Before it has reached criticality for the first time, its contents are less toxic than hydrazine. After criticality, the contents are more toxic than hydrazine

Before it has reached criticality for the first time, its contents are less toxic than hydrazine. After criticality, the contents are more toxic than hydrazine. Nuclear reactors are safe because so much engineering and procedural effort goes into keeping the fuel and its fission products contained. The fission products include extremely toxic materials. Strontium-90, one of the more common medium-lifetime fission products, has an acute gram-for-gram lethality comparable to the nerve gas sarin. The fast-decaying fission products like strontium-89 are even worse, though by their very nature they don't persist for long in discarded fuel.