[EDIT: It was 1.3.11 that I built it.]

I just built a fusion reactor in v.1.3.11. I found that when the pure water was passively flowing into the fusion reactor, it would sometimes/somehow get stuck at 499 on the input and wouldn't let anymore in even though the infill pipe was backed up. Couldn't figure out how to fix this other than to remove and replace the building, or put a pump to suck it out of the input. Both of those fixed it and it would start to run again. Jammed a few times on me. So I put a pump on the input to force the fluid in and it's been running smoothly. But probably a bug.

I will check it

1.3.14 I have insert a bigger space in the fusion reactor and a catalyst amout setting, I don't know if solve the problem but should reduce it enough. I think is a base game problem