Basic Components of PWR (part2)

We talk about some of internal components of Pressurized water reactor such as fuel pellets, fuel rods and fuel assemblies in the previous article.

We will continuo describing the internal components for PWR.

Cladding

 Function of the cladding:

1.    To maintain a barrier between the fuel and the coolant in order to maintain the primary circuit as clean as possible.

2.    To maintain a predetermined-geometry of the fuel element.

3.    The cladding is generally the only feature that stops the fuel and fission products from getting into the primary circuit.

4. be transparent to neutrons, so that it doesn’t absorb neutrons that could be used to induce further fission.

5. have a high thermal conductivity, and not have a high thermal expansion coefficient.

Requirements of cladding material:

1.    The cladding and duct materials should have the lowest possible neutron absorption cross section.

2.    The cladding is subjected to steadily rinsing stresses due to fuel swelling and fission gas release. To withstand these stresses the claddings must posses and retain good multi axial rupture strength, creep strength, and ductility.

3.    The wall thickness of the cladding should be as thin as possible for reasons of good neutron economy and minimum thermal stresses.

4.    The cladding material should be chosen to have melting point or phase transition point away from maximum temperature.

5.     The fuel and coolant may react slowly with the cladding so that a “wastage” allowance must be made. These reactions may also affect the mechanical properties and will have to be taken into consideration in safety analysis.

Note, In PWR we use zirconium has very attractive nuclear properties once it is separated from hafnium with which it generally occur in nature. The resistance of zirconium to corrosion is increased by addition of tin, iron, chromium and nickel.  Also stainless steels was used but has been totally superseded by zircalloy because of the high absorption cross section.