22.08.2015   Comprehensive nuclear materials

Effect of Radiolytic Weight Loss on Dimensional Change and Young’s Modulus

Dimensional change and modulus MTR data on small preoxidized samples, see Figure 51, appear to indi­cate that radiolytic weight loss would be expected, not only to increase dimensional change shrinkage and delay turnaround, but also to delay the structural

image464

Figure 50 Changes in Young’s modulus of Gilsocarbon due to fast neutron irradiation. Modified from Birch, M.; Brocklehurst, J. E. A review of the behaviour of graphite under the conditions appropriate for the protection of the first wall of a fusion reactor; UKAEA, ND-R-1434(S); 1987; Brocklehurst, J. E.; Kelly, B. T. Carbon 1993, 31(1), 155-178.

 

Подпись: Effect of Radiolytic Oxidation on Thermal Conductivity, Young’s Modulus, and Strengthincrease in Young’s modulus. This correlation has been used as the basis of models to predict dimen­sional change in radiolytically oxidized graphite.51

Подпись: 2Подпись: sПодпись: [48]Подпись:Подпись: [50]

4.11.17.2 Small Specimen Strength

Graphite is stronger in bend than in tension, and stronger in compression than in bend. Irradiated strength tends to be correlated with Young’s modulus. Graphite strength is significantly reduced by radio­lytic oxidation.

Losty and Orchard80 used the Griffith theory to try and demonstrate that the change in strength can be related to the square root of modulus as follows:

Failure stress s is proportional to the square root of the product of strain energy release rate g and Young’s modulus divided by the crack length c.

2gE

Thus, assuming that strain energy release rate, g, is not changed by irradiation and the critical crack,

s

E

_S0.

Eg.

Figure 52 was purported to support this relationship. However, statistical scrutiny of the data given in this figure revealed that there is not enough data to support the argument in favor of the square root law, or even a relationship to another power. There is a significant amount of data indicating that the relationship may be more appropriate as a direct relationship.81

Figure 53 illustrates that thermal conductivity, strength, and Young’s modulus are all significantly reduced by radiolytic oxidation.82 The data is usually fitted to a simple exponential decay83 of the form [P/P0] = exp(—lx). However, there must be a practi­cal ‘percolation limit’ to this law when all the porosity joins together and properties reduce to zero. This was recently highlighted in the statistical analysis of high weight loss PGA data by McNally eta/.84

4.11.19 The Use of the Product Rule

The product rule has been used for many years in the United Kingdom to combine changes due to fast neutron irradiation and radiolytic oxidation for strength, modulus, and thermal conductivity. Exam­ples are given below:

Strength

s

—

s

X

s

_S0.

.So.

irr

So.

oxidation

Young’s modulus

E

E

E

E

—

=

—

X

—

X

—

E0

E0

P

E0

S

E0

oxidation

Thermal conductivity

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Figure 51 Correlations between dimensional change and Young’s modulus structure term in Gilsocarbon. (a) Dimensional changes in pre-oxidized Gilsocarbon and (b) Young’s modulus structure terms in pre-oxidized Gilsocarbon. Modified from Schofield, P.; Brown, R. G.; Daniels, P. R. C.; Brocklehurst, J. E. Fast neutron damage in Heysham II/Torness moderator graphites (final report on 3-temperature zone rig); UKAEA, NRL-M-2176(S); 1991.