2a. Fuel, core & Cooling

Question 1

What are the 6 main requirements of fuel?

Answer 1

1. Mechanical stability - harsh conditions inside the reactor
2. Good heat transfer capacity - want to transfer heat easily to coolant
3. Retention of fission products - safer, easier clean, less spread of radioactive elements
4. High power density - more power small size
5. Tolerance of high temps - reduce long term creep and material deficiencies from high operating temps
6. Long life - minimise requirements of refuelling

Question 2

6 desired characteristics of a moderator

Answer 2

1. High scattering CS - similar mass to neutron, high chance of slowing them down rather than absorbing.
2. Strong tendency to reduce neutron energy upon scattering
3. Low absorption CS - don’t want to absorb neutrons for nuclear fission
4. High number of moderator atoms per unit volume - increases No. of scatterings
5. Sufficient structural strength - if solid
   Chemical compatibility

Question 3

Cladding - 6 main properties & main issue

Answer 3

1. Good thermal conductivity
2. Chemical compatibility with primary coolant - at operating and fault temps
3. High melting point - above fault temps
4. Adequate structural integrity - ductility, strength, creep resistance
5. Low Neutron Absorption CS
6. Low induced Radioactivity - neutron absorption causes the material to irradiate which contributes to material corrosion

Main issue - cladding carbonisation - fission of carbon in cladding matrix causes carbide precipitator

Question 4

Coolant - main features

Answer 4

1. High specific heat, thermal conductivity, convective heat transfer coefficient
2. Low viscosity and high density - requiring low power to pump
3. Thermal and irradiation stability
4. Small corrosive and erosive effects - inside and out of core materials
5. Small absorption cross-section
6. Freedom of impurities - would increase absorption CS
7. Availability and safe handling

AGR’s - use natural uranium and hence use CO2 as coolant as very low absorption CS required.

Question 5

General features of AGR Reactor Core

Answer 5

ARRANGEMENT: Reactor core & shielding hosed within an enclosed steel envelope called GAS BAFFLE —> Surrounding this are the boilers (12) & gas circulators (8) which are housed within PCPV

Size - 30m x 40m (high) external
Can be refuelled during operation

Question 6

General features of PWRs

Answer 6

Coolant: single phase, secondary loop generates steam
RPV - houses core, control rods etc only (no boilers)
Core barrel - slides down into reactor vessel which houses the fuel > hangs inside the reactor on internal supports > towards the bottom sits a lower core support plate where fuel assembly sits

Question 7

PWR fuel assembly

Answer 7

264 rods in a 17 x 17 array (289 total) - 24 slots used for control rods and 1 for instrumentation
193 fuel assemblies constitute the core (50,952 rods)
RCCA - Rod Control Cluster Assemblies have 16-20 rods and entree tubes in the assembly > these contain a set of control rods in a spider arrangement
53/193 fuel element contain RCCA’s
• these are arranged in three control banks and 6 shutdown banks > each banks control rods are moved equally

Question 8

Temperature limits, what are they?

Answer 8

UO2 - 2800C melting point
zircaloy clad - 1880C
Stainless Steel - ~1400C

AGR - have an assumed clad temp limit 1350C
• max temp of clad typically 860C during normal op > avoids rapid creep and oxidation
• inner bore of fuel typically 1200-1400C

PWR - central fuel temp ~1000C
Zircaloy clad - ~ 500C