Lesson plan

Topicm56f29ce5ec3e99e8_1528449000663_0Topic

Chain reaction

Levelm56f29ce5ec3e99e8_1528449084556_0Level

Third

Core curriculumm56f29ce5ec3e99e8_1528449076687_0Core curriculum

XI. Nuclear physics. The student:

9) describes the uranium ${}^{235}U$ fission reaction as a result of the neutron absorption; gives the conditions for the chain reaction.

Timingm56f29ce5ec3e99e8_1528449068082_0Timing

45 minutes

General learning objectivesm56f29ce5ec3e99e8_1528449523725_0General learning objectives

Describes the uranium ${}^{235}U$ fission reaction as a result of the neutron absorption.

Key competencesm56f29ce5ec3e99e8_1528449552113_0Key competences

1. Lists the conditions required for the chain reactionchain reactionchain reaction.

2. Writes the equation describing the fission processes.

Operational (detailed) goalsm56f29ce5ec3e99e8_1528450430307_0Operational (detailed) goals

The student:

- describes the process of the uranium ${}^{235}U$ fission,

- explains the conditions required for fission to occur.

Methodsm56f29ce5ec3e99e8_1528449534267_0Methods

1. Discussion.

2. Text analysis.

Forms of workm56f29ce5ec3e99e8_1528449514617_0Forms of work

1. Individual work.

2. Group work.

Lesson stages

Introductionm56f29ce5ec3e99e8_1528450127855_0Introduction

The teacher asks students to remind some facts about the nuclear reactions.

What the nuclear reaction is? What are the rules for writing the reaction equations?

Procedurem56f29ce5ec3e99e8_1528446435040_0Procedure

The teacher introduces the concept of nuclear fissionfissionfission and discusses its properties.

NuclearnuclearNuclear fission:
Nuclear fission is a type of nuclear reaction in which the nucleus splits into smaller fragments with smaller mass. In the fission process free neutrons and gamma rays are produced. The process is associated with a large amount of released energy.

Nuclear fissionfissionfission is carried out in a nuclear reactor or occurs spontaneously.

Two isotopes in common use as nuclear fuels are: uranium ${}^{235}U$ and plutonium ${}^{239}{Pu}$ .

The nuclear fissionfissionfission in chain reaction proceeds in several steps:

- the neutron strikes the nucleus.
- the nucleus absorbs the neutron,
- the nucleus undergoes deformation,
- the nucleus splits into two parts and releases two or three neutrons and energy,
- the newly formed fission products lose their kinetic energy and emit gamma rays,
- the fission products in the excited state lose their excess energy by radioactive decay and emit gamma rays and beta particle.

[Interactive graphics]

Chain reactionchain reactionChain reaction:
The released neutrons may hit other nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reactionchain reactionchain reaction. The chain reaction in nuclearnuclearnuclear reactors is controlled to stop it going too fast. The chain reaction is ongoing so long as the nuclei, that can undergo fission, are present in the sample.

Critical energycritical energyCritical energy:
Forced fissionfissionfission to occur requires the supply of a minimum energy.

The excitation energy of a nucleus determines how much the energy of the excited nucleus is greater than the energy of the nucleus in its ground state. The minimum excitation energy required for fission to occur must be above a certain level and is called the critical energy (EIndeks dolny c) or threshold energy. The critical energycritical energycritical energy depends on the nuclear structure.

Critical mass:
Critical masscritical massCritical mass is the minimum amount of fissile material necessary to achieve a self‑sustaining fission, chain reactionchain reactionchain reaction, characteristic of a given izotop.

Uranium fission:
Uranium is the most widely used fuel for nuclearnuclearnuclear fission. Nuclear power plants use uranium enrichedenrichedenriched in the isotope ${}^{235}U$ as the fuel. This isotope, in contrast to the more common uranium ${}^{238}U$ , can fission. The fission of uranium ${}^{235}U$ is triggered by the absorption of a moving neutron with low energy (below 10 eV) called slow neutron and unstable ${}^{236}U$ is formed. High energy neutrons (0,5 – 15 MeV), called fast neutrons, are not captured. Hence, these particles must be slow down in the process of moderation in the reactor. The average number of neutrons released per fission is 2,4.

The example reaction of ${}^{235}U$ fission:

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U * \to {}_{36}^{89}K r + {}_{56}^{144}B a + 3 {}_{0}^{1}n

is schematically presented in the figure below.

[Illustration 1]

In the uranium ${}^{235}U$ the nucleons are bound with an average binding energy per nucleon of about 7,6 MeV. In the fission product nuclei, the binding energy per nucleon is higher and amounts to approx. 8,5 MeV. The difference in binding energy is of 0,9 MeV per nucleon and ${}^{235}U$ has 235 nucleons. Hence, the energy of about 210 MeV is released in the single nuclear fission process. The released energy is the sum of the following components:m56f29ce5ec3e99e8_1527752256679_0In the uranium ${}^{235}U$ the nucleons are bound with an average binding energy per nucleon of about 7,6 MeV. In the fission product nuclei, the binding energy per nucleon is higher and amounts to approx. 8,5 MeV. The difference in binding energy is of 0,9 MeV per nucleon and ${}^{235}U$ has 235 nucleons. Hence, the energy of about 210 MeV is released in the single nuclear fission process. The released energy is the sum of the following components:

- kinetic energy of fission products is 175 MeV,
- kinetic energy of fission neutrons is 5 MeV,
- energy of the gamma radiation emitted during the fission is 7 MeV,
- energy of beta and gamma radiation during the decay of the radioactive fission products is 13 MeV,
- energy of the neutrinos is 10 MeV.

The students work in small groups on the uranium fission reactions.

Task 1

Some possible reactions resulting in different fission products are shown below. Figure out how many neutrons are produced in each reaction. Do not forget about mass number and atomic number conservation rule.

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{38}^{90}S r + {}_{54}^{144}X e + ... {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{35}^{87}B r + {}_{57}^{146}L a + ... {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{37}^{96}R b + {}_{55}^{137}C s + ... {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{52}^{137}T e + {}_{40}^{97}Z r + ... {}_{0}^{1}n

Answer:

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{38}^{90}S r + {}_{54}^{144}X e + 2 {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{35}^{87}B r + {}_{57}^{146}L a + 3 {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{37}^{96}R b + {}_{55}^{137}C s + 3 {}_{0}^{1}n

{}_{92}^{235}U + {}_{0}^{1}n \to {}_{92}^{236}U \to {}_{52}^{137}T e + {}_{40}^{97}Z r + 2 {}_{0}^{1}n

Some of the neutrons released during the fissionfissionfission may cause new fission processes. A part of them are involved in other processes.

[Illustration 2]

Task 2

A plutonium atom ${}^{239}{Pu}$ is bombarded with a neutron and produces an atom of strontium ${}^{96}{Sr}$ and four neutrons. In this reaction the other nuclide is formed. Identify it and write the corresponding equation.

Answer:

Barium, ${}_{56}^{140}B a$

${}_{94}^{239}{Pu} + {}_{0}^{1}n \to {}_{38}^{96}S r + {}_{56}^{140}B a + 4 {}_{0}^{1}n$

Lesson summarym56f29ce5ec3e99e8_1528450119332_0Lesson summary

Nuclear fission is a type of nuclear reaction in which the nucleus splits into smaller fragments with smaller mass. The process is associated with a large amount of released energy.
For the chain reaction to take place the critical energy must be supplied and the minimum mass of the sample, characteristic of a given element, known as critical mass, is necessary.m56f29ce5ec3e99e8_1527752263647_0Nuclear fission is a type of nuclear reaction in which the nucleus splits into smaller fragments with smaller mass. The process is associated with a large amount of released energy.
For the chain reaction to take place the critical energy must be supplied and the minimum mass of the sample, characteristic of a given element, known as critical mass, is necessary.

Selected words and expressions used in the lesson plan

fissionfissionfission

nuclearnuclearnuclear

critical energycritical energycritical energy

critical masscritical masscritical mass

chain reactionchain reactionchain reaction

power reactorpower reactorpower reactor

slow neutronsslow neutronsslow neutrons

enrichedenrichedenriched

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chain reaction1

fission1

nuclear1

critical energy1

critical mass1

enriched1

power reactor1

slow neutrons1

Scenariusz

Topicm56f29ce5ec3e99e8_1528449000663_0Topic

Lesson stages

Selected words and expressions used in the lesson plan