page_title: Post Irradiation Examination
section_title: Fuel cracking
hierarchical_section_title: Post Irradiation Examination / Fuel cracking
caption_attribution_description: English: Nuclear fuel temp calcs
context_page_description: Post Irradiation Examination is the study of used nuclear materials such as nuclear fuel. It has several purposes. It is known that by examination of used fuel that the failure modes which occur during normal use can be studied. In addition information is gained which enables the users of fuel to assure themselves of its quality and it also assists in the development of new fuels. After major accidents the core is normally subject to PIE in order to find out what happened. One site where PIE is done is the ITU which is the EU centre for the study of highly radioactive materials.
  Materials in a high radiation environment can undergo unique behaviors such as swelling and non-thermal creep. If there are nuclear reactions within the material, the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release:
context_section_description: As the fuel expands on heating, the core of the pellet expands more than the rim which may lead to cracking. Because of the thermal stress thus formed the fuel cracks, the cracks tend to go from the centre to the edge in a star shaped pattern.
  In order to better understand and control these changes in materials, these behaviors are studied. Due to the intensely radioactive nature of the used fuel this is done in a hot cell. A combination of nondestructive and destructive methods of PIE are common.
  In addition to the effects of radiation and the fission products on materials, scientists also need to consider the temperature of materials in a reactor, and in particular, the fuel.  Too high fuel temperatures can compromise the fuel, and therefore it is important to control the temperature in order to control the fission chain reaction. 
  The temperature of the fuel varies as a function of the distance from the centre to the rim. 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.
  Tₓ = TRim + ρ (rₚₑₗₗₑₜ² - x²) (4 Kf)⁻¹
  To explain this for a series of fuel pellets being used with a rim temperature of 200 ᵒC (typical for a BWR) with different diameters and power densities of 250 Wm⁻³ have been modeled using the above equation. Note that these fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in diameter.
caption: 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.
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