That's part of the design differences of land-based versus the space
constraints of naval application. They do have biological containment
and that serves as LOCA containment as well.
All US naval reactors have systems to protect the core in event of a
LOCA. In a sub in extreme emergency if those systems were to fail they
can open valves and allow seawater to simply flow over the core,
entering and leaving the sub by natural convection. Of course, the plant
is trashed, but the core doesn't melt down.
There has never been a meltdown on a US warship including several which
were lost at sea with reactors in operation. Avoiding a meltdown is a
matter of dropping the rods and providing sufficient cooling to deal
with the transient, which is difficult with land-based power plants but
not with power plants that have a whole ocean to use as a heat sink.
In any case, the power output of the largest naval reactor in the US
inventory is somewhere around 100 megawatts, 1/10 the output of a
typical base-load electric generating plant, and the ones used in
submarines are much lower capacity.
And this all leaves aside the different compromises that are made in
military vs civilian installations.
Satellites are mostly isotopic (Pu-238) decay heat powered
thermoelectric generators. The US has only had one experimental fission
reactor launched and that was ages ago while the Russians have used
quite a few altho I don't know just how recently.
Hmmm....this seems to be a fairly good article altho I didn't read it
carefully, skimming looks reasonable---
The primary reason for the size differential is the space reactors are
quite low power devices in terms of central generation requirements
(otoo 2-3 to 100-200 kw instead of 1000 MW). Also they don't require
much in the way of shielding onboard as there is no manned payload.
There's sufficient shielding in a commercial design that one can be in
containment but outside the biological shield area during operation even
though that is a rare event not done in normal operation as there is no
need for access there. We did do incore physics tests using
manually-controlled drives to insert probes in the calibration ports of
the fixed incore SPNDs (Rh-emitter self-powered neutron detectors)
during initial physics testing and follow-up at Oconee I to provide
verification data for the physics models and instrumentation to the NRC
for final approval of the design models back in the mid-70s. It was a
100F+/80%RH hellhole in the bottom of the incore termination tank and
miserable suited up but we did it. The thought that there was
2250psi/650F water just on the other side of a 1/2" diameter tube w/
only an end cap and weld was unnerving to say the least... :)
And, just like the boiler in a 1000 MWe coal-fired unit isn't all that
large, the reactor vessel containing the reactor core itself is only
roughly 25-ft tall (about twice the height of the fuel) and 12-15 ft in
diameter. All the rest is ancillary equipment. The reason containment
buildings are the size they are is that they must be large enough to
allow for adequate maneuverability of equipment inside and have ample
volume such that the design overpressure of a design LOCA is within the
ability of the containment to withstand. Years ago circle-W designed a
set of reactors w/ ice containment (a huge rack lining the upper reaches
of containment w/ blocks of ice and the ancillary ice-making equipment).
This did allow them to reduce the initial capital cost by making the
containment significantly smaller since the ice-melt during LOCA would
quench the steam, thereby holding down the maximum pressure but these
turned out to be high maintenance items and afaik the concept has been
dropped in current generation designs on the docket for licensing now.
*Any* power plant works by moving energy from one place to another,
usually as heat, sometimes (in the case of hydro), 'potential' energy
When you're tapping a 'heat flow', you have to have 'somewhere' for the
heat to go _to_. Nuke plants, being a closed system have to have *BIG*
cooling towers to transfer the heat from the circulating coolant to the
atmosphere. The actual "power-plant" at a nuclear generation facility
is relatively small.
Naval plants have the ability to use the 'external' water that they're
surrounded with as a heat dump. *also* _most_ of the output from the
'teakettle' is _not_ used to generate electricity, which vastly reduces
the size of the actual electrical generators. The marine nuke's primary
purpose is to generate _steam_, used to drive turbines that are connected
to the propellers. Yup, "modern" warships are *STEAM*DRIVEN*
Space-based units can simply 'radiate' the heat away. Keeping in mint
that any such units used in space generate _miniscule_ amounts of power
relative to a power utility plant.
For land-based power generation, the actual generators need the same
amount of space, regardless of where the steam that drives them
comes from (coal-fired, other fossil-fuel, nuke, concentrated solar).
A fossil-fuel plant has to have a _lot_ of space for fuel storage and
an automated feed-system that provides controlled continuous delivery
into the combustion area. 'Waste' heat is simply vented directly
to the atmosphere.
Nuke plants don't have _any_ of those space requirements for handling
incoming fuel. The steam-generator system is somewhat larger, because
f the self-contained fuel supply, the 'more extreme' operating
conditions, and mandated additional safety systems.
But, not significantly larger for conventional fossil plant of the same
thermal output (some, as nuclear units don't have as high a thermal
efficiency as do fossil), but not to the extent that one notices it.
And, of course, there are both fossil and nukes that also use water as
the ultimate sink and therefore don't have cooling towers (Oconee I, II,
III, ANO-I, ...)
In the same manner as a nuke...some have cooling towers, others have
lake or river water...whatever, it requires an ultimate sink somewhere
and the association of the cooling tower w/ the nuke plant is simply a
figment of the press and their penchant for the backlight steam plume w/
the red filter to create ominous mood.
OTOH, in the US owing to the political stalemate they have requirements
for spent fuel storage although that is counterbalanced by ash disposal
at coal-fired units that requires even more actual space.
I don't follow the logic/intent re: steam-generators; again there's
little difference although owing to the higher boiler outlet
temperatures compared to PWR exit temp's the thermal efficiency is
better for fossil. BWRs, of course, don't have external steam
generators, only separators before the turbines. I don't understand the
'more extreme' operating conditions at all...as noted, as for the
thermal cycle fossil is both higher temp and pressure than nuclear.
The additional safety systems do require some space but they're all on
the primary side HPI/LPI/etc., ... Makeup and so on are very similar.
Those statistics were for seven years of operation and were quite
consistent from year to year in the monthly peaks and valleys reflecting
climatological trends, not just a one-year aberration.
The data were only available on a monthly generation basis so the
extremes in availability would be greater as looked at shorter time
periods if that level of reporting were available. Last week in the
doldrums SIL came by the wind farm on way here for visit and reported
only 2-3 of the whole installation were turning.
While the fuel is free it isn't always being delivered and is a diffuse
source so takes a lot of infrastructure to concentrate it into useful
form. That translates to $$/kw on grid; I don't think there would be
any significant interest by utilities at all if it weren't for the
various State-legislated mandates for percentages of generation from
green sources passed onto the utilities and the various tax incentives
to subsidize part of the cost.
Whether it will be cost-competitive eventually w/o those is anybody's
guess; certainly C-taxes if introduced will change the playing field
immensely in foreseen and unforeseen ways (and I personally expect more
of the latter than former). Unfortunately, however much scale and
technology improvements benefit the capital cost/installed-MWe, the
fundamental nature of the intermittent fuel supply can't be improved or
eliminated so the required conventional reserve capacity will still be
required which essentially doubles the cost for every MWe that isn't
available or reduces grid reliability if not there.
Common steam plants in use today, built years ago - are in the 40-42
percent thermal efficiency range. Newer prototype plants have hit over
60 percent. I doubt the prototypes will ever be built full scale with
that level of efficiency.
An internal combustion engine typically averages 18 to 20 percent, but
can get over 40 percent at peak efficiency.
53 to 54 percent at perfect load for combined cycle now. 42 percent for
coal fired steam plants, again at perfect loading. None of these plants
get to stay at perfect loading very much of the time, but they try.
Well, the 42% of a current supercritical boiler isn't what I'd have
interpreted as "common and built years ago"...is about right for last 10
years or so, agreed.
The only real hassle w/ the combined cycle is that it's a misuse of NG
for baseload generation imo. Good choice for load following, etc., ...
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