VISUAL PHYSICS ONLINE

8.2 FROM THE UNIVERSE TO THE ATOM

THE NUCLEUS

P82 001

(a)

What is a fission reaction?

(b)

How can an uncontrolled nuclear chain reaction occur?

(c)

What is a controlled nuclear fission reaction?

View solution below only after you have completed the answering the question.

Solution

(a)

In nuclear fission, a heavy nucleus such as uranium splits into two intermediate-sized nuclei after being struck by a neutron. In the fission process other neutrons are released.

Typical fission reactions are:

²³⁵U₉₂ + ¹n₀ à ²³⁶U₉₂* à ¹⁴⁴Ba₅₆ + ⁸⁹Kr₃₆ + 3¹n₀

²³⁵U₉₂ + ¹n₀ à ²³⁶U₉₂* à ¹⁴⁰Xe₅₄ + ⁸⁹Sr₃₈ + 2¹n₀

(b)

When a nucleus splits due to a collision with a single neutron, other neutrons are released, so that a chain reaction is possible. To maintain the chain reaction a minimum mass of fuel (the critical mass) is required. Also, to enhance the chances of a neutron imitating a fission reaction, a moderator is used to slow down the released neutrons.

Chain reaction

Because each fission event releases two or more neutrons, while only one neutron is required to initiate the fission process, an avalanche of fissions can occur in a sample of fissionable material giving an uncontrolled chain reaction where an immense amount of energy is liberated in a very short period of time.

An uncontrolled chain reaction can cause an explosion of exceptional magnitude.

(c)

A chain reaction can be controlled, when one neutron per fission causes another fission reaction. In a controlled chain reaction, the energy output per second is constant. This occurs in a nuclear reactor. Movable control rods absorb neutrons to maintain a nuclear at a critical level to maintain a self-sustaining chain reaction and in the event of an accident can be dropped to be into reactor vessel to shut-down the fission process.