Fission release from fuel during transients

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

Some of the metallic fission products are plated out to a large extent onto the outer surfaces of the fuel rod claddings in the immediate vicinity of the defect this means that only small fractions of them are transported into the coolant. This is true in particular for tellurium, probably due to an electrochemical reaction on the Zircaloy surface resulting in the formation of the compound SnTe. The rather long-lived Te (halflife 76.3 h) decays there under the production and continuous release of its daughter to the coolant this mechanism is assumed to be the reason for the specific release behavior of this radionuclide, which is markedly different from that of the other iodine isotopes, both during constant load operation and during transients. 

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