Nuclear Fission (English)

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3-3: Nuclear Fission

On the previous page: 3-1:The Origin of the Nuclear Energy,

the fission of uranium-235

( )

was taken up as an example

of nuclear exothermic reaction,

in which a huge amount of energy

is released when neutrons

being bomberded against uranium-235

nuclei to split them into

several fragments.

One of the equations for these fission

processes is written as

Here

is the released energy

which is more than

200 MeV per one process.

"2n" on the right-hand side

means that two neutrons

are simultaneously emitted.

A schematic sketch

of this fission process

is drawn as the following figure.

[Schematic Drawing of the Fission Process of Uranium-235]

When neutrons are bombarded

against uanium-235,

the uranium-235 nucleus absorbs

a neutron to be a uranium-236

and it

splits into

two fragments of almost equal

masses

with emitting

some neutrons simultaneously

and evolving

a huge amount of energy

larger than 200 MeV.

This is nothing but

one of the examples

of fission of uranium-235.

It does not always

split into Ba and Kr

but usually into two fragments

with almost equal masses.

The number of emitted neutrons

is also not always constant

but it distributes over one

to several.

Then the emitted energy is

not always constant

but is almost 200 MeV.

The above equation is

one of the examples

of fission of uranium-235.

There are many other processes

of which examples are

as follows:

[The Discovery of Nuclear Fission]

Rutherford

carried out alpha-particle

scattering experiments

by bombarding the alpha rays

emerging from radioactive Polonium (Po)

or Radium (Ra) elements

onto various nuclei.

Thereby he succeeded

in the first artificial transmutation of elements (1919).

Since then, experiments

of alpha-particle scattering

on nuclei

had been repeated extensively.

Chadwick investigated

the mysterious penetrating radiation

emitted from a beryllium (Be) target

which is bombarded by

high-energy alpha particles

from a Po source.

Finally he concluded

that this mysterious radiation is

a neutral particle

with protonic mass.

This was the discovery of neutron

(1932).

O. Hahn

(Germany, 1879 - 1968) and

F. Strassmann

(Germany, 1902 - 68)

carried out the experiments

to bombard neutrons

from (Po-Be) source

against uranium.

They observed an important

phenomenon in which

the uranium nucleus

bombarded by neutrons

splits into two large

fragments.

Among the reaction products,

they found

Barium (Z = 56)

and Lanthanum

(Z = 57).

L. Meitner

(Germany, 1878 - 1968),

a former collaborator of Hahn,

and her nephew

O. R. Frisch,

who had fled to Sweden

from Nazi Germany,

were reported these experimental results

by a letter.

They analyzed these results

and immediately recognized

the significance.

They were convinced that

the experimental results

are due to

the neutron-induced nuclear fission

and they published this idea.

This was the first

discovery of nuclear fission.

[The Mechanism of Nuclear Fission]

As explained on the page:

3-1: The Origin of the Nuclear Energy,

if such a heavy nucleus

as uranium-235

splits into two fragments

of almost equal mass,

each of them would be bound more

tightly than the original nucleus.

In that case, an energy excess

is brought about

and the state after the reaction

becomes more favorable

than before the reaction.

Accordingly,

a heavy nucleus has

always a possibility of

nuclear fission.

Using the idea of

Liquid Drop Model,

let us explain

in what mechanism

such a heavy nucleus

as uranium-235 fission.

The nucleus is

a many-nucleon system

consisting of protons and neutrons.

It can be assumed to be

something like

a drop of water

or a raindrop.

A heavy nucleus

is therefore considered to be

very "soft" and "flabby"

and easy to deform.

However, since a strong nuclear force

works between nucleons

and it plays a role of

surface tension,

the ground state of a nucleus

is rather stable

and does not directly fission.

If some amount of energy is given

to a nucleus to be

in an exited state,

then it easily deforms

and fissions.

You would be able to understand

the fissioning mechanism

by the

following two figures.

[Fission Mechanism by Liquid Drop Model]

If such a heavy nucleus

as uranium-235

absorbs a neutron,

it is in an excited state.

The soft nucleus is easy

to deform.

Sometimes it becomes

dumbbells shape

and splits into

two droplets.

At that time

the energy excess

is released.

[Schematic Graph of Energy in Nuclear Fission]

This diagram schematically shows

the potential energy for nuclear fission

as a function of

the degree of nuclear deformation

or the average separation

of the two fission fragments.

The ground state of the nucleus

is confined within the valley

of potential barrier

and is stable.

When it is excited

to be close to

the top of the barrier,

it penetrates the barrier

with the tunnel effect

and run down along the potential

slope to fission.

[The Chain Reaction]

As shown in the equation at

the top of this page,

the fission of a uranium-235

is accompanied by the emission

of neutrons.

There are several types of

fission processes

in the uranium-235 fission,

in each of which one, two or three

neutrons are emitted;

on average, 2.5 neutrons

are released.

These neutrons are capable to

cause further fission

of uranium-235.

If each fission causes

an average of more

than one further fission,

an avalanche of fission reactions

will result.

This is called

the chain reaction.

As soon as Hahn and Strassmann

discovered nuclear fission,

many physicists

noticed the possibility

of the chain reaction.

If the chain reaction

occurs very rapidly

in an uncontrolled way,

a violent explosion follows.

Then it can be used for a military

purpose; i.e. an atomic

or strictly speaking a nuclear

bomb.

If the chain reaction

is well controlled,

then it can be used

as a power source.

The first experiment

of nuclear chain reaction

was carried out

in 1942

by E. Fermi

(Italy, USA, 1901 - 54),

who had fled to US

from Fascists Italy,

and his colleagues.

[Nuclear Reactor]

A device in which controlled nuclear

fission takes place

to produce heat or energy

is called a nuclear reactor.

It is the main facility

of a nuclear power station.

The first nuclear reactor

was constructed

by Fermi

and coworkers

at the University of Chicago

in 1942.

[Other Kinds of Fissionable Materials]

An elemental isotope that undergoes

induced fission

when being struck by a free neutron

is usually called "fissionable".

Of course, the most popular

fissionable material

is uranium-235

().

One of the other

well-known fissionable

materials is

pultonium-239

(),

which is usually produced

in a reactor

from uranium-238.

(Uranium-238 is not

fissionable, so that

it does not "burn"

in a reactor.)

While almost all pultonium

is artificially produced,

extremely tiny amount

is found naturally.

Another kind of

element

that can be used

as a nuclear fuel

in reactors

is thorium,

which occurs naturally.

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