Irradiation Creep in Graphite A Review Barry J Marsden The - PowerPoint PPT Presentation

Irradiation Creep in Graphite A Review Barry J Marsden – The University of Manchester Stephen D Preston – SERCO Assurance 1

Irradiation Creep in Graphite • Due to fast neutron irradiation • Significantly reduces stresses in nuclear graphite components • The difference in dimensions between a stressed sample and a sample having the same properties as that sample when unstressed 2

Irradiated Unstressed Graphite Changes • Dimensional • Modulus • Coefficient of Thermal Expansion -CTE 3

Irradiated Stressed Graphite Changes • Dimensional • Modulus • Coefficient of Thermal Expansion - CTE • Additional changes to CTE • Additional changes to modulus • Modified dimensional change rate 4

Irradiation Creep Rate (UK) • Proportional to stress • Inversely proportional to creep modulus γ = Irradiation dose • CREEP RATE = PRIMARY + SECONDARY d ε σ σ ( ) [ ( ) ] ( ) cr T exp b T = α − γ + β d E E γ c c 5

Coefficients • Primary and secondary ( ) T , b α ( ) T β • Secondary creep coefficient independent of temperature below 600 o C 6

Low Dose Creep • When expressed in elastic strain units E ε cr esu = c σ • Common law for tension and compression • Common creep law for all graphite types 7

16 14 12 Elastic Strain Units 10 8 6 4 2 0 0.E+00 2.E+21 4.E+21 6.E+21 8.E+21 Dose, EDND, n/cm 2 Pluto 300°C (Flux 4E13 n/cm2/s) BR2 300-650°C (Flux 3E14 n/cm2/s) Calder Hall 140-350°C (Flux ~1E12 n/cm2/s) UK Creep law 8

Creep Modulus E c • Unirradiated Young’s modulus • Creep rate not changed by pinning • However modified by: – Structural changes – Radiolytic oxidation • UK theoretical pinning / unpinning model appears to back this up 9

16 14 12 10 ESU 8 6 4 2 0 0.E+00 1.E+21 2.E+21 3.E+21 4.E+21 5.E+21 Dose, EDND n/cm2 Radiolytically oxidised sample 1 Radiolytically oxidised sample 2 Radiolytically oxidised sample 3 Unoxidised in compression Unoxidised pyrolytic Pre-irradiated to a high dose 10 UK Creep law

USA and Russian low dose data • Similar laws to the UK but has a different secondary creep temperature dependence 11

1.E-24 2 ) -1 (neutron/cm 2 ) -1 1.E-25 Creep Coefficient (Kg/cm 1.E-26 1.E-27 0 200 400 600 800 1000 Temperature °C Russian Graphite EGCR (American Graphite) CGB (American Graphite) 12

DRAGON Experience • Flux dependent creep coefficient (Verginga and Blackstone) exp ( Q / kT ) cont E φ 13

High Dose Creep • UK rule breaks down • Tension and Compression different • Kennedy, Cundy, Kliest σ � � K ε = � � crs E o � � V V ∆ � � K K 1 o ′ = − µ ( ) � V / V � ∆ o m � � 14

45 40 35 30 25 ESU 20 15 10 5 0 0 5 10 15 20 21 n.cm -2 EDN) Dose (x10 ATR-2E @ 300 °C (in tension) 300°C with x3.3 multiplier (in tension) ATR-2E @ 500 °C (in tension) 500°C with x3 multiplier (in tension) ATR-2E @ 900 °C (in tension) 900°C with x7 multiplier (in tension) ATR-2E @ 550 °C (in compression) 15

Creep strain, CTE and Dimensional change • By observation creep strain modifies CTE • However, dimensional change appears to be a function of CTE • Therefore creep strain should be expected to modify dimensional change • Kelly, Burchell model 1994 16

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Change in Modulus due to Creep Strain • For IG-110 graphite a 35% change in modulus for 0.23% creep strain - Oku -1998 18

Main Conclusions • Creep is important in the design of graphite components However: – High dose creep is not well understood – Interaction between creep strain, modulus, CTE and dimensional change is not well understood – Poisson's ratio and other lateral effects have not been well quantified – Need for a well characterised irradiation creep experiment 19