RRFM 2009 Transactions - European Nuclear Society

The distribution of gas pores of fission products in the fuel particles was studied with the
scanning electron microscope with magnification х3000. The distribution of the gas pores in
the fuel particles volume was analyzed by the stereometric metallography methods,
developed by S.A. Saltykov [3].
3. Results and Discussion
The dependence of the volumetric distribution of pores quantity in fuel particles on a pore
size is shown in Figure 1.
The fuel particles of specimen #135 with Bu=49.5 % have pores of the smallest size and their
diameters are within 90 to 700 nm (Fig. 1а). A mean diameter of these pores is equal to 255
nm. The dependence of the distribution of pores over sizes has one maximum, which
corresponds to a diameter of ~180 nm and equals to 1.53·10 9 mm -3 . It means that the entire
pore amount in the fuel particles of specimen #135 makes one poly-dispersed system
characterized by function N i =f(D i ), which is close to the normal Gauss distribution. In the fuel
particles of specimen #135 the concentration of pores is equal to 4.70·10 9 mm -3 , and the total
pore volume is 5.57·10 -2 mm 3 , which corresponds to a volume portion of pores of 5.57 %. In
the fuel particles of specimen #135 a maximum volume of pores is provided by sizes 350 and
455 nm making a portion of 22.9 and 22.7 % in the total pore volume, respectively (Fig. 2а).
An average velocity of porosity growth in the fuel particles as a function of a fuel burn-up
development rate is ΔV GFP /ΔBu=5.57/49.5≈0.11 % per 1 % fuel burn up.
The different manner of a pore size distribution is observed (Fig. 1а) in specimen #73, which
was sampled from the center of the FE and whose burn up was twice as large as that of
specimen #135. The first difference is that the curve of specimen #73 is shifted to the region
of larger sizes of pores, the pore diameters cover a wider range, which is from ~180 to 1400
nm. A maximum pore concentration is 3.54·10 8 mm -3 , which corresponds to sizes ~555 nm.
A mean diameter of pores is equal to 455 nm, that is by 200 nm larger than that in specimen
#135. The second difference is that there are two maxima on the curve of the dependence
N i =f(D i ). Hence the pores in the fuel particles of the specimen can be attributed, at least, to
two poly-dispersed systems. This dependence can be possibly explained by an occurrence
of pores in a grain body at high burn up, where the conditions of growth differ from those at
the boundaries, i.e. there would be two groups of pores: the one corresponding to the
boundaries and the other corresponding to the grain body, whose pores would have different
sizes due to the different time moment of their occurrence. This phenomenon may be also
explained by a more accelerated growth of pores of the larger size, which absorb the pores
of a smaller size. There are two circumstances capable to prove the above explanation.
The first one is that in the fuel particles of specimen #73 there are practically no pores of the
size D i