Nulcear - Fission Flashcards
What hold nuclei together?
The strong force
Nuclear Fission
Where heavy elements break apart into lighter ones.
Primary fission fuel…
U235
Fission energy compared to gasoline…
U235 release 200 MeV or 1 MeV per nucleon whilst gasoline about 1/2 eV per nucleon
How does a chain reaciton in fission work?
U235 releases neutrons which can induce further fission of other uranium
How can neutrons propogate fission?
They are neutrally charged so can be absorbed without repulsion
How do neutrons INDUCE fission?
Srong interactions formed hold several MeV enough to drive further fission
Why do heavier elements have more neutrons?
Despite strong force being stronger than electrostatic, there is a limit, where neutrons are required to balance
What does increased neutron content prevent?
Alpha decay or spontaneous fission
Why are emitted neutrons unlikely to induce fission?
They are fast moving and likely to be absorbed by other nuclei
How can neutron fission be promoted?
By slowing down the solution
How are neutrons slowed down?
A moderation like water or graphite
Why are Fission reactors often enriched with U235?
U235 only accounts for small portion of uranium naturally found.
Why are control rods important?
They are made of neutron-absorbing material that capture fission-released energy.
What are control rods made of?
Cadmium, hafnium or boron
Why is boron good in control rods?
Its 5 fermion neutrons prefer being paired so B10 absorbs to B11
Alpha Decay
Process of reduction of atomic mass, being the primary source of activity
Alpha decay depends on…
Quantum tunnelling and coloumb barrier penetration
Alpha particles
Helium nuclei emitted by radioactivty with very high lifetimes
Why is tunneling important in alpha decay?
It allows the alpha particle to escape the nuclei
Where is the coloumb barrier in decay?
Between alpha particles and nuclei
When was fission discovered?
In 1930, first controlled chain reaction in 1942, electrical generation in 1951 then plants in 1955
Cross Sections
Describes collision/reaction rate of particles
Equation for flux in cross-section….
phiB = dNb/dAdt - b is incidence particle, meaning amount of particles passing a unit area per unit time
dNc/dt
Rate of outomce or likelihood of a reaction taking place (c is outcome)
sigmaBT - > C = 1/phiB dNc/dt
Describes probaility of motion random motion collision reactions
Analogy of cross section…
A dart board where the larger surface area equate more points assuming random throwing
Why is nucleus cross-section so far extended?
Schrodinger equation as nuclei themselves have a a wavefunction
Example of a nuclei cross-section
Xenon135 cross-section 1000 times the size its actual radius
Why is U235 the best fission fuel?
It has low neutron absroption cross section with high fission cross-section, and high energy release of KE and Gamma Rays and low nucleon binding energy.
Why is low neutron cross-section important?
This means it is more likely to undergo fission when struck by a neutron rather than absorption(becoming U236
Gamma Rays
This is the highest energy photon arising out of nuclear events during radioactive decay
Why does U235 have low neutron absorption cross-section?
Resonance of 5 eV meaning neutrons close to this value more likely asborbed, whilst others less so.
Why does U235 asymmetry increase fission likkelihood?
Distribution of coulomb barrier differs, decreasing stability due to potential energy surface of nucleus lower with elongated shape, with imbalanced NSF
What is the coulomb force proportional to?
Inverse square of the distance between them, becoming weaker with increasing distance.
What distance does SNF act up to?
2-3 femtometers, whilst couloumb is long range.
Why does U235 have low binding energy per nucleon?
Proton repulsion so SNF of neutrons struggle to bind all nucleons together.
Why does U235 have high nuclear cross-section?
Nulcear properties(92 protons and 143 neutrons), both contributing to SNF but only protons to electric repulsion.
Why does nulceus becoming more unstable to larger it is?
Dense electrostatic repulsion counteracted by stronger binding of NSF.
Why does the odd neutrons in U235 improve fission?
Better buffer to reduce proton repulsion, thus instability occurs with neutron absorption.
Why does nucleus become less stable with increased neutrons?
SNF becoming weaker thatn proton repulsion at certain distances.
Fissile Isotope
These are elements that fission with low energy neutrons and capable of self-sustaining reactions
Binding Energy
This is the energy required to seperate a partcile from a system of particles or to dispsera all particles of the system.
Why does U235 have low binding energy per nucleon?
Because it is asymmetric in proton-neutron.
What does EV measure?
Electron energy gain when accelerated by a potential difference of one volt.
Equation for EV?
E = qV where q is electron charge and V is potential difference.
How many J is 1 eV?
1.602 x 10^-19 joules.
Potential Well
This is the region surrounding a local minium of potential energy.
What does increase of 1eV in an electron allow?
Escape of potential well of nucleus and free movement in space, escaping attractive force of the proton.
Work Fucntion
This is the minium amount of thermodynamic work required to remove and electron from a soild point in a vaccum to immediately outside the soild surface.
Why do fermions exist in pairs?
The spin nature of particles given by the pauli exclusion principles.
Pauli Exclusion Principle
This states that no two fermions can occupy the same quantum state simultaneously, meaning cannot have the exact spin at same positions.
Cooper Pair
This is an electron pair in a superconductor attractively bound with equal and opposing momentum and spin.
What happens when two fermions approach one another?
Wave functions overlap, with combined wave function describing behavior of both particles.
What does the PEP require of wave function combination?
It needs to be anti-symmetric.
Anti-Symmetric
This is a property of mathematical object descriing where the changing of signs occurs when any two of its indicies are interchanged.
Spin Paring
This is the energy associated with paired electrons sharing one orbital and its effect on the molecules surround ing.t
What is cross-section?
A measure of interactionf or reactions occuring between particles or atoms, calculabted based on their wave function.
Why is wave function important in cross-sections?
Describing quantum mechanical properties of the involved particle, that being position, momentum and energy.
Why is boron-carbide a good material for control rods?
High cross section for neutron absorption and resistance to environmental degradation to high radiation temperatures.
Why cant U235 absorb fast neutrions?
U235 has low cross section for thermal neutron capture, due to their unsufficient energy to induce fission.
Coeffcient of Void
This describes reactivity change due to change in amount of voides in the reactor core.
What is the process of void coeffcient?
Water as a coolant transfer heat from fuel externally, which causes water boiling forming steam and creating voids.
What does void creation result in?
Increase/decrease in rate of fission that would occur and heat generation with more loss of coolant.
What happens when coolant is voided?
Capacity to absorb neutrons decreasing, with increased reactivity, thus more steam and more reactivity, further decreasing coolant density.
What is Nuclear Binding Energy?
This is the difference between the mass of the nucleus and the mass of its constituent parts.
Semi-Empiracal Mass Formula
This calculates the binding energy of the groudn state of a nucleus.
Why is U235 unstable?
Total binding energy per nuckleon higher than other U isotopes
Why does U235 have higher total binding energy?
Balance of SNF and EF which act and counteract against each other.
Why does a neutron initaite fission in U235?
Only contribute SNF so when one is absorbed the binding energy increasese as no electrostatic force has been added.
What does neutron absorption lead to?
Formation of a compound nucleu due to increase in possible nucleon-nucleon interactions
Compound Nucleus
This is an unstable nucleus formed by coalescence of an atomic nucleus with a capturd particle.
Why doe nulceon-nucleon interactions become weaker with size?
Larger distances between them along side weaker EF repulsion(slower rate)
Why does rate of decay of EMF lower than SNF?
Mesons mediating SNF have mass whilst EF by mass-less, described by inverse square law.
Inverse Square Law?
This states that for a point source of waves that is capable of radiating omnidirectionally and with no obstcructions in the vicinity the intensity decreases with the square of the distances.
What is cross-section determined by?
Nuclear propreties
Resonance Energy
This is the energy level at which teh probabiltiy of neutron capture is greatly increased compared to neighbouring energy levels?
What does low neutron binding energy mean for?
Capture of neutron more likely than bouncing
When is neutron capture highest?
When neutron is at speed of the resonance energy
Why dpes asymmetry of U235 increased fission likelihood?
Elongated means imbalance of neutrons and protons increasing strength of EF
Elastic Scaterring
This is a mechanism by which fast neutrons lose their energy when they interact with atomic nuclei of low atomic number.
Why doesn’t U235 absorb fast neutrons?
The interaction time between neutron and nucleus is short, where absorption probabily depends on specific energy level availability.
Why does PEP require anti-symmetric?
Two electrons cannot have the same set of quantum numbers given by a wave function.
Why cant electrons be symmetric?
Fermions are identical in mass, charge and spin, so destructive interferences when peaks of WF overlap.
