Beginning with the elements first, some of which will tie into the radioactive changes:

Teflon

Polytetrafluoroethylene. Resistant to ACID, low heat conductivity. Resistant to most forms of damage, but not impervious. Does not turn into VIRS. TEFL or PTFE

Alpha Particle

Admittedly far-fetched, but an interesting concept. Emitted by radioactive elements like URAN and PLUT. Very faint and short-lived, only travelling a few pixels from the source. Stopped by ,ost non-conductive elements. Conduct electricity over a distance of several pixels with slightly warm arcs of LIGH to the closest particle or conductor. Creates PLSM when passing through a metal with SPRK. Splits into NEUT and PROT when hit by NEUT or PROT. Decreases STKM health by 1 when hit.

Gamma

Again far-fetched, but cool. Emmited by fusion and fission, as well as CAES, which I will cover below. Very penetrating like PROT, only stopped by LEAD, also covered below. Also rarely emitted when ELEC hits TUNG. Interacts with NBLE, scattering an ELEC particle. Decreases STKM health by 25. Kills PLNT, turning it into DUST.

Thorium Dioxide (ThO2)

White, light powder. Emits alpha particles. Turns into URAN and heats up when hit by NEUT. When next to TUNG, forms small plasma arcs when SPRK travels through TUNG. Glows white when heated by FIRE.

Caesium

Properties depend on tmp, or life, whichever works. When set to 133, the default value, CAES acts as a regular solid with a low melting point. Reacts violently with WATR and its various forms, generating heat and FIRE. When touching CAUS, both particles are destroyed and the reaction emits a lot of light and heat. When tmp or life is set to 137, the element functions in the same way, but emits gamma particles. CAES with a value of 137 is a product of PLUT fission. CAES-137 emits gamma particles, decreasing its tmp or life value with each, stopping at 133 at which point it becomes stable, generic CAES. Any value of CAES above 133 will emit gamma, but only 137 is a product of fission.

Lead

Dense metal with a relatively low melting point. Stops all forms of radiation in a similar way to INSL. When mixed with PSTE, the compound becomes PSTE (LEAD) and paints other elements with its deco color.

Radiation Changes

The addition of new radioactive particles with interesting interactions could breathe new life into the radioactive category. Instead of just bombs and reactors, materials could be used to make anything from X-Ray machines with ELEC and TUNG to MIG welders with Thorium Dioxide and TUNG. Electronics might take advantage of radioactive sources for alpha particles due to their unique conductive nature. PLUT bombs would have fallout even. It's a lot of addition, but I've been careful to keep it as non-destructive as possible. Any features listed here are just suggestions, so if you do decide to use any of these ideas, you have full creative liberty of course. Thank you for your time, even if nothing comes of it.