Patent Number: 043228534
Section: summary

This invention relates to the control of nuclear reactors and, more particularly, to the control of heat cycles in large nuclear reactors. The invention applies specifically to horizontal-column, natural-uranium, graphite-moderated, water-cooled nuclear reactors. Reactors of this general type are disclosed in U.S. Pat. No. 2,910,418 issued Oct. 27, 1959 to E. C. Creutz et al. Natural-uranium, graphite-moderated nuclear reactors are inherently all very large in size. In such reactors localized phenomena may result in heat and flux cycles wherein a surge of heat and flux, or "hot spot", occurs at some location within the reactor and the action taken to cool off the "hot spot" results in a hot spot at some other location in the reactor. One particular form of reactor to which the invention particularly applies which is now in operation is a graphite-moderated, water-cooled reactor operated with natural uranium as fuel for the production of plutonium. In these reactors process tubes containing the fuel extend through horizontal channels in the graphite. Cooling water passes through these process tubes over the fuel. The horizontal channels are overbored at the front and rear of the reactor to increase the temperature of the graphite at these locations. The graphite temperature is raised near the edges of the reactor where it is normally relatively low to reduce graphite distortion due to accumulation of stored energy. Finally, control rods which extend horizontally into the reactor at right angles to the process tubes are concentrated near the center of the reactor to obtain the maximum effect therefrom. When a reactor is operating at constant power, insertion of a control rod because of a hot spot requires withdrawal of a control rod at some other location in the reactor to maintain the power level constant. Insertion of a control rod at the hot spot causes a reduction in graphite temperature at that point. Reduction of the graphite temperature has a reactivity effect of its own independent of the control rods. Changing the graphite temperature changes the cross-section of the graphite as well as changing the thermal neutron energy. Since the graphite cools gradually, reactivity gradually decreases after movement of the control rod is completed. It is thus necessary to move the control rod out again to compensate for this decrease in reactivity. At the same time a control rod is inserted near the hot spot, a control rod is withdrawn at another location in the reactor to maintain a constant power level. Because of the control rod withdrawal, heating occurs at this other location. The reactivity increases after a delay because of the increased graphite temperature and the control rod must be reinserted. In other words, the action taken to counter a hot spot causes a hot spot somewhere else in the reactor and the action taken to counter the second hot spot causes another hot spot at the first or another location. The heating and cooling occurring in these heat cycles is reinforced by the xenon effect. When power is reduced in a given area, xenon-135 increases slowly since it is not being destroyed by radiation capture as fast as it is being produced by decay. This results in a further reduction in reactivity and further cooling of the graphite. Several hours later, reactivity again begins to increase, reinforcing the reactivity increase obtained by the countermovement of the control rod. These hot spots are most noticeable near the fringe of the reactor where no control rods exist and the temperature has been artificially increased by overboring the graphite channels. A hot spot may, for example, move from the front to the rear of the reactor or the reverse, thus establishing what may be called front-to-rear heat cycles. As a result of this problem, it has been necessary at times to decrease the power level of operation of the reactor to avoid uncontrollable heat cycles. This, of course, results in decreased production of plutonium. It is accordingly an object of the present invention to develop means for controlling a nuclear reactor. It is a more detailed object of the present invention to develop means for controlling heat cycles in a nuclear reactor. It is a still more detailed object of the present invention to develop means for controlling front-to-rear heat and flux shifts in a nuclear reactor. It is incidentally an object of the present invention to develop novel means for limiting the movement of the aforesaid control means. These and other objects of the present invention are attained by incorporating a control column in one or more of the reactor process tubes and providing means for varying the horizontal displacement of the control column. The control column contains fewer fuel elements than are in a normal fuel column and control elements disposed at at least one end of the control column. According to the preferred embodiment control elements are disposed at both ends of the control column so the total length thereof is greater than that of a fuel column and the amount of fuel and poison are balanced so that movement of the control column varies the over-all reactivity balance of the reactor only slightly.