Harnessing the power of the nucleus

Mass-Energy Relationships in Nuclear Reactions• It was observed that whenever transmutations

occurred, that the total atomic number and the total atomic mass number in the reaction remained unchanged.

• This suggested that both mass and electric charge was conserved in the nuclear reaction.

• Since nuclear reactions released a lot of energy, it appeared that the law of conservation of energy was violated. It should be noted that these reactions also absorb energy.

Mass-Energy Relationships in Nuclear Reactions• When Einstein published his Theory of Relativity in 1905, he

suggested that energy and mass were equivalent and summarized the relationship as

• E = mc2

• In 1932, Cockcroft and Walton used an accelerator to bombard lithium with protons. They created two alpha particles for each proton.

• Their calculations showed that the kinetic energy of the alpha particles was much greater than the kinetic energy of the proton, but that the extra energy of the alpha particles corresponded exactly to the loss of mass of the products as indicated by Einstein’s equation.

• Conservation of Energy was not violated, the lost mass was converted into energy.

• Because this energy originates in the nucleus, it is called nuclear energy.

• Two types of nuclear reactions of particular interest to us for their substantial release of energy are: nuclear fission and nuclear fusion

Nuclear Fission

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Nuclear Fission

• Heavier atoms such as uranium could be split into lighter atoms with the release of nuclear energy.

• A charged particle cannot be used to split the nucleus as it would be repelled or attracted, so neutrons were the particles used.

• Enrico Fermi may have succeeded in creating the fission in 1934 i.e. splitting (of the nucleus) reaction, but was unable to separate and identify the products.

• Otto Hahn and Fritz Strassmann successfully separated the products in 1939 and this was confirmed by Lise Meitner and Otto Frisch shortly afterwards.

• Frisch used the expression nuclear fission analagous to binary fission in biology as a method of cell division

• Nuclear fission releases about 10X as much energy as a normal nuclear disintegration and about a million times as much as in a chemical reaction

Nuclear Fission• In addition to the vast amount of energy released, two or three

additional neutrons were released. These could collide with other uranium atoms. If this process was allowed to continue, an increasing amount of energy would be produced and a nuclear chain reaction would be set up.

• Natural uranium is made up of 99.3%U-238 , 0.7%U-235, and trace amounts of U-234. Only U-235 is fissionable, so the uranium must be enriched to increase the proportion of U-235.

• The likelihood of neutrons colliding with U-235 nuclei is reduced if the speed of the neutrons is reduced. Materials used to slow down the neutrons are called moderators. The speed is decreased from about 4 x 106 m/s to 2 x 103 m/s. Typical moderators are graphite, ordinary water, heavy water, and beryllium. They slow down the neutrons without absorbing them.

• Control rods made of boron, or cadmium absorb the neutrons and alter the rate of the chain reaction as they are inserted or withdrawn from the reactor.

• A nuclear reactor controls the chain reaction and uses the heat to produce steam to drive a turbine and produce electricity.

• A nuclear bomb has no control rods so the chain reaction proceeds uncontrollably.

The First Reactor and Atomic Bomb Many European physicists moved to the US as

a result of WW II. These included Einstein, Fermi and Szilard. European , American ,Canadian, and British physicists collaborated on The Manhattan Project

The first successful nuclear reactor was constructed at The University of Chicago in December 1942.Einstein’s work was verified.

Three nuclear bombs were made in 1945. The first was tested in New Mexico on July 16, the second was dropped on Hiroshima on August 6, and the third (a plutonium bomb) was dropped on Nagasaki on August 8.

The First Reactor and Atomic Bomb Thousands of people were killed, the

cities were levelled by the heat, shock waves, gamma radiation and high energy neutrons

Many of the survivors later died of cancer and radiation exposure

Nuclear Power Reactors

Nuclear reactors use controlled fission reactions to generate heat (steam) which drives turbines and produces electricity

The Canadian Deuterium Uranium (CANDU) reactors use heavy water (D2 O) as the moderator and natural uranium as the fuel.

In an emergency, the heavy water can be drained out to stop the reaction

www.world-nuclear.org/info/inf32.html

Nuclear Fusion

Fusion is the process by which light elements combine to produce heavier elements. Must overcome repulsive forces. In order to get the nuclei to fuse when they collide, the temperature must be ridiculously-high e.g. a million degrees Celsius.

Fusion reactions use cheap, easily-obtainable fuels (deuterium from water) and the by products are not radioactive as those in fission ( He nuclei & neutrons)

www.asi.org/adb/02/09/he3-intro.html

Nuclear Fusion (from Physics 30 Saskatchewan Department of Education) Nuclear fusion occurs when two small nuclei join

together to form a larger nucleus. A large amount of energy is released during fusion. Nuclear fusion occurs within the sun and the stars.

Fusion in the sun is believed to occur in steps. Four protons produce one He atom, two positrons, and two neutrinos. The first two steps shown below must occur twice before the third step takes place:Fusion is believed to be possible only under extremely high temperatures. For this reason it is referred to as a thermonuclear reaction. The use of nuclear fusion for

commercial purposes is currently being investigated. 000

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Nuclear Fusion (from Physics 30 Saskatchewan Department of Education) One reaction that may eventually be able to produce

controlled fusion is shown at the bottom Fusion has the potential of providing an abundant supply

of energy. The fuel needed for fusion is readily available. Deuterium ( H 2:1) must be extracted from water. (About

0.015% of the hydrogen in water is exists as deuterium.) Tritium (H 3:1) must be made, since it does not occur naturally in sufficient quantities.

Tritium is radioactive (a beta emitter), with a half-life of 12.3 years. It is also toxic.

Complete fusion reactions produce no long-life products. Induced radioactivity is produced in the reactor container due to neutron flux. Also, tritium is radioactive and quite toxic. MeVnHeHH 6.171

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Nuclear Fusion (from Physics 30 Saskatchewan Department of Education) Fusion in stars produces all of the chemical elements

found on Earth. Sunlight is energy released from fusion reactions in the

sun. Examining the spectral characteristics of stars may

provide a better understanding of fusion.

Nuclear Fusion

Sustaining a fusion reaction may be possible by containing the reactants in a high temperature form of matter called plasma. Plasma particles can be contained within a magnetic field. This principle is referred to as magnetic confinement. The purpose of magnetic confinement is to avoid heat loss, not to prevent the walls of the confinement vessel from vaporizing, as often believed.

Another possible technique for sustaining a fusion reaction is inertial confinement, in which a fuel pellet containing the fusion reactants is bombarded by a high energy source such as a laser or an electron beam.

The fusion bomb, developed and first exploded in the early 1950's, was the first use of nuclear fusion.

Safety & Nuclear Reactors There is risk in all forms of power(i.e.

energy) generation-case in point The Gulf of Mexico Disaster (May 2010).

What is an acceptable level of risk? For nuclear reactors, biggest risk is if the

cooling systems fails , core quickly reaches temp of over 5000 ° C and melts.

Called “ China Syndrome” as molten core burns through the concrete foundation and into the earth releasing radioactive material into the environment

Safety & Nuclear Reactors Nuclear accidents in recent history: Chernobyl (Russia) 30 workers

killed evacuated 200 K people (entire city)

Three Mile Island (USA) no direct loss of life

Safety & Nuclear Reactors CANDU Reactors generally considered one of safest and most

efficient Has 3 systems to shut down reactor 1. Heavy water moderator can be dumped by gravity into storage

tank underneath reactor vessel. No heavy water means neutrons not slowed down and the fission reaction stops.

2. Boron can be injected into moderator absorbing enough neutrons to suppress the chain reaction- called “poisoning” moderator

3. Cadmium control rods held above reactor core by EM clutches. If power fails, they fall into core by gravity absorbing neutrons and stopping chain reaction.

If moderator tubes rupture, heavy water becomes superheated and quickly turns to steam. If pressure is not relieved, reactor building could rupture releasing steam and radioactivity into environment. A separate vacuum building can accommodate the steam and condense it with water, relieving the pressure and containing the steam and contents.