Lesson 3 ◆ Tutorial

Q: (a) Describe the four main steps in the manufacture of UO2_2 fuel pellets, starting from UO2_2 powder. (b) State the sintering temperature and explain why sintering is necessary. (c) Why are AGR fuel pellets annular (hollow centre) while PWR pellets are solid cylinders? (d) State a typical pellet diameter and explain why UO2_2 pellets are small.

A:

(a) Four steps: (1) Powder preparation and homogenisation — UO2_2 powder is blended to ensure uniform composition; (2) Pressing — powder is compacted in a die at high pressure to form “green” pellets; (3) Sintering — green pellets are heated to approximately 1,750∘^\circC in a reducing atmosphere (hydrogen) to densify them to ~95% theoretical density; (4) Grinding/finishing — sintered pellets are ground to precise dimensions.

(b) Sintering temperature is approximately 1,750∘^\circC. Sintering is necessary to bond the powder grains together, achieving the required density (~10.4 g/cm3^3, ~95% TD) and mechanical strength. Without sintering, the pellets would be friable and unsuitable for reactor use.

(c) AGR pellets are annular to provide a central void that accommodates fission gas release and fuel swelling, and to reduce the temperature at the fuel centre. This is necessary because AGR fuel operates at higher linear ratings. PWR pellets are solid because the linear rating is lower and the Zircaloy cladding provides adequate restraint.

(d) Typical pellet diameter is 8—14 mm. UO2_2 pellets are small because UO2_2 has poor thermal conductivity (~3 W/m⋅\cdotK at operating temperature). A small diameter limits the temperature difference between the pellet centre and surface, keeping the centreline temperature below the melting point (~2,865∘^\circC).