From the poster that gene-h linked:

> Why Aerogel?

> - Individual fuel particles must be micron sized to permit fission fragment escape

> - Accumulation of critical mass of micron dust particles presents a challenge (levitation not feasible)

> - Embedding in solid material could solve this, but need very low density to allow FF escape

> - High temperature, low density, IR-transparent aerogel is an ideal matrix for fission fragment engine core