Nuclear - BWR/CANDU/AGR

Overview of a BWR

• Under high pressure, boiling is stable and reactor controllable

• Steam generated in RPV

• Steam made >99% dry before sending into turbine

• Power density is smaller in BWR than PWR, so BWR need to be larger

• Water becomes radioactive so electricity generating system must be sheilded

• Typical inlet T 278 degrees C, outlet 287 degrees C

• Saturated steam produced at about 287 degrees and 7 MPa, efficiency ~33%

• Control rods come in from the bottom

Describe light water for BWRs

• Used as both coolant and moderator

  • 100-200 tonnes

• ADV

  • Excellent for slowing down neutrons

  • Readily available and cheap

• DIS

  • High vapour pressure therefore must operate at high pressure

  • Such a good neutron absorber that the fuel must be enriched to sustain chain reactions

Moderating efficiency for all three types of moderator

Describe a BWR fuel assembly

• Fuel rod: Pellets stacked inside, filled with helium at 0.3 MPa, Pellet clad gap = 0.23mm

• Tube: D=12.3mm, L=407mm, T=0.81mm, Pellets stacked to 380mm, leaving a top plenum

• Fuel rods are more spaced out and fewer per bundle than in PWR

• Min external force on fuel rods, each fuel rod is free to expland axially

• Possible to remove and replace an individual rod

Describe BWR control rods

Cruciform rods:

• Can fit snugly into the corners and along the edges of the fuel assemblies

• Good physical integrity

• Made of either solid neutron absorbing metal or hollow cross forms filled with rods of neutron absorbers

• Fewer used than PWR (~137 compared to 1000 in PWR)

Describe the fuel arrangement for a BWR

Control cell core:

• Different enrichments to reduce power peaking

• Low enrichment in corner rods and in the rods near the water gaps

• Higher enrichment in the central part of the fuel bundle

• Selected rods in each bundle are blended with gadolinium burnable poison

Describe BWR control

• Do not use soluble poisons

• Can still use burnable poisons

• Rely on control blade insertion pattern and core flow rate for control

Describe the difference between a convention and control cell core for BWR

Convention core

• Not allowing a blade to shadow a zone of the core too long, by rod swap at reduced power (85% to avoid pellet-clad interaction failures, no need to shut down reactor) Control rods swap every several months

Control cell core

• No rod swap and no power reduction. Use less reactive fuel assemblies in control cells, only allow fuel assembly to be in control cell for one cycle

What is Coastdown BWR?

At EOC, all blades withdrawn, core flow rate at max, to keep the power up

What is a HWR?

HWR - Heavy water moderated reactors

• 4 types

  • Pressure tube, heavy water cooled

  • Pressure tube, boiling light water

  • Pressure vessel, heavy water cooled

  • Gas cooled, heavy water moderated reactor

• PTHWC is most common

What is CANDU?

CANada Deuterium Uranium

• Coolant and moderator: D2O (>=99.75% pure)

• ADV - High neutron economy - Natural U can be used, uses 15% less U

• DIS - Expensive (20% of capital cost), larger core size than LWR

• Fuel pellet - UO2

• Online refuelling (10 channels, out of 300-500, per week)

• Steam produced at lower temp and pressure than LWR, lower efficiency

• Lower temp means neutrons more thermal in LWR than CANDU

CANDU fuel bundle

• Water temp ~70 degrees C

• Water pressure ~150kPa

• Each fuel channel consists of an inner pressure tube, which contrains rthe fuel bundles and the heavy water primary coolant, and an outer clandria tube

• Tubes are concentric, gap between them (8-9mm) is filled with a slow purge of CO2

Describe Gas Cooled Reactors

• Coolant - CO2, He, even air

• Moderator - mostly graphite

• Fuel - Natural U

• Examples

  • British Magnox and AGR

  • French NUGG

Primarily used for transport heat from core to boiler, rather than to cool the fuel itself

What are the pros and cons of gas coolant?

ADV

• Compressible, operating temperature can be chosen independent of pressure

  • Higher pressure can be used, increased efficiency

  • Pressure can be selected separately for safety and cost reasons

• No phase change as a result of T or P change under fault conditions

• Flow and temperature prediction is simple and more confident

• Lower burden of activated corrosion products, easier maintenance, lower radiation level and dosage

DIS

• Lower density and specific heat, larger size and building cost

What makes a good coolant?

• High specific heat capacity and high density

• Chemically stable, low corrosion

• Stable under radiation, low neutron-induced radioactivity

• Low absorption cross section, good neutron economy and to avoid rise in core reactivity in accidental loss of pressure

Pros and cons of CO2 and He in gas coolant

CO2 - Dense and cheap, but not chemically fully inert, moderate T, larger RPV

He - Inert, much higher specific heat capacity, but costly. Higher T and smaller RPV

Describe Magnox

Magnox - Magnesium non-oxidising) - Mg alloyed with Al and other metals, finned to improve heat transfer

ADV - Low neutron capture ~0.059 b (lower than Zr and Al)

DIS - Limits max temp - Reacts with water so cant be stored under water for too long

• Operating pressure - 2-27 bar

• Coolant - CO2

• Control rods - high Boron steel

• Use metallic U pellets rather than UO2 as fuel

• Fuel rods - D=28/29mm, L=48 to 128mm

• Use of natural U-235 Conc - frequent refuel

• Operate around 390 degrees C, some 360 due to corrosion effects of CO2 on steel

All used Magnox fuel has been reprocessed, compared to later gen, large amounts of waste and expensive to run (25-100% more costly)

Describe an AGR

Advanced gas reactor

• Descendant of Magnox, 4x the power densitiy, smaller heat exchangers

• Graphite moderated

• Coolant - CO2

• CO2 in core ~ 650 degrees C, 4 MPa

• Steam Production - 540 Degrees C and 16 MPa

• Fuel - lightly enriched UO2

• Higher overall efficiency ~42%