I've been reluctant to talk much about nuclear power here at The
Ergosphere because it's such a politically-charged topic. The
various issues of fuel availability, waste disposal and vulnerability of
reactors to attack attract a great deal of argument with little
agreement even on premises, let alone conclusions. This makes it a
singularly unfruitful area for discussion; it generates a lot of heat
but precious little light.
It might be more fruitful if some of the issues could be taken off
the table. Two of these issues are vulnerability of reactors to
terrorist attack and likelihood of leaks from other accidents.
Reactors are large, stationary (albeit fairly hard) targets; if the same
countermeasures could essentially eliminate the ability of terrorists
to hit the reactor while also confining most conceivable radioactive
accidents to the immediate area, both the real and perceived risks of
nuclear power would be greatly reduced.
One speculative vulnerability of reactors is aerial or artillery
attack, to breach the containment building and rupture the reactor
vessel to cause a meltdown. Leaving aside the extreme difficulty
of getting several bombers or a howitzer into the country and to the
proper position for attack, it's obvious that neither of these attacks
are even possible unless the reactor is above ground. An
underground installation is completely immune from attack by artillery
and would require nuclear bombs to damage with an aerial attack; a
terrorist attacker with a nuke has much better and softer targets than
reactors. It appears that underground construction (at an adequate
depth) is sufficient to eliminate most direct modes of terrorist attack.
The main issue with any such thing is the cost. Mining costs
money, construction in confined spaces is more difficult and expensive
than in open air, and engineering has to be done differently (and thus
separately) for structures intended to go underground. This would
make underground nuclear installations more expensive to build than
aboveground ones. But, I ask, are there compensatory benefits?
I can think of a few:
- There should be few issues with off-site liability insurance.
- Decommissioning means removing the fuel and locking the doors
(well, pouring concrete in the tunnels).
- As isolation is achieved with a physical barrier rather than
distance, plants can be located close to the points of use.
- Transmission losses are reduced.
- Plant waste heat can be used productively.
That last is the big one. If the typical plant is a pebble-bed
HTGR with a conversion efficiency of 40%, it increases the useful energy
from the plant by 150%.
District heating was once commonplace in cities, and the heat came
from the low-pressure steam output of generation plants (this is still
in use in some places, including many university campuses).
Unfortunately, the effort to remove pollution sources from cities also
caused all the byproduct heat to have to be dumped as waste, as heat
cannot be transported long distances without unacceptable cost and
losses. There is now an opportunity to reverse this trend and
capture that waste energy. But what's the value, and is it enough
to pay the extra costs?
Assume for the moment that the new reactors are 400 megawatt
pebble-bed HTGR's, the thermal efficiency is 40%, and T&D losses
for the typical above-ground unit are the average 7%. Further
asssume that the T&D losses for the underground unit are 3%, and
heat losses are 10%. The net product looks like this:
|Net to user
The ability to deliver "waste" heat in this case more than doubles the
total usable output from the plant. But the question still has
not been answered: what's the value of this new product?
The big answer depends on a bunch of smaller questions:
- How much of the rated heat output of the plant is used?
- What energy source is it replacing?
- In what form is it delivered?
- What is the cost of delivery?
- What is the backup in case of interruption?
For the sake of discussion I'll propose numbers that are not researched
and I hope aren't too unrealistic:
- Customers use 60 percent of rated output heat (the plant may
produce less than full output at times of low demand).
- This replaces natural gas for space heat and DHW, as well as
electricity for air conditioning (via absorption chillers).
- Heat is delivered as hot water or low-pressure steam at ~100 C.
- For a wild-assed guess, cost of delivery is 1 cent/kWh.
- The backup is electric resistance heat (used to supply service
when steam/water delivery is interrupted). Note that this is
better than current gas service, which provides no backup.
The retail price of natural gas is unlikely to go below $7/million BTU
in the next few years; if used at 95% efficiency, this corresponds to a
price of 2.5 cents/kWh of heat. The net value of the heat
delivered is the difference between this and the delivery cost, or 1.5
cents/kWh. For absorption A/C the energy replaced is electricity
rather than heat. The real cost of on-peak electricity for A/C is
at least 15 cents/kWh and sometimes much higher, so for this example I
will assume a flat 20 cent rate. The coefficient of performance
(CoP) of a good vapor-compression air conditioner is around 4, and the
CoP of an ammonia-water absorption-cycle chiller is approximately 0.5;
it takes about 8 kWh of heat to displace 1 kWh of electricity for
cooling, so the displaced cost of heat used for cooling is about
2.5 cents/kWh of heat (again).
|Net to user,
| Net value
| Net value
|| 0.4 (heating)
The total net value delivered is $42.6 million/year, or $106.50 per
kilowatt of electric capacity per year.
If natural gas goes up to $10/million BTU, the cost of heat from gas
goes up to 3.6 cents/kWh and the situation looks even better:
|Net to user,
| Net value
| Net value
|| 0.4 (heating)
The total net value delivered nearly doubles to $82.2 million/year, or
What kind of investment does this justify?
I'm no financial expert, but interest rates are fairly low at the
moment. If the investment in heat delivery infrastructure is
financed at 6% and amortized over 30 years, the heat stream is worth
about $1470/kWe at the $7/mmBTU cost of gas and a whopping $2830/kWe at
the $10/mmBTU price of gas! In contrast, the cost of
mass-produced pebble-bed reactors is
estimated at $1000/kWe
. It appears that the ability to make
use of plant waste heat is worth doubling or even tripling the cost of
What would it look like?
From the surface, not much; probably an access tunnel from a building
in an industrial or office park. During construction there would
be a lot of trucks taking soil and rock away and delivering concrete
and other materials. Cables would come to the surface in one or
more places to transmit power to electrical substations.
Underground it would be more interesting. The reactor proper
would lie at a safe (and perhaps considerable) depth, and its main
power turbines would be sited with it. But the heat distribution
network would radiate outward from it like a starburst, with pipes
carrying medium-pressure steam upward to local pressure-drop recovery
turbines in neighborhood manholes feeding the local steam/hot water
distribution pipes. Instead of a gas pipe coming into the house,
there would be a steam/HW supply pipe and a return pipe.
One curious feature is that the heat-distribution system would require
no pumps. Water coming down from the surface would arrive at a
depth of 1000 feet under more than 400 psi of pressure from gravity
alone; this pressure would have to be relieved through a throttling
valve or turbine to reduce it enough for the water to boil at less than
oven temperatures. Low-pressure steam has very low density, which
requires pipes too big to run long distances; the distribution network
would probably use steam at a moderate temperature and pressure.
Medium-pressure steam is far less dense than water, and would arrive at
the surface at not much less pressure than it left the underground; the
pressure could be used to drive another turbine. This convective
loop could generate power and provide fail-proof circulation.
Hardware at buildings would change too. Instead of a furnace,
you'd have a fan coil heated by steam or hot water; instead of a
boiler, you'd have a simple heat exhanger (with backup resistance
element). The water heater would look like an electric, but with
a water/steam coil in the bottom. But the big difference would be
in air condtioning systems. Absorption systems would be larger
than compressors, and would need to reject almost 3 times as much heat;
the outdoor units would be quite a bit larger than present compressors.
It might be worth putting them partially underground, leaving
only the condenser coils in the air. It might also be worth
installing the condensers in thermal chimneys, to cool them with
convective airflow and eliminate the need for fans. This would
have a definite and distinctive architectural impact.
Given such a heat distribution network, the reactor would not need
conventional cooling towers. The A/C chimney systems could be
employed as heat dumps when supply ran beyond demand.
Depending on the reactor design, the potential for damage or failure
seems very small.
- Pebble-bed reactors cannot melt down.
- Deeply buried reactors cannot be hit by aircraft, bombs or
- Gas-cooled reactors are unlikely to transfer radioactive
materials to cooling water or steam.
- Physical isolation of the reactor reduces or eliminates most
Worth doing? Depends what it costs to build something in a mine,
dig miles of tunnels and lay new piping networks. But if it is,
entire cities could be made independent of oil, coal and natural gas
for all their heating, cooling and electric requirements and do it
cleanly and quietly.
That's my kind of solution.