Fusion Engineering and Design 49–50 (2000) 383–388

Neutron effects on properties and annealingof low-Z materials

C.H. Wu a,*, J.P. Bonal b, H. Werle c

a The NET Team, Max-Planck Institut fur Plasmaphysik, D-85748 Garching, Germanyb Centre d’Etudes Nucleaires de Saclay, Laboratorire d’Etudes des Materiaux Absorbants, 91191 Gif-sur-Y6ette Cedex, France

c Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany

Abstract

Among the low-Z materials, carbon and beryllium are seriously considered as plasma facing materials (PFC) forthe next step fusion experimental reactor, because of their wide experiences as first wall and divertor plate protectionin present tokamaks. In addition, their excellent plasma performance has been demonstrated. Carbon based materialshave been chosen for protection of high heat flux components, in particular to off-normal operation, e.g. disruption,plasma excursion, whilst beryllium has been proposed as a PFC material for getting oxygen impurity in plasma.However, as next generation D/T plasma devices, i.e. International thermonuclear experimental reactor (ITER)producing intense neutron fluxes, substantial R&D is needed to elucidate the effects of neutron-induced damage onthe microstructure and critical properties of these materials (e.g. thermal conductivity, swelling, and tritium trapping)because they could limit the use of these materials in next generation fusion devices. Neutron induced changes inthermal conductivity, dimensional stability, mechanical properties as well as behaviour of tritium interaction arecrucial problems which need to be better understood. The assessed neutron flux of ITER will be �3.5–9.0×1014

cm−2 s−1 for the first wall, whilst the neutron flux for the divertor is �1–3×1014 cm−2 s−1, which leads to adamage of �10–20 dpa for the first wall and 3–6 dpa for the divertor for 1 full power year of operation. In theframework of European Fusion R&D Programs, an extensive effort on neutron effects of PFC materials is beingundertaken. This paper presents the first results of experiments performed to investigate the effects of thermalannealing on neutron damage of thermal conductivity and tritium inventory of various carbon based materials. Theconsequences are discussed. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Tokamak; Plasma; Beryllium

www.elsevier.com/locate/fusengdes

1. Introduction

Carbon-based materials and beryllium are con-sidered as attractive choices for the plasma facingcomponents (PFC), because of their low atomicnumber which leads to favorable plasma compati-bility. Carbon-based materials pose excellent ther-

* Corresponding author. Tel.: +49-89-32994232; fax: +49-89-32994198.

E-mail address: wuc@sat.rzg.mpg.de (C.H. Wu).

0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

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C.H. Wu et al. / Fusion Engineering and Design 49–50 (2000) 383–388384

mal shock resistance and are therefore indispens-able for high heat flux components in forthcomingfusion devices. The substantial advantage ofberyllium is the high capability of oxygen getter-ing. Beryllium is therefore considered to be usedin the forthcoming devices to control the oxygenimpurity in the plasma to the lowest level possi-ble. However, as next generation devices, i.e. theInternational thermonuclear experimental reactor(ITER) will produce intense neutron fluxes, sub-stantial R&D is needed to elucidate the effects ofneutron-induced damage on the microstructureand critical properties of these materials, e.g. ther-mal conductivity, swelling and tritium trapping,because they could limit the utilization of thesematerials in fusion devices. The crucial problemswhich need to be better understood is the be-haviour of thermal–mechanical properties,swelling and tritium retention in materials sub-jected crystallo-graphic damage and microstruc-tural changes induced by neutrons.

This paper presents the results of EuropeanFusion R&D on neutron effects of PFC’s. Theneutron induced changes in the thermal–mechani-

cal properties, tritium retention, as well as anneal-ing investigation as a function of temperature.

2. Results and discussion

2.1. Thermal property

2.1.1. Materials and irradiationTwo CFCs (Table 1) have been investigated:

A05 is a reference material which has been irradi-ated in the temperature range 400–1000°C and inthe damage range 0.4–1.9 dpa.g. MKC is a veryhigh conductivity CFC which has been irradiatedin the conditions 430°C/0.85 dpa.g.� A05: 2D felt-type CFC was supplied by Le

Carbone Lorraine. The elementary carbonfibers made from polyacrylonitrile (PAN) pre-cursor, are randomly distributed in a plane.The volume fraction of fibers is 25%. This feltundergoes a needling which orientates a frac-tion of the fibers in the direction perpendicularto the plane. The matrix densification is carriedout by chemical vapor deposition of pyrocar-

Table 1Thermal properties of the CFC’s before irradiation

Materials MKCTemperature (°C) A05

(//)a (Þ)b

1.81Density (g/cm3) 1.97

25 618Thermal conductivity (W/m K) 86324400 178 52 358600 139 42 290

252361188001000 105 32

400Average linear TEC (20-T°C) (10−6/K) 0.26 8.17 −0.668.430.56 −0.44600

800 0.79 8.61 −0.25−0.101000 0.95 8.73

25 665 665 669Specific heat capacity (J/kg K)400 1426 1426 1427600 158614751575800 1670 1670 1688

1000 1765 1765 –

a //, parallel to the fiber plane.b Þ, perpendicular to the fiber plane.

C.H. Wu et al. / Fusion Engineering and Design 49–50 (2000) 383–388 385

bon, followed by a pitch impregnation. Thefinal heat treatment of graphitization is per-formed at �2500°C.

� MKC: 1D CFC fabricated by Mitsubishi Ka-gaku. From its pitch based K 321 fibers; thefiber volume fraction is 60%

2.1.2. Determination of the thermal conducti6ityThe thermal conductivity of the irradiated sam-

ple (Ki) is determined before the thermal anneal-ing using the relation:

Ki=Di×ri×Cpi

where Di is thermal diffusivity of the irradiatedsample, ri the measured density of the irradiatedsample, and Cpi is the heat capacity of the irradi-ated material.

For A05, the evaluation of ri versus tempera-ture is performed using the following equation:

ri(T°C)=ri(25°)

1+ [(2ai(//)+ai(Þ))× (T°C-25°C)]

where ai(//) is the average thermal expansion co-efficient (25°C–T°C) of the irradiated material inthe parallel direction to the fibers plane and ai(Þ)is the same coefficient in the perpendicular direc-tion to the fibers plane. For MKC CFC, theevaluation of ri versus temperature is given by thefollowing equation:

ri(T°C)=ri(25°)

1+ [(ai(//)+2ai(Þ))× (T°C-25°C)]

The thermal diffusivity of the irradiated sample(Di) is obtained with Parker’s method using alaser pulse. The samples are discs (diameter:12mm; thickness: 5 mm). The method consists ofilluminating the front face of these discs with ashort heat pulse. The thermal diffusivity D isdeduced from the heat equation applied to thethermal transient of the rear face, namely a ther-mogram. The analysis of the thermograms ac-cording to Parker’s method allow one to solve theequation restricted to one dimension, in the totalabsence of heat exchange between the sample andthe furnace atmosphere, and to deduce the follow-ing result:

D=gL2

t1/2

where L is the thickness of the sample, t1/2 themoment when the temperature rise is half of themaximum, and g=0.13878. The thermal conduc-tivity after annealing (Ka) is calculated by therelation:

Ka=Da×ra×Cpa

where Da is thermal diffusivity of the irradiatedsample after annealing, ra is the measured densityof the irradiated sample after annealing. Aftereach annealing temperature, the density of thesample ra(25°C) is measured. For A05, the evalu-ation of ra versus temperature is performed usingthe following equation:

ra(T°C)=ra(25°)

1+ [(2ai(//)+ai(Þ))× (T°C-25°C)]

where Cpa is the heat capacity of the irradiatedsample after annealing. We have made the as-sumption that the heat capacity recovers at thesame rate as the thermal diffusivity. The heatcapacity after annealing is then given by therelation:

Cpa=Cp0+�DCp×

�D0−Da

D0−Di

�n=Cp0+DCp · d

with DCp=Cpi−Cp0 and d=D0−Da

D0−Di

.

We assume that: d=0 when Da\D0; and d=1when DaBDi.

2.1.3. Annealing of thermal conducti6ityThe irradiated samples are annealed in a ‘PY-

ROX’ oven. The annealing experiments are car-ried out under vacuum (10−4 mbar) up to2000°C, and with argon gas for the heating at2100°C. In order to determine the time necessaryto anneal the defects and recover the thermalconductivity at a given annealing temperature,isothermal annealings have been carried out dur-ing different annealing times. For example, a A05sample irradiated at 385°C/0.39 dpa.g, has beenannealed at 1200°C during a cumulative time of10, 30, 60 and 120 min, then the same sample hasbeen annealed at 2000°C during a cumulative time

C.H. Wu et al. / Fusion Engineering and Design 49–50 (2000) 383–388386

Table 2Isochronal annealing conditions

Annealing temperatures (°C)Materials Annealing time (h)Irradiation conditions

A05 500–2100430°C 1Step: 1000.85 dpa.g

700–200A05 1625°CStep: 1001.80 dpa.g

900–1000820°C 1A05Step: 1001.85 dpa.g

1100–2100A05 11005°CStep: 1001.85 dpa.g

500–2000MKC 1430°CStep: 1000.85 dpa.g

Table 3Coefficients of the curves (Ka)Tirr= f(T)

A (W/m K) C (W/m K) W(°C) Ki(W/m K)Irradiated materials Function

112.5 1231.4A05 (Tirr: 430°C) 171.8(Ka)400°C= f(T) 66.8A05 (Tirr: 625°C) (Ka)600°C= f(T) 64.3 1200.4 173.1 65.2

31.8 1590.7 268.5 87.7A05 (Tirr: 820°C) (Ka)800°C= f(T)33.8 1808.9(Ka)1000°C= f(T) 294.2A05 (Tirr:1005°C 73.4

262.5MKC (Tirr: 430°C) 1351(Ka)400°C= f(T) 216.3 112.5

of 5, 20, 50 min 1, 2 and 5 h, respectively. Theannealing temperature of 1200°C is associatedwith the annihilation of large dislocation loops ordefect clusters [1]; the annealing temperature of2000°C recovers the CFC’s initial thermal conduc-tivity after irradiation at 0.1 dpa.g/400°C [2].These two isothermal annealing experiments at1200 and 2000°C show that the annealing of thesample during 2 or 5 h does not improve thethermal conductivity. A total of 1 h of annealingtime is sufficient to recover the maximum of thethermal conductivity at a given temperature.Therefore, we have chosen the 1 h annealing timefor the annealing experiments (Table 2). For A05irradiated at 430°C/0.85 dpa.g, the thermal con-ductivity at 400°C after annealing ((Ka)400°C)does not change for annealing temperatures rang-ing between 400 and 800°C. At 900°C, (Ka)400°C

slightly increases, then a large increase occurs at1200°C due to the annihilation of large disloca-tion loops and defects clusters. The thermal con-

ductivity recovers its unirradiated value ((K0)400°C)at an annealing temperature of 1700°C. For thedifferent isochronal annealing experiments, thethermal conductivity (K0) curves versus annealingtemperature (T) can be fitted with a sigmoidalcurve (Table 3) whose equation is:

Ka=A

1+�

exp−�T−C

W�n+Ki

For all the investigated samples, the normalizedthermal conductivity at the irradiation tempera-ture (Ka/K0)T irr

becomes very close to 1 afterannealing at 2000 or 2100°C, respectively (Table4).

This means that major fraction of the defectshas been annealed at 2000/2100°C. However, thenormalized thermal conductivity at 25°C (Ka/K0)25°C reaches 1 only for the case: A05 irradiatedat 430°C/0.85 dpa.g. For A05 irradiated at 625,820, 1005°C and at high dose of 1.85 dpa.g., the

C.H. Wu et al. / Fusion Engineering and Design 49–50 (2000) 383–388 387

Table 4Normalized thermal conductivities at the irradiation temperature and at 25°C after annealing

Irradiation temperature (°C)Materials Annealing temperature (Ka/K0)T irrafter (Ka/K0)25°C after annealing

annealing(°C)

A05 2100430 1.02 0.96A05 625 2000 0.95 0.76

2000 0.98820 0.62A051005A05 2100 0.94 0.71

MKC 430 2000 0.99 0.89

ratio (Ka/K0)25°C ranges between 0.62 and 0.76;this means that all the defects are not yet com-pletely annealed at 2000/2100°C. This is likely dueto the substantial mobility of vacancies during theirradiation at 800/1000°C leading to defect clus-ters or large dislocation loops which are moredifficult to anneal and require annealing tempera-ture higher than 2000°C. In previous work [3], wehave shown that the ratio (Ki/K0)T irr

always in-creases with increasing irradiation temperaturebetween 400 and 1000°C. It is beneficial to irradi-ate CFC’s at relatively high temperature (\800°C). However, it is less effective to anneal thedefects at 2000/2100°C for irradiation at 1000°Cthan that for irradiation at 400°C.

2.2. Neutron effect on tritium retention

2.2.1. Influence of neutron damage on the tritiumretention in carbon

Before loading, the unirradiated specimens wereconditioned for 1 h at 850°C under vacuum toremove air and moisture. For the irradiated speci-mens, the conditioning temperature was adaptedto the irradiation temperature. The irradiatedsamples were loaded with tritium in a H2+5 ppmT2 atmosphere (2 bar, 1000°C, 6 h).

After loading, the surface adsorbed tritium wasremoved by purging at room temperature.Amount of the loaded tritium were determined bypurging with He+0.1% H2 and heating the sam-ples with �16°C/min up to 1100°C. Tritium re-leased as HTO is transformed to HT by a heatedZn-bed (�390°C) to avoid water adsorption. Thetritium activity of the purge gas is measured withproportional counter. After correction for back-ground, the count rate of the proportional coun-

ter is used to determine the rate (Bq/s) and thetotal amount of tritium released from the samples.To determine the total retained tritium the sam-ples were normally afterwards oxidized in oxygenat 850°C. The tritium trapped in the bubblers wasdetermined by liquid scintillation counting. Theexperimental procedure has been described in de-tail earlier [4]. The systematic investigation ofneutron effects has been carried out for the CFCmaterials AO5, N112, and CX2002, those wereirradiated at doses between 0.36 to 1.8 dpa andirradiation temperature at 385–1035°C.

The tritium retention of various CFC’s, afterirradiation at different doses and temperatures isshown in Fig. 1. It is seen, that the tritium reten-tion increases with doses. However, the tritiumretention reaches the maximum value at �0.3dpa. Above �0.3 dpa, the retention seems to beessentially independent of the doses. This confi-rmed the previous results.

The results also show, that at same doses, thetritium retention decreases with increasing irradia-tion temperatures.

Fig. 1. Tritium retention as a function of neutron damage andirradiation temperature.

C.H. Wu et al. / Fusion Engineering and Design 49–50 (2000) 383–388388

2.2.2. Thermal annealing effect on tritiumretention

To investigate the thermal annealing effect onthe neutron induced tritium trap site. variousirradiated samples: AO5, N112 and CX2002 wereheated at 1300°C for 2 h and than the tritiumretention is determined. For all three CFC’s, theamount of loaded tritium decreased by approxi-mately a factor of three after the 1300°C/2 hheating.

3. Conclusion

Isothermal annealing at 1200 and 2000 of A05irradiated at 385°C/0.4 dpa.g show that the ther-mal property has been effectively annealed after 1h.

The results show that by annealing at 2000–2100°C of irradiated CFC’s: irradiation tempera-ture 400–1000°C and 0.4/1.9 dpa.g, the thermalconductivity has almost been recovered. However,

not all of the defects are annealed, because theratio is still B1, except for A05 irradiated at400°C/0.85 dpa.g.

The recovery of the thermal conductivity byisochronal thermal annealing is almost the samefor standard CFC A05 and for very high thermalconductivity CFC MKC.

Tritium retention increases with increasing neu-tron damage. However, the tritium retentionreaches the maximum value at �0.3 dpa.

It seems that at temperature above 800°C, an-nealing of tritium traps take place. At 1300°C,�70% of the tritium traps are annealed within 2h.

References

[1] T. Maruyama, M. Harayama, J. Nucl. Mater. 195 (1992)44–50.

[2] C.H. Wu, J.P. Bonal, B. Thiele, J. Nucl. Mater. 212–215(1994) 1168–1173.

[3] J.P. Bonal, C.H. Wu, J. Nucl. Mater. 228 (1996) 155–161.[4] H. Kwast, H. Werle, C.H. Wu, Phys. Scripta 64 (1996) 41.