My general opinion is that no living creature could manipulate subatomic events on a cellular level.
Reasons to back that opinion:
- Every known biological event happens on molecular (or higher) level which is at least two magnitudes larger than subatomic.
- Every known biological structure is carbon based and any sort of nuclear reaction (I can't deal anything with "cold" in cold fusion, so I will assume it means "controlled" in contrary of hot as in "exploding".) happening inside a carbon-based cell would produce enough heat to burn the proteins of that cell.
- Every known creature utilities some sort of chemical energy. (Plants transform electromagnetic energy to chemical before utilizing.)
So, how could that work if it worked:
You would need an alternative digestive system to utilize fusion energy which has nothing to do with the regular one.
Creating the energy:
I would suggest to have one use only "reactor cells" which are cell sized hydrogen bombs, and a "reaction chamber stomach" where those cells would explode. (Please note that I don't have the slightest idea how a cell would be able to create fusion!) The stomach should have a very thick wall made out of non-organic material (preferably containing lots of heavy metals to reduce radiation) which would be consumed using the regular digestive system.
Protection from the heat:
Even if the reaction is under control, the "nuclear stomach" will undoubtedly heat up, therefore it must be bedded in an organ which is capable to cool it down (possibly by some liquid). The liquid will become extremely hot and somewhat radioactive so will be emptied from the body.
Utilising the energy:
At least this is something that is done in real nature: electromagnetic energy can be utilized using some version of photosynthesis using special energy draining cells which are injected to the nuclear stomach together with a few fusion cells. Those cells could transform the nuclear energy to some high energy-density chemical fuel which could be utilized in other parts of the body. Please note, that these cells will also die in the nuclear booms so they must be replaced after every "boom".
Cleaning up after using the "nuclear stomach":
The nuclear stomach is something that should be frequently replaced, because it would become radioactive very soon and consists of a material that can resist tiny nuclear explosions so the body will not be capable to decompose it. The tissues around the nuclear stomach will also need replacement for the same reason.
I'm neither a biologist nor a nuclear physicist and can't back my statement with more than common sense.