Fission

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Spontaneous & Induced Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
Isotopes of uranium and plutonium undergo fission and are used as fuels in nuclear power stations to convert nuclear energy into electrical energy.
During fission, a neutron collides with an unstable nucleus, which splits into two smaller daughter nuclei plus two or three neutrons; gamma rays are also emitted.
Energy is transferred from the nuclear potential energy store of the nucleus to the kinetic energy store of the fission fragments, which move away very quickly and carry energy away as heat.
Spontaneous fission is rare: it occurs without additional energy being put into the nucleus.
Induced fission usually requires the unstable nucleus to first absorb a slow-moving (thermal) neutron.
For example, uranium-235 has a very long half-life of 700 million years, so it has low activity and releases energy only if additional neutrons are added.
In induced fission, a neutron is absorbed by uranium-235 to make uranium-236, which is very unstable and splits almost immediately.

Nuclear fission

Energy Released in Fission Reactions

Daughter nuclei produced in fission have a higher binding energy per nucleon than the parent nucleus.
Energy is released due to the mass defect between the parent nucleus and the daughter nuclei.
Nuclear fission has the highest energy density of any fuel currently available (until fusion becomes feasible).
Calculations often use: density = energy density / specific energy, and number of nuclei = (mass × Avogadro's number) / molar mass.
The energy released per fission can be found from: energy released = (total binding energy of products) − (total binding energy of reactants).
For uranium-235 fission into technetium-112 and indium-122, the energy released is about 191 MeV per fission.
A 500 MW power plant at 35% efficiency requires about 1.58 kg of uranium-235 per day.
Over 2 million times more coal than uranium-235 is required to achieve the same power output.

Chain Reactions from Fission

Thermal neutrons have low energy and speed, allowing them to induce fission; neutrons with too much energy rebound away and fission does not take place.
Only one extra neutron is required to induce a uranium-235 nucleus to split by fission.
Each fission produces two or three neutrons which move away at high speed and can start further fission reactions.
A chain reaction occurs when each fission goes on to cause at least one more fission.
In a controlled chain reaction, only one thermal neutron from each fission is used to create another fission reaction.
The critical mass is the minimum mass of fuel required to maintain a steady chain reaction.
Using subcritical mass (less than critical) causes the reaction to eventually stop.
Using supercritical mass (more than critical) leads to a runaway reaction and eventually an explosion.

Operation of a Nuclear Reactor

In a nuclear reactor, a chain reaction is required to keep the reactor running; two factors must be controlled: the number of free neutrons and the energy of the free neutrons.
The main components of a nuclear reactor are: control rods, moderators, heat exchangers, and shielding.
The overall purpose of a nuclear reactor is to collect the heat energy produced from nuclear reactions.
Control rods absorb neutrons; they are made of a material that absorbs neutrons without becoming dangerously unstable.
The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods: lowering them decreases the rate of fission, raising them increases it.
Moderators slow down neutrons; they surround the fuel and control rods and are made from materials that are poor absorbers of neutrons, such as water.
Fast-moving neutrons slow down by colliding with moderator molecules, losing momentum until they are in thermal equilibrium with the moderator (thermal neutrons).
Heat exchangers transfer thermal energy efficiently between the water systems of a nuclear power plant, producing steam to drive electricity-generating turbines.
Shielding houses the reactor and absorbs hazardous radiation; it is made of steel and concrete nearly 2 metres thick.

Radioactive Waste Management

There are three main types of nuclear waste: low-level, intermediate-level, and high-level waste.
Low-level waste includes lightly contaminated clothing, gloves and tools; it is radioactive for a few years and is encased in concrete and stored a few metres underground.
Intermediate-level waste includes waste from decommissioned nuclear power stations; it has a longer half-life and is encased in cement in steel drums and stored securely underground.
High-level waste refers to unusable fission products from uranium-235 or from spent fuel rods; it is the most dangerous and remains radioactive for thousands of years.
Spent fuel rods are extremely hot and highly radioactive, so they must be handled and stored much more carefully than other types of waste.
Uranium-238 in fuel rods quickly decays into plutonium-239, which is high-level waste with a very long half-life of 24 000 years, presenting a long-term risk of contamination.
Treatment of high-level waste: placed in cooling ponds for years, plutonium and uranium are harvested for reuse, waste is mixed with molten glass (vitrification), encased in steel, lead or concrete, and stored very deep underground.
Isotopes with long half-lives must not enter water and food supplies; burial locations must be geologically stable, secure from attack, and designed for safety, but space for such locations is limited.

Advantages & Disadvantages of Nuclear Power

Advantages: nuclear power stations produce no greenhouse gases during operation (climate change friendly).
Advantages: uranium provides far more energy per kg compared to coal and other fossil fuels (high energy density).
Advantages: reserves of fissionable materials are much higher compared to fossil fuel reserves (availability of fuel).
Advantages: nuclear power is now regarded as one of the safest and most reliable processes for electricity production.
Disadvantages: production of radioactive waste is very dangerous and expensive to deal with, and stays hazardous for thousands of years.
Disadvantages: potential for catastrophic accidents, such as a nuclear meltdown (e.g. Chernobyl), with severe environmental and human consequences.
Disadvantages: potential for misuse of nuclear material and infrastructure in nuclear weapons and terrorist attacks.
Disadvantages: dangers associated with mining uranium, from handling to environmental effects.

Safety Measures and Society

Several measures reduce workers' exposure to radiation: fuel rods are handled remotely by machines.
The nuclear reactor is surrounded by very thick lead or concrete shielding to limit radiation exposure.
In an emergency, control rods are fully lowered into the reactor core to stop fission reactions by absorbing all free neutrons; this is an emergency shutdown.
Nuclear power can scare people if they do not understand it; it is dangerous if not handled properly and is invisible, which can be difficult for some people to comprehend.
With increased education on nuclear energy, society can use this knowledge to inform their own decisions and opinions.

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練習問題

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1.Which of the following correctly describes the neutrons that are used to induce fission in uranium-235?
Easy
- ASlow-moving thermal neutrons
- BFast-moving high-energy neutrons
- CProtons emitted by the moderator
- DGamma ray photons
2.A moderator slows down neutrons so that they become thermal neutrons.
Easy
True or false?
3.In a nuclear reactor, what is the purpose of the control rods?
Medium
- ATo absorb neutrons and control the rate of fission
- BTo slow down neutrons so they can induce fission
- CTo transfer heat to the coolant
- DTo shield workers from gamma radiation
4.Which of the following are products of the fission of a uranium-235 nucleus? (select all that apply)
Medium
- ATwo smaller daughter nuclei
- BTwo or three neutrons
- CGamma rays
- DA single heavy nucleus
- EProtons
5.Match each nuclear reactor component with its purpose.
Medium
- Control rods
- Moderator
- Heat exchanger
- Shielding
- Absorb neutrons
- Slow down neutrons
- Transfer thermal energy between water systems
- Absorb hazardous radiation
6.Place the stages of high-level radioactive waste treatment in the correct order, from first to last.
Medium
- Place waste in cooling ponds
- Harvest plutonium and uranium isotopes
- Mix waste with molten glass (vitrification)
- Encapsulate in steel, lead or concrete containers
- Store deep underground
7.In a controlled chain reaction, what is meant by the critical mass of nuclear fuel?
Medium
- AThe minimum mass required to maintain a steady chain reaction
- BThe maximum mass that can be safely stored
- CThe mass of fuel that produces a runaway explosion
- DThe mass of moderator needed to slow all neutrons
8.High-level nuclear waste remains radioactive for thousands of years.
Easy
True or false?

もっと問題

9.Which statement defines nuclear fission?
Easy
- AThe splitting of a large, unstable nucleus into two smaller nuclei
- BThe joining of two small nuclei to form a larger nucleus
- CThe emission of an α particle from a nucleus
- DThe absorption of a neutron by a stable nucleus without splitting
10.Which of the following are emitted during the fission of a uranium-235 nucleus? (select all that apply)
Medium
- AGamma rays
- BTwo or three neutrons
- CTwo smaller daughter nuclei
- DA single proton
- EAn electron
11.Spontaneous fission occurs when a nucleus undergoes fission without absorbing an additional neutron.
Easy
True or false?
12.What is the purpose of a moderator in a nuclear reactor?
Medium
- ATo absorb neutrons and control the rate of fission
- BTo slow down neutrons so they can induce fission
- CTo transfer heat to the coolant
- DTo shield the reactor from radiation
13.Which of the following are advantages of using nuclear power compared to fossil fuels? (select all that apply)
Medium
- AProduces no greenhouse gases during operation
- BHigh energy density
- CFuel reserves are plentiful
- DProduces hazardous waste that remains radioactive for thousands of years
- EPotential for catastrophic accidents
14.In a fission reaction, the total mass of the products is slightly less than the total mass of the reactants. What happens to this missing mass?
Medium
- AIt is converted into energy
- BIt is converted into extra neutrons
- CIt is lost as gravitational potential energy
- DIt becomes part of the moderator
15.A neutron is fired at a uranium-235 nucleus causing it to produce two smaller nuclei and two neutrons. What is this process called?
Medium
- ASpontaneous fission
- BInduced fission
- CRadioactive decay
- DNuclear fusion
16.A uranium-235 nucleus undergoes fission into two approximately equally sized products. Using the binding energy per nucleon curve, which of the following is closest to the energy released?
Hard
- A2 × 10^-11 J
- B4 × 10^-11 J
- C8 × 10^-11 J
- D1 × 10^-10 J
17.Which of the following statements about a thermal neutron are correct? (select all that apply)
Medium
- AIt has low energy and speed
- BIt is in thermal equilibrium with the moderator
- CIt is produced directly by fission
- DIt can induce fission in uranium-235
- EIt is highly radioactive
18.Which of the following are reasons why the energy released in fission is approximately equal to the difference in binding energy between the parent nucleus and the daughter nuclei? (select all that apply)
Hard
- AThe daughter nuclei have higher binding energy per nucleon than the parent
- BThe total mass of the products is less than the total mass of the reactants
- CThe number of nucleons is conserved
- DThe energy is released as kinetic energy of the fragments
- EThe parent nucleus has higher binding energy per nucleon
19.In a nuclear reactor, the coolant transfers heat from the reactor core to the heat exchanger.
Easy
True or false?
20.Spent fuel rods are classified as low-level radioactive waste.
Medium
True or false?
21.A teacher demonstrates moderation using a golf ball (mass m) hanging vertically and just touching a stationary hockey ball (mass 3m). The golf ball is pulled aside and released; after the collision the balls move in opposite directions with equal speeds. Which feature of this demonstration best models the action of a moderator in a nuclear reactor?
Medium
- AA light particle transfers energy to a much heavier particle in a collision
- BA heavy particle absorbs a light particle completely
- CTwo particles of equal mass collide and stop
- DA particle rebounds with more kinetic energy than it had before
22.In the golf-ball/hockey-ball moderation demonstration, the collision is not a perfect model of neutron moderation in a reactor. Which two statements describe real differences between the demonstration collision and neutron–moderator collisions? (select all that apply)
Medium
- AIn the reactor the neutron may be absorbed or scattered by the moderator nucleus rather than simply bouncing elastically
- BThe demonstration uses macroscopic balls so energy losses to sound and deformation are not present in the reactor
- CIn the reactor the neutron collides with a nucleus many orders of magnitude smaller than itself
- DIn the reactor the neutron–nucleus collision is governed by the electromagnetic force only
- EIn the demonstration the balls move in one dimension only, whereas in the reactor collisions are three-dimensional
23.A fission reaction releases 191 MeV per fission. Using 1 MeV = 1.6 × 10^-13 J, which expression correctly gives the energy released per fission in joules?
Hard
- A191 × 1.6 × 10^-13 J
- B191 / (1.6 × 10^-13) J
- C191 × 1.6 × 10¹³ J
- D191 × 10^-13 / 1.6 J
24.A 500 MW nuclear power plant operates at 35% efficiency. Which of the following steps are required to calculate the mass of uranium-235 consumed per day? (select all that apply)
Hard
- ACalculate the input power as P_out / efficiency
- BCalculate the number of U-235 nuclei in 1 kg using Avogadro's number and molar mass
- CCalculate the specific energy of U-235 in J kg⁻¹
- DDivide the total energy output per day by the speed of the neutrons
- EMultiply the number of nuclei per kg by the energy released per fission
25.Place the steps in the treatment and storage of high-level nuclear waste in the correct order, from first to last.
Hard
- Place waste in cooling ponds of water
- Harvest isotopes of plutonium and uranium for reuse
- Mix waste with molten glass and solidify (vitrification)
- Encapsulate in steel, lead or concrete containers
- Store deep underground in geologically stable sites
26.In a nuclear reactor, what is the purpose of the shielding?
Medium
- ATo absorb hazardous radiation and house the reactor
- BTo slow down neutrons so they become thermal
- CTo absorb excess neutrons to control the rate of fission
- DTo transfer thermal energy between water systems
27.In a nuclear reactor, the coolant is pumped into the reactor at a cold temperature to extract heat released by fission reactions.
Easy
True or false?
28.Match each safety measure with its purpose in a nuclear power plant.
Medium
- Handling fuel rods remotely
- Thick lead or concrete shielding
- Fully lowering control rods in an emergency
- Storing waste in geologically stable sites
- Reduces workers' exposure to radiation
- Limits radiation exposure around the reactor
- Stops fission by absorbing free neutrons
- Prevents long-lived isotopes entering water and food supplies
29.A reactor is operating normally. Exactly one neutron from each fission goes on to cause another fission. What happens if the control rods are lowered further into the core?
Medium
- AMore neutrons are absorbed, so the rate of fission decreases
- BFewer neutrons are absorbed, so the rate of fission increases
- CNeutrons are slowed down more, so the rate of fission increases
- DThe moderator becomes more effective, so the rate of fission increases
30.In a nuclear reactor, what is the function of the heat exchanger?
Medium
- ATo transfer thermal energy from the coolant to water in a separate system, producing steam
- BTo absorb neutrons and control the rate of fission
- CTo slow down fast neutrons so they become thermal neutrons
- DTo absorb hazardous radiation emitted by fission products
31.Intermediate-level nuclear waste includes items such as clothing and gloves that are lightly contaminated.
Medium
True or false?
32.Which of the following statements about the management of high-level nuclear waste are correct? (select all that apply)
Hard
- AIt remains radioactive for thousands of years
- BIt is initially stored in cooling ponds of water
- CIt is mixed with molten glass and solidified (vitrified)
- DIt can be safely disposed of with regular waste after a few years
- EIt is encased in steel, lead or concrete containers
33.In a nuclear reactor, why must fast-moving neutrons produced by fission be slowed down?
Medium
- ASlow neutrons are more likely to be absorbed by uranium-235 and induce fission
- BSlow neutrons are more easily absorbed by control rods
- CSlow neutrons carry more energy to the coolant
- DSlow neutrons are less likely to escape the reactor core
34.How is nuclear fission defined?
Easy
- AThe splitting of a large, unstable nucleus into two smaller nuclei
- BThe joining of two small nuclei to form a larger nucleus
- CThe emission of an α particle from a nucleus
- DThe capture of an electron by a proton in a nucleus
35.Which particles are released when a uranium-235 nucleus undergoes fission?
Easy
- ATwo or three neutrons and gamma rays
- BTwo protons and two electrons
- COne α particle and one β particle
- DOnly gamma rays
36.Spontaneous fission occurs without the nucleus absorbing an additional neutron.
Easy
True or false?
37.What is a thermal neutron?
Easy
- AA neutron with low energy and speed that can induce fission
- BA neutron with high energy that rebounds away from the nucleus
- CA neutron that has been absorbed by a control rod
- DA neutron emitted as gamma radiation
38.Which material is commonly used in the fuel rods of a nuclear reactor?
Easy
- AUranium-235
- BWater
- CSteel
- DConcrete
39.What property must a material have to be used as a moderator in a nuclear reactor?
Easy
- AIt must be a poor absorber of neutrons
- BIt must be a strong absorber of neutrons
- CIt must be highly radioactive
- DIt must be a good conductor of electricity
40.What is the function of the control rods in a nuclear reactor?
Medium
- ATo absorb neutrons and control the rate of fission
- BTo slow down neutrons to thermal energies
- CTo transfer heat to the steam generator
- DTo shield workers from radiation
41.State the name of the process in which a large, unstable nucleus splits into two smaller nuclei.
Easy
42.Complete the sentence about the components of a nuclear reactor.
Easy
The ____ surrounds the fuel rods and slows down neutrons, while the ____ absorb neutrons to control the fission rate.
43.A fission reaction releases 191 MeV of energy. Given that 1 MeV = 1.6 × 10^-13 J, calculate the energy released in joules. Give your answer in scientific notation.
Medium
44.A fission reaction releases 191 MeV of energy. Given that 1 MeV = 1.6 × 10^-13 J, calculate the energy released in joules. Give your answer in scientific notation to 2 significant figures.
Medium
45.A nuclear power plant has a power output of 500 MW and operates at 35% efficiency. Given that the specific energy of uranium-235 is 7.83 × 10¹³ J/kg, calculate the mass of uranium-235 required per day. Give your answer in kg to 2 significant figures.
Hard
46.Complete the sentence about the operation of a nuclear reactor.
Medium
In a nuclear reactor, the ____ absorbs neutrons to control the rate of fission, while the ____ slows down neutrons to make them thermal.
47.Determine the value of x, the number of neutrons released.
Hard
48.Calculate the energy released in this reaction, in MeV.
Hard
49.State the material commonly used in the fuel rods of a nuclear reactor.
Easy
50.State the property a material must have to be used as a moderator in a nuclear reactor.
Easy
51.State what is meant by a thermal neutron.
Easy
52.Complete the sentence about the function of the control rods.
Medium
Control rods are made of a material that ____ neutrons, and their depth in the core is adjusted to control the ____ of fission.
53.Outline the difference between spontaneous and induced fission.
Hard
54.Explain how the binding energy per nucleon curve shows why fission of uranium-235 releases energy.
Hard
55.Explain the difference between radioactive decay and the process of induced fission.
Hard
56.Explain what happens to the energy released by fission in a nuclear reactor.
Hard
57.A student sets up the golf-ball/hockey-ball demonstration to model moderation. Explain how the demonstration relates to the moderation process in a reactor, and state two ways in which the collisions in a nuclear reactor differ from this demonstration.
Hard
58.A fission reaction releases 191 MeV of energy. Given that 1 MeV = 1.6 × 10^-13 J, calculate the energy released in joules. Give your answer in scientific notation.
Medium
- A2 J
- B1 J
- C3.056e-11 J
- D-1 J
59.A fission reaction releases 191 MeV of energy. Given that 1 MeV = 1.6 × 10^-13 J, calculate the energy released in joules. Give your answer in scientific notation to 2 significant figures.
Medium
- A1 J
- B3.1e-11 J
- C2 J
- D-1 J
60.A nuclear power plant has a power output of 500 MW and operates at 35% efficiency. Given that the specific energy of uranium-235 is 7.83 × 10¹³ J/kg, calculate the mass of uranium-235 required per day. Give your answer in kg to 2 significant figures.
Hard
- A1.6 kg
- B2.6 kg
- C3.6 kg
- D0.6 kg
61.Determine the value of x, the number of neutrons released.
Hard
- A5
- B3
- C4
- D2
62.Calculate the energy released in this reaction, in MeV.
Hard
- A229 MeV
- B172 MeV
- C210 MeV
- D191 MeV

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