Neutron:rradiation Effects in Graphite for Fusion Reactor App:ications‐Hydrogen Absorption,Diffusion and Annea:ing Effectsロ

marerlals rOr ρノasma racゎ g wa//s,n a rtysノ Oη reacrο r ′f,sascerra′ ηed′ ηめ,s stυ dyめ arfwο 々ゎds OFカ ノdroger fraρ ρわgslres eχ lsr a/7d be addrrlora1/y ρ″οdt/Cecy dt/r1/7g 77eυ frOn′″adrar10r7 Tわ e Crearlο /7 ar7d aη ηea′ ′ηg ο′ヵりese rν γοrraρ ρlr7gs〃 es s/7ο w 9υ lre d′ rrerer7f be/7aソ ′0̀

Graphte and carbon lber7carbon compostes(CFCs)are︵０ 工 ¨・０一×︶⊆ｏ５⊂①´①に ＣＯ Ｏ﹂０ゝエBυ ノた /7/drOgeη rere/7f10/7 and absο ″ρl10η k1/7etrCS わa1/ebeer7 Sft/dled Oη graρ わ′re, 14//7′ C/71S Oη e ο
rr力 e cand,dare□Trap 2(C″ stJIte edge surface)
J cLster boリ□Trap l onters● ■輌¨tt樵

principal rnaterials for current fusion experimental devices

Annea‖ ng Effect(a)Change Of Hydrogen Retent on

due to high therrnal conductivity and therrnal shock resist vity

:詰 1胤 詰胤 12i3K● おK おK1873K
馳0常 ‖Hovvever,these mater als degrade such various advantages

Fig 2 Change of hydrogen retention and the pre―by irradiation of neutrons originated from the fusion reaction

effect ln neutron― irradiated graphite shown for each trappingof deuterium and trtium ヽloreover, since carbon has aSle(abSOrplon temperature:1273K,Equllibrium pressurel〜

chemical affinity to hydrogen, it may cause high tritium
inventory in plasma facing vva‖ s and a large amount offuel

10 kPa)gas recycling betvveen fusion plasma and wa‖ materials tthesites for hydrogenauthors also have found out that neutron irradiation

annealing

Graphite and CFC samples were irradiated in the」

apan

significantly raised hydrogen retention in graphite in previous

Matenals Tesln9 Reactor(」 MTR)at vanOus luences up tostudles From the vlevypoint of plasma density control andreducing the radioactive tritium lnventory in plasma facing
components,it ls important to clarify the hydrogen behavior

54x1024n/m2(>l MeV),WhiCh COrresponded to 0 65 dpa,

in these materials

different neutron fluences in Fig 2(a)The tOtal retention

Fi9 1 shOWS a rnodel of hydrogen trappin9 and transport
in graphite proposed bythe authors ln an absorption process,fluences below o oll dpa,the increase is mainly due to thehydrogen molecules penetrate easly through open pores

increase oftrapping in trap 2,while trap l retention ls clearlyand grain boundaries of graphite filler 9rains and reach thesurface ofthe grain Then,hydrogen vvi‖ migrate into the filler

grain apparently as molecule (ln praCtiCe, sequence ofdissociation and recombination)cOntrO‖ ed by the diffusion
process vvith an activation energy of 1 3 eV The hydrogen
be trapped at outside edge surface of crystallites by acovalent bond with the adsorption enthalpy of 2 6 eV tthis

belovv 473 K Hydrogen retention in neutron― irradiatedgraphite determined for each trapplng site is shown forincreases with the irradiation fluence At lower neutronincreased at higherfuences above 0 017 dpa Atthe hi9hest
fluence of 0 65 dpa,the total retention became 6 3 tirnes as
lar9e as that for 0 047dpa, and the retention consisted ofnearly the same quantty oftrap l and trap 2 retention

The retention obtained for the samples vvhich vvere

site has been named as trap 2 1f the trap 2 sites areannealed before hydrogen absorption is shown in Fig 2(b)The retention which correspOnds to trap 2 tends to decrease
with the increase of pre― annealing temperature Thisindicatessufficiently fulfilled with hydrogen atoms, hydrogen canthat most ofthe trap 2 sites induced by neutron lrradiation wi‖

perrneate into the crysta‖ ite along the graphite intercalations

be annealed out at high temperatures However,the trap 2

The hydrogen absorption,or atrnospheric hydrogen pressure
decrease,in this stage is contro‖ ed by first― ordered chemicalvvas sti‖wi‖

sites rnay not be completely annealed out,since the retention

reaction, which caused by dissociation at a surface orlargerthan the amount obtained forthe unirradiatedsamples, which vvas also pre― anneaied at 1873 K On thedetrapplng from trap 2 inside a crysta‖ ite,there are interstitial

other hand,retention in trap l tends to remain relatively highciuster loops or zigzag structures, and they should be theother trapping site for hydrogen(trap l)丁 he trap binding
energy for hydrogen vvas estimated to be 4 4 eV ln the caseof unirradiated samples,the number oftrap l is below 10,6

even afterthe heattreatment at 1873 K One can concludeof the total number of trapping sites,and trap 2 is the rnalor

irradiated graphite and their pre―the trap l sites should be therrna‖ y more stable than trap 2
Although tis not mentioned in the article,hydrogen diffusion
coellicients and reaction rate constants at trap 2 in neutron―

annealing effects have been

investigated tthese results are described in refs[ll and[2]H2H2Referencesill H Atsul))A N71u la l、 I )T Tal)(lbl̲・ ′lncl T SI]kanlaヽ
Nに ,cl 1/1nler 386‑388 379(2009)
121 H Aistin〕

T Tを

390‑391 581(2009,'nalDe anCl T Sh kanna」」 Ntc ,、 latol

Key Words
FtlsiOln Rcε lctor lN/atel als Grap1lto rladiat orn EIiects

Contact toHisao Atsumi(Department of Electnc and Electlcal●Hydrogen atom Trap 2 Site ttrap l Site

Engineer ng, Faculty of Sclence and En9ineering, Kinki∪niversly)e―

Fi9 1 Schematic i‖ ustration of hydrogen trapplng sites andhydrogen transpOrtin a graphite materialイθ

ma i atsumi@ele kindal ac ip

丁atsuo Sh kama(Nuclear Matenals Sclence D vislon)e―ma ish kama@imrtohoku acJp

KIヽ OKEヽO Res.・ olch

H19b 9hls 2009