Fuel Changes during Reactor Operation

Throughout the operation of a nuclear reactor, the physical changes take place to the fuel which affects its capability to transfer heat to the coolant. The precise changes which take place are dependent on the kind and form of fuel. Some of the reactors use fuel assemblies which consist of zircalloy tubes having cylindrical ceramic pellets of uranium dioxide. During construct, a small space or gap is left among the fuel pellets and the zircalloy tube. This gap is filled with pressurized helium. Since the reactor is operated at power, numerous physical changes take place in the fuel which affect the gap among the pellets and clad. One change takes place due to high pressure in the coolant exterior the clad and the relatively high temperature of the clad throughout reactor operation. The high temperature and pressure causes the clad to be pushed in on the pellets by a process termed to as creep. The other physical change is caused by the fission procedure. Each fission event generates two fission product atoms from a fuel atom. Even however each fission product atom is approximately half the mass of the fuel atom, the fission products receive more volume than the original fuel atom. Fission products which are gases can collect altogether and form small gas bubbles in the fuel pellet. Such factors cause the fuel pellets to swell, increasing them out against the clad. Therefore the two procedures of pellet swell and clad creep both works to decrease the gap among the fuel and clad.

This change in the gap among the pellet and clad has important impact on heat transfer from the fuel and operating fuel temperatures. Primarily an important temperature difference exists across the gap to cause heat transfer to occur by convection via the helium gas. Since the size of the gap is decreased, a smaller temperature difference can sustain the similar heat flux. Whenever the fuel pellets and clad come in contact, then heat transfer by conduction substitute's convection and the temperature difference among the fuel surface and clad reduces even more. Due to the procedures of pellet swell and clad creep, the fuel temperatures of some reactors reduce slightly over time whereas the heat flux from the fuel and thus the power of the reactor stay constant.

Not all changes which take place to the fuel throughout reactor operation work to enhance heat transfer. When the chemistry of the coolant is not cautiously controlled in suitable limits, chemical reactions can occur on the surface of the clad, resultant in the formation of a layer of corrosion products or crud among the metal of the clad and the coolant. Usually, this layer will have a lower thermal conductivity than that of the clad material; therefore it will act as an insulating blanket, decreasing heat transfer.

When this corrosion layer is permitted to form, a larger temperature difference will be needed among the coolant and fuel to maintain the similar heat flux. Thus, operation at similar power level will cause higher fuel temperatures after the build-up of corrosion products and crud.