NUCLEAR CHEMISTRY

Chapter 19

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Radioactivity

• Nuclear reactions occur when the nuclei of an unstable isotope (radioisotopes) gain stability by undergoing changes.

• In 1896, Antoine Henri Becquerel, Marie Curie and Piere Curie discovered radioactivityradioactivity, the , the process by which materials give off process by which materials give off rays, and rays, and radiationradiation, the penetrating , the penetrating rays and particles emitted by a rays and particles emitted by a radioactive source.radioactive source.

Pierre, Marie, and Henri won the Nobel Peace Prize

in physics in 1903.

The principle of radioactivity disproved Dalton’s theory of invisible atoms and further lead to the concept that radioisotopes have unstable nuclei (too many or too few neurons relative to the number of protons).

Dalton

Radioactive decay is the spontaneous emission of radiation by an unstable nucleus.

Ionizing/Non-ionizing Radiation

• Ionizing radiation: Causes ions to form. Examples include: x-rays, ultraviolet,& gamma

• Non-ionizing radiation: Does not cause ions to form. Essentially not dangerous. Examples include: radio, radar, microwaves, visible light

Types of Radiation-that are radioactive!

There are 3 types of radiation emitted during radioactive decay:

Alpha- helium nucleus; 2 neutrons+2protons; paper & clothing and skin stop penetration

Beta- electron; metal foil stops penetration

Gamma- electromagnetic radiation; no charge; 6 inches lead & 4 ft. concrete stop penetration

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Alpha, Beta & Gamma Radiation

• Alpha radiation is blocked by skin, but is dangerous in the lung.

• Beta radiation can penetrate the body but is blocked by a thin sheet of aluminium.

• Gamma radiation can penetrate the body, and needs more than a cm of lead to stop it.

Nuclear Stability and DecayOf the 1500 different isotopic nuclei that are known, only 264 are stable and never change.

Elements with a low atomic number (below 20) is about 1:1. Above 20, stable nuclei have more neutrons than protons.

On a neutron vs.. proton graph, the band of stability is the location of stable nuclei.

Depending on the neutron to proton ratio, the type of decay differs:

Alpha emission- mass # decreases by 4, atomic number decreases by 2.

Beta emission, a proton converts to a neutron- the atomic # decreases by 1, and the # of neutrons increases by 1.

A positron is a particle with the mass of an electron but has a positive charge.

All nuclei with an atomic number greater than 83 are radioactive.

Alpha, Beta & Gamma Radiation

• Alpha radiation is blocked by skin, but is dangerous in the lung.

• Beta radiation can penetrate the body but is blocked by a thin sheet of aluminium.

• Gamma radiation can penetrate the body, and needs more than a cm of lead to stop it.

Half- Life

Every radioisotopes has a characteristic rate of decay measures by its 1/2 life of the nuclei of a radioisotope sample to decay to products.

1/2 lives may be as short as a fraction of a second, or as long as billions of years!

In nuclear medicine, many artificially produced radioisotopes have short 1/2 lives, a positive feature since the rapidly decaying isotopes do not pose long-term biological radiation hazards to the patient.

Refer to page 849 to see one isotope that has a long 1/2 life.

Transmutation Reactions

The conversion of an atom of one element to an atom of another element is called transmutation, and can occur through radioactive decay and when high energy particles bombard the nucleus of an atom.

Transmutation elements are elements in the periodic table with atomic numbers above 92.

Nuclear Fission

When the nuclei of certain isotopes are bombarded with neutrons, fission occurs.

Fission is the splitting of a nucleus into smaller fragments.

A chain reaction can occur when neutrons strike the nuclei of other atoms. This is similar to the toppling of dominoes.

Nuclear fission can unleash enormous amounts of energy.

Critical Mass: Minimum amount of fuel to sustain chain reaction

If uncontrolled= nuclear bomb

If controlled= nuclear power plant

Fission Chain Reaction

Power Plant design

...Power Plants...

Control Rods

-absorb neutrons

-made of cadmium

-Pre-Chernobyl 30

-Post-Chernobyl 80

-during Chernobyl 6

Water

-produces steam-> turn turbine-> generator= electricity

-coolant around core

ChernobylDifferences between Soviet and US reactors...

Control Rods- Only 6 rods in during disaster; US had at least 30.

Cooling System- Soviet used graphite (pure form of Carbon), this heats up by itself, unlike water (US).

Containment Dome- Absent in the Soviet, used i.e. in Seabrook.

“Grade” of Uranium- The Soviet used poor (and therefore cheap) uranium-> doesn’t work as well

Nuclear Fusion

When nuclei combine to form one large nucleus:

Deuterium+Tritium-----> Helium+neutron+energy

You need 40000000 degrees Celsius for fusion to take place.

Recently, scientists invented cold fusion, involving palladium.

Applications to RadiationAlthough radiation can be harmful, there are many ways in which it can be useful in the field of medicine.

Neutron activation analysis is a procedure used to detect trace amounts of elements in samples.

Radioisotopes are used to detect cancer. For example, Iodine 131 is given to a patient in a slurry, to detect thyroid problems. The amount of iodine uptake is measured by a scan of the patient’s throat with a radiation detector.

Phosphorous-32 is used to detect skin cancer; technetium-99 detects brain tumors and liver damage.

Effects of Radiation• Dose in rems Physiological Effect

• 0-25 No effect is observed.

• 26-50 A decrease in white blood cells is observed.

• 51-100 Lesions occur and white blood cell loss is significant.

• 101-200 Nausea and loss of hair occur.

• 201-500 Ulcers and hemorrhaging occur.

• over 500 Death

RADIOACTIVE WASTE - Production, Handling & Disposal• Introduction• Alpha, Beta & Gamma• 3 Types of Disposal• Low, Medium, High Level

Waste• nuclear fuel cycle• waste from the Front-End• Waste from the Back-End• Reprocessing• Waste Disposal• Layers of Protection• Summary

Introduction to Nuclear Disposal

• In any industry, the thermal generation of electricity produces waste.

• Nuclear power is the only industry which takes responsibility for all its waste and costs it into the product.

• Nuclear power produces a very large amount of energy for very little fuel, however the waste is radioactive.

• Radioactivity is caused by the decay of isotopes.

Radioactive Waste management

• Concentrate and Contain - concentrated, and then isolated.

• Dilute and Disperse - dilute to acceptable levels, then release into the environment.

• Delay and Decay - radioactive waste that has a short half-life can be allowed to decay in a container until its radioactivity is minimal.

Types of Radwaste

• Low-level Waste - mostly short-lived radioactivity, not dangerous to handle. Disposed of in shallow landfills, often compressed.

• Intermediate-level Waste - higher amounts of radioactivity requiring special shielding. Usually solidified, and buried - short-lived waste in landfills, and long-lived waste deep underground.

• High-level Waste - only 3% of the volume of all radioactive waste but 95% of the radioactivity. Usually the spent fuel, it needs cooling before disposal, and special shielding. Sometimes reprocessed.

Nuclear Fuel Cycle - The process of producing electricity from nuclear fuel.

‘Front End’ Waste Disposal

• Mining - dust from mining is controlled to reduce inhalation of radioactive material, and is collected, and put back into the process. Radon gas concentrations are reduced with good ventilation, the gas is diluted and dispersed into the atmosphere in large amounts of air.

• Milling - waste from milling such as radium is put into tailings dams designed to retain the solid and stop the liquid seeping out. These may be put back into the mine or covered with concrete.

• In Situ Leach (ISL) mining - dissolved material other than uranium is put back into the ground where it came from.

‘Back End’ Waste Disposal

• Reactor - This is where most of the radioactive waste is produced. 99% of the radioactive waste produced in the fissioning process is retained in fuel rods.

• Reprocessing - the spent fuel can be regarded as waste like USA and Canada do, or the fuel can be reprocessed like in Europe.

Reprocessing• Dissolve, and separate chemically into

uranium, plutonium, and high level waste.

• 97% of the spent fuel can be reused, it is mainly in the form of depleted uranium.

• The 3% of high level waste must be isolated for a long time. The liquid is stored in stainless steel tanks and covered in concrete until it is solidified.

• Immobilizing high level waste - Liquid waste evaporated and mixed with glass forming materials, melted and poured into containers.

Final Disposal of high level waste

• Forty years after removal from the reactor, less than a thousandth of its original radioactivity remains.

• The ultimate disposal of vitrified waste requires its isolation from the environment for a long time. This is usually done by burying them in deep shafts.

• After 1000 years the radioactivity will be similar to naturally occurring radiation from uranium ore.

Layers of Protection

• Immobilize waste in an insoluble matrix e.g. borosilicate glass.

• Seal inside a corrosion resistant container e.g. stainless steel.

• Surround containers with wet rock (bentonite clay) to inhibit ground water movement.

• Locate deep underground in stable rock structure.

• Site the repository in a remote location.

Summary

• Radioactive waste is produced in nuclear reactor.

● This can then be reprocessed or disposed of.

• The waste is buried underground with many layers of protection.