a nuclear chain reaction that is able to continue by itself, or the conditions under which this happens

A criticality can occur when too much radioactive material is placed together. 当过多放射性物质放置在一起时，就可能出现核连锁反应的临界条件。

the fact of being extremely important

Without denying the criticality of this particular problem, there are also others that need to be considered. 这个具体问题的严重性毋庸置疑，但同时也还有其他问题需要考虑。