Radionuclides in the fuel pellet - cladding gap

Besides the fuel, the closed chemical system fuel rod includes the fuel pellet -cladding gap and the upper and lower gas plenum, which are filled with helium overpressure (about 2 MPa at ambient temperature) in the course of fuel rod fabrication. During reactor operation, a certain gap inventory of radionuclides is generated which is of interest in the event of an operational fuel rod failure as well as in a loss-of-coolant accident. In typical LWR fuel rods, this gap inventory is mainly formed by fission product recoil from the fuel pellets. According to Wise (1985), one quarter of the fission fragments generated within a recoil length i from the 

Investigations reported by Neeb (1978), in which the inner surface of the cladding of a typical PWR fuel rod was subjected to a selective leaching procedure. 

The chemical conditions in the gap are controlled by the oxygen potential, po, which can be calculated by  

In the analyses reported by Bleier et al. (1984), C was not detected in the gas phase of the fuel rod, either as CO2 or as CO/ CH4. From this result, one can calculate that the fraction of C present in the gap amounts to less than 10 % of the fuel rod inventory, at the maximum. 

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