Different thermal hydraulic limiting phenomena's are important to be studied during the natural convection cooling of a hypothetical irradiated Low Enriched Uranium (LEU) plate's target. The irradiated LEU targets are used for the production of the medical isotope molybdenum −99 (Mo-99), whose decay product, technetium −99 (Tc-99) is used in the majority of medical diagnostic imaging procedures. After the irradiation of LEU targets, they are acquired a high level of radioactivity, so the direct handling and maneuvering for them is quite difficult. The targets should stay a period of time for cooling either at the reactor core or at the storage basket inside the reactor main pool. So, this study was performed to insure the thermal-hydraulic limiting phenomena's (Onset of Nucleate Boiling, Pulsed Boiling Power ration, Onset of Net Vapor Generation and Burn — out Power ratio) during the natural convection cooling mode with variant power ratio (plate decay power at the time of establishment of natural convection regime to the steady state plate full power). The study was performed at two different positions, the first position was inside the reactor core where a long chimney exist while the other was in the storage basket. The study was aimed to evaluate the optimum power ratio, that is a function of time, at which the targets could transfer from the reactor core into the storage basket. The LEU targets had two possibilities during the irradiation, half load and full load irradiations. Both of the possibilities were discussed and analyzed at different power ratios. All the limiting phenomena's were satisfied the thermal-hydraulic safety margins except the Onset of Nucleate Boiling Ratio, it had some considerations.
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