Nuclear fusion and fission

Question 1

Explain what is meant by a chain reaction, naming the materials and particles involved

Answer 1

U-235 captures a neutron and splits into 2 smaller nuclei releasing more neutrons
At least one of these neutrons go on to cause more splitting

Question 2

Explain the purpose of a moderator in a thermal nuclear reactor

Answer 2

The moderator reduces the kinetic energy of neutrons so they are absorbed by the uranium

Question 3

Explain why the shielding around the core becomes radioactive

Answer 3

Neutrons are absorbed by the nuclei in the shielding converting these nuclei into unstable isotopes

Question 4

Explain what is meant by a thermal neutron

Answer 4

Slow moving electrons
In thermal equilibrium with the moderator (300K)
Have energies of order 0.025eV
Range of KE similarly to a gas at room temp, 300K

Question 5

Name the substance used as the moderator in a pressurised water reactor

Answer 5

Water

Question 6

Collisions in reactors are what type?

Answer 6

Usually elastic, not always

Question 7

State what is meant by the binding energy of a nucleus

Answer 7

The amount of energy required to separate a nucleus into its separate nucleons

Question 8

Explain why fission fragments are unstable and explain what type of radiation they are likely to emit initially

Answer 8

Likely to be above/to the left of the line of stability - larger N/Z ratio than stable nuclei

Question 9

Describe the changes made inside a nuclear reactor to reduce its power output and explain the process involved

Answer 9

Insert control rods into reactor while will absorb more neutrons reducing further fission reactions

Question 10

State the main source of the highly radioactive waste from a nuclear reactor

Answer 10

Fission fragments / used fuel

Question 11

Explain the nature of the radiation that may be emitted from an excited nucleus of the moderator

Answer 11

Gamma/EM radiation
As the energy gaps are large in a nucleus as the nucleus de-excites down discrete energy levels to allow the nucleus to to allow the nucleus to get to the ground state

Question 12

Describe what happens to the neutrons as a result of the subsequent elastic collisions of a neutron with the moderator

Answer 12

Momentum/kinetic energy is transferred to the moderator atoms meaning the neutrons eventually reach speeds associated with thermal random motion / reaches speeds which can cause fission

Question 13

Nuclear fission process in nuclear reactors

Answer 13

Splitting up: large nuclei with at least 83 protons are unstable and some can randomly split into two equally smaller, more stable nuclei. Energy is released during nuclear fission because the new smaller nuclei have a higher binding energy per nucleon. Each fission event released energy, many lighter elements and two or three neutrons.

Question 14

What is meant by a spontaneous and an induced process

Answer 14

The process is spontaneous if it occurs by itself or induced if it is encouraged to occur.

Question 15

The larger the nucleus, the more _____ it is
This means that larger nuclei are more likely to
This means that _____ limits…

Answer 15

unstable
spontaneously fission
This means that spontaneous fission limits the number of nucleons a nucleus can contain/possible elements

Question 16

Fission can be induced when U-235 is

Answer 16

bombarded with neutrons making it very unstable. Only low energy neutrons (thermal neutrons) can be captured in this way.

Question 17

If each fission event releases two neutrons on average, after n events there would be

Answer 17

2^n fission neutrons.

Question 18

Explain how a chain reaction occurs

Answer 18

Fission neutrons, the neutrons released in a fission event, are capable of causing further fission events as a result of a collision with another U-235 nucleus causing a chain reaction where fission neutrons produce further fission events which release fission neutrons and cause further fission events etc.

Question 19

Each event takes less than one second and releases around ____MeV therefore releases loads of energy in the long run.

Answer 19

200

Question 20

Energy is released when a fission event occurs because

Answer 20

the fragments repel each other (as positively charged) with sufficient force to overcome the strong nuclear force trying to hold them together. The fragment nuclei and the fission neutrons therefore gain kinetic energy. The two fragment nuclei are smaller and therefore more tightly bound than the original U-235 nucleus. So they have more binding energy so they are more stable than the original nucleus. The energy released = change in binding energy. Binding energy of each nucleon increased by 1 MeV

Question 21

What two isotopes can be used for fission

Answer 21

U-235 (enriched uranium)

Pu-239

Question 22

How is Pu-239 formed

Answer 22

formed by bombarding U-235 with neutrons therefore artificial

Question 23

Particle equation for fission

Answer 23

n + U-235 –> n + n + y + nucleus1 + nucleus2

Question 24

Fusion of light nuclei

Answer 24

Nuclear fusion means joining nuclei together. Two light nuclei can combine to create a larger nucleus. The product nucleus has more binding energy per nucleon than the smaller nuclei so the binding energy per nucleon also increases in this process providing the nucleon number of the product nucleus < about 56. A lot of energy is released during nuclear fusion because the new, heavier nuclei have a much higher binding energy per nucleon. The binding energy curve shows that if two light nuclei are combined, the individually nucleons become more tightly bound together. The binding energy per nucleon of the product nucleus is greater than that of the initial nuclei so the nucleons become even more trapped in the nucleus when fusion occurs meaning the energy release = the increase in binding energy

Question 25

Q = Energy released =

Answer 25

Q = Energy released = change in binding energy = delta m * c^2 where delta m is the difference between the mass before and the mass after the event

Question 26

Fusion particle equation

Answer 26

isotope 1 + isotope 2 –> fusion –> isotope 3 + energy

Question 27

Change in binding energy per nucleon:
fission =
fusion =

Answer 27

0.5MeV

>5MeV

Question 28

All nuclei are positively charged so there will be an _______.

Answer 28

electrostatic force of repulsion between them

Question 29

Solar energy is produced as a result….

Answer 29

of fusion reactions inside the sun

Question 30

Nuclear fusion can only take place if the

Answer 30

two nuclei that are to be combined collide at high speed so that they overcome this electrostatic force due to their charge and get close enough for the attractive force of the strong interaction to hold them both together.

Question 31

About ____ of kinetic energy is needed for fusion - a lot of energy

Answer 31

1 MeV

Question 32

Nuclei require to fuse.

Answer 32

lots of energy

Question 33

Protons i.e. Hydrogen nuclei inside the sun’s core fuse together in stages to form… releasing…

Answer 33

helium nuclei releasing energy

Question 34

At very high temperatures, atoms are stripped of their electrons to form matter called ___

Answer 34

plasma

Question 35

The nuclei of the plasma move at very high speeds in the sun because of

Answer 35

the enormous temperature