I wondered that also.
Of course, Organic Peroxides “DON’T PLAY NICE”. It might be that the risk of venting was viewed as greater than the risk of explosion. Things like that are a nightmare for hazard control, and playing the “Oh Crap” game sometimes leads management to say “That will never happen! Go back to your hole and drink more coffee.”
As a process guy, I keep asking myself why people install emergency generators below grade. I am a very paranoid soul, and have had more than one “Oh Crap” moment to make sure the emergency power is properly secured.
Nothing quite like watching a potential runaway reaction and hoping the back up power is able to open the cooling lines to convince you that while it may be expensive, planning for every event is great insurance.
Similar “oh shit” at Fukushima. I was scratching my head after I saw how easily that disaster could have been averted.