Posted by daestrom on December 12, 2008, 12:09 am
Neon John wrote:
Actually, when operating a low-enrichment reactor at full power 7% is from
decay of fission products, not 10%.
PWR folks refer to this as "-1/3 StartUp Rate". Power initially drops down
to very low in the power range almost instantly then decays at a negative
one-third of a decade per minute. Since 'power range' is about 6 to 8
decades above the stable level in the source range, that works out to about
15 to 30 minutes. But essentially all heat production has stopped much
sooner. Heat production is down to decay heat levels within a minute.
Having been around a few and reviewed post-trip data, yes on a log scale it
draws a very nice straight line for quite a while.
BWR folks, because we use a different label on the meters refer to it as a
"negative 80 second period". Power decreases by a factor of e every 80
seconds. But it works out to the same rate of decrease.
It's spelled HPCI (High Pressure Coolant Injection), but you have the
pronounciation right. I believe it is PWR folks that have a HPSI (High
Pressure Safety Injection) as one of the modes of operating the CVCS
(Chemistry/Volume Control System).
The smaller steam-driven system used on BWR-3/4/5 is called RCIC (rick-sy),
Reactor Core Isolation Cooling. LPCI (Low Pressure Coolant Injection) are
electric-driven pumps that are for emergency use only and don't normally
operate on a typical SCRAM (unless water level drops down to level '1' which
is only a few feet above the fuel).
But on *most* SCRAMs, HPCI and RCIC are not used. They may start if level
drops severely but not necessarily inject. On most SCRAMS in a BWR, the
normal feed-water system supplies makeup water to the reactor and
main-turbine bypass valves dump reactor steam to the main condenser.
The early BWR's (Big Rock, Oyster Creek, and Nine Mile) used a wonderful
system called "iso-condensers" (not to be confused with 'ice-condensers'
used by some PWR containments) or "emergency-condensers". Simply system
where steam is taken from the hot reactor, condensed in a heat exchanger
mounted higher than the reactor and gravity drained back to the reactor.
Cooled by boiling water out of a storage tank, these could keep the reactor
cool for more than a day with just two air-operated valves failing open to
initiate the system. The ESBWR design is bringing back a similar passive
Condensate storage tanks in BWR 3/4 are the primary supply to HPCI/RCIC, but
the suppression pool is the ultimate source of water for those systems.
IIRC, our CST's are only about 360 kGAL. They hold water for normal
steam-plant make up and the initial operation of HPCI/RCIC.
A BWR containment's suppression pool on the other hand holds over a million
gallons and although the water is not as clean as we'd like for use in the
steam plant normally, it is available for HPCI/RCIC once the CST's are
emptied or for LPCI injection if warranted.
In BWR's the RHR heat exchangers are long-term cooling and supplied by
Emergency Service Water. But ESW is motor driven pumps from the lake /
river that the plant uses for cooling, not a tank. Without ESW cooling, the
water in the suppression pool could reach 200 F within about a day of a
total loss of other cooling. Of course it would take quite a while to boil
that much water, but if it even gets to 200 F then we can't pump it because
of NPSH requirements on the LPCI pumps.
And of course like all good nucs, there are even provisions for fire system
cross tie to supply reactor from fire main. But that's a spool piece and
some fittings that takes a while to rig into place.
Posted by David Hansen on December 10, 2008, 10:39 pm
On Tue, 09 Dec 2008 12:22:04 +0000 someone who may be The Natural
Yawn. Still making a fool of yourself I see. Do keep it up.
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
Posted by dennis@home on December 7, 2008, 9:10 pm
Good theory but its a shame they use the pumped storage to smooth out peaks
so there isn't any pumped capacity available to cover nuclear drop outs if
one happens after east enders has finished.
Posted by Neon John on December 10, 2008, 7:56 am
actually, I'm going to take exception with both of you guys. 100%
availability is, in fact, possible, or at least so close as to be
statistically so. Most of our nuclear plants demonstrate that fact day in and
day out. Running continuously from refueling shutdown to refueling shutdown
is now the norm. The less-than-100% capacity factor shown in industry
statistics is accounted for by the normal 18 month refueling intervals and the
typical six-week refueling outage.
The industry is now making a major push towards high burn up fuel and extended
refueling intervals. Doubling the interval to three years is the goal. I
think it's doable. The industry has also gotten much better at the refueling
process itself, cutting outages from months during the 70s to mere weeks
Also, because the equipment in a nuclear plant is so highly redundant, major
maintenance on individual pieces of equipment is done while the plant is
online. Only one single item maintenance, such as the turbine or the
generator, becomes necessary is the plant shutdown for maintenance. And, of
course, that pesky little problem we had back in the 80s with water chemistry
and steam generator corrosion that necessitated many steam generator
replacements. Even then, the Tennessee Valley Authority set new records with
their steam generator replacement program.
So over the extremely long term, and depending on how you define it, nuclear
plants will never achieve exactly 100% availability, but they will come very
very close. Once the industry completes its conversion to high burnout fuel
and extended outages, I would expect the capacity factor to be in the high
90s, perhaps as high as 98%.
footnote for the pedantic: I'm retired and have been for some time. My
contemporary knowledge comes from industry statistics and from talking to
still-employed colleagues. And, of course, knowledge that the nuclear industry
changes very slowly so my first-hand experience is still mostly valid.
Undoubtedly I may be off by a bit on some figures quoted to establish order of
magnitude (an average refueling interval may be 6.9 weeks, for example) but
frankly, I'm not going to research a Usenet article like I would an academic
paper. For you readers, that's what Google's for.
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
Vegetarian - Indian word for "poor hunter".
Posted by The Natural Philosopher on December 10, 2008, 9:16 am
Neon John wrote:
Thats what I meant. Overall you have to shut down for refuelling at some
point, and that's not an operation to be hurried. Six weeks every 18
months gives you 92% uptime. Pebble beds might do better, as you might
be able to shovel new pebbles in and remove old ones more or less
continuously without shutting down. But here since these are planned
outages, it would be more like doing it annually or bi-annually in the
march to october period, when demand is lower.
Yes, thats what is plaguing or current reactors. A heritage of old
plabnt that wasn't that well put together in the first place.
similar position here..