For the life of me i cannot find any rules for radiation in any of the core rule books, maybe its just the corrosive environment section i should use but i can remember there being rules for radiation, can anyone point me in the right direction?

It's a hazardous environment. Just give it a rating and call it done.

I'd make it more like Vacuum personally. The core mechanic for sucking Vac isn't all that different from how most people interpret radiation* if you just remove the suffocation part...

If you want to go old school, blasters, especially low powered ones, don't react well to radiation. Blasters with a damage of 6 or less can be damaged one step on 2 Threat, 7+ three threat.

*Having worked a little with radiation I've learned that most people don't really know anything about radiation, with the majority of what you see in movies being totally and completely wrong. What kind of radiation (yeah there's different kinds), it's power, your exposure time, and other factors can change a lot. So having it work like vacuum one minute, and fire the next isn't all that crazy if the story calls for it. Likewise you can have different species react differently. An area with an exposed gamma source might be an 8 for a human player, but only a 3 to an Ugor simply because his biology is different.

Normally, I'd treat ionizing radiation as a kind of poison or illness, "radiation poisoning" or "radiation sickness" resp. actually, as it affects the whole body on a cellular level.

Concerning the kind of radiation, as a simplification alpha can be ignored easily, as long as you don't inhale or ingest the emitter; any soak 2 full body armour should protect from beta; so only with gamma things will get really interesting.

Myself, I think I'd go along these lines: a certain amount of damage (1 to 10), ignoring soak from brawn but not armour, setting in periodically (every minute to every few hours), a resilience check (2purple/1red perhaps) will reduce the damage for each success, threats will cause strain, despair will trigger long term effects.

Concerning the kind of radiation, as a simplification alpha can be ignored easily, as long as you don't inhale or ingest the emitter; any soak 2 full body armour should protect from beta; so only with gamma things will get really interesting.

Sometimes it's just easier on everyone if we stick to Theta radiation

Neutron radiation would also be a problem along with gamma. I work in radiation safety so I'm going to suspend everything I know for a minute and suggest you have them suffer wounds each turn 1 to 10 depending on the amount.

well half of my party have fallen into the undercity of taris during the rebellion period so i was thinking that certain areas still suffer from radiation even after being rebuilt

Radiations effects can be either deterministic or stochastic. We've mostly been talking about deterministic effects from large doses of radiation so far. You seem to be wanting something slower and less prescriptive. Stochastic effects have a chance of occurring but are not gaurenteed. They often have more long term effects as well. You could represent this with resilience checks. Failures could reduce strain threshold until a medicince check is made. Threat or despairs could cause critical. The check would make the effects less prescribed and the difficult can be adjusted to account for different levels of contamination. In this case I would give boost dice for using shielding, protective equipment, etc...

well half of my party have fallen into the undercity of taris during the rebellion period so i was thinking that certain areas still suffer from radiation even after being rebuilt

I would just use the rating system for the hazardous environment rules and decide how fast you want damage to accumulate for your players, as well as, if soak or breathers mitigate some or all of it.