Anyone who has seen pictures of an exploded gun barrel or receiver that resulted from an accident involving a charge of half-a-teaspoon of smokeless powder - might have doubts about who ultimately wins the battle of pressure versus steel.
The thing is that they would bleed pressure off from the containment vessel. That’s what almost all of the radiation released from Three mile Island was about, keeping pressure in the containment vessel down.
If the containment vessel is cracked, there would be no pressure buildup. so you would have fumes going right into the air, but no risk of explosion.
What happened at Chernobyl was that the reactor exploded from just the sort of pressure problem you’re describing, almost before the reactor operators realized they had a big accident on their hands.