradiated HTR graphite grades M.C.R. Heijna, J.A. Vreeling NRG, Petten The Netherlands INGSM September 2013 Seattle

Outline Introduction –HTR graphite –INNOGRAPH irradiation programme Tensile strength from diametric compression –Why? –How? Results –Movies! –Trends for coke, grain size, production process Conclusions 2

HTR Graphite 3 Two types of (V)HTR: pebble bed prismatic block Graphite as moderator Structural material Images from Wikipedia

INNOGRAPH programme 4 European Commission Framework Programmes: Retain (V)HTR knowledge Screen properties of modern graphite grades

INNOGRAPH programme 5 INNOGRAPH-1A: 750°C, 5-10 dpa

INNOGRAPH programme 6 INNOGRAPH-1B: 750°C, dpa INNOGRAPH-2A: 950°C, 4-8 dpa INNOGRAPH-2B: 950°C, 8-15 dpa

INNOGRAPH programme 7 INNOGRAPH-1B: 750°C, dpa INNOGRAPH-2A: 950°C, 4-8 dpa INNOGRAPH-2B: 950°C, 8-15 dpa

INNOGRAPH programme 8 INNOGRAPH-1C: 750°C, 1-2 dpa Irradiation will be concluded this month

INNOGRAPH design Selected irradiated samples from “1A”and “2A” were further irradiated in “1B” and “2B” INNOGRAPH-1B and -2B have been assembled in the NRG hot-cells

INNOGRAPH design 10 Each experiment contains 24 thermocouples and 9 dosimeter sets Temperature controlled by gas mixture Temperature control within ±30°C In the order of samples per experiment Sample geometry: 6 mm high, 8 mm diameter Flat side with sample coding Samples in inert environment

Graphite grades 11 Major grades for this programme

Irradiation in HFR, Petten 12 Tank-in-pool type MTR 40 MW thermal Operated by NRG

Post Irradiation Examination 13 Data on 800 graphite samples Dimensional change Density Young’s modulus Coefficient of Thermal Expansion Thermal conductivity Electrical resistivity sample recovery measurements data set

Test matrix strength measurements 14 Irradiation temperature 750°C Fine grains and coarse grains Pet & pitch coke Various production processes Different levels of dpa’s / volume change Sample selection limited by: Reloaded samples Microscopy samples

Diametric compression 15

Why diametric compression? 16 Typical tensile test geometry INNOGRAPH geometry

Tensile strength from diametric compression 17 S.P. Timoshenko, J.N. Goodier, “Theory of elasticity”, 1970 H. Awaji, J. Soc. Mat. Sci. Jap. 27 (1987) C. Berre et al., unpublished Tensile strength < Compressive strength

Tensile strength from diametric compression 18 z = unique direction

Experiment! Coarse grain graphite 19

Experiment! Fine grain graphite 20

Strength vs dpa; Grain size 21

Strength vs dpa; Grain size 22  V/V 0

Scanning Electron Microscopy 23 Fine grain, 0 dpa Grade C Coarse grain, 0 dpa Grade A Fine grain, 13.4 dpa Grade C Coarse grain, 12.8 dpa Grade A

Strength vs dpa; Coke 24

Strength vs dpa; Process 25

Conclusions Successful irradiation and PIE campaign concluded by demolishing samples Tensile strength vs. dpa trends derived from diametric compression At turn-around, strength doubles –Regardless of grain size, coke, production process Fine grained graphite stronger than coarse grained –Regardless of irradiation Coke and process have negligible effect on tensile strength 26

Conclusions Successful irradiation and PIE campaign concluded by demolishing samples Tensile strength vs. dpa trends derived from diametric compression At turn-around, strength doubles –Regardless of grain size, coke, production process Fine grained graphite stronger than coarse grained –Regardless of irradiation Coke and process have negligible effect on tensile strength Thank you for your attention! 27