The chain of failures illustrates the capacity of events such as this massive earthquake and tsunami to overwhelm systems that are designed to be "redundant" - to have more than one means of doing the same thing.
The earthquake caused Fukushima Daiichi and other power stations to shut down - taking away the electricity driving the reactors' cooling systems.
Back-up was supposed to come from diesel generators.
They cut in - but then cut out again after about an hour, probably due to being overwhelmed by water from the tsunami, although Tepco has not confirmed this.
The diesels themselves were backed up further by batteries, but these were designed to function only for eight hours.
When they ran out, nothing else was available.
Reports say that five fire pumps were then deployed to provide water, but that the explosions in buildings 1 and 3 knocked four of them out of action.
Meanwhile, devastation from the tsunami as well as the fear of aftershocks means simply driving new pumps or fuel to the site is much more difficult than it would be under normal circumstances.
All this is already providing material for anti-nuclear groups to argue that no nuclear facility can be designed to be completely safe.
This is manifestly correct; but the same is true for any industrial operation.
Supporters of nuclear power will point to the fact that so far casualties number just a few, that engineers have so far - however desperately - been able to confine the problem, and that far fewer people die each year from nuclear accidents than in coal-mining.