I wonder if there is a yet unidentified action. I am not a chemist or even high school physics teacher, so I have no background to comment, but I will ask the following.
What if the materials form some type of energy antenna? Perhaps turn magnetic fields into heat? Kind of like a thermocouple but works on a different (non nuclear) law of physics? Would there be a way to structure that as a lab experiment?
Just speculation on my part.
The observation has been that deuterium or hydrogen interact with some metals to produce heat. There are known chemical pathways to produce heat, but what has been observed is a level of heat well above what can be expected with ordinary chemistry. I like to think of it as a super-duper resonating chemistry.
When there’s radiation as high as gamma rays, it is a proof that this thing is nuclear.