Supplementary Reactivity Control

The control requirements of the initial core are designed to be considerably in excess of the equilibrium core requirements because all of the fuel is fresh in the initial core. The initial core control requirements are met by use of the combined effects of the movable control rods and a supplemental burnable poison. 

Only a few materials have nuclear cross sections suitable for burnable poisons. An ideal burnable poison must deplete completely in one operating cycle so that no poison residue exists to penalize initial U-235 enrichment requirements. It is also desirable that the positive reactivity from poison bumup matches the almost linear decrease in fuel reactivity from fission product buildup and U-235 depletion. A self-shielded burnable poison consisting of Gd203 dispersed in a few selected fuel rods in each fuel assembly provides the desired characteristics. Gd203 depletes as a cylinder with decreasing radius to provide a linear increase in reactivity. The concentration is selected so that the poison essentially depletes in the operating cycle. It is possible to improve power distributions by spatial distribution of the burnable poison. 

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Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

Answer It can be re-otorted almost too easily. Additional control is necessary to keep the reactor subcrltlcal during the outage and provide supplementary reactivity control so that the safety rods may be wlthdrawTi and the pile re-startod. 

The cold or hot reactivity of the water-cooled loading Is not to exceed the coinblned negative reactivity worth associated with the Horizontal Control Rod System the xenon present In the reactor Inserted supplementary control devices and the poison pieces or poison columns under Irradiation. That Is the reactivity In excess of that coiqpensated by Inserted supplementary and permanent poisons Is not to exceed the strength of the Horizontal Control Rods, 1 5 per cent k/k In the... 

Supplementary control In the operating reactor shall be limited In use to compensate for not more than i4>.0 per cent excess reactivity. Supple-... 

Supplementary solid burnable poisons are used to assist in providing reactivity compensation for fuel burnup. For all operating cycles, the supplementary control is provided by gadolinium mixed into a portion of the UO2 reload fuel rods. 

Poth, N., Frey, Th., Stichlmair, J., MINLP Optimization of Kinetically Controlled Reactive Distillation, 11 Symposium on Computer Aided Process Engineering, Supplementary Proceedings Volume, 2002, 79-84. 

Answer After about two days the reactivity gain will exceed the capacity of the horizontal rod system in the older piles, and the reactor must be kept subcrltlcal with the vertical safety rods and/or supplementary control or temporary poison. 

Reactivity Transient - The variation of available excess reactivity with time. Since the iiactor is normally operated with a nearly constant power level (tha.t is, Kex equal to 1.000) the reactivity transient is the variation with time of the amount of control red, plus any supplementary control in the reactor. 

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