NYGH 304 Physics SIA: Nuclear Energy-

Friday, June 20, 2008

Nuclear energy produces 16 percent of the world’s electricity, compared with coal, which produces 39 percent and hydropower, which produces 19 percent. Just how green is nuclear energy?
NOT
1.Every step (except reactor operation) in the long chain of processes that makes up the nuclear fuel “cycle” - mining, milling fuel fabrication, uranium enrichment, construction and decommissioning of the reactor, and waste management - burns fossil fuels and hence emits carbon dioxide (CO2)
2.CO2 emissions from the nuclear fuel cycle are only small when high-grade uranium ore is used
3.However, there are very limited reserves of high-grade uranium in the world and most are in Australia and Canada
4.Worldwide electricity demand is predicted to grow by 85 percent by 2030, nuclear power would have to almost double its capacity just to maintain its current share of the energy mix, therefore it is not a feasible solution for an energy source
5.Nuclear power produces toxic, radioactive waste, for which no safe method of disposal has been approved. U.S. nuclear plants have produced over 40,000 metric tons of high-level radioactive waste, which is deadly for hundreds of thousands of years. It is produced at an annual rate of about 10,000 tonnes in nuclear power reactors worldwide.
6.In addition to the perennial problems of plant malfunction and human error, terrorism looms large as a threat to nuclear plants and everyone working and living in their vicinity. The 1986 Chernobyl disaster is an example of a disastrous catastrophe that killed many.
7. The cost of building nuclear plants is too expensive. The world's 440 operating power reactors, with about 364,000 MWe of total capacity, produce about 16% of the world's electricity. Coal, gas and oil account for four times that amount — about 64%. So to replace fossil fuel generated electricity with nuclear power would require a five-fold increase in the number of reactors, from 440 to about 2200. The cost of the additional 1760 reactors would be several trillion dollars.

BUT!
THERE'S AN ALTERNATIVE!
"Physicists and engineers at Beijing's Tsinghua University have made the first great leap forward in a quarter century, building a new nuclear power facility: a pebble-bed reactor (PBR) – sometimes also known as a Pebble Bed Modular Reactor (PBMR). This reactor is small enough to be assembled from mass-produced parts and cheap enough for emerging economies. Its safety is a matter of physics, not operator skill or reinforced concrete. This reactor is meltdown-proof.

What makes it so safe is the fuel: instead of conventional fuel rods made of enriched uranium, PBRs use small, pyrolytic graphite coated pebbles with uranium cores. As a PBR reactor gets hotter, the rapid motion of atoms in the fuel decreases probability of neutron capture by U-235 atoms. This effect is known as Doppler Broadening. Nuclei of heated uranium move more rapidly in random directions generating a wider range of neutron speeds. U-238, the isotope which makes up most of the uranium in the reactor, is much more likely to absorb the faster moving neutrons. This reduces the number of neutrons available to spark U-235 fission. This, in turn, lowers heat output. This built-in negative feedback places a temperature limit on the fuel without operator intervention.

PBRs use high-pressure helium gas, not water, for cooling. Reactors have been “run dry” – without cooling gas. Result: they simply stabilize at a given temperature – lower than the pebbles’ shell melting point. No meltdown can occur.

Safe Nuclear Power and Green Hydrogen Fuel, Pic 2

PBR from www.pbmr.co.za

South Africa may have the most modern PBR on the drawing board. With the help of German scientists – acknowledged leaders in the field - they have planned to build several reactors within the next five years. Images in this article come from their design.

The reactor core is a bin of uranium fuel pebbles. Each tennis ball-sized pebble is rotated and/or checked for reactivity by removing them from the bottom of the funnel shaped reactor core. Spent pebbles are replaced by adding new ones at the top of the stack. Used ones that are still reactive also go to the top of the bin. The reactor can be re-fueled without stopping power production – not possible in conventional rod reactors which requires a full shut down.

Pebbles, because of their round nature, allow the cooling gas to be introduced at the bottom and pass freely through the stack. The heated gas is removed to perform work like spinning a turbine to generate electricity then recycled in a closed loop back to the reactor core.

PBRs use helium, which has high thermal conductivity and inertness (read: fireproof and noncorrosive) for cooling. This makes them more efficient at capturing heat energy from nuclear reactions than standard reactor designs. The ratio of electrical output to thermal output is about 50%.

Safe Nuclear Power and Green Hydrogen Fuel, Pic 3

Reactor Interior – pebbles in red: www.eskom.co.za/ nuclear_energy/ pebble_bed/ pebble_bed.html

The high-temperature gas design also has a silver lining – it can produce hydrogen. Think about that – fuel cell vehicles need expensive-to-produce hydrogen to run on – this reactor could make hydrogen as a byproduct.

Generation of hydrogen has been the biggest stumbling block to it adoption as a clean fuel. Hydrogen, found primarily in water, is expensive to extract as a gas. While the technical problems of handling, storage and use as fuel are largely solved, the high energy cost to produce hydrogen has made it an energy transport medium, not a source.

These new reactors run at high temperatures which are perfect for cracking abundant water or helium gas into hydrogen which can then be used as a green fuel – burning hydrogen just produces water vapor.

PBRs could produce cheap hydrogen that could be piped to areas of need or used in the local communities.

Plant sites are much smaller than traditional nuclear power plants. Their modular design allows for smaller plants that can grow with needs. A single PBR reactor would consist of one main building covering an area of about 1,300 square meters – less than half a football field. It would be about 42m high (6 stories), some of it below ground level. Billion dollar steel reinforced concrete containment vessels are not required – any coolant leak would be in the form of nonradioactive helium gas which would quickly disperse with out causing any ill effects.

Safe Nuclear Power and Green Hydrogen Fuel, Pic 4

Internal functioning with cooling diagram: www.eskom.co.za/nuclear_energy/pebble_bed/pebble_bed.html

Safe Nuclear Power and Green Hydrogen Fuel, Pic 5

Fuel Spheres: www.eskom.co.za/nuclear_energy/pebble_bed/pebble_bed.html

Each PBR would produce between 100 and 200 MW – small, in comparison to light and heavy water reactors which typically product around 1,000 MW. But they could easily be scaled up by adding reactors.

Ten PBR reactors producing 1,100 MW would occupy an area of no more than three football fields. Each PBR could serve about 30,000 to 40,000 homes.

Control rooms - much simpler than standard ones - would have a few PCs and extra monitors instead of banks of valves and dials. Each control room could monitor and manage up to 10 reactors.

One of the key features to this technology, especially important in China where energy demand is exploding, is its modular nature. While conventional reactors in operation today are all one of a kind – although many are based on the same designs – PBR reactors could de built with standard rail-movable components. When a new power plant is needed, they simply load the parts on a train with a construction crew and can have it delivering power in short order. Traditional plants in the US were sunk principally by long construction times and cost overruns, not environmental regulations.

Nuclear waste disposal has become a hot-button issue. Standard nuclear waste is very radioactive for 10,000 years or more. It must be transported to and stored in special containment facilities – normally underground. It can also be reprocessed but this is costly and technically difficult. There are only 3 reprocessing facilities worldwide: Thorpe in England, Cogema in France and Myakrt1 Chemical Combine in Russia. Far away from most of the world that needs clean, inexpensive power.

Fuel pebbles have 4 caps of containment built in. Many authorities consider pebbled radioactive waste stable enough it can be safely disposed of in geological storage – without any additional shielding or protection. Even in tests where pebbles were exposed to very high heat without coolant for long periods, they showed no outward damage. If one did manage to break a pebble it would only release one tiny (0.05mm) uranium dioxide particle. This particle is too heavy to be wind borne and so could not be blown into other areas like the fallout from the explosion at Chernobyl.

PBR proponents state they plan to store all waste products on the plant site – avoiding costly and dangerous radioactive material movement. "

1.According to the Keystone Center’s “Nuclear Power Joint Fact Finding” released last year, failing to replace existing nuclear power plants over the next half-century would actually increase carbon emissions by 12.5 gigatons.
2.The World Health Organization (WHO) carried out several studies after the 1986 Chernobyl disaster; one, conducted 19 years later, concluded that 75 deaths could be directly attributed to the accident. Other WHO findings: 28 deaths among first-responders in the year after the accident could be directly linked to acute radiation sickness; there was a large increase in highly treatable tyroid cancerns among young people and no clearly demonstrated increases in leukemia or other non-thyroid solid cancers; and the lifetime risk of cancer deaths among those exposed to Chernobyl radiation was about 3 to 4 percent higher than average
3. By comparison, the health impacts of the 1979 Three Mile Island accident in the U.S. were minuscule, with no attributable illnesses or deaths. The Keystone Center’s “Nuclear Power Joint Fact Finding” last year said the average dose of radiation to the region’s 2 million people was about 1 millirem, with the maximum exposure to individuals right outside the site at less than 100 millirem. By comparison, a full set of chest x-rays delivers 6 millirem of radiation, and a year’s exposure to natural background radiation gets you 100 to 125 millirem.
4. Participants in the Keystone Center “Nuclear Power Joint Fact Finding” all conceded that “on balance, commercial nuclear power plants in the U.S. are safer today than they were before the 1979 accident at Three Mile Island.” In fact, an industry study in 2003 found that even a direct-side impact by a large commercial airliner wouldn’t cause a loss of coolant at a nuclear power plant.
5. A National Academy of Sciences study found a low risk of widespread harm from either a terrorist attack or a serious accident involving spent nuclear fuel. And the Keystone Center’s “Nuclear Power Joint Fact Finding” found that “the risk of a major accident at a nuclear facility is not seen as a significant risk by investors today.”
6. A 2001 study by the Paul Scherrer Institute in Switzerland (quoted in “The Revenge of Gaia”) found that, beteween 1970 and 1992, nuclear power had the best safety record of all major energy sources, both in terms of total deaths and deaths per terawatt of energy produced each year. The results for the top four sources were coal: 6,400 total deaths, 342 deaths per terawatt per year; hydro power: 4,000 total deaths, 884 deaths per terawatt per year; natural gas: 1,200 total deaths, 85 deaths per terawatt per year; nuclear power: 31 total deaths, 8 deaths per terawatt per year.
7. A life-cycle assessment by Meier Engineering Research found that nuclear fission energy actually had a lower life-cycle greenhouse gas emission rate than solar (using an eight-kilowatt, building-integrated photovoltaic system for the assessment): 15 tons of carbon dioxide-equivalent per gigawatt-electric of electricity, compared to 39 tons for photovoltaic. Of course, those rates were considerably higher for fossil-fuel sources like natural gas (469 tons) or coal (974 tons).
8. Nuclear power makes economic sense. According to the Energy Information Administration, operation, maintenance and fuel costs per kilowatt-hour for nuclear plants are more than twice those for hydroelectric, but nearly a third less than those for fossil steam energy and two-thirds less than either gas turbine energy or small-scale photovoltaic or wind energy.
9. During the nuclear testing heyday of the Cold War era, the superpowers set off numerous nuclear weapons; in 1962 alone, test bombs equaled the output of 20,000 Hiroshima warheads. Such tests, Lovelock argues, released radioactive materials into the air equal to two Chernobyls a week for a whole year … yet no proven health damage to humans was observed in subsequent years. (For more details, see “The Revenge of Gaia,” pages 94 - 95).

CONCLUSION
Though there are indeed many disadvantages to nuclear power, however, as compared to the alternative energy sources like solar energy or fossil fuels, it seems that nuclear power is the best choice out of the rest. Therefore I conclude that with current technologies, the green energy for the future can only be nuclear energy, unless a technological breakthrough occurs.

REFERENCE
Clayton, Mark. (2007). How Green is Nuclear Power?. Retrieved June 1, 2008 from http://www.csmonitor.com/2007/0307/p01s04-sten.html?page=1

Dunn, Philip. (2005). Safe Nuclear Power and Green Hydrogen Fuel. Retrieved June 3, 2008 from http://www.physorg.com/news8956.html

Jim, Green. (2005). Global warming: Nuclear power no solution. Retrieved May 31, 2008 from http://www.greenleft.org.au/2005/622/34954

Rainwater van Suntum, Lisa. (2005).Spinning Nuclear Power into Green.Retrieved June 1, 2008 from http://www.prwatch.org/prwissues/2005Q1/nuke2.html

Gregory, Shirley Siluk. (2008) . Devil's Advocate: 10 Green Arguments for Nuclear Power. Retrieved June 5, 2008 from http://planetsave.com/blog/2008/01/15/devils-advocate-10-green-arguments-for-nuclear-power/

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