Chapter 13

Nuclear Reactions

Radioactivity• The spontaneous emission of particles or energy from an

atomic nucleus as it disintegrates.• The particles emitted are: alpha particles ( ) 2 protons and 2 neutrons beta particles ( ) a high energy electron gamma rays ( ) electromagnetic energy only, the highest

possible energy.• The amount of protection needed for nuclear radiation is:

gamma (piece of lead)> beta (thin layer of metal) > alpha (sheet of paper).

• There is often gamma radiation emitted along with alpha and/or beta particles.

• The disintegration of a radioactive nucleus is called radioactive decay.

Radioactive Decay

HeThU 42

23490

23892

He42 Is the same as an particle.

UHePu 23892

42

24294

Worksheet

Radioactive Decay

e N C 01-

147

146

e01- is the same as a particle.

LaeBa 14157

01

14156

When a beta particle is emitted, a neutron gets converted to a proton. As a result, the mass number doesn’t change, but the atomic number increases by one and the next element in the periodic table is obtained.

Nuclear Fission and Fusion• Nuclear fission occurs when an unstable

massive nucleus splits into smaller, more stable particles through the emission of alpha or beta particles.

This occurs rapidly in an atomic bomb and slowly in a nuclear reactor.

• Nuclear fusion occurs when less massive unstable nuclei come together to form more stable and more massive nuclei.

This occurs rapidly in hydrogen bombs and occurs continually in the sun, releasing energy essential for the continuation of life on earth.

Nuclear Fission• Some nuclei are unstable because they are too large (atomic

number greater than 83), because they have an odd number of protons or neutrons, or because they have an unstable neutron-to-proton ratio (larger ratios in elements with more than 83 protons are more stable in general).

• The unstable nuclei undergo radioactive decay, eventually forming products or larger stability.

• When nuclear decay occurs a tiny amount of mass, called a mass defect, is converted to energy, according to Einstein’s equation:

E = mc2.

• The mass of the unstable large nucleus is higher than the masses of the resulting stable nuclei after nuclear fission occurs. This is the mass which is converted to energy according to Einstein’s equation. A little bit of mass produces a large amount of energy.

• This is the energy which is released when nuclear fission occurs. It is the binding energy and it is equal to the mass defect.

Nuclear Fusion• For smaller nuclei, with atomic number 20 or less, if the

proton : neutron ratio is 1:1 the isotope is more stable. • When two small unstable nuclei are joined together the

mass of the unstable nuclei is slightly more than that of the resulting more stable nucleus.

• This is called the mass defect and it is converted to energy according to Einstein’s equation:

E = mc2

• The energy released when a nucleus is formed is called the binding energy.

• This energy is released when nuclear fusion occurs.

The maximum amount of binding energy released during formation of the nucleus occurs around mass number 56. It decreases in both directions. Fission and fusion both release energy.

The fission of U-235 produces 2 or 3 neutrons along with the other products.These neutrons can then each move to other U-235 nuclei where theyare absorbed.

Fission occurs again with the release ofMore neutrons, which move on toother U-235 nuclei to continue theprocess.A reaction where the productsAre able to produce More. Reactions on a self-sustaining series is called a chain reaction.

Fission of U-235• Natural occurring uranium is mostly U-238, an isotope

that does not fission easily.• Only about 0.7% of the natural uranium is the highly

fissionable U-235. • This low ratio of readily fissionable uranium 235 nuclei

makes it unlikely that a stray neutron would be able to achieve a chain reaction in naturally occurring uranium. This is a sub critical mass.

• A critical mass is a mass of sufficiently pure U-235 (or Pu-239) that is large enough to produce a rapidly accelerating chain reaction is called a supercritical mass.

• Atomic bombs use a small, conventional explosive to push sub critical masses of U-235 or other fissionable materials into a supercritical mass. Fission occurs almost instantaneously in the supercritical mass and tremendous energy is released in a violent explosion.

http://www.howstuffworks.com/nuclear-bomb.htm

Nuclear Power Plant

Nuclear Power Plants

The compositionOf the nuclear fuelIna fuel rod:

A) before use

B) after use

Nuclear Power Plants• After a period of time the production of fission products

in the fuel rods begins to interfere with effective neutron transmission, so the reactor is shut down annually for refueling.

• The spent fuel rods contain an appreciable amount of usable uranium and plutonium.

• The spent fuel rods are stored in cooling pools at the nuclear plant sites. In the future the spent fuel may be reprocessed to recover the U and Pu through chemical processing or put it in terminal storage.

• The spent fuel rods represent an energy source equivalent to more than 25 billion barrels of petroleum. Six other countries do reprocess the spent fuel.

Fission and Fusion

• Nuclear energy is released when: 1. massive nuclei such as U-235 undergo

fission 2. less massive nuclei such as hydrogen

come together to form more massive nuclei through fusion.

Nuclear Fusion• Nuclear fusion is responsible for the energy the

energy released by the sun an other stars.• At the present half way point in the sun’s life,

with about 5 billion years to go-the core is now 35% hydrogen and 65% helium.

• Through fusion, the sun converts about 650 million tons of hydrogen to 645 million tons of helium every second. The other roughly 5 million tons of matter are converted into energy.

• Even at this rate the sun has enough hydrogen to continue the process for an estimated 5 billion years.

Nuclear Fusion• The reactions which take place in the sun are to convert

H-1 to H-2 (deuterium) and H-3 (tritium), and to then converted the tritium to He-4.

• Fusion appears to be a desirable source of energy on earth because:

1. There are massive amounts of H-2 available from all the water of the oceans. 2. Enormous amounts of energy are released with no radioactive by products.• There are problems, however, since the temperatures

required are in the order of 100 million degrees celsius, and the concentrations of H-2 need to be huge so that many reactions occur in a short time, so the pressure needed is huge.

Half-Lives

• A half-life is the length of time required for one-half of the unstable nuclei to decay.

• Each isotope has its own characteristic half-life, ranging from fractions of a second to billions of years.

• The half-life is independent of the amount of the radioactive isotope which is present.

• A sample of Bi-210 has a half-life of 5 days. How much is left after 10 days?

a) 100 %b) 50 %c) 25 %d) 0 %

Other uses of radioactivity

• Food preservation: Radioactive Co-60 or Cs-137 emit gamma radiation which kills bacteria and other pathogens.

• Nuclear medicine: Radioactive iodine is used to treat thyroid cancer patients by diagnosing the disease. It is also used for many other diagnostic tests.

• Cancer treatment: To destroy cancerous cells. Unfortunately this has side effects.

Big Bang Theory• A “big bang” of energy created a plasma of protons and neutrons.

This led to the formation of H nuclei.

• The H nuclei were pulled together by gravity into masses that would become the stars. The H nuclei fused into He nuclei, releasing enough energy that the star began to shine.

• The fusion process continued for billions of years, releasing energy as heavier and heavier nuclei were formed. Eventually the star materials were fused into nuclei around iron, the element with the lowest amount of energy per nucleon and the star used up its energy source.

• Larger, more massive dying stars explode into supernovas. Such an explosion releases a flood of neutrons, which bombard medium weight nuclei and build them up to more massive nuclei, all the way from iron up to uranium. These elements are then spread into space as dust which became the materials of which planets were made.

• The force of gravitational attraction provided the initial energy for the whole process.

Review Exercises

• Applying the Concepts p. 13-26 to 13-25:# 1, 2, 3, 4, 19, 20, 22• Parallel Exercises, Group A:# 1, 2, 4, 5, 6.New Book: p. 380-383 # 2, 3, 4, 5, 6, 8, 9,

16, 17, 18, 19, 24, 25, 26, 27, 29, 30, 31, 32, 33, 41, 42, 44, 45, 46, 49.

Summary• Alpha, beta and gamma particles-what each of them is.• Reactions where alpha and beta particles are emitted.• Distinction between nuclear fission and nuclear fusion.• Binding energy and mass defect- The shortage of mass

which is converted to energy when unstable isotopes undergo fission or fusion.

• Advantages and disadvantages of nuclear fusion.• Half lives: what they are and determining amounts based

on half lives.• Uses of radioactivity.