Andy Dingley wrote:
On Mar 19, 5:19 pm, The Natural Philosopher
wrote:

PWR reactors are capabe of cooling the cores by convection alone in a
shutdown situation.

Firstly they're not. Only some recent designs, very few of which have
yet been built (at commercial sizes)


I think you will find that any new plant is that way,

Secondly the reactors at Fukushima are BWRs, not PWRs.

Which was exactly my point.

BWRS are inherently more dangerous because the primary cooling circuit
goes through the turbine halls and that means any issues there are
outside the containment area. PWRS keep the primary circuit within a
containment.

They cat boil, because they are at pressure.

Under shutdown conditions the wont lose water for circulation, because
there is enough convection through the core to at least keep the primary
circulating.

You still need to get that heat out of the primary, yes, but if the tank
of secondary water is large enough, its no worse than getting the heat
out of spent fuel rods. I.e. natural evaporation and boiling can do
that. It wont go bang because there is no hydrogen in te secondary
circuit, or indeed in the primary, because thats still fill of
pressurized water.
..
Furthermore, the hotter that primary water gets, the more it damps down
the reactions as I understand it.


There are still some potential failure modes, but nothing like as likely
as a BWR.

And I am sure that we will see designs full able to cope with a
powerless shutdown emerge in due course.

After all, in the final analysis, its simply a means to et rid of heat
till nuclear reactions slow enough. A bloody great tank of water is in
principle, all it takes.