Which Equation Represents a Fission Reaction

Students demand to move beyond the thought that nuclear changes are represented solely by blastoff, beta and gamma decay. There are other decay processes, and there are other events that occur when a nucleus absorbs a particle and becomes unstable.

Lesson Summary

• Discussion: Transmutation of elements (15 minutes)
• Educatee questions: Balancing equations (xxx minutes)
• Discussion: Induced fission (10 minutes)
• Demonstration: The nucleus equally a liquid driblet (x minutes)
• Discussion: Fission products and radioactive waste (10 minutes)
• Worked example: A fission reaction (10 minutes)
• Discussion and demonstrations: Controlled concatenation reactions (15 minutes)
• Discussion: The possibility of fission (10 minutes)
• Student questions: Fission calculations (twenty minutes)

Discussion: Transmutation of elements

Start past rehearsing some causeless knowledge. What is the nucleus made of? (Protons and neutrons, collectively know as nucleons.) What two natural processes change one element into another? ( α and β decay). This is transmutation.

Using a Periodic Table, explain that α decay moves two places downwardly the periodic tabular array. What almost β
^–
decay? (Moves one place up the periodic tabular array.) Introduce the idea of β
⁺
disuse. (Moves i place downwards the periodic tabular array.)

Write full general equations for these processes.

In that location is another manner in which an element may be transmuted; for example, the production of radioactive

14
6C
used in radio-carbon dating in the atmosphere by the neutrons in cosmic rays.

14
7Northward
+


1n
→

fourteen
viC
+

1
1H

Rutherford constitute that protons be in the nucleus by bombarding nuclei with alpha particles. Patrick Blackett carried out further experiments and showed that the bombarded nucleus had transmuted. Enquire your students to complete the following nuclear equation that summarises the transmutation of nitrogen into oxygen:

He +

fourteen
7Due north
→  O +

i
aneH

They should become:

4
iiHe
+

xiv
7Northward
→

17
8O
+

1
1H

Cockroft and Walton were the beginning to
split
the atom, past bombarding lithium with protons from their accelerator.

1
1H
+

vii
3Li
→

8
4Be
→  ii

4
2He

Pupil questions: Balancing equations

Students can practise balancing equations.

Episode 527-one: Isotope production (Word, 50 KB)

Discussion: Induced fission

In the examples higher up, small parts are
chipped off
nuclei. The behaviour of the heaviest natural chemical element, uranium, is dissimilar. It breaks up into ii big chunks – into ii elements nearer to the heart of the periodic tabular array – so-chosen
induced fission
. The two lighter elements are referred to as
fission fragments
.

How practice the 2 common isotopes of uranium

235
92U
and

238
92U
differ? (
238
92U
has three more neutrons than

235
92U.) It is the

235
92U
not the

238
92U
that fissions. Information technology absorbs a neutron, and so splits into fission fragments, i.e. any ii smaller nuclei that can be fabricated from the 235 nucleons of the

235
92U.

Episode 527-two: Nuclear fission (Word, 123 KB)

Sit-in: The nucleus as a liquid drop

In many ways, nuclei behave like a drop of liquid. Show a water filled airship – a practiced model for a nucleus. After the assimilation of the neutron, the nucleus of

238
92U
wobbles. As soon as the electrical charge distribution departs from the spherical (pinch the airship into a dumbbell like shape) the mutual coulomb repulsion between the 2 ends drives the fission process. An alternative is to grease a plate and put a big drib of water on information technology. Wobble the plate about and sentry the drop carve up.

Word: Fission products and radioactive waste

Most of the energy released is carried off in the loftier speeds of the fission fragments. Because they have a relatively high fraction of neutrons, they are unstable, and decay with short half-lives. They course the
high-level
radioactive waste that cannot exist but tending of; it has to be stored somewhere for a minimum of 20 half lives.

By what factor volition the activity autumn after 20 half lives? ( ½
²⁰
is about 10^-half-dozen
, or ane-millionth)

137
55Cs
has a half life of thirty.23 years: twenty half lives is 605 years

90
38Sr
has a one-half life of 28.ane years: 20 half lives is 562 years

Call up about the consequences if waste product disposal has to be engineered to remain intact for many centuries. (Which applied science structures accept existed for the last 600 years?)

Worked examples: A fission reaction

Hither is the nuclear equation for a typical fission process:

1due north
+

235
92U
→

236
92U
→

138
53I
+

95
39Y
+ ?

What is required to balance the equation? (3 neutrons)

Why are there some neutrons left over? (Relate this to the
N–Z
curve. The heaviest elements have the largest neutron excess to remain stable. The 2 lighter fission fragments have a higher fractional neutron excess; hence some are
left over.) These
left over
neutrons are the vital key to unlock nuclear power using fission.

Word and demonstrations: Controlled chain reactions

If at least one surplus neutron tin can induce fission in another

235
92U
nucleus and so on, then a self sustaining release of energy is possible. For a power station a
controlled
chain reaction is needed. Should each fission effect in
more
than one farther fission, then the chain reaction is said to diverge. In a bomb the aim is to get the chain reaction to diverge as fast equally possible.

Blow up two balloons; let one fly off; release the other slowly, to illustrate the difference between uncontrolled and controlled energy release.

There are a number of analogues of concatenation reactions that tin can exist demonstrated at this bespeak, using matches or lines of dominoes.

Episode 527-3: Chain reactions (Word, 173 KB)

Episode 527-4: Fission analogues (Discussion, 26 KB)

Give-and-take: The possibility of fission

What are the chances that a neutron will strike another nucleus? First call up that atoms are generally empty space. The nuclei of two adjacent uranium atoms are typically 10,000 nuclear diameters autonomously. Emphasise this by picking a student in the eye of the course, and estimating her/his width (0.3 m?). Where volition the next
pupil nuclei
be situated? (iii km
away.) A fast-moving neutron will travel a long fashion before information technology strikes some other nucleus.

In fact, most neutrons are absorbed by

238
92U
nuclei, which are much more common than

235
92U, and quite good at absorbing fast neutrons. Instead of fissioning they transmute into

239
94Pu
which is fissile, the favourite explosive cloth for making nuclear bombs. Pure natural uranium is incapable of sustaining a fission reaction –
less
than i fission neutron succeeds in inducing a further fission.

Enquire your students how this problem might be overcome in gild to accept a controlled chain reaction. (The respond is the introduction to the next episode.)

Student questions: Fission calculations

Calculations of free energy released in fission events.

Episode 527-5: Fission – do questions (Discussion, 36 KB)