Dimensional changes in polycrystalline graphites under fast-neutron irradiation

Abstract

The changes in dimensions and linear thermal expansion coefficients of several polycrystalline graphites irradiated at 170 and 200 °C are presented. The results are compared with the changes in these parameters which were observed in very highly oriented pyrolytic polycrystalline graphite and were discussed in the preceding paper. The relation between dimensional changes occurring in polycrystalline aggregates and those in a monocrystal is discussed in terms of structural parameters Ax. The variation of these parameters with crystal strain is derived for the different graphites which have been investigated. The analysis shows that, at low crystal strains, c axis crystal growth is absorbed by porosity so that the bulk volume changes in the polycrystalline materials are less than those in a single crystal. However, as crystal strain closes the porosity, the rate of bulk volume change approaches that of the single crystal and eventually equals it. At still higher crystal strains nett porosity generation can occur so that the bulk expands at a faster rate than the single crystal. The results also show that the relationship between linear growth rate and the linear thermal expansion coefficient, which has been well established at low crystal strains, breaks down at high strains. However, the results have been used to develop a new model. This enables the crystal dimensional changes to be determined as a function of fast-neutron dose at any irradiation temperature, using bulk dimensional change data from experiments on pile grade A graphite. The crystal strains determined from the new model can be used to predict the dimensional changes of other graphites at other irradiation temperatures from their behaviour at 200 °C.

Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)

“…The bulk volume changes in polycrystalline graphite during irradiation, on other hand, are less than those in a single crystal at low crystal strains because the c -axis crystal growth is absorbed by accommodation porosity. Once the crystal strain closes the porosity, the bulk volume change approaches that of the single crystal (HOPG) and eventually equals it [2].…”

“…Intensive efforts have been made to measure the irradiation induced dimensional changes of graphite and to understand the mechanism of these changes [1][2][3]. Because naturally occurring single crystals of graphite are too small for use in the measurements of irradiation induced dimensional change, highly oriented pyrolytic graphite (HOPG) has been used as an approximant of single crystal graphite [1,4].…”

Advantages of natural microcrystalline graphite filler over petroleum coke in isotropic graphite preparation

“…Nuclear graphite components are considered to reach their lifetime when the dimensional change in either the against-grain or with-grain direction returns to zero. For anisotropic graphite such as PGA graphite [22,26], the turnaround occurs at a much lower dose in the againstgrain direction, resulting in a short lifetime for the graphite. In contrast, isotropic graphite (such as Gilsocarbon graphite) shows similar dimensional stability in both directions [22,27], thereby giving the graphite a much longer lifetime.…”

Theory of radiation-induced shape-change of graphite

“…In terms of the principal axes, only the diagonal components ε α = ε αα and P α = -σ αα of the deformation and radiation effects tensors are different from zero, and the elastic energy becomes (6) Here K is the bulk modulus, and µ α is the shear modulus in the direction α. In terms of the principal axes, only the diagonal components ε α = ε αα and P α = -σ αα of the deformation and radiation effects tensors are different from zero, and the elastic energy becomes (6) Here K is the bulk modulus, and µ α is the shear modulus in the direction α.…”

“…Since the elastic stresses can be transmitted only through direct contacts with neighboring crystallites, only such contacts contribute to the last term in expression (6). According to the lattice model, the internal stresses P α in Eq.…”