From having cut up a home heating oil tank. The sparks from a grinder or cutoff wheel will ignite diesel. But without atomization diesel will not explode like gas.
I have cut into several tanks that could no be emptied completely, I drilled a hole into the tank, and then using a metal cutting blade on a sawzall to open a window big enough to service the tank. After thorough cleaning, the tanks were repaired with welding, painted inside with tank liner, and welded shut. I used this process to repair the keel and saddle tanks in my steal sailboat.
By the way, If you keep the blade speed down, a sawzall blade will cut a long way through steel. Run the saw at about 1/3 of it's full RPM. My personal record is almost 4 feet through 1/4" plate before the blade needed to be replaced. Run the saw at full speed and you will only get a few inches. You want a blade with a tooth spacing that will allow 2 teeth in the thickness of the steel of possible. If not, the finest you can get.