Nuclear Energy Flashcards
There is a much higher probability that a thermal neutron will cause a fission event than a fast neutron, what is a thermal neutron?
A neutron that is in thermal equilibrium with its surroundings and has a KE «_space;1eV
When is energy known as decay energy outputted?
During radioactive decay. In pure Alpha decay, the energy appears as kinetic energy of the daughter nucleus and of the alpha particle
What are neutron source rods?
The rods that provide the neutrons required to initiate the chain reaction. They usually contain a material that generates neutrons by spontaneous fission, e.g. californium-252
What are the technical issues that need to be considered to make sure that sufficient heat energy is generated in the reactor and then extracted to make high-pressure steam?
- Probability of fission occurring must be high to ensure that enough fission events take place every second to generate the required energy
- Enough fission neutrons go on to create more fission reactions, self-sustaining chain reaction
- Number of free neutrons in the reactor can be controlled
- Heat energy generated in the reactor can be extracted efficiently
- People working at the power station are shielded from the radiation emitted from the reactor
Where and why are metastable states useful?
In nuclear medicine because if the parent nuclei can be separated from the excited daughter nuclei, then radioactive material that emits only gamma rays can be produced. This can then be used as a tracer.
What is induced fission?
When fission fragments and a variable number of neutrons and gamma photons are released as a result of bombarding heavy nuclei with neutrons. Typically 150-200 MeV released per fission event
Where does the energy released from fission events appear? How can it be found?
1) In the form of kinetic energy of the fission fragments and neutrons
2) Can be found by calculating the mass lost in the fission reaction
Why is the mass of separated nucleons higher than that of the original nucleus?
Work needs to be done to overcome the strong nuclear force and separate the nucleons. Doing work transfers energy and therefore, according to Einstein, transfers mass
Describe a graph of neutron number against proton number?
For nuclides with proton number 20 or less, the graph follows an approximate straight line for which N is roughly equal to Z. For stable nuclei with proton number is greater than 20, the ratio N/Z > 1 and continues to increase for the heavier nuclei
The scattering of Electrons when being fired at the nucleus is due to…
… their charge and the resulting attractive force between the electrons and the positively charged protons in the nucleus
What happens to spent (used) fuel?
It is transferred to cooling ponds, where the water acts as a coolant and a shield for emitted beta and gamma radiation. The rods are then transported by train to a reprocessing plant. Fuel rods are then cooled further in ponds before being reprocessed
What are the characteristics of a coolant?
Must have good heat transfer characteristics, either a high specific heat capacity (liquid water) or the ability to be pumped around the system very quickly (CO2 gas)
Why do all light (ordinary) water reactors require enriched uranium fuel rather than natural uranium?
Despite its excellent heat transfer properties, it is over 100 times more likely than CO2 to absorb neutrons
Describe low-level waste, how is it treated/stored?
It has low levels of radioactivity and doesn’t generate heat, e.g. contaminated clothing. It is compacted to a fraction of its original volume and stored in steel drums in concrete vaults
What happens when spent fuel is reprocessed?
It is dissolved in nitric acid, and the uranium and plutonium content is chemically removed and stored for later use. The remainder contains the fission fragments from the fission of uranium-235 and their decay products, it is highly radioactive and lots of heat energy is generated. This is high level waste
What enables a more compact design in pressurised water reactors (PWRs)? How does it remain liquid at such high temperatures?
1) The fact that water acts as both the coolant and the moderator. The water flows around the primary cooling system, extracting heat from the reactor core and transferring heat to the secondary cooling system where high-pressure steam is generated
2) It is kept under very high pressure, about 150x atmospheric pressure
What is a moderator?
The material used to slow down fast neutrons in a nuclear reactor
What is a fast neutron? How are fast neutrons slowed down so the likelihood that they create a fission event is increased?
1) A typical neutron produced in a fission event with about 2MeV of KE, travelling at about 2x10^7 m/s
2) The reactor contains a material made up of nuclei that the fast neutrons can collide with, causing there speed to be significantly reduced at each collision
What is a critical chain reaction?
One where on average one of the neutrons produced in a fission event goes on to produce another fission event
To maintain a self-sustaining chain reaction in a nuclear reactor, it is important that…
… the materials in the reactor core have a low probability of absorbing neutrons
The splitting of a uranium-235 atom is asymmetrical, what does this mean?
One fragment is significantly larger than the other
What do control rods do?
Control the rate at which fission occurs by controlling the neutron density, they therefore control the power output of the reactor
What is the critical mass of fuel
The minimum mass required to establish a self-sustaining chain reaction
Why is zirconium used as cladding in fuel rods?
It has a very low probability of absorbing neutrons
What is 1u equivalent to?
931.5 MeV
When an element is described as being in a metastable state, what does this mean?
That the excited nuclei of that element return to their ground state with a half live longer than 1 ns
Why does fission of heavy nuclei produce energy?
When the heavy nuclei split into two lighter nuclei, both with nucleon numbers > 56, there is an overall increase in binding energy per nucleon and therefore a loss of mass. This corresponds to energy being transferred to the surroundings
What is the binding energy per nucleon?
The energy required per nucleon to separate the nucleons in a nucleus
The more tightly bound a nucleus…
… the greater the reduction in mass and the more energy transferred to the surroundings
What is a coolant?
The material that passes through the reactor core and absorbs heat energy provided by fission
Why do alpha particles emitted from a particular radioactive isotope have a distinct energy?What are they described as?
The kinetic energy is shared in a fixed ratio between the alpha particle and the daughter nucleus. They are described as being monoenergetic
What is fissile material?
Nuclei that undergo induced fission
When two nuclei fuse together, what condition must be satisfied?
The fusion process must release energy to the surroundings, therefore the two nuclei that fuse must have a lower binding energy per nucleon than that of the nucleus formed. This is because the tighter bound a nucleus, the higher it’s B.E. per nucleon
Describe intermediate-level waste, how is it treated/stored?
It has the high to medium levels of radioactivity but doesn’t generate enough heat to require cooling as a part of its storage, e.g. cladding that is removed from the outside of spent fuel. Can be stored in vaults or encapsulated in cement inside stainless steel drums. Thick concrete walls are required to shield operators from high levels of radiation
What are the different things that could happen to a fission neutron?
- It could pass through the cladding of the fuel rod, undergo a series of collisions with the moderator atoms causing it to slow down, before passing through the cladding of another fuel rod to cause a fission event
- It could be absorbed but the control rod
- It could be absorbed by the fuel cladding or the material of the moderator of coolant
- It could be absorbed by a uranium-238 nucleus (resulting in plutonium)
- It could be absorbed by a fission fragment nucleus or by a nucleus formed from the decay of a fission fragment
- It could escape the reactor and be absorbed by the concrete shield
How do you measure the radius of nuclei of different elements?
By directing a beam of high-energy electrons at a thin solid sample of the element and measuring the number of electrons scattered through various angles in a specified time interval
Describe high-level waste, how is it treated/stored?
It has both high levels of radioactivity and heat, therefore it requires cooling as a part of its storage, e.g. Reprocessed spent fuel. It is stored as a liquid in water cooled tanks, or it can be evaporated to form a powder and contained within glass in the process called vitrification. Requires thick concrete walls to shield operators from high levels of radiation
What is binding energy per nucleon an indicator of?
Nuclear stability
What is the job of shielding in a power station?
To absorb any escaping neutrons and gamma photons and protect the workforce
How is energy harnessed from controlled fission in a nuclear reactor?
Energy released is used to heat water and produce steam at high pressure which drives turbines, which in turn drives a generator and produces electricity
Why does the accelerating of electrons through 400Mv influence the scattering results when firing these electrons at a nucleus?
The de Broglie wavelength of the electrons is approximately 10^-15m, which is roughly the radius of a nucleus. Therefore the wave behaviour of the electrons affects the scattering results - a diffraction pattern showing maxima and minima is superimposed
What is a self-sustaining chain reaction?
Where for every neutron absorbed that causes fission, at least one of the neutrons produced goes on to produce another fission event
What does the critical mass depend on?
The concentration of the radioactive substance and the geometry of the core and fuel rods, as this affects surface area to volume ratio
What are the problems we face when trying to generate energy from nuclear fusion?
Generating the high temperatures needed to make the nuclei travel fast enough to overcome the electrostatic repulsion between them/ the strong nuclear force
What is a nuclide?
A type of nucleus with a specific number of protons, a specific number of neutrons and a specific energy state
What axis would you use to show that the nuclear radius (R) is proportional to the cube root of the nucleons number (A^1/3) when plotting electron diffraction results?
R against A^1/3, graph is a straight line through the origin with a gradient of 1.05fm (the constant of proportionality)
What are the three types of nuclear waste?
High-level, intermediate-level and low level, all of which require different handling and storage
What is the binding energy of a nucleus?
The work that would need to be done to separate all its nucleons
What are control rods made of?
Cadmium or boron, materials that have a high probability of absorbing neutrons
What is a fuel rod made of?
A column of uranium oxide pellets clad and sealed into zirconium alloy tubes typically 4-5m long. Zirconium is used because it has a very low probability of absorbing neutrons
What element has the most binding energy per nucleon?
56^Fe
Does the density of a nucleus depend on the nucleon number?
No
Nuclear changes that involve an increase in binding energy per nucleon result in…
… energy being transferred to the surroundings
What should the mass difference after a decay be equivalent to?
The total kinetic energy and/or electromagnetic energy of the products
Why is water commonly used as a moderator?
A fast neutron is most likely to lose a lot of its KE if it collides with something if similar mass. Water contains protons which have almost the same mass as neutrons
Why does a bigger mass of fuel mean a smaller percentage of neutrons produced are likely to escape?
The bigger the mass, the smaller surface area to volume ratio
What is one atomic mass unit or 1u?
1/12 the mass of a carbon-12 atom
What are the conditions for a fast neutron to become a thermal neutron in one collision?
The neutron must undergo a head-on elastic collision with a stationary particle of the same mass so the neutron loses all its KE. This is shown by the conservation of momentum and by the conservation of KE for the collision
Why are electrons more suitable for bombarding particles than alpha particles?
1) Electrons, being leptons, only interact with the nucleus via the electromagnetic force which is well understood. Alpha particles, being hadrons, also interact with the nucleus via the strong nuclear force which is less well understood
2) Electrons also have much smaller masses, allowing to be accelerated to very high speeds, giving them a de Broglie wavelength similar in size to the nuclear radii and therefore create diffraction patterns which can be used to determine the size of the nucleus
How can you calculate the energy released to the surroundings during fusion?
By calculating the change in binding energy or by calculating the mass lost
What is a decay series?
A series of decays from one element to another element which is stable
