12.106. Before considering fission product transport, mention should be made of the physical barriers in reactor systems to the escape of radio­activity. The first barrier is the fuel material itself, which in the case of uranium dioxide retains the solid fission products and inhibits release of the volatile radionuclides. The fuel rod cladding is the second barrier. During service, the cladding does develop small pinholes and cracks, which allows some radioactivity to escape into the cooling water. The third bar­rier, the primary coolant system boundary, contains the normally small amount of radioactivity in the circulating coolant, which is continuously monitored. The final physical barrier is the containment structure. How­ever, as we discuss shortly, certain chemical and physical processes along the transport path also inhibit the fission product release.