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Tuesday, June 7, 2011

Know More Nukes

It seems pretty likely that the Fukushima nuclear plant accident is going to send ripples (if not tsunamis) through the nuclear power industry.  Unfortunately, as is so often the case, the ripples are likely to be misguided, focus on the wrong issues and recommend untenable solutions.  Such is the controversy over nuclear power.  Rather than focus on the emotionally-laden issues that tend to drive the nuclear debate, I’d like to look at some of the practical issues and tradeoffs that characterize nuclear power.

Nuclear energy was developed after World War II as a peaceful use for the devastating power of atomic fission.  It was based on the simple idea of using the heat generated by controlled fission reaction to make steam that could be used to drive a turbine and make electricity.  Because it does not rely on the combustion of any fuel, nuclear fission was an ideal fuel for ships – particularly submarines, which can remain submerged almost indefinitely.  When used to operate power plants, nuclear energy was characterized in the 1950 and 1960s as providing electricity too cheap to meter.  It has substantial advantages over other electricity sources, particularly coal.  Characterizing coal-fueled generation as “burning dirt” is not far from the truth.  The table below compares the annual requirements for a 1,000 MW nuclear generator versus the same size coal plant (burning Power River Basin coal), both running base load, about 90% of the time:

Fuel required
5,000,000 tonnes
27 tonnes
GHG output – CO2 equiv
7,884,000 Tonnes
NOx emissions
14,000 Tonnes
Radiation released
490 person-rem/year
4.8 person-rem/year

That’s right, a coal power plant requires over 600 tons of coal per hour while a nuke needs to be refueled once every 18-24 months.  It’s no wonder that nuclear power plants might have been seen as an incredible boon to the industry.  But, of course, none of the nuclear opponents are suggesting replacing the nukes with coal plants.  Instead, the solution is usually to build more renewables, they don’t pollute, the fuel is “free,” and they create more jobs.  The renewable resources of choice are primarily wind and solar.  Time for another reality check.

Wind Power

Wind turbines are probably the most widely recognized renewable resources.  They must be located in areas where sufficient wind is present and only operate when the wind is blowing.  Even sufficiently windy areas can only support generation about 30% of the time.  As a result, to replace a baseload nuclear generator of 1,000 MW, would require over 3,000 MW of wind generation plus 1,000 MW of storage that can operate the 70% of the time when the wind is not available.  Wind turbines require about 30-50 acres/MW, so 3,000 MW would require about 120,000 acres or 187.5 square miles.  This is in addition to the storage facility, which with currently available technology, would need to be a pumped hydro storage facility which would also have a substantial footprint and cost.

Solar, Maybe?

Solar power is another favored alternative to nuclear power.  It has the same kind of limitations as wind, though it is possible to have storage incorporated into a solar resource (by using concentrating solar thermal and molten salt storage rather than photovoltaics).  But here again, many more MW would be needed to produce the same energy as a baseloaded nuke.  Solar can only produce energy when the sun is up and high enough in the sky to be collected.  A 25% capacity factor is typical for middle latitude installations, so 4,000 MW of solar would be needed to provide the same energy as our 1,000 MW nuclear plant.  Solar plants need about 7 acres per MW, so about 44 square miles of land would have to be covered by collectors to be displace a single nuclear unit.  Because of the negative impact of cloud cover, these plants would be best sited in relatively sunny areas like the desert. 

Natural Gas Generation

The most likely replacement for nuclear power is probably natural gas combined cycle generation.  It is a fossil fuel that is combusted in the generation process, but because combined cycle generation is more efficient than steam cycle generation (<7MMBtu/MWh versus 10MMBtu/MWh for coal) and natural gas is less carbon intensive than coal (117#CO2/MMBtu versus 213 for coal), it produces about 40% as much CO2 per unit of electricity as coal.  Natural gas is much cleaner burning than coal and delivered via pipeline rather than unit train, gas plants are more scalable than coal or nuclear generation.  Thanks to advances in drilling technology that are allowing access to shale gas (yet another anathema for environmentalists), natural gas availability is increasing and prices are remaining fairly stable.  The United States could replace its entire nuclear fleet with gas-fired generation and increase total gas consumption by less than 25%, a significant increase but a potentially viable one.  Besides the fact that this conversion would increase GHG emissions by about 300 million tonnes per year, if the gas were used instead to replace coal power plants, it would reduce GHG emissions by about 450 million tonnes per year.

Distributed Generation

For those who prefer to think outside the central power plant box, distributed generation is the answer.  A combination of rooftop solar with storage for residential loads could have some promise, though it would be a major undertaking.  A fairly large residential solar system would cover about 800 square feet of roof and produce a maximum of 10 kW.  Being fixed panels, these systems would produce less energy than a centralized tracking system, with approximately a 20% capacity factor.  That would mean 5,000 MW to replace a baseloaded nuclear plant.  That would be about 500,000 rooftops.  Local battery storage could be accomplished with a battery pack about the size of one used for an all electric car, about 70 kW.  To replace the entire 101,000 MW of nuclear generation in the US would require solar panels on 50 million roofs with a comparable number of electric car batteries.  Should electric car batteries and photovoltaic panels continue to drop in price, this could become a viable option – in a decade or two.  By piggybacking on electric vehicle development, this approach could actually significantly reduce reliance on imported oil.  Should fuel cells ever become a cost-effective alternative, they could also prove to be a game-changer.

The Real Problems with Nuclear Power

When they work the way they’re supposed to, nuclear power plants are very impressive.  They don’t pollute the air, don’t create greenhouse gases, require virtually no fuel, can fit in a fairly small space, and like to run flat out all the time.  They put less radiation into the atmosphere than coal and produce vastly smaller quantities of waste.  Their primary problem, when operating as designed, is the large amount of heat that must be removed from the process. Nuclear plants produce steam at a lower temperature and pressure than generators that rely on combustion can produce.  As a result, more lower temperature heat must be removed from the steam to achieve efficient operation.  That is why nuclear plants have those huge iconic hyperbolic cooling towers, or are located adjacent to bodies of water into which they transfer heat.  The amount of heat they transfer can impact local ecosystems, not to mention the organisms destroyed in pumps and screens as they are sucked through the cooling system.  The US EPA and at least one state (California) are developing regulations to reduce or mitigate the impacts of this once through cooling process.
Another problem with nuclear power is what happens when things aren’t working the way they’re supposed to.  Because of the potential problems when something does go wrong, nuclear power plants are pretty much uninsurable.  Instead, governments legislate liability limits for nuclear plant owners or take responsibility beyond a certain level.  While this has been necessary to make investment in nukes commercially viable, it eliminates or at least mutes signals to engineer changes that would reduce the potential risks associated with something going wrong. 
What about the radioactive waste generated by these plants?  While the “preferred” solution of hauling spent nuclear fuel to a geologically stable location where it can be stored for the thousands of years needed for it to reach safe levels of radioactivity has not come to pass, dry cask storage systems make it possible for a nuclear plant to store all its spent fuel on site in a passively safe manner.  According to the World Nuclear Association[1], worldwide, there are about 270,000 tonnes of used nuclear fuel currently in storage with an additional 12,000 tonnes added annually.  Compare this to the 125 million tons of combustion by-products produced annually by coal power plants in the US.  Nuclear plants could actually store all their spent fuel on site for their entire operating life in containers that can be safely shipped to centralized storage or reprocessing facilities when and if they become available.

The Bottom line

Nuclear energy is an attractive base load generating resource that can produce large amounts of electricity without the pollution problems and global warming impact of plants that rely on combustion of fossil fuels.  Nukes require much less real estate than solar or wind generation and provide a much more predictable energy supply than these intermittent resources.  When the smaller scale, passively safe, factory built nuclear generators currently under development are licensed and become available, they may have an important role to play in our energy future.

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