SCEN102: Chapter 13

Nuclear Fission Reactors

A fission reactor contains four componants: nuclear fuel, control rods, moderator (to slow neutrons, which is required for fission), and liquid (usually water) to transfer heat from the reactor to the turbine and generator. A significant disadvantage of fission power is the generation of radioactive waste products.

The Breeder Reactor

A reactor specifically designed to breed more fissionable fuel than is put into it is called a breeder reactor.

Chain reaction

A self-sustaining reaction in which the products of one reaction event stimulate further reaction events.

Three types of radiation:

Alpha particle Beta particle Gamma ray

Beta particle

An electron ejected from a nucleus; is indistinguishable from an electron in a cathode ray or electrical circuit, or one orbiting the nucleus; the difference between an electron and a beta particle is that a beta particle originates from inside the nucleus, from a neutron; normally faster than an alpha particle and carries only a single negative charge (-1); can penetrate light materials such as paper or clothing.

What happens where neutrons in a nucleus change into protons?

As neutrons in a nucleus change into protons, the stability of the nucleus decreases because the repulsive electric force becomes increasingly significant. (More neutrons are needed to help balance between the protons.)

Thermonuclear fusion

Nuclear fusion produced by high temperature.

How do you carry out a fusion reaction in controlled conditions?

One solution is to confine the reaction in a nonmaterial container. A nonmaterial container is a magnetic field, which can exist at any temperature and can exert powerful forces on charged particles in motion.

Alpha particle

The combination of two protons and two neutrons; relatively easy to shield because of their relatively large size and their double positive charge (+2); they do not usually penetrate through light materials such as clothes or paper.

How are older, nonliving things dated?

The dating of older, but nonliving, things is accomplished with radioactive materials such as uranium.

Strong nuclear force

The force of interaction between all nucleons (protons and neutrons); this force is very strong but only over extremely short distances (about 10^-15 m, the diameter of a typical atomic nucleus). Therefore, it only works well with elements under 82 on the periodic table.

The relationship between the masses of nucleons:

The greatest mass per nucleon occurs for the proton alone, hydrogen, because it has no binding energy to pull its mass down. Progressing beyond hydrogen, the mass per nucleon is smaller, and is least for one in the nucleus of the iron atom. Beyond iron (Fe with atomic number of 26), the process reverses itself as nucleons have progressively more mass in atoms of increasing atomic number.

Why does it get hotter down below the Earth?

The main reason it gets hotter down below the Earth is because Earth contains an abundance of radioactive isotopes and is heated as it absorbs radiation from these isotopes.

How are the masses of isotopes measured?

The masses of the isotopes of various elements can be very accurately measured with a mass spectrometer. This important device uses a magnetic field to deflect ions of these isotopes into circular arcs. The greater the inertia (mass) of the ion, the more it resists deflection, and the greater the radius of its curved path. The magnetic force sweeps lighter ions into shorter arcs and heavier ions into larger arcs. The slope curves because there are proportionally more neutrons in the more massive atoms.

Critical mass

The minimum mass of fissionable material in a reactor or nuclear bomb that will sustain a chain reaction.

Relationship between energy and mass of a particle:

The more energy associated with a particle, the greater the mass of the particle.

Why are more neutrons needed the more protons there are in the nucleus?

The more protons there are in a nucleus, the more neutrons are needed to help balance the repulsive electric forces. For larger nuclei, more neutrons than protons are needed.

Nuclear fusion

The combination of the nuclei of light atoms to form heavier nuclei, with the release of much energy.

What do neutrons attract by the strong nuclear force?

Neutrons have no electric charge and so only attract other protons and neutrons by the strong nuclear force.

What is the function of neutrons in the atomic nucleus?

Neutrons serve as a "nuclear cement" holding the atomic nucleus together.

What needs to happen for fusion reactions to occur?

For a fusion reaction to occur, the nuclei must collide at a very high speed in order to overcome their mutual electric repulsion. The required speeds correspond to the extremely high temperatures found in the Sun and other stars.

Gamma ray

High-frequency electromagnetic radiation emitted by radioactive elements; is a pure energy; has no mass or electron charge; can penetrate through most materials, but not usually dense materials such as lead, which absorbs them; generally more harmful to us than alpha or beta particles.

Artificial Transmutation

In a cloud chamber, moving charged particles show a trail of ions along their path in a way similar to the ice crystals that show the trail of jet planes high in the sky. Artificial transmutation produces the hitherto unknown synthetic elements from atomic number 93-118; all of these artificially made elements have short half-lives; if they ever existed naturally when Earth was formed, they have long since decayed.

E=mc²

Mass and energy are two sides of the same coin. E stands for the energy that any mass has at rest. m stands for mass. c is the speed of light - c² is the proportionality constant of energy and mass.

What happens to neutrons when they are by themselves?

Neutrons are not stable when they are by themselves. A lone neutron is radioactive, and spontaneously transforms into a proton and an electron. A neutron seems to need protons around to keep this from happening.

What do protons repel by the electric force?

Protons also repel other protons by the electric force.

What do protons attract by the strong nuclear force?

Protons attract both protons and neutrons by the strong nuclear force.

Radiation Dosage

Radiation dosage is commonly measured in rads. One rad = .01 J of radiant energy absorbed per kilogram of tissue. Some forms of radiation are more harmful than others. The unit of measure for radiation dosage based on potential damage is the rem. In calculating the dosage in rems, we multiply the number of rads by a factor that corresponds to different health effects of different types of radiation determined by clinical studies. Lethal doses of radiation begin at 500 rems. All the radiation we receive from natural resources and from medical procedures is only a fraction of 1 rem. The average person in the United States is exposed to about 360 mrem a year. The human body is a significant source of natural radiation, primarily from the potassium we ingest. When radiation encounters the intricately structured molecules in the watery, ion-rich brine that makes up our cells, the radiation can create chaos on the atomic scale. Some molecules are broken, and this change alters other molecules, which can be harmful to life processes. Cells can repair most kinds of molecular damage caused by radiation if the radiation is not too severe. When radiation is sufficient to kill cells, the dead cells can be replaced by new ones. Sometimes a damaged cell survives with a damaged DNA molecule—causing mutations (can lead to cancer in some cases or if a reproductive cell is damaged, it can spread to offspring).

Radioactive Tracers

Radioactive isotopes used to trace pathways are called tracers. In a technique known as medical imaging, tracers are used to diagnose internal disorders. This technique works because the path the tracer takes is influenced only by its physical and chemical properties, not by its radioactivity.

Repulsive electrical force

Repulsive electrical interactions are relatively long-ranged. In other words, after 82 on the periodic table, the nuclei of elements become very unstable.

Carbon-14 dating

Scientists can calculate the age of carbon-containing artifacts, such as wooden tools or skeletons, by measuring their level of radioactivity, referred to as carbon-14 dating.

Transmutation

The changing of one chemical element to another. There are two kinds of transmutation: natural and artificial.

Half-life

The rate of decay for a radioactive isotope is measured in terms of a characteristic of time, the half-life. This is the time it takes for half of an original quantity of an element to decay. Half-lives are remarkably constant and are not affected by external conditions. The half-life of an element can be calculated at any given moment by measuring the rate of decay of a known quantity. - Done using a radiation detector. In general, the shorter the half-life of a substance, the faster it disintegrates, and the more radioactivity per amount is detected.

Nuclear Fission

The splitting of the nucleus of a heavy atom into two main parts, accompanied by the release of energy. In the nucleus of every atom is a delicate balance between attractive nuclear forces between protons. In all known nuclei, the nuclear forces dominate. In uranium, however, this domination is tenuous. If a uranium nucleus stretches into an elongated shape. If the elongation passes a certain point, electrical forces overwhelm strong nuclear forces, and the nucleus splits, which is referred to as nuclear fission.

How do you calculate the average mass of nucleon?

To obtain the average mass per nucleon, you divide the total mass of a nucleus by the number of nucleons in the nucleus.

Examples of things caused by radioactivity:

Volcanoes, geysers, hot springs, and the drifting of the continents are all caused by radioactivity.

Natural Transmutation

When an alpha particle leaves the nucleus of an element, the mass number of the resulting atom is decreased by 4, and its atomic number is decreased by 2 → resulting in 2 spaces back in the period table. When a beta particle leaves the nucleus of an element, the mass is unaffected, but its atomic number increases by 1 → resulting in 1 space forward in the periodic table. When gamma radiation happens, there is no change in mass or atomic number (stays same element in periodic table).

When do we say an element is radioactive?

When the nuclei is unstable (over 82 on the periodic table).

Radioactivity

When unstable nuclei break down and emit radiation. • Because it involves the decay of the atomic nucleus, it is often called radioactive decay.