1Chemistry 2C Lecture 25: May 28 th, )Binding Energy Revisited 2)Fission 3)Fusion Lecture 25: Nuclear Power

2Chemistry 2C Lecture 25: May 28 th, 2010 Binding Energy Nuclear energy can be extracted by generating nuclides with greater binding energy from those of lesser (thus releasing energy that can be used for constructive uses. Fe is the most stable nuclide with greatest binding energy Fusion works on this side of Fe (taking small nuclides to make bigger ones) Fission works on this side of Fe (taking big nuclides to smaller ones)

3Chemistry 2C Lecture 25: May 28 th, 2010 Fission During the search for new elements in the 1930’s, it was discovered that bombarding a nucleus with neutrons could cause a nuclear to absorb a neutron and then split into two large fragments. This gives overall increase in energy as the sum of the binding energies for Ba and Kr are higher than for U!  E=8.2x10 7 kJ/g of 235 U = 3 tons of coal combustion More neutrons are produced as products faster than being absorbed! This is a chain reaction that exponentially increases the rate = explosion!!!!

4Chemistry 2C Lecture 25: May 28 th, 2010 Nuclear Fission Chain Reaction Step 1. A 235 U atom absorbs a neutron, and fissions in two new atoms (fission fragments), releasing three new neutrons and some binding energy. Step 2. One of those neutrons is absorbed by an atom of 238 U, and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However one neutron does collide with an atom of 235 U, which then fissions and releases two neutrons and some binding energy. Step 3. Both of those neutrons collide with 235 U atoms, each of which fission and release between one and three neutrons, which can then continue the reaction.

5Chemistry 2C Lecture 25: May 28 th, 2010 Nuclear Fission Chain Reaction There are 443 licensed nuclear power reactors in the world, of which 441 are currently operational operating in 31 different countries. Together they produce about 17% of the world's electric power. The chain reaction is the basis of both an atomic bomb and of a nuclear fission reactor. The latter is a controlled chain reaction that generated heat, to make steam and run a turbine, which generates electricity. Components of a controlled chain reaction: Fuel rods, Control rods and moderators….

6Chemistry 2C Lecture 25: May 28 th, 2010 However, the chain reaction must be controlled! The Chernobyl disaster occurred at 01:23 a.m. on April 26, 1986 at the Chernobyl nuclear power plant in Pripyat, Ukraine. It is regarded as the worst accident in the history of nuclear power

7Chemistry 2C Lecture 25: May 28 th, 2010 Radioactive fallout 137 Cs after Chernobyl 137 Cs was the major long-living radionuclide that were released in aerosol form and hence traveled by wind to other nations. At different times after the accident, different isotopes were responsible for the majority of the external dose.

8Chemistry 2C Lecture 25: May 28 th, 2010 Fuel rods Fuel rods are filled with pellets of uranium oxide in which the 235 U has been enriched to ~3% (natural abundance =0.8% Step 1: Uranium ore - the principal raw material of nuclear fuel Step 2: Yellowcake - the form in which uranium is transported to an enrichment plant Step 3: UF 6 - used in enrichment Step 4: Fuel rod - a compact, chemically inert, insoluble U oxide rod While electric power reactors require only enrichment to about 3% 235 U, the weapons applications required enrichment to over 90% 235 U.

9Chemistry 2C Lecture 25: May 28 th, 2010 Control rods A control rod is a rod made of chemical elements capable of absorbing many neutrons without decaying themselves. They are used to control the rate of decay of uranium and plutonium Control rods are commonly composed of silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, gadolinium, and europium.

10Chemistry 2C Lecture 25: May 28 th, 2010 Moderators The moderator slows down the neutrons to the right speed to be absorbed preferentially by 235 U. Commonly these are D 2 0 (heavy water) and graphite. A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. This differentiates it from a heavy water reactor, which uses heavy water as a neutron moderator. In practice all LWRs are also water cooled.

11Chemistry 2C Lecture 25: May 28 th, 2010 All put together The fuel rods have enough U235 to sustain a chain reaction. This chain reaction won’t get out of hand by using control rods that can absorb the neutrons and slow the process down. High energy neutrons (fast neutrons), don’t get captured as easily as slow (or thermal) neutrons, hence a moderator is used.

12Chemistry 2C Lecture 25: May 28 th, 2010 What afterwards Eventually, the nuclear fuels is spent– when to fraction of fissionable material is too low – and it must be disposed of every 1-2 years. The fuels are not recycled in this country, but there are advances in this front. Temporary Storage of Fuel Rods: There is, as of now, no permanent storage site of spent fuel rods. Dry storage: entails taking the waste and putting it in reinforced casks or entombing it in concrete bunkers. Wet storage: The spent rods are placed in the pool, where they can cool down.

13Chemistry 2C Lecture 25: May 28 th, 2010 Nuclear Waste Spent fuel rods are changed every 18 months and are generally not fissionable material. They however, are very radioactive for thousands of years. High level waste: Low level waste: Contaminated tools, protective clothing, etc. Fission products and depleted fuel: includes fission products of 235 U and the fission products of those nuclei –including a wide range of radionuclides: Nuclide : 99 Tc 237 Np 239 Pu 240 Pu 242 Pu 235 U Halflife: 2.1 x 10 5 y 2.1 x 10 6 y 2.4 x 10 4 y 6.8 x 10 3 y 3.18 x 10 5 y 7.13 x 10 8 y

14Chemistry 2C Lecture 25: May 28 th, 2010 America’s permanent solution The most promising option so far is burying the waste in the ground -- "deep geological disposal" Because a spent fuel rod contains material that takes thousands of years to become stable (and non-radioactive), it must be contained for a very, very long time. Yucca Mountain

15Chemistry 2C Lecture 25: May 28 th, 2010 Reprocessing Nuclear Waste Spent fuel discharged from reactors contains appreciable quantities of fissile and other radioactive materials, including reaction poisons (the reason the fuel had to be removed). These fissile and fertile materials can be chemically separated and recovered from the spent fuel. The recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel. Currently, plants in Europe are reprocessing spent fuel from utilities in Europe and Japan. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States due to nonproliferation considerations.

16Chemistry 2C Lecture 25: May 28 th, 2010 Reprocessing Nuclear Waste Nuclear fuel cycle begins when uranium is mined, enriched and manufactured to nuclear fuel (1)which is delivered to a nuclear power plant. After usage in the power plant the spent fuel is delivered to a reprocessing plant (if fuel is recycled) (2)or to a final repository (if no recycling is done) (3)for geological disposition. (4)In reprocessing 95% of spent fuel can be recycled to be returned to usage in a power plant (4).

17Chemistry 2C Lecture 25: May 28 th, 2010 Fusion Nuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus. It is accompanied by the release or absorption of energy depending on the masses of the nuclei involved. The fusion of two nuclei lighter than iron or nickel generally releases energy while the fusion of nuclei heavier than them absorbs energy To overcome the mutual electrical repulsion of positively-charged nuclei, a fusion processes require extremely high temperatures and densities. These conditions are found only in the core of a star. Under these extreme conditions, particles move fast enough to get close enough for strong nuclear force to overcome electrical repulsion.

18Chemistry 2C Lecture 25: May 28 th, 2010 Fusion in the stars… The net result is the fusion of four protons into one alpha particle, with the release of two positrons, two neutrinos, and energy, but several individual reactions are involved, depending on the mass of the star.

19Chemistry 2C Lecture 25: May 28 th, 2010 National Ignition Facility (Livermore) * The facility is very large, the size of a sports stadium * The target is very small, the size of a BB-gun pellet * The laser system is very powerful, equal to 1,000 times the electric generating power of the United States * Each laser pulse is very short, a few billionths of a second Current estimates put the ultimate cost between $3.5 and $6 billion.

20Chemistry 2C Lecture 25: May 28 th, 2010 Fusion The easiest and most immediately promising nuclear reaction to be used for fusion power is: The only fusion reactions thus far produced by humans to achieve ignition are those which have been created in hydrogen bombs The H-bomb released more energy than an A-bomb, and with not as dirty products-- mostly He gas.