11. Fission, Decay, Waste Flashcards
Briefly describe the two types fission
Spontaneous - Happens without external energy
Induced - Initiated by addition of energy
What does SEMF predict about the splitting of nuclie in fission, and what is observed?
SEMF predicts equal size splitting
However asmmetry is observed due to shell structure
From SEMF, what nucleus size can spontaneous fission occur
Typically A > 100
What typically happens to daughter nuclei after spontaneous fission?
They are still neutron rich so undergo several beta decays until they reach a stable nuclide
At which point does spontaneous fission have a larger probability, to where it competes with alpha decay?
When you have very heavy nuclei, A > 270
- Z^2 / A = 49 for an ellipsoid
What type of problem is spontaneous fission, and describe it
A potential barrier problem like alpha decay
- Daughter nuclei needs to get through the barrier
- About 5-6MeV below the top potential (Activation energy)
- Spontaneous fission occurs when this disappears with a large probability
What is the probability of a daughter nucleus tunnelling through the potential barrier and why?
Very small because we are dealing with very large objects
How can we induce fission in terms of the activation energy?
Add energy to the nucleus typically by absorption of neutrons as they don’t feel the EM force
Describe the energies involved in induced fission between even N and odd N nuclei
Odd - Adds the energy of the neutron as well as the pairing energy (often > activation energy)
Even - Capturing neutron movees to a less energetically favourable state by a change in pairing energy
Describe and explain the differences in crosssections for U235 and U238
U235 is odd N so very low energy (thermal) neutrons can induce fission
U238 needs neutrons with 1.2MeV
State the multiplication factor
k = (no. neutrons produced in n+1 stage) / (no. neutrons produced in n state)
Describe what happens to the product nuclei after U235 Fission
- Most of the released energy is carried away by fragments
- 2.5 neutrons created which have a reasonable high energy
- Some fragments give off further neutrons with a mean delay time of 13s
What are the k values of interest (multiplication factor)
k = 1: Critical, constant chain reaction rate
k < 1: Subscritical, rate goes down
k > 1: Supercritical, rate goes up
What is the equation for the mean free path of a neutron between two collisions
L = 1/(ρ_nucleus * mean σ_total)
- Density of nucleus and mean total cross section
- Mean cross section if U235 and 238 present
- Typically 3cm
Describe the critical mass for U235
1 in 6 neutrons cause fission, so will travel Lsqrt(6) = 7cm and then induce fission
- Critical mass corresponds to a radius of 7cm on fission starts
What is the equation for the power output of a reactor?
W_f = J N σ_fission
J - Neutron flux
N - Number of nuclei that can undergo fission
W - Fission rate
What is the energy released by a single fission?
200MeV for U235
Why do we slow the neutrons down, and how are they slowed?
For U235, the cross section is much higher at low neutron energies
- Slow down with a moderator to increase fission efficiency
What are actinides?
Radioactive elements with an atomic number between 89 and 103
- Mainly Uranium itself with other elements
- Have very long lifetimes (000s of years)
What are the two forms of nuclear waste, and state their lifetimes
Actinides (> 000s of years) and fission products (decades)
What is the general cycle of nuclear waste after it is removed from a reactor?
- Stored near reactor for cooling as it has a very high activity even without fission
- Once activity decreases, longer term storage
What are the three forms of long term storage for nuclear waste?
- Deep underground
2. Transmutation + induced fission in accelerator systems
What must happen QMically for fusion to occur
The two nuclei have to overcome the Coulomb potential barrier in order to get close enough to fuse
