Fast neutron reactors absorbent elements

The Advanced Fuel Recycle Program is concerned with the safe reprocessing of mixed plutonium and uranium oxide fuels, characteristic of fast reactors. The safe handling and storage of these fuels hin on calculations and these in turn depend on clean, well-deflned experiment data for validation. Benchmark experiment data have been. acquired for fast test reactor (I R)-type fuels for impoisoned systems and systems intermixed with soluble poisons. However, there, are no data now available, fliat explore the criticality of these fuels intermixed with solid neutron absorbers (poisons). In this paper, we will present the results of experiments performed at the Pacific Northwest Laboratory (Critical Mass Laboratory) on fast test reactor fuel elements intermixed with solid neutron absorbers. The isons used were Bbral and cadmium plates and gadoliniuth cylindrical rods. Each absorber was separately examined to see its reactivity effect on lattices of FTR fuel pins in water none was intermixed. 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

The First Shutdown System (FSS) is designed to shut down the core, when abnormal or deviated from normal situations occur, and to maintain the core sub-critical during all shutdown states. This function is achieved by dropping the neutron-absorbing elements into the core by the action of gravity when the water flow in the CRD mechanism is interrupted, so malfunction of any powered part of the hydraulic circuit will cause the immediate shutdown of the reactor. Six out of twenty-five absorbing elements are part of the Fast Extinction... 

Boron has been included as a neutron absorber in various materials in addition to concrete. For example, borated graphite, a mixture of elemental boron and graphite, has been used in fast-reactor shields. Boral, consisting of boron carbide (B4C) and aluminum, and epoxy resins and resin-impregnated wood laminates incorporating boron have been used for local shielding purposes. Boron has also been added to steel for shield structures to reduce secondary gamma-ray production. In special situations, where a shield has consisted of a heavy metal and water, it has been beneficial to add a soluble boron compound to the water. 

It has to withstand fast power increases, so that reactor power may follow demand. Fuel elements must accept neutron absorbing rods and burnable poisons. 

The construction of a fast reactor requires pure, or nearly pure, fissile material, so the construction of ZEPHYR could not be undertaken until this material was available. The fuel used is, in fact, plutonium enclosed in a metal can. The core of a fast reactor, i.e. the central portion containing the fuel elements, is very small and in the case of ZEPHYR only about the size of a top hat. The core is surrounded by a more massive envelope or blanket of uranium which either reflects back any neutrons escaping from the core or absorbs them so as to produce fresh plutonium. 

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