W13 - Physics & Fuel prerequisites

Question 1

What are the 5 main terms of the SEMF, what do they address?

Answer 1

1. Volume term: increase in volume means increase in mass number and thus binding energy (+ term)
2. Surface term: SNF weaker for nucleons on outer surface, hence counters the overestimation of the 1st term (- term). This effect has a smaller impact for larger sized nuclei.
3. Coulomb term: deals with the force required to overcome the repulsion between protons in the nucleus (- term). Can be Z^2 or Z(Z-1) (second technically more accurate as a single proton wouldn’t have anything to repel against).
4. Asymmetry term: relates to Paulis exclusion principle “ no two protons or neutrons can exist in the same state of the atom”. Without charge (coulomb energy) the SNF is the dominant contribution to nuclear potential and acts both equally on protons and neutrons. Hence equal number of protons and neutrons results in a lower energy state (-ve if Z & N are not equal)
   • spacing between states inversely proportional to the size of nucleus — hence larger nuclei have smaller BE loss for asymmetries in N&Z
5. Pairing term: because N & Z levels are slightly different, it is favourable to complete the Z or N level (even-even +, even-odd 0, odd-odd -). Maximum overlap for two identical nucleons in the same spatial state, hence higher BE for same paired nucleons. Hence lowest BE for single paired nucleons.

Question 2

Why is a nucleus more stable with both N and Z and not just one type?

Answer 2

A nucleus is a bound state where the nucleons are confined by nuclear potential.

Particles that are confined have discrete energy levels (quantised)

The pairing term illustrates how a mass number comprised of both types of nucleons would be more energetically stable than the same mass number comprised of either Zs or Ns.

Question 3

What is the nuclear energy potential well and why does it differ between nucleons?

Answer 3

An energy well describes how deeply bound a nucleon is within the nucleus, the lower down the nucleus the more stable it is.

The SNF affect both Z and N equally. The EMF affects Z’s only and is a repulsive force, hence the proton potential well is higher for protons (see diagram). This is also why the proton well has a raised lip, which needs to be overcome for protons to be added to the nucleus.

Particles have the tendency to fill the lowest energy levels first, with the difference between Z and N growing more pronounced for larger nuclei.

Well (below 0) = attractive

Outside well (above 0) = repulsive

Question 4

What is the front end of the fuel cycle?
Help - mining, milling, conversion, enrichment, fabrication

Answer 4

Mining - removes ore from the earth
Milling - chemically processes the ore into concentrated ’yellowcake’ U2O3
Conversion - transforms the processed ore into uranium hexfluoride gas UF6
Enrichment - separates and concentrates the amount of U-235 isotope in the UF6 gas
Fabrication - Enriched UF6 is converted back into UO2 oxide and fuel pellets are fabricated

Question 5

What are the different types of ores and what happens once they’re mined?

Answer 5

Pitchblende - UO2
Uranite - U3O8 or UO2 + 2UO3
Zeunerite - Cu(UO2)2(AsO4)2*10-16H2O

Converted into Urania (yellowcake) at the end of the mining, leaching and filtering process

Yellowcake consists of 70-90% UO2 at natural abundance (99.3% U238)

Question 6

What are the main enrichment and fabrication techniques?

Answer 6

Enrichment - (1) gaseous diffusion (2) Centrifuge enrichment (3) laser enrichment

Fabrication - (1) conversion of UF6 —> UO2 (2) re-enrichment of processed uranium (3) MOX production

Question 7

What is the cause of degradation of fuel in the reactor and what are the main effects?

Answer 7

Cause - higher burn up results in more damage | rim region of pellet becomes increasingly damaged

Effects - change in thermal conductivity, production of fission products, Xe bubbles, swelling leading to cladding failure

Question 8

Detail Materials, volume and radioactive content as a % between high, intermediate and low level waste

Answer 8

High-Level Waste:
M - fission products, transuranic elements generated in core
V - 3%
R - 95%

Intermediate-level Waste:
M - fuel cladding, chemical sludge, decommissioned components
V - 7%
R - 4%

Low-level Waste:
M - paper, rags, clothes, tools etc
V - 90%
R - 1%