RRFM 2009 Transactions - European Nuclear Society

of the crack of pillowing). It showed a
different distribution of pores over
sizes as compared to #135 and #73
(Fig. 1.b). The difference is in two
maxima available there, which are an
evident proof of the presence of two
poly-dispersed systems of pores in the
fuel particles of specimen #74-1. The
first maximum corresponds to the
pores quantity of 4.86·10 8 mm -3 and is
related to a pore size of ~280 nm. The
second maximum is related to size
445 nm and equals to 4.19·10 8 mm -3 .
However, a volumetric distribution of
size group pores has one maximum on
its curve and it is located close to the
maximum of specimen # 73 (Fig. 2a).
A pore size in specimen #74-1
changes in a manner similar to that of
#73 but it changes in a wider range,
i.e. from 180 nm to 1400 nm. At the
same time a mean size of pores in
specimen #74-1 equals to 415 nm and
is ~10 % less than that of #73. A
specific pore volume in fuel particles of
#74-1 is 0.12 mm 3 /mm 3 , which is
~10 % more than that of #73. Besides,
in fuel particles of specimen #74-1 the
concentration is even higher, it is equal
to 2.11·10 9 mm -3 , which is ~1.4 times
more than that in specimen #73.
Depending on the rate of attaining
burn up the average speed of the
porosity development in fuel particles
of #74-1 equals to ΔV GFP /ΔBu=
=12.0/96.4≈0.12 % per 1 % burn up,
which is by ~10 % higher than that of
specimens #135 and #73 and is likely
to be within the uncertainty limits of
both the determination of a pore
volume and the calculation of a burn
up value.
Specimen #74-2 was cut out of a
swelling area to study its three regions
i.e. the first was at a distance of
~0.5 mm from the crack, the second
was next to the crack, at a distance of
~300 µm, and the third was directly at
the crack mouth, at a distance of
~30 µm. The dependence of the pore
quantity and volumetric distribution on
the size is shown in Figures 1b and 2b.
Fig. 1. Size distribution of gas pores in the areas of
normal swelling (а) and near the pillowing (b
Fig. 2 Volume distribution of gas pores in the areas
of normal swelling (а) and near the pillowing (b)
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