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Conference Article: ''Peu a peu'': A new fuelling mode of the pebble bed reactor

Teuchert, E.; Gerwin, H.; Haas, K.A.; Ruetten, H.J. (Kernforschungsanlage Juelich G.m.b.H. (Germany, F.R.). Inst. fuer Reaktorentwicklung)

Abstract

An increase of the core size of pebble bed HTRS and its power output is possible by an annular configuration of the core with a column of graphite blocks in the middle. The need of a more involved pebble flow control and disloading technique can be avoided by a new mode of fuelling: For the initial reactor loading the fissile enrichment is chosen to achieve criticality when a portion of only 30% of the core is filled with the fuel elements. Continuous onload filling provides for the burnup, but disloading of the elements is omitted until the core is totally filled up. Thereupon the reactor is shut down, cooled down, depressurized, and entirely unloaded by means of a new device being inserted from the top, e.g. by a suction dredge. Such a device could be constructed transportable and ambulantly applied for several reactors of that type, because it is required once in two years (full power). This mode of ''peu a peu'' filling of the reactor approximately corresponds to the batch fuelling mode of other reactors, but does not require control poison to cover the excess reactivity. A target design has been studied achieving 250 MWsub(th). In all stages of filling the temperature transient of accidental core heat up is limited as for the conventional MODUL. No disloaded fuel has to be handled during reactor operation, which represents a contribution on safety and protection against fuel diversion. The capital costs must be expected to be lower than for the MODUL, and fuel cycle costs are approximately the same

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key words: burnup; depressurization; high temperature; loss of flow; modifications; pebble bed reactors; performance; reactor fueling; temperature distribution; accidents; homogeneous reactors; reactor accidents; reactors; solid homogeneous reactors
Reference:
Specialists' meeting on safety and accident analysis for gas-cooled reactors. Oak Ridge, TN (USA). 13-15 May 1985
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--358, pp:299-311