content: Amount of fuel available to sustain the reaction.  Like gas in a car, the fuel has to be pumped in for it to work. If one of these blows up it can only burn up the fuel currently in the system, where fission reactors have ALL of the fuel available the entire time, which allows for run away reactions like fukushima and chernobyl. Any explosion in a fusion system will be a very bad thing of course and if the shielding fails a pulse of radiation will get out and destroy a bunch of stuff, but it won't leave a pile of long term pollution behind messing up the place for centuries. A fusion system blowing up would not be much different than an oil refinery blowing up, and that happens all the time.