Nuclear fusion and fission Flashcards
Explain what is meant by a chain reaction, naming the materials and particles involved
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
Explain the purpose of a moderator in a thermal nuclear reactor
The moderator reduces the kinetic energy of neutrons so they are absorbed by the uranium
Explain why the shielding around the core becomes radioactive
Neutrons are absorbed by the nuclei in the shielding converting these nuclei into unstable isotopes
Explain what is meant by a thermal neutron
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
Name the substance used as the moderator in a pressurised water reactor
Water
Collisions in reactors are what type?
Usually elastic, not always
State what is meant by the binding energy of a nucleus
The amount of energy required to separate a nucleus into its separate nucleons
Explain why fission fragments are unstable and explain what type of radiation they are likely to emit initially
Likely to be above/to the left of the line of stability - larger N/Z ratio than stable nuclei
Describe the changes made inside a nuclear reactor to reduce its power output and explain the process involved
Insert control rods into reactor while will absorb more neutrons reducing further fission reactions
State the main source of the highly radioactive waste from a nuclear reactor
Fission fragments / used fuel
Explain the nature of the radiation that may be emitted from an excited nucleus of the moderator
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
Describe what happens to the neutrons as a result of the subsequent elastic collisions of a neutron with the moderator
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
Nuclear fission process in nuclear reactors
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.
What is meant by a spontaneous and an induced process
The process is spontaneous if it occurs by itself or induced if it is encouraged to occur.
The larger the nucleus, the more _____ it is
This means that larger nuclei are more likely to
This means that _____ limits…
unstable
spontaneously fission
This means that spontaneous fission limits the number of nucleons a nucleus can contain/possible elements
Fission can be induced when U-235 is
bombarded with neutrons making it very unstable. Only low energy neutrons (thermal neutrons) can be captured in this way.
If each fission event releases two neutrons on average, after n events there would be
2^n fission neutrons.
Explain how a chain reaction occurs
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.
Each event takes less than one second and releases around ____MeV therefore releases loads of energy in the long run.
200
Energy is released when a fission event occurs because
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
What two isotopes can be used for fission
U-235 (enriched uranium)
Pu-239
How is Pu-239 formed
formed by bombarding U-235 with neutrons therefore artificial
Particle equation for fission
n + U-235 –> n + n + y + nucleus1 + nucleus2
Fusion of light nuclei
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
Q = Energy released =
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
Fusion particle equation
isotope 1 + isotope 2 –> fusion –> isotope 3 + energy
Change in binding energy per nucleon:
fission =
fusion =
0.5MeV
>5MeV
All nuclei are positively charged so there will be an _______.
electrostatic force of repulsion between them
Solar energy is produced as a result….
of fusion reactions inside the sun
Nuclear fusion can only take place if the
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.
About ____ of kinetic energy is needed for fusion - a lot of energy
1 MeV
Nuclei require to fuse.
lots of energy
Protons i.e. Hydrogen nuclei inside the sun’s core fuse together in stages to form… releasing…
helium nuclei releasing energy
At very high temperatures, atoms are stripped of their electrons to form matter called ___
plasma
The nuclei of the plasma move at very high speeds in the sun because of
the enormous temperature
