Post Irradiation Examination

After major accidents the core (or what is left of it) is normally subject to PIE in order to find out what happened.

Materials in a high radiation environment (such as a reactor) can undergo unique behaviors such as swelling[1] and non-thermal creep.

At distance x from the centre the temperature (Tx) is described by the equation where ρ is the power density (W m−3) and Kf is the thermal conductivity.

To explain this for a series of fuel pellets being used with a rim temperature of 200 °C (typical for a BWR) with different diameters and power densities of 250 Wm−3 have been modeled using the above equation.

Radiochemistry and Nuclear Chemistry, G. Choppin, J-O Liljenzin and J. Rydberg, 3rd Ed, 2002, Butterworth-Heinemann, ISBN 0-7506-7463-6

Temperature profile for a 20 mm diameter fuel pellet with a power density of 250 W per cubic meter. Note the central temperature is very different for the different fuel solids.
Temperature profile for a 26 mm diameter fuel pellet with a power density of 250 W per cubic meter
Temperature profile for a 32 mm diameter fuel pellet with a power density of 250 W per cubic meter
Temperature profile for a 20 mm diameter fuel pellet with a power density of 500 W per cubic meter. Because the melting point of uranium dioxide is about 3300 K, it is clear that uranium oxide fuel is overheating at the center.
Temperature profile for a 20 mm diameter fuel pellet with a power density of 1000 W per cubic meter. The fuels other than uranium dioxide are not compromised.