New Models for UO2 Fuel Structure Evolution under Irradiation in Fast Reactors

On the base of analysis of experimental observations and critical assessment of existing models for oxide fuel structure evolution under operation conditions of fast reactors, new models for fuel restructuring and coring are proposed. The restructuring model describes coherent motion in the temperature gradient of various voids (gas bubbles, sintering pores and large lenticular pores) and grain boundaries, to which the voids are attached. As a result, the model explains elongation of thermally growing equiaxed grains and formation of columnar grains, and predicts a rapid formation of extended columnar grain zone during a relatively short initial period of fast reactor irradiation. The coring model describes formation and growth of the central void in the fuel pellet, activated by mass transport from the inner to the outer zone of the pellet under stresses induced by inhomogeneous fuel densification in the initial period of irradiation.Comment: 17 pages, 7 Figure

Behaviour of oxide layer of zirconium-based fuel rod cladding under steam starvation conditions

Das Verhalten der Oxidschicht von Zirkonium-basierten Brennstabhüllrohren unter Dampfmangelbedingungen Es werden die Ergebnisse von Dauerglühversuchen mit voroxidierten Zircaloy-Hüllrohren in Inertgasatmosphäre bei Glühtemperaturen zwischen 1250 und 1500 °C und ein entsprechendes Modell dargestellt. Ziel der Tests war die Untersuchung der Kinetik der Oxidschichtreduzierung während der Dampfmangelphase bei der Trockenlegung des Reaktorkerns während eines schweren Störfalles. Neben der Abnahme der Oxidschichtstärke wurden die homogene Entwicklung von α-Zr(O) Ausscheidungen innerhalb der Oxidschicht und die Entwicklung einer α-Zr(O) Schicht an der äußeren Oberfläche des Hüllrohres nachgewiesen. Das Phänomen sollte einen starken Einfluss auf die intensive Wasserstofffreisetzung während der folgenden Abschreckphase haben

Application of Mechanistic Criteria of Cladding Oxide Shell Failure to the Analysis of Core Degradation Simulated in Bundle Meltdown Tests

In this paper, the strong influence of the clad failure criteria on the calculated scenario of bundle degradation was demonstrated on the basis of comparative analysis of Phebus FP and PBF-SFD tests using the SVECHA/QUENCH code. The main peculiarity of the proposed mechanistic criteria is that they are based on the actual geometry of degraded fuel rods and not on a constant, user-defined failure temperature. This allows better modelling concerning the formation of local molten pool(s) in the upper part of bundle and the timing of subsequent materials relocations from the local molten pool(s). In general, the mechanistic criteria developed allow one to reasonably calculate the bundle degradation progression in both tests. Therefore it is recommended to use these new, mechanistic cladding failure criteria in the severe accident system codes as well.JRC.F.4-Safety of future nuclear reactor