Robatoy wrote:
....

... They have tried super-
critical systems, but the plumbing had to be sooo huge as to be cost
prohibitive, like a steam line with 12" ID and 36" OD, just think of
the flanges.

....

That's not so...there are some 600 supercritical plants in operation
dating to as early as the 60's. TVA's Bull Run went on line in 1967 and
routinely has had one of the best if not leading heat rate and
availability in the US since, winning the annual efficiency rating
fairly frequently until some of the newer units came on line. It's
still routinely in the top five.

Bull Run generates more than six billion kilowatt-hours of
electricity a year, enough to supply about 430,000 homes. It has been ranked the
most-efficient coal-fired plant in the nation 13 times and is
consistently in the top five each year. In 2005, the plant achieved its
best summer reliability ever, and in November of that year it set a
plant record for continuous operation when it ran nonstop for 189 days
with no unscheduled outages for maintenance or repairs.

I don't know the actual steam line dimensions, but while it is very
thick-wall as compared to normal Sch 40/60/80 indeed, I'm pretty sure
12" walls are extreme (and 12" ID is way too small, I think they're more
like 30" ID).

I looked in my old B&W _Steam_ book; they don't have any typical
supercritical plant steam line dimensions unfortunately, but state that
the Barberton fabrication facility could manufacture up to 8" wall
thickness. There are almost no flanges in a supercritical facility;
it's virtually all welded (for obvious reasons). I'll ask one of my old
buddies what is a typical steam line dimension. (BTW, at least in the
olden days, thick-wall pipe of these dimensions was made by boring solid
material; I presume probably still is).

I'm not up to date on current statistics; quite a number of the recent
and current boilers being built in China are supercritical units so
they're certainly not out of style.

As an aside, an unfortunate disadvantage of nuclear units of all types
(other than the oddballs that did not prove out like the HTGR or
liquid-Na) are limited as compared to fossil owing to the limitation of
core power density required to prevent either DNBR (PWRs) and/or
centerline fuel melt (both) of the fuel. This limits them to lesser
thermal efficiency than fossil units. One reason for the B&W OTSG was
its ability to have 30-40F of superheat that compensated somewhat (as
compared to conventional SG's). I'd have to look up CANDU but I don't
think it's power density rates any higher than that of conventional
LWRs; it's advantage is low-enrichment cost and the continuous refueling
facility.

So, the supercritical boiler is alive and well... (At least outside
the US where progress hasn't had the plug pulled, anyway....)

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