26 March 2021

Portable Nuclear Power Plants And The Alternatives

Nuclear power has lots of benefits that this proposal for a 1-5 megawatt portable nuclear power plant illustrates well. I'm not convinced that it is a great solution near war zones, however.

Portable Nuclear Power Plants

The Pentagon has selected two companies to move forward with developing small, portable nuclear reactors for military use in the field.

BWXT Advanced Technologies and X-energy were chosen by the department’s Strategic Capabilities Office to continue on with Project Pele, which seeks to develop a reactor of 1- to 5-megawatt output that can last at least three years at full power. In addition, the reactors must be designed to operate within three days of delivery and be safely removed in as few as seven days if needed.

The two companies, along with Westinghouse Government Services, were each given preliminary contracts of less than $15 million in March 2020 to begin design work. The final design is due to the Strategic Capabilities Office in 2022, at which point the Defense Department will make a decision on whether to move forward with testing the systems.

“We are thrilled with the progress our industrial partners have made on their designs,” Jeff Waksman, Project Pele’s program manager, said in a statement. “We are confident that by early 2022 we will have two engineering designs matured to a sufficient state that we will be able to determine suitability for possible construction and testing.”

The Pentagon has long eyed nuclear power as a potential way to reduce both its energy cost and its vulnerability in its dependence on local energy grids. According to a news release, the Defense Department uses “approximately 30 Terawatt-hours of electricity per year and more than 10 million gallons of fuel per day.”

According to an October 2018 technical report by the Nuclear Energy Institute, 90 percent of military installations have “an average annual energy use that can be met by an installed capacity of nuclear power” of 40 MWe (megawatt electrical) or less. 

. . .

That effort, ordered in the 2019 National Defense Authorization Act, involves a pilot program aimed at demonstrating the efficacy of a small nuclear reactor in the 2- to 10-MWe range, with initial testing at a Department of Energy site around 2023.

While Project Pele is focused on the potential for deployable nuclear reactors, the acquisition and sustainment effort is focused on domestic military installations, with the goal of being operational by 2027.

From here.

A one megawatt power plant is enough to power about 500 homes.

If the illustration showing one unit fitting into a C-130 transport aircraft or a single semi-truck load or a single rail car is correct, the size would be on the order of 20 short tons to generate 1 megawatt of electricity for three years. Refueling in place for another three years, however, could involve a much smaller payload.

This is, at least, a factor of twenty better than the next best alternative (natural gas) in terms of logistics burden, and much better than that relative to diesel, coal, wind power, or solar power. The advantage is proportionately greater if the 20 ton portable nuclear power plant provided more than 1 megawatt of power. If it could produce 5 megawatts of power, the advantage would be a factor of 100 better.

The cost of such a power plant isn't clear.

If the power plant unit cost scaled proportionately to larger nuclear power plants, the cost of the power plant would be on the order of $1-$6 million for a 1 megawatt plant, but it isn't obvious that this is a reasonable assumption. It would have a useful life of several decades.

According to the same source, nuclear power plant fuel costs about $1,390 per kilogram and a kilogram of fuel generates about 360 megawatt hours of electricity. So, it needs about 73 kilograms of fuel for three years for a 1 megawatt plant at a fuel cost of $101,470 (far lower than any of the alternatives in addition to being much easier to deliver in a single trip).

Put another way, it takes about 1 gram of nuclear power plant fuel to power one house for one gram. The fuel cost for that one week is about $1.39.

Nuclear power plants also involve high level nuclear waste disposal issues that aren't currently resolved in the U.S. and would add additional cost. It would generate no air pollution, however. There are environmental issues involved in mining uranium, but because so little uranium is needed for fuel, these concerns shouldn't be overstated.

The national security concerns involved in keeping 73 kilograms of uranium based nuclear fuel safe from adversaries and preventing proliferation of uranium that could be used in nuclear weapons is a serious concern in a front line military application.

But the same concerns would not apply in civilian applications in remote domestic areas like small Alaskan villages or Hawaiian islands, where grid based power generation could be a problem.

Fossil Fuel Alternatives

Fossil fuels are far less portable, even without considering the power plants themselves.

Natural Gas Generators

How does natural gas compare?
A high efficiency, natural gas-fired combined-cycle power plant might consume about 7000 Btus of gas to produce one kilowatt-hour of electricity. That would be about 7 cubic feet of natural gas. It would therefoe take about 7000 cubic feet of gas to produce one megawatt-hour. In the U.S. today, one thousand cubic feet of gas sells at wholesale for about US$7.00 (seven dollars. So it would cost about $49 (wholesale price) just for the fuel to make 1 MW-hr of electricity this way.
So, 184 million cubic fee of natural gas would be needed to run a 1 megawatt power plant for three years. The fuel cost at U.S. natural gas prices would be about $1,288,000 over three years (in addition to the cost of the power plant). 

A fully loaded semi-truck can carry about 11 tons of liquid natural gas (which means that 184 million cubic feet of natural gas is about eighteen truckloads shipped in liquid natural gas form).

The power plant costs about $215,000 (used without installation or delivery). See also hereThe generator itself isn't huge (certainly less than one truckload, and probably less than about 20 tons). 

Additional materials (and weight) to store the natural gas until needed might be required and might take another truckload or so.

The combined weight would be about 20 truckloads to operate it for three years.

A much more powerful 42 megawatt gas turbine electrical generator weighs about 151 short tons with a width of 170 inches, a length of 650 inches, and a height of 194 inches, in addition to a need for 90 square feet of duct flow areas at the inlet and 57 square feet of exhaust area. This would be 4.6 short tons if it scaled linearly, although it probably would actually weigh more than that because there are probably efficiency of scale.

This is far and away the lowest logistical burden of the fossil fuel alternatives, in addition to being much cheaper than the diesel generator option.

Natural gas produces the least air pollution, by far, of the fossil fuels, although the environmental impacts of producing natural gas through fracking are material.

Diesel Generators

It would take 2,102,400 gallons of petroleum liquids (50,058 U.S. barrels of oil, weighing about 7,359 short tons, or about 180 semi-tanker trucks full) to run a 1 megawatt power plant for three years. At about $3 per gallon for diesel fuel (commonly used by the military) it would cost about $6,307,200 to run diesel powered electricity generators producing 1 megawatt of power for three years (in addition to the cost of the generator).

A one new megawatt diesel generator (fuel not included) weights about 8.7 short tons and costs about $180,000 (including shipping but not including installation) with dimensions of about 169" x 79" x 93" but without fuel tanks. One twice that capacity with everything included weighs about 21.25 tons:
A 2,000 kW set can be housed in a 40 ft (12 m) ISO container with fuel tank, controls, power distribution equipment and all other equipment needed to operate as a standalone power station or as a standby backup to grid power. These units, referred to as power modules, are gensets on large triple axle trailers weighing 85,000 pounds (38,555 kg) or more.

The combined fuel and generator weight for a 1 megawatt diesel power plant would be about 7,380 short tons in 181 truckloads to operate it for three years.

Diesel fuel produces lots of air pollution, although not as much as coal, producing petroleum and refining it to produce diesel fuel has major environmental impacts, and so does the harm caused by inevitable oil spills when it is transported.

Coal Fired Power Plants

It would take about 14,454 short tons of coal to to run a 1 megawatt power plant for three years (at a U.S. delivered price of $38.52 per short ton), for a cost of $556,768 over three years. This is about 627 semi-tractor loads of coal (see here and here).

This is in addition to the cost of the generator and the size of a coal fired 1 megawatt power plant is less easily available since a typical coal fired power plant has a 600 megawatt power producing capacity.

The cost of a fully installed 1 megawatt coal fired power plant, meeting utility grade emissions standards, and ignoring economies of scale, is about $4 million. It would probably take more than 1 truckload to ship a one megawatt coal fired power plant. Generously estimating that it would take three truckloads, the total shipping requirement is 630 truckloads to operate it for three years.

Coal produces an immense amount of highly damaging air pollution, producing it has catastrophic environmental consequences in the area it is produced, and coal has not insignificant amounts of toxic and radioactive materials in it that make storing coal and disposing of coal waste a major environmental threat that can leach into ground water as well. Coal spills, however, are trivial threats compared to oil spills and do not present nearly as much of an explosive risks of a natural gas transportation incident (although coal dust can explode).

Renewable Power Comparisons

Wind Power

A single large wind turbine can generate 1.5-1.8 megawatts of electricity, and weighs 164-334 tons, in addition to a concrete anchor of more than 1000 tons. The total height, including the blades is 328-410 feet.

The turbine itself would require 10-20 truckloads (given how awkward it is to ship), and about 50 truckloads of cement would be required. Since two turbines would be required about 120-140 truckloads of materials would have to be shipped to build a power plant with annual generating capacity comparable to a 1 megawatt nuclear power plant, although it would obviously operate for much more than three years.

You need about 50 acres of land area per industrial sized wind turbine, but unlike the land used for a solar farm, the land at the base of a wind turbine can be used for other purposes. They cost about $1.5 million to $2 million per megawatt installed, although obviously, with no fuel costs.

Because wind turbines don't generate electricity at peak capacity 24/7 so it takes more than 1 megawatt of power production capacity to match the annual electrical output of a 1 megawatt fossil fuel or nuclear power plant, although the disparity of about 3-1 is less than for solar power. It takes about two big wind turbines to match the annual power generation of a 1 megawatt fossil fuel or nuclear power plant.

Also, in the absence of a larger power grid, some sort of battery or electricity storage system would need to be in place at additional cost and logistical demands to provide 24/7 power. It would take a roughly 2.5 megawatt hour capacity battery system (2500 kWh) to make a 3 megawatt capacity wind turbine system provide 24/7 power equivalent to a 1 megawatt fossil fuel or nuclear powered system.

Projected electric car scale battery costs as of 2024 are about $100 per kWh, which would imply about $250,000 for a battery support for a 24/7 megawatt capacity wind power system. This is fairly modest relative to the $4.5 million to $6 million installation cost. They would also not be all the huge in size at about 6 kilograms per kWh, which is about 15 metric tons (or about 16.5 short tons) for a 2.5 megawatt battery. The battery unit adds about 1 truckload to the total weight of the system.

Wind power has a mild and possible to mitigate impact on birds. It generates no air or water pollution. The materials used to build wind power turbines are mostly mundane and not particularly environmentally problematic (e.g. cement and steel) compared to many other possibilities. A few exotic materials used in wind turbine electronics create some environmental impacts when produced, however.

Solar Power

It takes roughly 5 megawatts of peak solar power generation capacity to produce the same amount of average electricity per day as a 1 megawatt fossil fuel or nuclear power plant, because a solar power plant doesn't generate electricity 24/7.

It costs about $22 million to build a 5 megawatt solar farm, although it would generate power for more than three years and only needs to be serviced 3-4 times a year (another source says closer to $5 million, but this may be after subsidies), although it would obviously operate for much more than three years. 

It would take about 35 acres of land, and isn't remotely portable. 

It isn't entirely obvious how many truckloads of materials would be required for a solar farm of this scale, but it would be considerable. The solar panels themselves weigh about 133 pounds per kilowatt of power generation capacity, which would imply that a one megawatt solar farm would have 133,000 pounds (62.5 short tons) of solar panels, which would be about three truckloads. But, a great deal of additional weight would be required for mountings, cement pads upon which to place the mountings, and the electrical grid connecting the panels.

Also, in the absence of a larger power grid, some sort of battery or electricity storage system would need to be in place at additional cost and logistical demands to provide 24/7 power. It would take about a 1.5 megawatt hour battery capacity for a 5 megawatt solar farm to provide 24/7 power comparable to a 1 megawatt nuclear power plant. This would cost about $150,000 and weight about 8 metric tons (about 8.8 short tons).

Still, the total number of truckloads of materials involved in a solar farm would be quite competitive with diesel generators, coal fired power plants or wind power, and maybe even with natural gas, although the footprint of the solar system on the ground would be vastly greater than any of the other alternatives.

Solar power generates no air or water pollution. The materials used to build solar panels, however, create some environmental impacts when produced and if they have to be discarded. They also take up a lot of open space, which may or may not have had more desirable uses such as farmland, parks, housing, and open space/natural habitats.

My understanding is that solar panels require more annual maintenance on average (e.g. to replace or repair damaged panels from hail storms) than wind turbines do.

Operating Costs

None of these estimates consider operating costs other than fuel, which are implicitly assumed to be comparable. But, the operating costs other than fuel (mostly utility personnel) would not be immaterial and would not necessarily be all that similar between the options.

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