Qualification of selected graphites for a

future HTR Mike Davies, AMEC Nuclear UK Technical Meeting on High-Temperature Qualification of High Temperature Gas-Cooled Reactor Materials,

10-13 June, IAEA Vienna

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Content

Qualification envelope for a High Temperature Reactor (HTR) and a Very

High Temperature Reactor (VHTR) – peak fluence and temperature range

Fluence range of the INNOGRAPH experiments

Sample shapes/sizes/orientations

Pre- and post-irradiation measurements and spreadsheets developed

Comparisons of the behaviour of the different graphites at 750oC and 950oC

in terms of dimensional change, dynamic Young’s modulus (DYM), coefficient

of thermal expansion (CTE) and thermal conductivity

Summary of observations on the graphite behaviour

Further planned work/experiments

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Qualification envelope for a HTR (1)

750700650600 950900850800

Graphite temperature (deg C)

Fluence

550 1000

Graphite temperature range of interest

Peak fluence

position

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Qualification envelope for a HTR (2)

Decided on an initial irradiation temperature of 750oC (corresponding to the

that at the peak fluence) and a peak fluence of 25 dpa (~200 x 1020 n/cm2

Equivalent DIDO Nickel Dose)

Decided that the irradiation to the peak fluence should be done in two

stages, the first stage to ~1/3 peak, and the second stage to ~2/3 peak.

~50% of the samples from the first stage would be transferred to the second

stage after full PIE, so they would see up to the peak fluence. Flux buckling

gives a range a fluences (65-100%) for each stage so that the ‘full curves’

could be reasonably well defined

First stage of the irradiation to ~1/3 peak fluence was initiated in the 5th

Framework Programme (FP5), and was referred to as INNOGRAPH 1A

Limited data was also obtained at a lower temperature of 650oC (due to one

capsule running at this lower temperature)

In FP6 (RAPHAEL), a decision was made to aim for a Very High

Temperature Reactor, with a higher gas outlet temperature and

consequently higher graphite temperatures (up by 50 – 100oC)

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Qualification envelope for a VHTR (1)

750700650600 950900850800 1050

Graphite temperature (deg C)

Fluence

HTR VHTR

550 1000

Graphite temperature range of interest for HTR

Graphite temperature range of interest for VHTR

Peak fluence

positions

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Qualification envelope for a VHTR (2)

Another two stage experiment was therefore undertaken at 950oC, but to a

lower peak fluence of 16 dpa, as shrinkage ‘turnaround’ and ‘cross-over’ to

positive growth were expected to happen much earlier than at 750oC

One capsule was again run at a lower temperature of 850oC (to provide a

limited amount of data at the intermediate temperature)

The second stage of the 750oC experiment (INNOGRAPH 1B) and the two

stages of the 950oC experiment (INNOGRAPH 2A and 2B) were all completed

in FP6

These three experiments also included two vibro-moulded graphites as they

were being considered for PBMR and ANTARES

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Selected graphites

GrafTech PCEA Petroleum Extrusion

PPEA Pitch Extrusion

PCIB-SFG Petroleum Iso-moulding

BAN Needle Extrusion

SGL NBG-10 Pitch Extrusion

NBG-20 Petroleum Extrusion

NBG-25 Petroleum Iso-moulding

NBG-17 Pitch Vibro-moulding

NBG-18 Pitch Vibro-moulding

Toyo Tanso IG-110 Petroleum Iso-moulding

IG-430 Pitch Iso-moulding

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Fluence range for each INNOGRAPH experiment

INNOGRAPH-1A: 750oC, medium fluence

INNOGRAPH-1B: 750oC, high fluence

INNOGRAPH-2A: 750oC, medium fluence

INNOGRAPH-2B: 750oC, high fluence

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Materials test reactor (Petten, Holland)

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Samples used in irradiation experiments

- Eight drums with three channels each

- Specimen width 8 mm

- Specimen height 6 and 12 mm

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Sample shapes/sizes/orientations (1)

Samples have a machined flat parallel to length axis (shown exaggerated in

left figure)

Sample identification is engraved on the flat

Great care has to be taken when assessing the anisotropy of the graphite i.e.

the differences between the with-grain (WG) and against-grain (AG)

directions.

Z axis of specimens (perpendicular to flat) is carefully aligned to correspond

with the major WG and AG directions

7.7 mm

8.0

mm

3.0

mm

z

z

xy

L

x

D

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Sample shapes/sizes/orientations (2)

WG

AG

G AG

x (WG)

l (AG) d (AG+WG)

Extrusion

WG

AG

WG

Iso-moulding

l (WG)

d (AG) x(AG)

WG sample

AG sample

x (AG)

l (WG) d (AG+WG)

l (AG)

d (WG) x(WG)

WG sample

AG sample

Samples are defined

according to orientation of

main axis (i.e. L direction).

D and x could be both WG,

or both AG or one of each

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Pre- and post-irradiation measurements and

spreadsheets developed

Pre- and post-irradiation measurements concentrated on dimensions (‘L’, ‘D’

and ‘x’) and mass (from which volume and density were obtained), as well as

dynamic Young’s modulus (DYM), coefficient of thermal expansion (CTE) and

thermal diffusivity (from which thermal conductivity is obtained)

NRG developed a single spreadsheet to capture all the measurements and

their uncertainties, along with the calculated values, and these data have been

presented at various INGSMs

In parallel, AMEC developed a spreadsheet for each graphite which determines

the property changes at each temperature due to the irradiation (expressed

either as a fractional change or a percentage change).

Curves are fitted through these data for each property variation, differentiating

between WG and AG directions where appropriate

Aim was to allow selection of the ‘best’ graphites and to provide design data

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Comparison of dimensional change – 750oC (1)

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Comparison of dimensional change – 750oC (2)

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Comparison of dimensional change – 750oC (3)

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Comparison of dimensional change – 950oC (1)

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Comparison of dimensional change – 950oC (2)

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Comparison of dimensional change – 950oC (3)

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Comparison of extruded graphites at 750 and 950oC

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Comparison of moulded graphites at 750 and 950oC

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Comparison of DYM changes

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Comparison of CTE changes - 750oC

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Comparison of CTE changes - 950oC

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Comparison of thermal conductivity changes - 750oC

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Comparison of thermal conductivity changes - 950oC

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Summary of observations (1)

Despite being classed as ‘isotropic’ or ‘near-isotropic’ grades (based on the ratio

of CTE in the perpendicular and parallel directions), all graphites tested

exhibited significant anisotropic behaviour with respect to dimensional change

For all graphites tested, the maximum shrinkage and fluence to both turnaround

and cross-over are all less for the AG direction

For 750oC, the cross-over for WG direction is in the range 15 to 22 dpa, and for

the AG direction is in the range 9 to 19 dpa

For 950oC, the cross-over for the WG direction is in the range 9 to 11 dpa, and

for the AG direction is in the range 7 to 10 dpa

For 750oC, the maximum volume change is in the range 7 to 19 % at 23 dpa

For 950oC, the maximum volume change is in the range 17 to 33 % at 14 dpa

For extruded graphites the peak shrinkage is the same but occurs at a much

lower fluence at 950oC compared to 750oC i.e. the initial shrinkage rate is higher

For moulded graphites the initial shrinkage rate is the same at 750oC and 950oC

but the graphites continue to shrink more at 750oC than at 950oC

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Summary of observations (2)

Moulded graphites generally shrink less, turn around at a lower fluence and

cross over to positive growth at a lower fluence than extruded graphites

The ‘same’ graphite produced by two different manufacturers shows very

similar dimensional change behaviour at both 750oC and 950oC

The scatter on most of the linear dimensional change data is generally higher

than for the volume change

At 750oC, the initial DYM increase for extruded graphites is by a factor of ~1.4

with the maximum increase being by a factor of ~2.8, whereas for moulded

graphites the factors are larger at ~1.8 and 3.4 respectively

At 950oC, the factors for the extruded and moulded graphites are similar to

the corresponding values at 750oC, but the changes occur more rapidly

At 750oC, the CTE variation for extruded and moulded graphites are similar

with an initial increase followed by a decrease to a plateau

At 950oC, the trend is initially similar to that at 750oC, but there is a noticeable

increase at higher fluence

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Summary of observations (3)

At 750oC the expected large initial decrease in thermal conductivity is

observed followed a plateau

The largest change occurring at the lower measurement temperatures.

The initial large spread in thermal conductivity over the temperature range

reduces considerably when the plateau is reached.

There is a further reduction at higher fluence and this appears to be

happening earlier for the moulded graphites.

The thermal conductivity behaviour at 950oC is similar to that at 750oC

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Further planned work/experiments

7th Framework Programme, ARCHER, finishes in February 2015

Some strength tests have been performed and the results will be used to

establish the variation with fluence and temperature, and to see how this

relates to the DYM variation – square root relationship, linear, other

Since the measurements are related to individual samples, an assessment

will be carried out to look for any correlations e.g. between DYM and

dimensional change

An irradiation experiment at 750oC to a low fluence (1-2 dpa) has already

been completed and will be used to better define the early part of the curves

for specific properties – dimensional change, DYM, CTE, thermal conductivity

The ‘best’ graphites for future HTRs will be identified

Property variation ‘curves’ for core design purposes will be produced