Nuclear : Nuclear Reactions

Describe fission

• Take a large fissile nuclear (eg. U235 or Pu239)

• Interact with a neutron

• Nucleus becomes unstable, vibrates deforms and splits

• Forms

  • Lighter nuclei (fission products)

  • Kinetic energy

  • Gamma

  • Free neutrons (key for chain reaction)

Advantages and disadvantages of fusion

ADV

• Fusion materials are more plentiful

• Much less radioactive waste

• Waste decays away much quicker

DIS

• Thermonuclear fusion - ~100 million degrees

• Costs a lot

• Structural materials need to endure extreme conditions

• Configuration to prevent plasma from touching reactor walls

• Energy input vs output

Define fissile

Capable of sustaining a chain reaction of nuclear fission, with neutrons of any energy (any - slow thermal neutrons)

Define fissionable

Capable of undergoing fission

• Fissile is a sub set of fissionable (low energy neutrons)

Define fertile

Can be converted to fissile with bombardment of neutrons in a reactor

Describe how U-235 and U-238 relate to fission terms

U-235 can undergo thermal fission (fission induced by absorption of slow moving neutrons

U-238 is both fissionable and fertile (can be converted into Pu-239) but not fissile only fission with high energy neutrons

Describe fission neutrons released after fission

On average around 2-3 released per fission (2.4 avg), this increases with the incident neutron energy, one additional neutron is emitted for every 6-7 MeV increase in incident neutron energy

• Prompt neutrons - >99% emitted instantly (~10^-14s)

• Delayed neutrons - <1% released comparatively long after the fission event

  • Delayed neutrons are from beta decay of some fission products

  • Delayed neutrons are key to control nuclear reactors

Describe the fission fragments

• Almost always splits into roughly equal parts but never exactly equal

• Highly energetic and highly ionised atoms

• Average charge of lighter group ~ +20e

• Average charge of heavier group ~ +22e

• Travel short distances

• Need added cladding

What is a barn?

Chance of a physical collision between a neutron and an atom

What is the mean free path of a neutron?

Average distance of travel before neutrons interact with the fission nuclei to trigger fission

Equation for collision rate

Sigma = cross section

I = Intensity flux (nv) [n = neutron number density, v = neutron speed]

X = distance

A = area

N = number density

Describe the cross section

• Proportionality constant in the collision rate expression

• Number of collisions with one nucleus/time/intensity

• Sigma/A = probability that a neutron in the beam will collide with the nucleus

• Sigma = micro cross section for nuclei

• Sigma N = macro cross section for matrerials

Describe the two types of neutron interactions with matter

What are the equations for the 4 different adsorption reactions with neutrons?

What is the neutron multiplication factor , k?

If q =k-1 >0, number of neutrons will increase by nq within time tp

What is prompt neutron lifetime?

Average time of prompt neutrons between their emission and absorption in a reactor or escape from the reactor

• Equal to diffusion time plus deceleration time

• For an infinite thermal reactor, diffusion time dominates therefore lifetime ~ diffusion time

What are the diffusion times for water, graphite, heavy water, fast reactor?

What are the mechanisms of radiation damage?

• Transmutation - product of helium

• Frenkel defects

• Wigner energy (a radiation induced effect in graphite)

Describe transmutation as an effect of radiation