Nuclear Fuel Flashcards
What is burnup?
The energy produced per unit mass of fuel. Has units of GW days/tonne of uranium
What are the functional requirements of nuclear fuel?
Provide physical location for fuel
Prevent relocation of fuel to a more reactive configuration
Protect fissile material from erosion and corrosion due to coolant
Provide a primary barrier to radiation
Facilitate the transfer of heat from fuel to coolant
Provide a compact, transportable structural unit
What is a fuel element?
The smallest fuel component that satisfies all functional requirements
What is a fuel assembly?
Consists of fuel elements, usually in rods, to facilitate handling
Benefits of metal fuels?
Easy to manufacture (a lot of experience)
Very high thermal conductivity
Moderate melting point (but lower than ceramic)
High density (lower volume to transport)
Benefits of ceramic fuels?
Much higher melting point that metal fuels
Good chemical compatibility with coolants
Less affected by radiation than metals
Why is UO2 the dominant fuel type?
High melting point (makes up for lower conductivity)
Chemically compatible with most coolants (water, CO2, zirconium alloys)
Easy to fabricate
Oxygen has low neutron capture cross section
Is structurally stable over long periods of time
Long period of good experience with UO2
What materials are the most commonly used for nuclear cladding?
Stainless steel and Zirconium alloys. Iron, however, absorbs neutrons so Zr is preferred.
Alternatives include Ceramics
What process is mainly used to convert UF6 to UO2?
The Integrated Dry Route (IDR)
Describe the Integrated Dry Route process
UF6 is evaporated and fed into the top of an inclined, rotating kiln. UF6 reacts with steam which is fed at the upper end of the kiln, and then proceeds to react with steam and hydrogen gas, which is fed in at the bottom end. UO2 is produced at the bottom.
Outline the chemical reactions which occur in the IDR process
UF6 + 2 H2O -> UO2F2 + 4 HF
4 UO2F2 + 2 H2O + 2 H2 -> U3O8 + UO2 + 8HF
U3O8 + 2 H2 -> 3UO2 +2 H2O
Describe the neutron flux distribution and temperature distribution across a cylindrical fuel rod.
The neutron flux is maximum at the outer radius of the rod as neutrons are moderated outside of the rod. Flux is axisymmetric so is at a minimum at the centre of the rod
Temperature flux is a maximum at the centre of the rod and varies as a quadratic across the radius. Temperature is minimum at the outer radius as the rod is in contact with the coolant.
Quote the temperature variation equation
q’’ = -k ∇T
∇.(k ∇T) + q’’’ = 0
q’’’ = ∇.q’’
k = Thermal conductivity (W/m/K) q'' = heat flux (W/m2) q''' = heat production per unit volume (W/m3)
What is the radial temperature distribution across a fuel rod?
T(r) = T_w + q’’‘/4k * (R^2 - r^2)
Tmax = T(0) Tmin = T(R)
How does the temperature variation affect the structure of a fuel pellet?
The temperature distribution varies across the rod’s radius, hence, the thermal expansion of the pellet is greater at the centre than at the outer radius.
