Basically equivalent to a dirty nuclear bomb going off with an under-powered dispersing mechanism. That building is not easy to clean up if at that is at all possible.
Cesium-137 cleanup isn’t especially difficult compared to some other radioactive agents. A rather painful vigorous scrubbing (speaking from experience) is effective for external exposure and Prussian Blue is a moderately effective antidote for internal exposure that will likely prevent loss of life. No radiation contamination is “safe” and Cesium-137 is particularly nasty, but cleanup is definitely possible without long term effects (like at the Chernobyl site) generally.