Nuclear energy

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Content – Forms of energy

Nuclear energy is the energy in the nucleus, or core, of an atom. Energy is what holds the nucleus together.

Nuclear energy can be used to create electricity, but it must first be released from the atom. In nuclear fission, atoms are split to release the energy.

In a nuclear power plant nuclear fission takes place at a controlled manner to produce electricity. The feed is pellets of uranium. In the reactor, atoms of uranium are broken apart. As they split, the atoms release small particles. The particles cause other uranium atoms to split, starting a chain reaction. The energy released from this chain reaction creates heat.

Fission and Fusion

There is basically one nuclear process currently used for commercial energy production, that is fission.

Fission

Fission implies splitting of large atoms normally uranium or plutonium into two smaller atoms,. To split an atom, it needs to be hit by a neutron. Several neutrons are then released splitting other nearby atoms, producing a nuclear chain reaction releasing substantial energy, generating heat that is normally turned into electricity.

Fusion

Fusion is combining two small atoms such as Hydrogen or Helium to produce heavier atoms and energy. These reactions can release more energy than fission without producing as many radioactive byproducts. Fusion reactions occur in the sun, generally using Hydrogen as fuel and producing Helium as waste. This reaction has not been commercially developed yet.

The table shows the energy density of a few materials. When uranium undergoes nuclear fission it attains a very high energy density.

 Material Energy Density (MJ/kg) Wood 14,5 Ethanol 27 Coal 33 Crude oil 41.9 Diesel 43,4 Natural Uranium (LWR) 5.7×105 Thorium (breeder) 7.9×107

Nuclear binding energy

The energy required to break down a nucleus into its component nucleons is called the nuclear binding energy.

Nuclear binding energy is usually expressed in terms of kJ/mole of nuclei or MeV/nucleon.

Formula – Nuclear energy

Mass defect and nuclear binding energy
The basis for calculating the nuclear binding energy for a substance is the equation.

$E= mc^2$ or
$m= \dfrac{c^2}{E}$

We first need to calculate the mass defect to to be able to calcutlate the potential for releasing energy when fission takes place.

To calculate the mass defect we subtract the nucleus mass of the base material from the combined mass of the base material components:

Mass c (combined mass)    = MP + MN (Mass Neutron)

MP =Mass Proton = nP*amuP

MN =Mass Neutron = nN*amuN

Dm = Mass c – MassBM

Then to convert the mass defect into energy we first need to convert the mass defect into the unit Kg and then into its energy equivalent:

Dm(amu) * 1.6606 x 10-27 kg/nucleus

1amu = 1.6606 x 10-27 kg

c = 2.9979 x 108 m/s

E = mc2 = (Dm(amu) *1.6606* 10-27 kg/nucleus) * (2.9979 x 108 m/s)2

E = DM*1,4924483 *10-10 J/nucleus

To convert this into KJ/Mol the following conversion applies:

E= DM*1,4924483 *10-10 J/nucleus * 6.022 x 1023 nuclei/mol* (1 kJ/1000 J) * = DM*8,9875 1010 kJ/mol of nuclei.

Avogadro’s Number = 6.022 x 1023 nuclei/mol

Nuclear fuel

After mining, uranium has to undergo four main steps to make it useable as nuclear fuel.
Those are:

• Milling
• Conversion
• Enrichment
• Fuel fabrication

The main suppliers of uranium are:Kazakhstan, Australia, Canada, Namibia, Niger, Russia and the United States.

To enable the chain reactions necessary for continuous operation of a nuclear reactor a high concentration of the isotop, uranium-235 is required. This is obtained by “enrichment ” of the uranium.

The main fuel enrichment facilities are located in: France, Germany, the Netherlands, Russia the United Kingdom and the United States.

When the enrichment has taken place the uranium is converted into powder which is then pressed into pellets. The pellets are loaded into metal tubes which are inserted into the nuclear reactors as fuel.

The average useful lifetime of nuclear fuel (uranium) is about five years. This is the time the fuel spends inside the reactor which is powering the electrical generators.

The replacement of fuel(uranium tubes) is normally sequenced such that all is not replaced at one time. The replaced units are placed in a pool of water for cooling. These units are highly radioactive. After cooling the used units are stored in containers usually made of steel-reinforced concrete.

Nuclear waste

Due to the large amount of highly radioactive waste created during production of nuclear power, waste management is one of the main concerns related to nuclear power generation. In addition the radioactivity of the waste remains at high levels for extremely long periods, therefore there are considerable technical issues related to handling and storage of the waste material.

There are various fission products generated from nuclear fission, some of them extremely long lived:

• Technetium-99, half-life 220,000 years
• Iodine-129 half-life 15.7 million years
• Neptunium-237 half-life two million years and
• Plutonium-239 half-life 24,000 years

Despite that many waste management solutions for a long time have been considered by the governments, the progress on this matter has been limited in respect of viable long term solutions. Most of the strategies being considered involves deep geologic placement. The long timeframes, in the range of 10 000 to millions of years before the waste represent safe levels of radiation is of course a significant reason for this.

Research is being carried out related to reactor types that may use the nuclear waste as fuel, reducing the timespan necessary to reach safe levels of radiation down to a few hundred years rather than thousands and millions of years. These are typically the American Fast Reactor and the Molten salt reactor.

Another type of reactor being considered is the Thorium reactor using thorium without mixing it with uranium or plutonium as fuel. The waste from this reactor type remains radioactive for a few hundred years.