Sounds like a high water casualty or pipe failure in the super-heated steam area for the boiler based on the description.
Though later they make it sound like a low-water casualty....
High water is worse for the turbines, low water is more likely to fail the boiler piping. They make it sound like the low water isn’t such a bad thing, but both take down the plant catastrophically. One requires a turbine refit, one a boiler piping refit. If steam broke out its was most likely a complete failure in one or the other.
On Independence two boilers per shaft/propeller. 8 boilers total.