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Physics Paper 2 (Content) > Nuclear Physics (Reactors) > Flashcards

Nuclear Physics (Reactors) Flashcards

1
Q

Which number on the elemental symbol shows the relative atomic mass?

A

The top one (aka biggest number)

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1
Q

What is the relative atomic mass unit equivalent to in MeV?

A

1 u = 931.5 MeV

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2
Q

What is the mass of a proton in terms of u?

A

1.00728u

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3
Q

What is the mass of a neutron in terms of u?

A

1.00867u

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4
Q

What is the mass of an electron in terms of u?

A

0.000549u

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5
Q

What is meant by the mass defect of a nucleus?

A
  • The actual mass of a nucleus is NOT equal to the mass of the added constituents
  • The difference between the two is the mass defect

== Note: mass of constituents > mass when a nucleus

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6
Q

How do you calculate mass defect?

A

Calculated mass - Actual mass = Mass defect

Note: calculated mass is the added mass of constituents

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7
Q

What is meant by binding energy?

A
  • When constituents bind = mass is lost
  • Lost mass must be turned into energy

(The mass lost is technically mass defect = binding energy)

If we want to separate nucleus into constituents = must put energy back in to regain mass

Energy needed is binding energy

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8
Q

Definition of binding energy?

A

Energy required to separate a nucleus into its constituents

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9
Q

How do you calculate average binding energy per nucleon?

A

B/A

Binding energy/ Nucleon number

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10
Q

Describe the relationship between average binding energy and stability

A
  • Higher average binding energy = more energy needed to separate nucleus
  • Decay is less natural = more stable nuclei
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11
Q

Describe the graph of average binding energy against nucleon number

A
  • A sharp increase at beginning (For light nuclei)
  • Levels out into maximum point
  • Slow decrease (For heavy nuclei)
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12
Q

At which point on the graph of average binding energy against nucleon number, does the most stable nuclei occur?

And at which element

A

The most stable nuclei occur around the maximum point on the graph

  • since higher binding energy = more stable

Hence for Iron

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13
Q

How do you calculate binding energy?(if given mass defect)

A

Mass defect (in terms of u) x 931.5

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14
Q

How do you convert mass defect into standard units (Kg)

A

1 u = 1.661 x 10^-27

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15
Q

What is meant by nuclear fission

A

Breaking apart a large nuclei into two smaller daughter cells, causing a release in energy due to an increase in average binding energy

16
Q

Describe how Uranium undergoes nuclear fission

A
  • Fission is induced by firing a thermal neutron at Uranium atom causing it to be more unstable
  • The nuclei breaks apart into two daughter cells, and three neutrons are also ejected from the nucleus
  • Energy is released because the new smaller nuclei will have a higher average binding energy
17
Q

Explain why energy is released during nuclear fission

Talk about both binding energy and mass defect

A
  • Total binding energy has increased
  • Since the nuclei “decays” it becomes more stable
  • Decay of this more stable daughter cell will take more energy
    == Smaller daughter cell has higher average binding energy

AND

The total mass of products is less than parent nucleus therefore the “lost” mass is converted to energy according to Einsteins equation E=mc…

This energy is released in the form kinetic energy of the fission fragments AND as gamma radiation

18
Q

Explain how induced nuclear fission can be used in nuclear thermal reactors to generate power

A
  • Fuel rods contain pellets of Uranium 238
  • Thermal Neutrons fired rod to rod inducing fission in adjacent rod == undergoes chain reaction
  • Coolant absorbs the heat released during fission reactions in the core of the reactor
  • This is heat is them used to make steam which powers electricity-generating turbines
19
Q

Explain the use of moderators in a thermal nuclear reactor

A
  • When neutrons are traveling too fast to be absorbed by Uranium pellets
  • A moderator is used to slow neutrons released in fission reaction
  • Increases amount of fission as more neutrons can be absorbed to induce fission
20
Q

Explain the use of control rods in thermal nuclear reactors

A

MUST have control rods

  • Absorb neutrons so that they stop fission when too much is occurring
  • Allowing a chain reaction to be controlled in a safe manner
21
Q

What substance is usually used as a moderator and why?

A
  • Water
  • Pure water
  • Contains hydrogen (You want moderator atom nuclei to be same size almost as neutron)
  • Inexpensive
  • Not very reactive
22
Q

What substance is usually used for control rods and why

A
  • Boron
  • As this does not undergo fission
23
Q

Explain how fission can be induced by a thermal neutron specifically

A
  • Firing thermal neutron into the Uranium nucleus causing it to become extremely unstable
  • Thermal neutrons have a low energy meaning they can induce fission

Oppose to neutrons with high energy = as they would just rebound away from nucleus (not causing fission)

24
What is meant by critical mass?
- Minimum mass of fuel required to maintain a steady chain reaction Example: Using EXACT critical mass would give one single fission reaction as a chain (one after other) But using a mass lower than critical mass would lead to the reaction eventually stopping
25
Explain how a moderator is able to perform its function
- Slows down neutrons to thermal speeds through elastic collisions (between the nuclei of the moderator atoms and the fission neutrons) - Closer the moderator atoms are in size to a neutron == Larger proportion of momentum is transferred (therefore lowing number of collisions needed until begins to slow neutrons speed)
26
Explain why water is a good substance for a coolant?
- High specific heat capacity therefore can transfer large amounts of thermal energy (Can also be used as moderator for same reason)
27
Explain how shielding is used as a safety aspect in nuclear reactors and the problem associated with it.
- Around the nuclear reactor is a very thick concrete shielding - This blocks radiation from escaping from the reactor and affecting workers at power stations However may become radioactive after some time, as neutrons that escape reactor enter the nuclei of concrete Thus they become unstable and emit beta minus decay
28
Explain how control rods are used in emergencies
- Dropped into the reactor core entirely - In order to stop fission reactions from occurring by absorbing all the free neutrons "emergency shut down"
29
What is high level waste and give two examples
Most dangerous type of waste remains radioactive for thousands of years - Spent fuel rods - Unusable fission products from fission of Uranium 235
30
What is enriched Uranium and how does it help with safety
U-238 enriched with U-235 U-235 is the isotope that actual undergoes fission U-238 helps absorb fission neutrons thus helps control reaction
31
What is intermediate level waste, how is it treated and give an example
- Daily used items - Example fuel rods themselves - Waste will have a longer half life than the low level waste - encased in cement in steel drums and stored underground
32
What is low level waste, give example and how is it treated?
- Lightly contaminated ex: clothing and tools - will be radioactive for a few years - encase in concrete and store few metres underground
33
How can high level waste be treated?
- Initially placed in cooling ponds of water close to the reactor for a number of years - Isotopes of plutonium and uranium are harvested to be used again - Waste is mixed with molten glass and made solid (vitrification) - This is encased in containers made from steel, lead or concrete - Stored VERY deep underground
34
Benefits of nuclear power
- Nuclear power stations produce no polluting gases - Highly reliable - Require less fuel as uranium provides far more energy per kg compared to coal
35
Risks of nuclear power
- Production of radioactive waste is very dangerous - Expensive to dispose of waste - Nuclear meltdown such as Chernobyl could have catastrophic consequences to environment and people
36
What is meant by a thermal neutron
- Slow moving neutrons (low kinetic energy) (They need to be slow to induce fission hence moderators) - In thermal equilibrium with moderator/other material - Have energies of order 0.025 eV
37
What is meant by a fission fragment?
- Particles released during fission - Two daughter nuclei and neutrons
38
Difference between normal elastic collisions and elastic collisions due to moderator
- KE loss is variable - Collisions not always elastic - Collisions not always head on

Physics Paper 2 (Content) (8 decks)

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