How would you cause an explosion in a underground facility? A small amount of explosives smuggled in each day..to build a critical mass? Or maybe a shot of natural gas into a vent system? Or a flammable liquid introduced into the water supply? Or..just a run away of some electrical equipment?
However it was done..if it was done..great.
I suspect that a computer virus (a "Trojan") that plays havoc with the experimental set-up, causing things to "go off" prematurely, would be the most cost-effective method.
Regards,
Remember - miners have to work at not causing an explosion by accident. Underground facilities may have explosive gasses build up naturally. With uranium enrichment, you also have a range of potentially unpleasant gasses in use. The fluorine in uranium hexafluoride has to come from somewhere, and if they Iranians are morons, they might mess that up. The centrifuges rotate at a high rate and can generate static electricity or in other ways cause a spark if they are not maintained correctly, which could be colorful if there were explosive gasses present. The other processes needed for weapons components might intentionally (through virus commands or through direct physical sabotage) or inadvertently release the explosive gasses without intentionally smuggling in bulk explosives.
Causing something like this, if it was caused, is not hard. Bakeries can explode from airborne flour. Most homes have natural gas, which can be made to explode. Gasoline and propane can be made to explode. Chemical labs almost always have bulk solvents, which are often volatile and can become explosive. Tanks of high pressure hydrogen, oxygen, or other simple gasses are found in a surprisingly large proportion of labs. From the perspective of those who don't want nuclear war, it's not "how do we get the explosives in there" but "which ones shall we use".