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The modelling of fuel freezing and sticking to unmelted steel surfaces and the possible subsequent release of frozen crusts is an important part of the Material Dynamics (MDYN) module of the European Accident Code-2 (EAC-2). The latter is a detailed new development for the improved analysis of unprotected accidents in LMFBRs. The modelling of fuel freezing and sticking in accident analysis codes is difficult because the coolant channels are usually not completely filled with molten fuel as in clean experiments and, additionally, fission-gas bubbles play a role in accident calculations. Based on recent findings in the Geyser experiments at Grenoble, a thin (100-200 micron) crust should form relatively slowly. Additional fuel can then freeze on such crusts in a bulk manner. If the temperature of moving molten fuel drops below a melt-fraction of about 30% (mobility limit) it slowly starts to form a thin crust underneath all the moving fuel in a mesh cell. If the underlying cladding or structure melt, their respective crusts do not get released and added to the moving fuel. In cells where the crusts do not get released, additional fuel can abruptly freeze once its temperature drops below the solidus temperature (bulk freezing). A further important aspect of this new model is the treatment of fission-gas porosity in the molten crusts. With the new freezing approach, the extent of fuel relocation in the TREAT L8 experiment is somewhat overpredicted. However, by raising the melt-fraction at which crust formation starts (mobility limit) the degree of fuel relocation is reduced. Moreover, bulk freezing should probably also be initiated at the fuel mobility limit.

Additional information

Authors: WIDER H U, JRC Ispra (IT);DEVOS J J, JRC Ispra (IT)
Bibliographic Reference: Paper presented: 14th Liquid Metal Boiling Working Group Meeting, Brasimone (IT), April 16-18, 1991
Availability: Available from (1) as Paper EN 36160 ORA
Record Number: 199110746 / Last updated on: 1994-12-02
Original language: en
Available languages: en
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