Nuclear Energy Nuclear fission is when a heavy nucleus splits into two smaller nuclei. The total mass of the products is less than the original mass. The mass difference results in an energy gain, about 200MeV for each fission event. 0 1 n U-----> Ba Kr n The intermediate stage, U, which is very unstable is when the nucleus elongates and compresses until finally splitting. The three neutrons are available to bombard other nuclei.

Nuclear Reactors When a nucleus undergoes neutron bombardment and fission occurs, the neutrons released bombard other nuclei and if not controlled result in a chain reaction releasing a large amount of energy in a chain reaction, an explosion. A nuclear reactor is a system designed to maintain a self sustained chain reaction releasing controlled energy.

Cont. The average number of neutrons produced per fission event is 2.5, defined as the reproduction constant K. In reality K is less than 2.5. Self sustain chain reaction occurs when K = 1, then the reactor is said to be critical. When K is less than 1 it is subcritical and the reaction dies out. If it is greater than 1 it is super critical and the reaction is runaway.

Cont. When neutrons are released they have high kinetic energy and need to be slowed down to be absorbed by 235 U. This is done by the use of a Moderator, originally Carbon later heavy water. The moderator absorbs the kinetic energy of the neutrons allowing it to be captured by the nucleus causing fission. Not all neutrons are captured some leak out of the core before capture. A large surface area to volume ratio in the reactor will reduce the leakage. To much leakage and the reaction will stop.

Control of Power Level If nuclear bombardment is not controlled the reactor becomes too hot and will melt and/or the chain reaction is too fast and an explosion may occur. Temperature if often controlled by cooling water that removes excess heat from the core. In addition it is necessary to use control rods that absorb excess neutrons before being captured by the fission process. These rods made of either graphite, boron or cadmium are inserted between the fuel rods and adjusted to control the amount of neutron absorption.

Cont. The heat from a nuclear reactor is used to produce steam that is used to run a turbine that in turn powers a generator. Power production whether by nuclear reactors, burning fossil fuel or some other means from the point of turning a turbine to powering a generator are virtually all the same.

Nuclear Fusion Fusion is the opposite to fission in that two smaller nuclei are combined to form a heavy nucleus which also results in the release of energy from the conversion of matter. The sun provides heat due to nuclear fusion of protons (hydrogen ions) to deuterium, then a hydrogen ion and deuterium to 3He and finally either hydrogen and 3He to 4He or two 3He to 4He and two hydrogen ions

Cont. First 1 1 H H---> 1 2 D + e + + v then 1 1 H D ---> 2 3 He +  then either 1 1 H He ---> 2 4 He + e + + v or 2 3 He He ---> 2 4 He + 2( 1 1 H) The energy liberated is carried by the gamma rays, positrons and neutrinos. The gamma rays are absorbed increasing the temperature, the positrons combine with electrons forming gamma rays and are absorbed, the neutrinos escape the star taking with them energy.

A Star Formation A star is formed by a shock wave from a supernova collapsing a cosmic dust cloud upon itself. As the particles collapse they accelerate and generate huge gravitation force and heat. As protons collide they repel and gravity continues to attract. The heat generated is in the order of 10 7 K. As fusion occurs the energy liberated prevents the star from collapsing under its own gravity and the cycle continues.

Fusion Power A large amount of energy is released in a thermonuclear reaction (fusion) and comparatively few radioactive by-products are formed. However proton-proton reaction are unrealistic due to the need of extremely high temperatures and pressures. The sun functions because of the extreme density of protons at its core. Reaction of deuterium with tritium seems promising but for the confinement of plasma at temperatures 10 8 K and the time required.

Magnetic Field Confinement It is the combination of two magnetic fields either helical or spiral in design used to confine plasma in a chamber and prevent it from touching the vessel walls. Temperatures 30 times hotter than the sun are produced by introduction of high kinetic energy neutral particles into the plasma or by the addition of energy from intense lasers

Elementary Particles Atom from ‘atomos’ indivisible remained until the early 20th century, the proton neutron and electron until 1937 and then collision of high energy particles discovered new particles. From 1960 on except for the electron, photon and a few others subatomic particles matter is made of quarks.

Forces of Nature Force relative range of mediating strength force field particle Strong 1 short 1fm Gluon electro long  1/r 2 Photon magnetic Weak 10 -(6) short fm nucleus Gravitational long  1/r 2 galaxy

Antiparticles For virtually every particle there is an antiparticle, having the same mass but opposite charge. Electron-positron, proton-antiproton, neutron-antineutron Exceptions are the photon and neutral pion which are their own antiparticles

Mesons Two mesons of slightly different mass, the muon (  ) has weak and electromagnetic interaction and plays no role in strong nuclear forces. Its life time around 2.2x10 -6 s decays to an electron, a neutrino and an antineutrino. The other, pi meson, pion (  ) comes in three varieties  -  +  o and decays into muon and an antineutrino in 2.6x10 -8 s. The pion is involved in transferring energy between nucleons

Particle Classification All particles except the photon can be classified into two group.1) Hadrons which include mesons and baryons are distinguished by mass and spin. All mesons decay to electrons, positrons neutrinos and photons. Baryons have masses equal to or larger than a proton and except for the proton all decay and produce a proton. Protons are composed of quarks.

Cont. 2) Leptons (meaning light), include electrons, muons and a tau and all have neutrinos associated with them. They have weak interaction and are considered truly elementary. Hadrons are composed of two or three fundamental constituents called quarks, (u,d,s). Quarks have fractional electronic charge and each have an antiquark of opposite charge.

Colored Quarks The three quarks are colored red blue and green. The antquarks are anti-red anti-blue and anti- green. Baryons consist of the three different colored quarks and mesons consist of one color and an anti quark of the anti-color. Both baryons and meson are colorless. The strong force between quarks is called color force and is carried by massless particles called gluons. Different colored quarks attract each other as opposites oppose. When gluon are absorbed or released the quarks color changes.