HEAT GENERATIONHeat TransferVolumetricThermalSourceStrengthThe total heat output of a reactor core is called the heat generation rate. The heat generationrate divided by the volume of fuel will give the average volumetric thermal source strength. Thevolumetric thermal source strength may be used to calculate the heat output of any section of fuelrod, provided the volume of the section is known.Volumetric Thermal Source StrengthQcoreVfuelFuelChangesDuringReactorOperationDuring the operation of a nuclear reactor, physical changes occur to the fuel that affect its abilityto transfer heat to the coolant. The exact changes that occur are dependant on the type and formof fuel. Some reactors use fuel assemblies that consist of zircalloy tubes containing cylindricalceramic pellets of uranium dioxide. During manufacture, a small space or gap is left betweenthe fuel pellets and the zircalloy tube (clad). This gap is filled with pressurized helium. As thereactor is operated at power, several physical changes occur in the fuel that affect the gapbetween the pellets and clad. One change occurs due to high pressure in the coolant outside theclad and the relatively high temperature of the clad during reactor operation. The hightemperature and high pressure causes the clad to be pushed in on the pellets by a process referredto as creep. Another physical change is caused by the fission process. Each fission event createstwo fission product atoms from a fuel atom. Even though each fission product atom is roughlyhalf the mass of the fuel atom, the fission products take up more volume than the original fuelatom. Fission products that are gases can collect together and form small gas bubbles within thefuel pellet. These factors cause the fuel pellets to swell, expanding them out against the clad.So the two processes of pellet swell and clad creep both work to reduce the gap between the fueland clad.This change in the gap between the pellet and clad has significant impact on heat transfer fromthe fuel and operating fuel temperatures. Initially a significant temperature difference existsacross the gap to cause heat transfer to take place by convection through the helium gas. As thesize of the gap is reduced, a smaller temperature difference can maintain the same heat flux.When the fuel pellets and clad come in contact, heat transfer by conduction replaces convectionand the temperature difference between the fuel surface and clad decreases even more. Due tothe processes of pellet swell and clad creep, the fuel temperatures of some reactors decreaseslightly over time while the heat flux from the fuel and therefore the power of the reactor remainconstant.Not all changes that occur to the fuel during reactor operation work to enhance heat transfer.If the chemistry of the coolant is not carefully controlled within appropriate limits, chemicalreactions can take place on the surface of the clad, resulting in the formation of a layer ofcorrosion products or crud between the metal of the clad and the coolant. Typically, this layerwill have a lower thermal conductivity than that of the clad material, so it will act as aninsulating blanket, reducing heat transfer.HT-02 Page 50 Rev. 0
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