Fuel Element Changes

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

The neutron kinetics was described with due regard for six groups of delayed neutrons. The core reactivity changes were determined by temperature effects of reactivity of fuel elements and the alloy as well as by water penetration into the core. 

Maintenance of criticality. As each fuel element in a reactor is irradiated, its composition changes, as does its contribution to overall reactivity. To maintain criticality in the face of these composition changes, it is necessary either to move control poison or change its concentration or to move fuel or change its concentration. Because reactivity changes caused by... 

The chemical composition of fission products in discharge fuel is controlled by the long-lived and stable species. The amounts of most of the fission-product chemical elements change but little for thousands of years after discharge. Those elements that do change significantly in amount over long decay periods include ... 

Analyze fuel processor catalysts to determine structural and elemental changes... 

Radioactive materials such as exposed fuel elements have continually changing composition because of radioactive decay. 

Changing of stack dimensions due to irradiation of the graphite presents a very s erious situation from all aspects - main--tenance, operation, safety. The continued distortion is affecting operation of the rods and in some cases, limits fuel element charging. Much of the on-reactor maintenance revolves around alleviation of problems caused by graphite distortion. Future problems with the thermal shields and Ball 3X system may result. All in all, graphite distortion is the major key to continued operation of the reactor because of its effect on all other reactor systems. Since the graphite and the shields defy replacement or major modification, their deterioration becomes a matter of serious ct ncern. 

The most obvious case in point is that of the experimental critical assemblies wherein fuel mixtures are often simulated by stacking small plates of the individual isotopes. Although there are not supposed to be any large temperature changes in such a reactor, one must consider accident situations, and in this case the heat generated would stay mainly in the fissionable isotopes. A second case can be imagined if for a power reactor the fuel element fabrication involves a step where the isotopes are... 

An important point for the safety of reactors is the influence of the core temperature on fc, i.e., on the reactivity. Water-moderated reactors can be built to have a negative temperature coefficient an increase in temperature/power will lead to steam bubbles near the fuel elements and to a decrease in moderation. If the reactor has been designed a little undermoderated, a drop in moderation will bring about a drop in reactivity (fc). Such a reactor will be naturally stable against undesired changes in power. One may recall the discussion of the Oklo reactor at the beginning of the chapter. 

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