David Hansen wrote:
It convinces all but the Faithful Greenpissers.
This arises because the nuclear plants are run as "base load". as
they can't be quickly started and shut down. Ie they run all the
time. In the UK peak loads are met with gas turbines which can be
started up in minutes.
The world's first commercial / nuclear power plant was in the UK. we
know all about nuclear power and are still the world's leading
reprocessor of fuel.
Thank you very much for this description.
My limited amateur understanding was that without water to slow down
neutrons, fission would decrease and decay would be the major heat
You're most welcome.
That is true. However, realize that a core with a few million megawatt-hours
on it produces about 10% of its full rated power from decay heat at the
instant of SCRAM. This power output decays rather rapidly so that in a few
hours it is not significant in terms of a percentage of full output. Still a
lot of heat, of course.
Another thing to be aware of is that a core does not instantly shut down when
SCRAMed. The control rods can't absorb even a small fraction of the available
neutrons - just enough to shut the core down with a great deal of margin. It
takes a little while - minutes - for the nuclear reaction to taper off. The
decay is exponential and it nearly asymptotes for quite some time from delayed
neutron emissions as part of the short lived fission product decay process.
This has no effect on the core's behavior during an accident, for it will be
cooled passively for longer than that but it is a bit of knowledge that is
handy to have. That is, understanding that a nuke is neither turned on or off
instantly as if a switch were thrown.
In a PWR, the turbine and reactor trips are intertied (post-TMI
lessons-learned) so that the initial decay heat is handled by the condenser
bypass, or if the condenser vacuum is lost, via atmospheric vents (spectacular
in both sight and sound). As the decay heat declines, other systems take over
until ultimately the RHR (residual heat removal) system assumes the duty of
carrying away the little heat still being produced when the system is
In a BWR during a SCRAM, part of the steam drives the HPSI (pronounced
"Hipsy") pump (that takes water from the torous or suppression pool, depending
on the MK number) and injects it directly into the reactor. This is a steam
driven pump and all the valves fail open so it cannot fail to run. Plus there
is quadruple redundancy. As the decay heat lessens and the pressure drops,
cooling is transferred to the LPSI ("Lipsy") pump. Pretty much identical in
function to the HPSI, those turbine driven (and one electric for each type for
diversity) pumps provide cooling water as long as steam is being made.
When the steam production stops, the core is cool enough for the RHR system to
take over. The RHR consists of simple water to water heat exchangers and
pumps, one side receiving primary water and the other side river water from
the Vital Service Water supply. This is a multiply redundant source of
cooling water that can be drawn from the river or from the condensate storage
tanks (typically 2 - 1 million gallon tanks per unit).
Well, enough brain dumping for one day.
John De Armond
See my website for my current email address
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Multitasking: Reading in the bathroom!
Neon John wrote:
When working on a coal fired 500 MW set, I was fascinated to see
the sizeable explosion diaphragms fitted to the LP turbine
casings. Basically a foil disk about 300 mm dia, with a sharp
point poised to pierce it if it ever bulged upwards. Later I was
commissioning a relay rack which controlled the turbine hall
windows. They were opened just before all condenser vacuum was
lost. I would not have wanted to be there.
Chris J Dixon Nottingham UK
Have dancing shoes, will ceilidh.