Sunday, October 31, 2010

Nuclear Energy

Nuclear energy actually describes two different types of energy, fission and fusion. Nuclear fission is the form already in use, while no nuclear fusion power plants yet exist. Nuclear fission is the process of splitting heavy atomic nuclei, such as uranium, plutonium, or thorium into lighter products, turning a small amount of the mass into energy in the process (associated with the nuclear binding energy of the nuclei). Nuclear fusion involves combining lighter atomic nuclei into heavier ones, retrieving a small amount of the mass as energy once again. As a general rule, fission releases energy for nuclei much larger than iron, while fusion releases energy for nuclei much smaller than iron.

Energy Potential

Nuclear energy is unlike most sources of energy that are considered renewable, because in any given year we are able to extract as much energy from it as we choose to use in fuel. Nuclear energy isn't limited by the amount of solar radiation falling on the surface, the speed the winds blow, or the amount of rain that falls. The limiting factor is the amount of material available to be used in nuclear reactions.



Uranium has two isotopes currently used to produce energy from nuclear fission. The first of these, uranium-235 makes up only about 0.7% of worldwide uranium, but can be used much more easily. The second isotope of uranium, U-238, can be turned into plutonium-239 within fast breeder reactors, which can then extract about 60 times as much energy from the same amount of uranium ore.

Uranium is currently mined from uranium ores. There are presently about 4.7*10^9 kilograms of uranium stored in these ores, enough at our present rate of use of just nuclear energy to last us a mere 85 more years according to the International Atomic Energy Agency. If we started exclusively using fast breeder reactors however, this same supply of uranium could provide the same power output for over 2500 years.

Much more uranium is stored in phosphates; an estimated 3.5*10^10 kilograms. Accessing this would come at a few times the cost of mining uranium ore, but since the cost of the fuel is such a tiny fraction of the cost of running a nuclear power plant, fuel extraction costs can afford to increase several times without causing a significant increase in the price of the electricity we receive.

The vast majority of the world's known uranium is stored in the oceans however. A total of 4.5*10^12 kilograms exists dissolved in our sea water, enough, if used in combination with fast breeder technology to continue current nuclear energy production for nearly 2.4 million years. Extracting uranium from sea water has been done in the past; however, never on an industrial scale, most likely due to the lack of economic incentive at current uranium prices.

Nuclear energy only accounts for 5.8% of the world's present energy supply. If we wished to replace the energy from all sources with energy produced from fast breeder reactors, the world's known uranium resources could last roughly 140,000 years.


The other major fuel currently used in fission reactors is thorium. Thorium reactors are similar to fast breeder reactors, but rather than turning uranium-238 into fissile plutonium-239 they turn thorium-232 into the fissile uranium-233. Known worldwide thorium reserves which are easily recoverable amount to 2.6*10^9 kilograms. Using this easily recoverable thorium with current reactor technology, this could supply all world energy demand for roughly 66,000 years.


Fusion is the form of energy that powers our sun. Currently fusion reactions have been done in lab conditions on earth, and energy has been gathered from these reactions, but the energy put into starting these reactions has always been greater than the energy recovered. In 2008 construction began on an international project, ITER, that hopes to be able to sustain a fusion reaction with an output of 500 million watts (Joules per second) for at least 1000 seconds at a time with an energy input of only a tenth of this amount. ITER plans to react deuterium and tritium creating helium and energy as its product.

Deuterium is a naturally occurring isotope of hydrogen, and about 33 grams of deuterium can be found in every ton of water. Tritium does not occur in large quantities naturally, but is made from lithium. The amount of lithium is the limiting factor in this reaction, so we will investigate its total potential.

According to the USGS, “The identified lithium resources total 760,000 tons in the United States and more than 13 million tons in other countries.” This is much less than the estimated 230 billion tonnes that are dissolved in our oceans. A reasonable estimate for energy gained per kilogram of lithium would be around 8*10^12 Joules. Assuming this technology pans out, this would mean nearly 2*10^27 Joules of potential energy, or enough to replace earth's current supply for 3.7 million years.

The deuterium-tritium reaction is not the only one which has been proposed. Deuterium-deuterium is a reaction that has been thoroughly tested in the past although also not yet implemented. For this reaction energy output is roughly 3.5*10^14 Joules per kilogram of deuterium. With about 2.4*10^16 kilograms of deuterium in our oceans, this would mean roughly 8.3*10^30 Joules, or enough to replace our current rate of supply for 16 billion years. This is longer than the current age of the universe and well past the anticipated death of our sun. We can only assume we will be able to harness a fraction of this energy, but this is an energy source with billions of years of potential in any case.



Much public fear exists over the potential for a nuclear meltdown. This public fear isn't entirely misplaced as a full scale nuclear meltdown has happened in the past in Chernobyl,Ukraine, and another near meltdown at Three Mile Island in Pennsylvania.


Fission reactors generate large amounts of heat even when they are not producing electricity, and the Chernobyl nuclear facility required coolant to be flowing at all times even when not generating electricity. In the case of a power outage, three generators would come online in order to keep coolant flowing. Starting these generators up took 15 seconds, and they took an additional 60-75 seconds to reach their full operating capacity. This was thought to be unacceptable, and thus it was proposed that the energy remaining in the plant's steam turbines could be used to power the coolant pumps for roughly 45 seconds while the generators started up. The reactor went online in 1983 without this system being tested successfully.

On April 25, 1986 conditions were planned to run the test at last. The day shift had been instructed on how the test was going to proceed, and the reactor had reduced its power output in the early morning on April 26. However, another power plant unexpectedly went offline that day and the Chernobyl reactor was ordered to return to full output. At around 11PM, the reactor was allowed to reduce its power output once again. The day shift had long since departed at this point, and the night shift was given little opportunity to prepare before being thrust into this test.

The reactor was supposed to be turned down to 700 million watts of power, but the production of neutron absorbing xenon in the reactor core caused the power to continue to fall down to roughly 500 million watts. The senior engineer overseeing the experiment then inserted the control rods into the reactor core much too far causing the power to drop to a mere 30 million watts.

When the control room finally decided to restore power, it took several minutes for power generation to begin to rise again, and it stabilized at roughly 200 million watts of output due to the amount of xenon that had accumulated during this time. At this point the control rods had been maximally withdrawn from the reactor core.

After the reactor power had stabilized near 200 million watts for some time the engineers decided to increase water flow through the reactor as part of the test, despite this being well below the intended test conditions. Increasing water flow dropped the power output of the reactor once again, causing the controllers to manually remove control rods from the reactor in an attempt to restore power.

It was under these conditions that the test began. Steam to the turbines was shut off and the water coolant was pumped using the energy remaining in the generator. As the generator slowed however the coolant began to flow more slowly, allowing more of it to boil and decreasing its ability to cool the reactor causing the reactor to heat more and boil more of the coolant.

The system handled this situation quite well actually, reinserting control rods to handle the increase in temperature. As the test was winding down, the emergency shutdown for the reactor was manually initiated, possibly as an attempt to turn off the reactor after the experiment. The system took several seconds to fully insert the rods however, which were poorly designed with graphite tips, causing them to displace coolant before inserting the neutron absorbing material.

As this happened the core power spiked, with none of the usual control rods in, little to no coolant flowing and steam already decreasing the efficacy of the coolant. Under high pressure steam exploded, fracturing some of the control rods and preventing others from inserting properly. A few seconds later a full nuclear meltdown occurred exploding a significant portion of the facility and causing the remainder to burn for several days.

In addition to having gone online without the proper safety tests having been conducted, the facility was in non-compliance with numerous standards of the time for nuclear facilities, most importantly the positive feedback the steam had on the core temperature, and the improper design of the control rods.

This full scale meltdown occurred at one of the world's largest nuclear facilities and represents the worst possible scenario for any nuclear power malfunction. In addition to the 57 direct deaths as a result of the accident, it is estimated that the total number of deaths attributable to this incident could amount to several thousand.

Several thousand deaths is a tragedy without a doubt, but it is on par with estimates for the deaths attributable to the pollution emitted from coal power plants alone every single year. In relation to power output, nearly all estimates put nuclear energy as one of the safest energy sources regarding its cost in human lives.

Spent Fuel

The radiation from the fuel itself isn't the main concern when dealing with spent nuclear fuel. The fuel was mined out of the earth in the first place and it would be hardly any effort to dilute it down to its original concentration and dispose of it underground. The main concern is the radiation from shorter lived isotopes created as products of the fission reaction. If uranium-238 with a half life of 4.5 billion years is split into two smaller isotopes each with half lives of 4.5 million years, then the expected rate of decay at present will have increased 2000 fold (although the type of radiation emitted may have changed). Many of the fission products have much shorter half lives, and once spent fuel has been allowed to cool for a few decades only a tiny fraction of the original radioactivity remains.

Within roughly a thousand years, the radioactivity of the spent fuel is no higher than that of the original ore.


Nuclear energy is by far the cheapest energy source with the potential to replace fossil fuels in the long term. It also has the ability to keep up with a growing population and an increasing standard of living. If we wish to make nuclear power last in the long term, more sustainable fast breeder technologies will need to be adopted on a large scale, but eventually nuclear fusion technology will need to be explored. Current projects such as ITER and its proposed successor DEMO show a great deal of promise, but have yet to become realities. If these technologies pan out as hoped, we may finally have found a solution to long term energy independence.

Additional Reading

Sustainable Energy — without the hot air

No comments:

Post a Comment