14.2.1 Fission-induced heating

As the reactor is brought to power at the start of irradiation, fission of the fissile isotope atoms in the fuel (primarily 235U for uranium fuels, 233U for thorium — uranium fuels, and 239Pu and 241Pu for uranium-plutonium or thorium-plutonium fuels) commences. The kinetic energy of the fission fragments is converted to thermal energy as the fragments come to rest in the fuel matrix as a result of inelastic scattering. This causes heating of the fuel bars (for metallic fuel employed in Magnox reactors and in some fast reactors) or fuel pellets (for ceramic fuel employed in all other cases). Thermal equilibrium (i. e. a steady-state condition) is rapidly established at any given reactor power level, such that the rate of heat generation by fission is balanced by the rate of heat dissipation to the reactor coolant. The heat flux from fuel pellets/bars to coolant via the cladding (also known as the sheath or can) is associated with a radial temperature gradient, with the maximum temperature occurring at the centre of the fuel pellets/bars.

The radial temperature distribution is complicated by the non-uniform generation of heat within the fuel pellets. In fast reactors the non-uniformity is minimal. However, in thermal reactors there is significant neutron flux depression as the thermalised neutrons diffuse from the moderator into the fuel pins, leading to preferential fissioning and heat generation in the outer regions of the fuel pellets/bars. In uranium-bearing fuels this is accentuated by the generation of fissile 239Pu from epithermal neutron capture in fertile 238U as irradiation proceeds (Carlsen and Sah, 1981).

Heat transfer is primarily by conduction through the fuel pellets/bars and cladding (and any pellet-cladding, or bar-cladding, gap), and by convection at the cladding-coolant interface. Radiation heat transfer from fuel pellets to cladding is also important at high fuel temperatures (such as occur in fast reactor fuel pellets, or at high power in LWR, AGR and CANDU fuel).

If the fuel becomes overly hot (which is generally only possible during severe accidents), the fuel pellets or bars, or even the cladding, can melt. In the case of metallic fuel, a phase change can also occur whereby there is a rapid and significant increase in fuel bar volume, which can rupture the cladding. For an unalloyed uranium fuel bar (as is effectively used in Magnox reactors) this (first) occurs at a temperature of 660 °C, when there is a transition from the alpha phase to the beta phase (Greenough and Murray, 1962).