I've been reading the SRD for Diaspora, especially the section on personal weapon design. I've found that the rules handle most of my cases very well, but I can't seem to find a way to model grenades. I did find the following reference:

Brawling and Close Combat Weapons

Brawling and Close Combat weapons are blades, clubs, and other designed melee weapons. They also include thrown weapons such as spears, shuriken, and (at higher technologies) grenades.

and the thrown weapon stunt

Thrown

weapon may only be thrown, using the Agility Skill, and uses the range 1-2. Normal penalties for exceeding this range (-2 per band) apply. Weapon has the, “Out of ammo” Aspect, which may be compelled. Increases base cost by 1. 1bp.

But no rules for designating a weapon single use. Am I missing something here?

Diaspora takes a strong stance on counting bullets, hence the Out of ammo aspect mechanic of firearms. And the approach in the rulebook assumes that the grenades are not weapons but ammunition. When you list grenades in your character's weapon manifest, he is assumed to have a number of grenades ready, and he's out when someone compels the aspect.

In that vein, you may just tack on the Out of ammo aspect to any single-use weapon of which you can carry around a few of them.

If you want to model a weapon that is single-use and too unwieldy or complicated to be considered just ammunition (a shoulder launched surface-to-air missile comes to mind), just define it as a stunt.

Single use: Weapon may only be used once and is then discarded or expended. -2bp

The ruleset is flexible enough to allow things like this. Use it :)

Given the level at which Diaspora usually handles things, I'd model grenades not as individual objects ("I have two grenades"), but as a weapon type representing a collection of exploding things. ("I have a belt of grenades" or "I have a grenade launcher".)

Generally, Diaspora doesn't bother to track individual ammunition, and neither should you - FATE systems are an abstraction level up from that. The "Out of ammo" aspect then represents running out of grenades.

Use thrown weapon for range, and obviously it should have "Explosive" and "Harm". (A grenade launcher will work better modelled as a slugthrower-type weapon with explosive ammo; give it 'Versatile' if the grenades can be thrown separately.)