Tuesday, July 13, 2010

Isn't energy from solar/wind better then nuclear power?

I personally think solar and wind have some place in our energy future. The sun is the source of most of the energy available to us, and in some places in the country the wind blows with pretty good regularity. However solar and wind can not be the solution to our energy problems alone, they just don't have the energy density.

The sun outputs roughly 400 watts per square meter to the surface of the earth. With 25% efficiency, that yields 100 watts per square meter. The yearly average number of daylight hours is around 12 hours per day. The peak output of a solar plant is thus 1200 watt hours (1.2 kwh) on average per square meter in the southwest. This is variable over the year, on the summer solstice the number of daylight hours is 14.2 hours per day and 9.8 hours per day for the winter solstice. Even at 50% efficiency, we would only double that to 2400 watt hours (2.4 kwh) per square meter. To reach the same power output of a nuclear power plant with 25% efficiency solar panels, we need to match 24 gigawatt hours per day. This means we need 7.7 square miles of panel area and a technology that can store 12 gigawatt hours of energy without significant losses, so that it can be released over the night. This is a very simplified analysis, and any real installation will be larger. The storage technology that we have now also imposes significant losses and will carry a large penalty in required area.

Wind is much worse, and needs around 270 square miles of area, according to an analysis published by the office of Senator Lamar Alexander, to produce 1 gigawatt of reliable electricity. To replace all our coal fired plants with wind farms would require an area a little bit bigger then the state of Michigan (our 12th largest state at 86,943 square miles).

Solar and Wind just can't compete with nuclear power because of the energy density. With advanced technology and investment they might help generate some more of our electricity in the future. However, they just aren't feasible replacements for baseline loads. To make a significant dent in carbon dioxide emissions and air pollution we need energy that is reliable 24 hours a day, 365 days a year.

Saturday, July 10, 2010

Aren't Nuclear Reactors Dangerous?

In June 1959, Niels Bohr pressed a switch to rapidly remove all the control rods from the first Inherently Safe Nuclear Reactor – the Triga. The reactor did not meltdown, but after a very brief power spike of a few thousandths of a second the reactor quieted down. If such a feat was attempted in a normal Pressurized Water Reactor, it would surely cause a catastrophic accident. The Triga is a very different kind of reactor, it is built on principles which guarantee safety by the laws of physics, not just engineering cleverness. Most nuclear reactor designs currently in use today, and most of our advanced nuclear designs are not Inherently Safe Nuclear Reactors. As we deploy more nuclear reactors, we should develop systems that are safe by the nature of the laws of physics. This should be our long term goal as a society for nuclear power. One design that is particularly promising in this regard is the Liquid Fluoride Thorium Reactor, it can be built to have passive safety guaranteed by the laws of physics. That design also has many other highly desirable features, such as high usage of nuclear fuel, low proliferation risk, and higher thermodynamic efficiency.

Even without Inherently Safe Nuclear Reactors, the nuclear industry has the best track record of all power producing industries as far as safety is concerned. There has been only one severe accident in the history of nuclear power usage that caused loss of life. This accident occurred at Chernobyl in the Ukraine, and resulted in the loss of 56 lives.

While the accident at Chernobyl was horrible and something we want to avoid in the future, it pales in comparison to the loss of life from other energy sources. Chernobyl was caused by what could be described as an unsafe experiment in reactor physics done by ill trained personal. The Chernobyl reactor was also an unsafe design that does not lose moderation when the coolant is lost. The Chernobyl reactor used graphite as a moderator, while western designs typically use the coolant itself as a moderator. When a design that uses the coolant as a moderator experience loss of coolant, the nuclear reaction slows and becomes less intense. The opposite happened in the Chernobyl reactor, and without the containment that is standard in Western designs there was release of radiation into the environment. We can definitely do better, and avoid this sort of accident in the future.

In the United States and the European Block, we have never had a commercial nuclear accident that caused the loss of life. There have been studies and analysis that considered the replacement of the Barseb├Ąck nuclear reactor in Sweden with coal power. For the same generating capacity the study concluded with a high probability that around 200 lives per year would be lost by replacing one nuclear reactor with a coal fired plant producing the same energy output.

Here is a chart detailing deaths per terawatt hour:

One nuclear power plant typically produces 1 gigawatt continually. Over an entire year if run at 100% capacity it generates 8760 gigawatt hours of electricity, this can also be expressed as 8.76 terawatt hours.

The mean average number of deaths per terawatt hour for coal is 25 deaths for the European Union. The mean average number of deaths per terawatt hour of generating capacity for nuclear is 0.02 deaths (An ExternE report, a research project done in the European Union to determine the external costs for energy generated was the source for the numbers used in the following calculation.)


8.76 terawatt hours * 25 deaths per terawatt hours = 219 deaths per year from coal.

8.76 terawatt hours * 0.04 deaths per terawatt hours = 0.3504 deaths per year from nuclear.


The study makes a further statement that the risk of death from a nuclear accident was very unlikely and the risk of death from coal was very high, essentially 100%.

The chart above came from the following report available on the internet:

http://manhaz.cyf.gov.pl/manhaz/strona_konferencja_EAE-2001/15%20-%20Polenp~1.pdf

Additional information about the ExternE project in general is available at:

http://www.externe.info/


Nuclear power is the safest power producing technology we currently possess. To call nuclear power unsafe just does not make sense given the numbers. Additionally with basic fundamental research, we will only further improve the situation.

Thursday, July 8, 2010

Isn't Spent Nuclear Fuel a Problem?

We have been told that spent nuclear fuel is a huge problem. It is something that is currently a difficulty, but with investments in technologies that we have known about for 40 years the problem becomes much more manageable. Even in its current state, nuclear waste is significantly better then our other energy choices. The coal plants that currently produce around 50% of our power have a much nastier waste problem. Based of statistics from 2005, there is approximately 336 gigawatts of generating capacity from coal in the United States. In total these plants burn 1.05 billion short tons of coal, and generate 120 million short tons of toxic waste. To put things in perspective that is roughly equivalent to having something the size of a 1990 Honda Civic for every family in America. (A 1990 Honda Civic weighs 1 metric ton, 120 million short tons is approximately 108,862,000 metric tons and as of the 2000 census we have 105,480,101 households in the United States.)

The amount of arsenic alone in this amount of coal is enough to kill every man woman and child on the planet ten times over. In the 1.05 billion tons of coal there is 7884 tons of arsenic, 109 tons of mercury, 1167 tons of beryllium, 8810 tons of chromium, 750 tons of cadmium, and 2587 tons of selenium, and 9339 tons of nickel. Coal combustion waste is the second largest waste stream in the United States.

If we average out the 120 million tons of coal waste by gigawatt of electricity generated for a year, we get an average of 357,000 short tons (323,994 metric tons) of toxic waste per gigawatt / year. A single large nuclear power plant generates only 35 metric tons of spent nuclear fuel per year, while producing 1 gigawatt over that year. If we could replace all of our coal generating capacity with nuclear reactors tomorrow, we would produce approximately 11,760 metric tons of spent nuclear fuel per year. This is still a large amount of waste, but it is about 1 / 10,000th the amount of waste.

Most of the spent fuel is not waste, but in the United States we do not reprocess our fuel. Around 1 ton of the spent fuel consists of fission products, and 0.3 tons is plutonium. The remainder of the fuel is primarily a mix of U-235 and U-238. With an efficient reprocessing technology we can reduce the waste portion of spent nuclear fuel significantly. If we remove the Uranium metal from our spent nuclear fuel, we are left with only 437 metric tons of waste to match the generating capacity of coal. The plutonium is the longest lived component of the waste portion, and the 101 metric tons produced of it can be burnt in fast reactors as fuel. By reprocessing our fuel and burning it in fast reactors we are left with 437 metric tons of fission products. Many of these fission products are useful industrially, but even if we consider it all as waste we have 4 / 100,000th the quantity of the coal waste. An efficient and responsible nuclear industry would produce 437 honda civic's worth of nuclear waste per year. Nuclear waste processed in this manner would decay in about 300-500 years to the activity level of Uranium ore.

Why are we burning coal when we could do so much better?