Nuclear Fuel Cycle Front End

Fuel, cladding, moderator and coolant of PWR

Fuel: enriched UO2

Cladding: zircaloy

Moderator: light water

Coolant: light water

Fuel, cladding, moderator and coolant of BWR

Fuel: enriched UO2

Cladding: zircaloy

Moderator: light water

Coolant: light water

Fuel, cladding, moderator and coolant of CANDU

Fuel: natural UO2

Cladding: zircaloy

Moderator: heavy water

Coolant: heavy water

Fuel, cladding, moderator and coolant of AGR

Fuel: enriched UO2

Cladding: stainless steel

Moderator: graphite

Coolant: CO2

What is the benefit of the two loop system in a PWR?

Second loop protected from cladding failure.

In the two loop system, pressurised water carries heat from core to boiling water circuit.

How many loops in a BWR?

One loop - fuel in core directly boils water

Basic concept of how a nuclear reactor generates electricity

• Controlled nuclear chain reaction generates heat

• Heat boils water

• Boiled water produces steam

• Steam drives a steam turbine to generate electricity

Why do we choose UO2 as fuel?

U-235 is the only naturally occurring fissile material

(Fissile = undergoes fission with thermal neutrons)

High melting point

Dense pellets obtained through sintering

What are the processes in the front end of the nuclear fuel cycle? What is the AIM?

Mining uranium ore

Milling ore to produced yellowcake

Conversion to uranium hexafluoride gas

Enrichment to increase concentration of U-235

Deconversion to UO2

Fuel fabrication to make pellets

AIM: To produce a pure UO2 pellet enriched in U-235 for fission

What are the conventional mining approaches, and how do they differ?

Open pit - uranium ore located near surface; uses drilling and explosives; benches to prevent collapse; destructive with high risks of contamination

Underground mining - uranium ore located deeper underground; less waste rock removed hence less environmental impact; exposure to radon gas

What is the milling process? What is the product of the milling process?

Milled to separate uranium from other constituents. Ore ground into powder and goes through a series of leaching and filtering processes to remove unwanted elements.

Converted into uranium concentrate U3O8 aka yellowcake.

What are the conventional milling approaches, and how do they differ?

Heap leaching - crushed ore laid on a gradient on thick plastic; sulphuric acid sprayed on ore for 30-90 days as the uranium is extracted as a liquid and treated; slower but cheaper than traditional milling

In-situ leaching - leaching solutions pumped into drill holes to dissolve ore minerals; uranium-rich fluid pumped to surface and uranium compounds extracted; minimal surface environmental disturbance; no waste rock; must avoid contaminating groundwater

Why do we enrich uranium fuel?

Mined natural uranium contains -0.7% U-235 (fissile isotope)

Enrichment increases this to 3-5%

Fission occurs when U-235 absorbs a neutron. Can make this more likely by slowing the neutrons down, and enriching the uranium.

What are the two routes for conversion? What do they produce?

Wet process and dry process.

Make UF6.

What is the conventional enrichment process?

Centrifuge

UF6 gas rotated at high speed

Slightly lighter U-235 stays in the middle

Heavier U-238 pushed by centrifugal force to the edge of the centrifuge.

Require a cascade of multiple centrifuges to gradually enrich the UF6 in U-235

What are the two steps in converting enriched UF6 back to UO2? (deconversion)

Describe the process of fabricating uranium dioxide pellets.

Pressed into green pellets

Sintered to remove pores

Rectified (to ensure same size and shape)

Want pellets as dense as possible: more fuel and safer

Define burnup

Amount of energy produced by tonne of fuel

Describe current burnup trends and implications

Average burn for PWRs in the range 40-50 GWd/tHM

BWRs in range 35-45 GWd/tHM

New plant designs aim to reach 60 GWd/tHM

GWd/tHM = gigawatt-day per metric tonne of heavy metal

Advantages of high burnup

Less fuel needed, less waste produced, operate for longer between refuelling.

Disadvantages of high burnup

Higher enrichment levels required, higher irradiation, zirconium cladding becomes brittle, more radioactive products, more decay heat.

What is the purpose of fuel cladding?

• Acts as barrier between radioactive fission and activation products in the fuel and the coolant

• Fuel must NOT make direct contact with coolant

• Cladding prevents corrosion of fuel and accelerated release of fission products

• Prevents potential for radioactivity to be released into the environment

• Also acts as structural component of fuel assembly

Key criteria for fuel cladding materials

• Low neutron absorption cross section

• High melting point

• Low thermal expansion coefficient

• No dangerous activation products

• Safe waste material

What is the most common fuel cladding material?

Zirconium alloy (zircaloy)

Key processes in zirconium production

Use chlorine chemistry to separate zirconium from hafnium.

Kroll process to get Zr metal from Zr ore

What is corrosion?

Deterioration of a material due to its environment.

Balance between thermodynamics and kinetics.

• Thermodynamically, corrosion is inevitable (metals oxidise to return to a lower energy state).

• Longevity of a material depends on kinetics (rates) of corrosion reactions.

Describe electrochemical corrosion as a redox reaction.

Oxidation = removing electrons from metal

Reduction = generating negative ions

Results in anodic and cathodic regions, with a potential difference between them due to the transfer of electrons.

What is the redox potential?

Measure of the tendency of a chemical species to acquire/lose electrons.

The more negative a materials potential, the more energetically favourable it is to oxidise (corrode).

Name the four requirements for corrosion

Anode

Cathode

Electrical connection

Ionic connection

Describe corrosion protection approaches

Corrosion inhibitor

Protective coatings

Cathodic protection

Name and distinguish between key corrosion phenomena

General - uniform loss of material over the entire surface at an approximately uniform rate

Galvanic -

Crevice - intensive, localised corrosion, occurring within shielded areas

Pitting - intensive, localised corrosion that results in holes/pits

Intergranular attack - localised corrosion that occurs when the microstructure of the grain boundary is more susceptible to attack, so corrosion spreads along the edges of the grain

Stress corrosion cracking - progressive fracturing that occurs in metals as a result of the combined influence of tensile stress and a corrosive environment

Describe corrosion control parameters implemented in nuclear power plants.