RRFM 2009 Transactions - European Nuclear Society

small in a plate having a low fission rate and, hence, typically a low temperature. These
observations suggest that Si accumulation in the ILs occurs primarily during a heating process of
the fabrication stage and additionally by fission enhanced diffusion when the fission rate is high.
It is fortunate that the Si diffusion is fast in a plate with a high fission rate and therefore high
temperature in which the effectiveness of Si addition is needed the most.
The IL thicknesses observed in R2R040 and other plates with 2wt% or more Si addition is small,
smaller than in U 3 Si 2 /Al at the same condition. This remarkable effectiveness of Si addition is
similar to the test results by Ugajin [12], although he added Mo in U 3 Si 2 . Ugajin observed that
U 3 Si 2 +0.5wt%Mo/Al dispersion grew thinner ILs than U 3 Si 2 /Al dispersion. This observation and
the present result show that Si has a superior effectiveness when combined with Mo.
As discussed earlier, no porosity was observed in the ILs in plates with the addition of 2wt% or
more Si to a burnup close to 90%. This stability improvement by the presence of Si in the IL was
theorized by Ryu [13]. Ryu showed that the enthalpy of formation of IL decreases, or becomes a
larger negative number, when Si is added to the IL. The average bond strength typically
increases with decreasing enthalpy of formation, by which the viscosity of the IL increases to
better retain fission gas.
However, as the IL thickness increases, Si deficiency eventually will occur in the IL. Then, the IL
growth rate increases and pores may form in the ILs as observed in R2R088 and R6R018 from
the RERTR-9B. In order to estimate the minimum as-fabrication Si content to avoid the Si
deficiency, we assume the following: (1) Fuel particles have a uniform size (~90 μm) and
distribution in the meat. (2) Thermal diffusion during fabrication reaches equilibrium. (3) For
conservative results, fission enhanced diffusion is neglected. (4) By analogy to UAl 3 [14], the
minimum Si concentration is 5at% in the IL. (5) The amount of Si diffused into the ILs is equal to
the amount in the ILs.
Figure 4 shows the estimated Si contents in ILs for two IL thicknesses that have as-fabrication Si
content of 2% in the Matrix. As shown in Fig. 4(b), the estimated Si content in ILs is slightly
higher for the high U-loading cases than the minimum Si content. This overprediction suggests
that some of the assumptions for the model may be unrealistic. The inhomogeneity of fuel
particle size and distribution seems to be the most sensitive assumption.
1.0
10
1.0
10
IL volume fraction
0.8
0.6
0.4
0.2
8
6
4
2
Si content in IL (%)
IL volume fraction
0.8
0.6
0.4
0.2
8
6
4
2
Si content in IL (%)
0.0
0
5 6 7 8 9
U-loading (gU/cm 3 )
0.0
0
5 6 7 8 9
U-loading (gU/cm 3 )
(a) IL = 5 μm and 2% as-fabrication Si in Al (b) IL = 9 μm and 2% as-fabrication Si in Al
Fig. 4 Estimated Si content in IL for Al-2wt% as-fabrication Si for typical RERTR test
condition. The broken lines indicate the minimum Si content in the IL necessary for
the Si addition to be effective.
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