Fusion Engineering and Design 29 (1995) 428-434 ... a Department of Nuclear Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan.
Fusion Engineering and Design 29 (1995) 428-434
ELSEVIER
Radiation
Fusion Engineering and Design
damage of graphite: degradation of material parameters and defect structures
T. Tanabe a, T. Maruyama
b,‘, M. Iseki ‘, K. Niwase d, H. Atsumi e
a Department of Nuclear Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan ’ Research Laboratory for Nuclear Reactors. Tokyo Institute oj. Technology, 0-okayama, Meguro, Tokyo 152, Japan Department of Nuclear Engineering, Faculty of Engineering, Nagoya Univeristy. Furo-cho, Chikusa, Nagoya 464, Japan ’ Department of Physics, Hyogo University of Teacher Education, Yashiro-cho, Hyogo 673-14, Japan e Department of Nuclear Reactor Engineering, Kinki University. Kowakae, Higashi-Osaka 577, Japan
Abstract On the basis of our previous work, we have discussed the relation between damage structure and the degradation of the material parameters of neutron-irradirited graphites. The defects produced in a basal plane and/or in between the basal planes (in-plane or two-dimensional defects), which are appreciable in the early stage of irradiation, seem to play a critically important role both in the reduction in the thermal conductivity and increase in the hydrogen retention. They also cause a dimensional change through increase in lattice spacing between the basal planes of graphite. Although the in-plane defects are rather easily annealed out, there seems to be no way to avoid their production under neutron irradiation, particularly at low temperatures. After heavy irradiation, the defects grow into three-dimensional clusters, probably accompanying some sp*-to-sp3 transition. They play an important role in volume expansion and result in complete loss of the layered structure of graphite (amorphization), which is very difficult to anneal. Considering the annealing behaviors of the thermal conductivity, lattice constant and electrical resistivity, we propose a new model based on the sp*-to-sp3 transition that can explain the observed effect for both damage and annealing processes without any contradiction.
1. Introduction Radiation damage is the most important critical issue for application of graphite as a plasma facing material [I]. In previous work [2-41 it has been shown that the thermal conductivity of the neutron-irradiated graphite
’ Present address: Power Reactor and Nuclear Fuel Development Ibaraki
Cooperation, 31 l-13, Japan.
0-arai
Engineering
Center,
0-arai,
is reduced to around a tenth of the initial value after the irradiation of as small as 0.01 displacements per atom (dpa) [2], while hydrogen retention increases by several orders [3,4]. Both make the lifetime of the first-wall graphite very short and force frequent replacement. Therefore an improvement in the tolerance to neutron irradiation and the optimization of operation conditions are urgent tasks for the research and development of fusion graphite. Up to now, however, little has been known about the relation between the changes
0920-3796/95/$09.50 c 1995 Elsevier Science S.A. All rights reserved SSDlO920-3796(94)00303-3
T. Tunabe et al.
Table I Irradiation
conditions
of graphite
specimens
Fluence (E > 1 MeV) (neutrons m-*)
dpa
8.30 1.38 1.92 5.40 7.40
0.01 0.02 0.25 0.70 0.82
x x x x x
429
/ Fusion Engineering und Design 29 (1995) 428-434
in the Japan
Materials
Temperature
Testing
Reactor
Specimen
(K)
lo*’ lo= 10z4 IO24 1oz4
Number Density
