It's (a) mine!

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:

1. There should be few issues with off-site liability insurance.
2. Decommissioning means removing the fuel and locking the doors (well, pouring concrete in the tunnels).
3. As isolation is achieved with a physical barrier rather than distance, plants can be located close to the points of use.
   1. Transmission losses are reduced.
   2. 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:

Unit	Output	Deliverable   fraction	Net to user
Remote aboveground	400 MWe	0.93	372 MW
	600 MWth	0	0
Local underground	400 MWe	0.97	388 MW
	600 MWth	0.90	540 MW

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:

1. How much of the rated heat output of the plant is used?
2. What energy source is it replacing?
3. In what form is it delivered?
   
4. What is the cost of delivery?
5. 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:

1. Customers use 60 percent of rated output heat (the plant may produce less than full output at times of low demand).
2. This replaces natural gas for space heat and DHW, as well as electricity for air conditioning (via absorption chillers).
3. Heat is delivered as hot water or low-pressure steam at ~100 C.
4. For a wild-assed guess, cost of delivery is 1 cent/kWh.
5. 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).

Output	Deliverable  fraction	Capacity used	Net to user,  average	Delivery  cost	Cost  displaced	Net value per unit	Units  per year	Net value  delivered
600 MWth	0.90	0.4 (heating)	216 MW	$.01/kWh	$0.025/kWh	$0.015/kWh	1.892*109 kWh	$28.4 million
		0.2 (cooling)	108 MW	$.01/kWh	$0.025/kWh	$0.015/kWh	9.406*108 kWh	$14.2 million

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:

Output	Deliverable  fraction	Capacity used	Net to user,  average	Delivery  cost	Cost  displaced	Net value per unit	Units  per year	Net value  delivered
600 MWth	0.90	0.4 (heating)	216 MW	$.01/kWh	$0.036/kWh	$0.026/kWh	1.892*109 kWh	$68.0 million
		0.2 (cooling)	108 MW	$.01/kWh	$0.025/kWh	$0.015/kWh	9.406*108 kWh	$14.2 million

The total net value delivered nearly doubles to $82.2 million/year, or $205.50/kWe/year.

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 construction.

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.

Risk factors

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 artillery.
• Gas-cooled reactors are unlikely to transfer radioactive materials to cooling water or steam.
• Physical isolation of the reactor reduces or eliminates most other hazards.

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.  ¶ 4/28/2005 06:06:00 PM