Just two things wrong with what you said. First of all, you said there is no way radiation could effect the interior of a ship with duranium armor except if its already blown up, but the ships don't all have duranium armor. My ships (right now) are using composite ceramic armor, which isn't rad-proof last I checked, as well as if there are holes in the armor (from battle damages) the radiation can soak through anyway.
Anything is radiation proof when used in large enough quantities. Modern Battle Tanks are ceramic armour with DU inserts, which provides limited protection from gamma and x-rays. Most nuclear reactors use H2O or H2 to capture radiation. Six feet of deuterium will caption nearly 100% of the radiation emitted by submerged fuel rods. Any ship that travels in deep space would need to be protected from cosmic radiation, as well as any radiation from stars, black holes, supernovas, enemy weapons, planetary radiation, etc. It would be very odd to have a ship be unprotected against such a common danger.
And I never said anything about radioactive material, I was talking about weaponized radiation weapons (specifically radiation enhanced lasers, which by the definition of laser: light amplification by stimulated emission of radiation, with normal lasers in game doing a lot already) which are far more than radioactive materials. Secondly, radiation has nothing to do with atmospherics. What your thinking of is radioactive dust, what I'm talking about is radiation which still exists even in a vacuum (as evident of energy from the sun in the form of light, which is radiation, reaching us). Plus, the current rules of the game in itself are already breaking the laws of physics.
I think you're mixing up ionized radiation with non-ionized radiation.
Non-Ionized Radiation is everything from radio waves, infrared, light and up to certain UVs.
Non-Ionized Radiation would damage something by transferring it's energy through radiation (as opposed to conduction or convection). Lasers are
Non-Ionized and therefor will either burn the target or not burn the target.
Ionized Radiation is radiation which is powerful enough to damage individual strands of DNA by simply colliding with it, or damage chemical bonds in molecules. When people mention radiation, they usually think of
Ionized Radiation. Each particle (either an alpha, beta or neutron) or wavelength (far UV, gamma, x-rays) is able to damage the target without needing to heat the target in any meaningful way.
While I would love radiation weapons, the fact is that any radiation weapon powerful enough to penetrate an armoured starship would also be able to destroy the ship through heat alone. Using against light skinned ships, fighters and missiles would be interesting, but would be a highly situational weapon to the extreme.