Axial Xenon Transient After a Load Reduction

Before the load reduction, the reactor was near equilibrium xenon with a slightly bottom-skewed flux shape, indicated by AFD = -2%. When power is reduced, the neutron flux and the xenon burnup rate decrease quickly, but iodine-135 continues to decay into xenon-135 with its approximately 6.6-hour half-life. Xenon therefore builds up preferentially in the region that had the higher flux before the transient, which is the bottom half in this case. The increased bottom-half xenon depresses bottom-half flux, shifting the flux shape upward and driving AFD positive. That is consistent with the observed swing from -2% to +7% over four hours. If no action is taken, xenon will continue to accumulate in the bottom half until the local xenon peak is established, and AFD may continue positive for a time. Later, as the xenon decays and the flux redistribution changes the local burnup and production rates, the perturbation will reverse and AFD can swing negative. This is the classic axial xenon oscillation: the initial tilt is not a simple reactivity effect that can be trimmed once with rods; it is a spatial xenon redistribution that evolves over many hours.

The +7% AFD exceeds the plus or minus 5% technical specification limit, so the crew cannot simply ignore it; the required action to restore AFD within 1 hour or reduce power must be met. However, inserting 30 steps as a single aggressive move is a poor choice. Control rods enter from the top, so a large insertion will sharply reduce top-half flux, create a local flux depression, and change the xenon production and burnup pattern in a way that can amplify the next half of the oscillation. It may bring AFD below +5% quickly, but it risks a larger negative AFD later when the xenon distribution reverses. A better approach is to use small, measured rod moves with pauses to observe the AFD response, or to reduce power as allowed by the technical specification if AFD cannot be controlled smoothly. Boron is not a good tool for rapid AFD correction because it is spatially uniform and mixes slowly; it changes bulk reactivity rather than directly correcting axial shape. The supervisor should direct verification of AFD with incore and excore indications, confirmation that rod position is as indicated, and a controlled rod motion strategy that restores compliance without chasing the xenon transient. The crew should also anticipate the later reversal and plan rod withdrawal rather than continuing to insert.

Boron worth is -7 pcm/ppm, so 50 pcm requires 50/7 = 7.1 ppm of boron concentration change. If reactivity must be added, boron would be diluted by about 7 ppm; if reactivity must be removed, boron would be added by about 7 ppm. The calculation assumes the boron worth is constant over that small range, which is reasonable for a planning estimate but should be verified with the cycle-specific curve. Boron is slower than rods because it must be injected, mixed, and sampled, and the RCS turnover time delays its effect. It is also spatially uniform, so it does not directly correct an axial flux tilt. That is why boron is preferable for bulk reactivity changes and long-term xenon compensation, but not for a fast AFD excursion that must be corrected within 1 hour. The correct division of labor is to use controlled rod motion or power reduction for the AFD technical specification action, and use boron for the slower bulk reactivity balance so that the rods do not remain deeply inserted and distort the axial flux shape.