Nuclear Mini-unit investigation

Nuclear stability

What is the nuclear stability? Nuclear stability means that nucleus is stable meaning that it does not spontaneously emit any kind of radioactivity (radiation). On the other hand, if the nucleus is unstable (not stable), it has the tendency of emitting some kind of radiation, i.e., it is radioactive. Therefore the radioactivity is associated with unstable nucleus:

Stable nucleus – non-radioactive

Unstable nucleus – radioactive

Keep in mind that less stable means more radioactive and more stable means less radioactive.

We want to know why there is a radioactivity. What makes the nucleus a stable one? There are no concrete theories to explain this, but there are only general observations based on the available stable isotopes. It appears that neutron to proton (n/p) ratio is the dominant factor in nuclear stability. This ratio is close to 1 for atoms of elements with low atomic number and increases as the atomic number increases; and ratio close to 1.5 for elements between Sc to Po, as seen in the figure below (also known as the band of stability)

FUSION v. FISSION

In Chemistry, molecules that are unstable tend to react with other molecules. When this happens, lots of energy is released, so the reaction is called an exothermic reaction. The same thing is true for nuclear reactions. Reactions that would tend to change a nuclide from an unstable condition to a more stable condition would also be exothermic.

If you recall from the first installment of the lectures, there was an image that talked about binding energy that looked like the image below. According to the chart, there are two kinds of unstable nuclides; Very small and very large nuclides. There are two types of reactions that take advantage of these two kinds of unstable nuclides and form the intermediate mass numbered nuclides which are more stable.

Nuclear Fission and Nuclear Fusion result from the collision of two particles to form a new element. These nuclear reaction occur where either two nuclei or a nucleus of an atom with a subatomic particle like a proton, neutron, or high energy electron from outside the atom, collide and produce a new elements.

As illustrated in the image below, nuclear fission occurs when a large unstable atomic nucleus decomposes when it is bombarded by subatomic particles (i.e. nuetron) and forms smaller more stable nuclei (new elements). This is extremely exothermic and is responsible for the energy produced by the Atom bomb.

Nuclear fusion occurs when two unstable small nuclei join together to form a more stable and larger nucleus. The energy produced by the formation of a larger atom is more than produced by nuclear fission and is responsible for the energy produced by the Hydrogen Bomb.

Both Fission and Fusion take advantage of instability resulting in an exothermic reaction.

NUCLEAR REACTIONS

The same can be said for other types of nuclear reactions such as: Alpha, Beta, Gamma, Electron Capture and Positron decay. Now, the nuclear equations below occur due to the unstable nuclei of radioisotopes and are known as Radioactive Decay.

Types of Nuclear Decay:

Alpha decay occurs when the nucleus of an unstable isotope has way more protons than neutrons. Therefore, a helium nuclei comes out from the parent nucleus to form an isotope with a smaller mass. This helium nucleus is known as alpha particles.

Beta Decay occurs when there are way more neutrons than protons in the nucleus of an unstable isotope. Therefore, the neutron converts into a proton and a beta particle (mass of an electron) is ejected from the nucleus.

Positron Emission occurs when there are too many protons in the nucleus. Therefore, a proton is converted into a neutron and a positron (mass of an electron, charge of a proton) is ejected from the nucleus.

Electron Capture occurs when there are too many protons in the nucleus. Therefore, an electron is captured and combines with a proton to form a neutron.

Gamma Decay occurs in ALL decay processes. When an atom goes through nuclear decay, high energy electromagnetic waves are emitted.

NUCLEAR HALF LIFE

The Following is an explanation of half-life and how half-life is used in calculations. The calculations video for this portion of the notes is at the bottom of the page.

Half-life is the amount of time elapsed for the isotope amount to reduce by half when it goes through radioactive, or nuclear decay. Different radioisotopes have different half-lives. Half-life is given the symbol t_½.

Example: 100g of Carbon-14 reduces to 50g after 5730 years have elapsed (t_½=5730 yrs).

100g of Uranium-232 reduces to 50g after 68.9 years have elapsed (t_½=68.9 yrs).

Calculations with half-life

The number of half-lives an isotope goes through can be determined if you know the amount of time elapsed and the t_½ of the isotope.

Example: 25g of Carbon-14 remained after 11,460 yrs. If the half-life of Carbon-14 is 5730 years, how many half-lives did Carbon-14 go through?

n=number of half-lives

The amount of radioactive isotope remaining can be calculated:

N_f = N_i (0.5)ⁿ

Where:

N_f = amount of radioisotope remaining

N_i = original amount of radioisotope

Example: Strontium-90 has a half-life of approximately 28 years. If you begin with 100% of Strontium-90, calculate the percentage of strontium-90 remaining after 280 years.

N_f = N_i (0.5)ⁿ

N_f = ? %

N_i = 100%

number of half-lives = time ÷ half-life = 280 ÷ 28 =10

N_t = 100 x (0.5)¹⁰ = 0.098%

The half-life can be calculated. If you are not familiar with natural log, familiarize yourself with natural log rules through this link.

Example: The original amount of Carbon-14 was 100g and it decayed to 12.5g after 17,190 yrs. What is the half-life of Carbon-14.

N_f = 12.5 g

N_i = 100 g

time elapsed = 17,190 years

half-life = ?

The time elapsed (or age) of isotope can be calculated:

Example: A sample of Uranium-232 was found in a sealed, labeled bottle. According to the label, the mass of the Uranium in the sealed container had a mass of 100. g, but when weighed, it had a mass of 30. g. How long has the uranium been sealed in the bottle if uranium-232 has a half life of 68.9 years?

N_f = 30. g

N_i = 100. g

half life = 68.9 years

time elapsed = ?

APPLICATION OF NUCLEAR REACTIONS

Nuclear Fission Reactor

Nuclear fission reactors are device used to produce large amount of energy which can be utilized for some good purpose. These reactors based on the nuclear fission reaction and generally used uranium and polonium isotopes for nuclear fission reaction.

There are several types of nuclear reactor made up of some general components like

Fuel

• Generally Uranium and polonium used as a basic fuel in nuclear fission reactors.
• Uranium is taken as pellets of uranium oxide (UO₂) which are arranged in tubes to form fuel rods.

Moderator

• There must be some material to decrease the speed of neutron produced as fissionable product.
• These materials used to slow down the speed of neutrons are called as Moderators.
• Generally heavy water and graphite rods serve as a good moderator in reactors.
• Other moderators are helium at 100 atm and 1273 K, beryllium at high temperature, sodium at 773 to 873 K used in breeder reactor or BeF₂ + ZrF₄ used in gas-cooled reactors.

Control rods

• As name implies, these rods used to control the fission reactions.
• They are made up of some neutron-absorbing material like cadmium, hafnium or boron.
• During nuclear fission reaction, they are inserted or withdrawn from the core to control the rate of reaction.

Coolant

• Coolant is a liquid or gas which circulates through the core to transfer the heat from core to coolant.
• Water is a best primary coolant which forms steam after absorbing heat from the core.

Pressure vessel or pressure tubes

These are series of robust steel holding the fuel and conveying the coolant through the moderator.

Steam generator

It is a part of the cooling system where water used as a primary coolant and produce steam for the turbine.

Containment

It is a meter-thick concrete and steel structure around the reactor core which is designed to protect core from outside intrusion as well as to protect those outside from the effects of radiation in case of any malfunction inside.

On the basis of different coolant, moderator and fuel, nuclear fission reactor can be different types.

For example,

Advantages of Nuclear Fission

1. In nuclear reactor the nuclear chain reaction is carried out in a controlled manner and liberated heat is converted into electricity.
2. The fuel used in nuclear reactor is relatively expensive and available in trace amounts but fuel used in very little quantity to produce such a large amount of energy.
3. For example; about 28gm of Uranium releases the same amount of energy as the energy released by 100 metric tons of coal.
4. The nuclear fission is not contributes in global warming or other pollution effects which mainly associated with fossil fuel combustion.
5. Because of small quantity required for fuel in nuclear reactors, it can be easily transported.
6. The large heat produced in nuclear reactors can be used to produce cheap electricity which can be further used for other benefits.
7. The waste product formed during the fission process is very less and nuclear reactors are very reliable source of energy.
8. The nuclear reactor can be active for approx. 40 to 60 years.

Advantages and Disadvantages of Nuclear Fission

Uses of Nuclear Fission

• Nuclear fission energy appears promising for space propulsion applications and in the generation of high mission velocities with less reaction mass. This is because of production of large amount of energy, around 106 times more than any chemical reactions and can be used for the current generation of rockets.
• Nuclear fission is generally used in radioisotope thermoelectric generators for space mission.
• Just like conventional thermal power stations which generate electricity by using the thermal energy released from burning fossil fuels, nuclear power plants follow the conversion of the energy released from nuclear fission in nuclear reactor.
• The heat produced during the nuclear fission removes from core by using coolant and used to generate steam which drives a steam turbine. These steam turbines are connected to a generator to produce electricity.

• Nuclear technology is widely used in diagnostics and radiation treatment like radiation therapy for cancer.
• Nuclear fission is used to produce some less common radioisotopes like cesium-137 (Cs-137) by using uranium-235 which is used in photographic sources.
• Nuclear fission energy is also use as a power source for propelling submarines and some type of surface vessels.
• Another application of nuclear fission reactors is their high neutron fluxes which can be used for studying the structure and properties of materials.

taken from: http://chemistry.tutorvista.com/nuclear-chemistry/nuclear-fission.html