IDK how many real-world casuistics we actually have, but Wikipedia says that absorbed dose over 30 Gy results in severe headache, severe fever, nausea, vomiting and seizures, with death taking up to two days.

I linked it in another comment, but I'll put it here as well - here's an IAEA report about an accident where a worker walked into an active irradiation room of a commercial irradiator, he was there only for about a minute total but had to leave because of intense pain which made him realize something is wrong. The total dose is estimated at 10-20Gy but it was still enough to make him very unwell almost immediately and caused death about a month later:

https://www-pub.iaea.org/MTCD/publications/PDF/Pub925_web.pd...

Yeah, we are pretty lacking on high end data but my understanding was somewhere around 50Gy was expected to knock out brain function. And picking up the hot stuff from a reactor is way above that.