Blocks placed in worlds running recommended versions of Forge (14.23.5.2768) or above don't emit any radiation, however reloading world in older version of Forge (14.23.4.2767) fixes them.

Placing reactor core in world with Forge 14.23.5.2768 (no radiation):

Reloading same world with same mods and Forge 14.23.4.2767 (core starts emitting radiation):

Mod version: 1.12.2-3.1.0b44

No errors in the log at all.
Also after several tests i've noticed that radiation can break randomly even on Forge 2767 (log) but reloading same world in 2768 (log) somehow fixes it for already placed blocks! (but breaks again if i replace them)

I've had a few others report this issue as well. Still have yet to replicate it in the dev environment. Any chance you spotted errors in the console. Such as null pointer exceptions from one of the atomic science threads.

Latest build (46) with Forge 2768 (deleted configs, recreated the world) - still not working, no errors in the log.

Strange, I tested with the dev environment and our Voltz modpack. In both cases, it worked even when removing rods and inserting them again. What I can say is try the latest alpha, if that doesn't work I'll talk you through turning on debug. That way I can see what is happening.

Add "-Ddevelopment=true" to your run arguments, without the ". This should enable some extra debug data in the log file.

If you're using the twitch or vanilla launcher. You can add the line to the JVM arguments in the screenshot. To get to this page, click launch options at the top of the MC launcher. Then select the modpack you want to edit. Twitch should make a new MC profile/modpack per modpack you have.

I managed to replicate the problem with the latest forge. Going to add some debug code and hunt down the issue.

Just tested, seems to be fixed. Thanks!

Fixed in build 50 https://minecraft.curseforge.com/projects/atomic-science/files/2630719

Re-open if issue still exists.

Cause: Data Handler was checking if the thermal or radiation source was still valid. The valid check would then look for a fuel rod in the reactor. Empty reactors would result in no registry to the handler.
Fix: Switched over to does source exist check. Which replaced the usage of validcheck in most cases