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PROBLEM

NUCLEAR REACTORS AND REACTIONS

Fission means to divide. In nuclear fission,a nucleus

divides into two nuclei. Nuclear fission is a nuclear

change. When a uranium­235 atom is bombarded with

neutrons, it may split into two nuclei. The nuclei are about equal in size. For example, a uranium-235 nucleus may gain a neutron. It divides to form barium 144 and krypton 90. In this nuclear fission, two neutrons are released:

₉₂²³⁵U + ₀¹n → ₅₆¹⁴⁴Ba +₃₆⁹⁰Kr + 2 ₀¹n

one atom of uranium-235 plus one neutron yields one atom of barium 144 plus one atom of krypton 90 plus two neutrons.

PROBLEM

· Why is nuclear fission a nuclear change?

Uranium fission may result in a chain reaction. A nuclear chain reaction is a

series of rapid nuclear fissions. First, one uranium-235 nucleus is split by a neutron.

This releases two neutrons. The two neutrons released in the fission are captured

by two more uranium nuclei. This causes them to split. These two nuclei each emit

two neutrons. They are captured by four more nuclei, causing them to split. The

fission of nuclei and emission of neutrons continue through the uranium sample.

A chain reaction occurs if enough uranium is present. Other­wise, the neutrons escape from the uranium without striking nuclei. Then there are no fissions and a chain reaction does not begin. A neutron must travel at least 10 cm before it slows down enough to be captured by a uranium nucleus. If it escapes before it has gone this distance, the neutron will not cause an atom to split. The smallest amount of an element needed for a chain reaction is called the element's critical mass.The critical mass of uranium-235 is about 24 kg.

During fission, mass is converted to energy. The energy is given off as gamma radiation. The total mass of the two nuclei and two neutrons produced in fission is less than the mass of the original uranium-235 nucleus. The energy released by nuclear fission can be calculated by the equation:

E = mc²

(energy equals mass times the speed of light squared)

The energy released through the fission of a single uranium-235 nucleus is about 200 million electron volts. An electron volt is the energy required to move an electron between two points with a potential difference of one volt. For comparison, the energy in a photon of visible light is about 2,5 electron volts. The electrons hitting the screen of a television set have an energy of about 20000 electron volts. Fantastic amounts of energy are released in nuclear fission! Energy equal to about 25 million kilowatt-hours is released through the fission of only one gram of uranium. A piece of ura­nium no heavier than a loaf of bread and smaller than a golf ball can provide the energy of 1000 metric tons of coal.

1.What is needed before a nuclear chain reaction will occur?

2.What is critical mass?

3.What quantity is repre­sented by each letter in E = mc²?

nuclear fission – ділення ядер chain reaction – ланцюгова реакція

strikе – бомбардувати capture – захватити

electron volt – електрон-вольт critical mass – критична маса

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