Righti didn’t realize It was explosive like that but it really is

Flash point 2 °C (36 °F; 275 K) Autoignition temperature 435 °C (815 °F; 708 K) Explosive limits 1.7%-8.2%

Drilling is too risky then. What about dumping liquid nitrogen on the thing until it’s doused?

It's possible to drill or cut without creating sparks. Just need to control the speed of the cutter.

Besides, tanks like these have various portholes, valves and drains already. The article mentions an "inoperable valve" so maybe that's the problem but I'd be surprised if there were just one. They must have been getting the contents out of the tank and into the manufacturing process somehow.

The standard choice for MMA processing is actually stainless steel, because carbon steel/iron/etc scavenge the inhibitor used to prevent thermal runaway. HDPE would work, and often how it is transported in drums, but for actual processing, everything would normally be spec'd as stainless steel.

Which, of course, is pretty spark resistant to begin with.

Even if this wasn't true, this is not a hard problem, you can use non-sparking tools, proper coolant, lots of things to avoid sparks.

Or you know, we could require that highly flammable materials subject to thermal runaway have "drill here in case of emergency" patch of non-sparking material or something.

The cost of ATEX/Class 1 Div 1 compliance would not really go up if you required this.

Inert gas?