i made a creative Singleplayer world to test builds of reactors and turbines and solve some issues with the build i had. then i found out i can't place yellorium in the access port. then i made a survival world and there i could place the yelorium in the access port.
Forge: 10.13.0.1208
Big reactors: 0.4.0rc3

Sorry, I'm not 100% sure I understand the issue you're trying to report.

Basically, you created one world and were unable to put yellorium ingots into the access port's inlet slot, and then you created a second world and it worked fine?

Can you try creating a second/third creative world?

Are you testing this in singleplayer or multiplayer?

after multiple client restarts it started working again. so you can scrap this. no clue what caused it.