Enter web-comic celebrity Randall Munroe (of xkcd), and his friend Ellen, Senior Reactor Operator at the Reed Research Reactor, who have put together some charts which might help us make sense of all this radiation talk.

FORBES: Measuring the Radiation Threat

Hours earlier, the plant operator switched off a reactor cooling system after discovering rats near critical equipment.

BBC: IAEA calls for improvements at Japan's Fukushima plant

The plant's operator, Tepco, had intended to cool reactor 1 by filling the containment chamber with water.

BBC: Japan nuclear: Tepco halts Fukushima cooling plan

The plant's operator Tokyo Electric Power Co (Tepco) said exposure to radiation levels in the No 3 reactor for four-and-a-half hours would exceed the emergency safety limit for power plant workers.

BBC: Robots record high radiation levels at Japan reactors