Xenon transient

Axial Xenon Transient - Equilibrium Cycle (Fuel Zoned Axially -... 

Figure 4.2-11 shows the axial power distribution for an equilibrium cycle. This distribution indicates 65 percent of the power in the top zone, 25 percent in the middle zone, and 10 percent in the bottom zone. This distribution is expected to minimize peak fuel temperatures. The selection of the active core height of ten fuel elements was made to yield a maximum power rating while maintaining an axial power shape that remains stable with burnup and stable to axial xenon transients. 

AXIAL XENON TRANSIENT -EQUILIBRIUM CYCLE (FUEL ZONED AXIALLY - TEMPERATURE COEFFICIENT 3.5x10 5 rc)... 

If the reactor is shut down, however, a second aspect occurs. The xenon is now not being removed by neutron interaction but is still being produced, faster than it decays, by the decay of the previously produced iodine. Thus the xenon concentration rises sharply to several times its previous value (depending on the operating power level) before it subsequently decays with the drop in iodine production after such a shutdown. If the reactor is to be restarted in this interval, of 20-40 h, yet further reactivity will be needed. If the reactor is partially shut down, a partial xenon transient can occur, calling for addition and subsequent removal of reactivity to compensate if the reactor is to continue to be operated. 

The control rod calibration problem under study in the present discussion is concerned with a special situation where it is desired to calibrate a control rod during a xenon transient. What is meant by a xenon transient is explained briefly in what follows. When a reactor is in operation, certain nuclei with large neutron absorption cross sections are produced, so that they act as poisons. Of these poisons, xenon-135 is the most troublesome. In a reactor operating at power a balance is eventually achieved between rates of formation and loss of the absorbing nuclei, so that an equilibrium concentration is attained in the reactor. However, when a reactor operating at power is shut down, the xenon continues to increase [1, p. 335] without a sufficient neutron flux available to hum out the xenon, so to speak. Thus, the xenon will eventually disappear by radioactive decay, but not before it builds up to a maximum of substantial proportions. The maximum concentration will occur at about 12 hours after shut-down, the magnitude of the peak concentration depending on the power level before shut-down. This explains why, whenever it is necessary to be able to restart a reactor at any time after shutdown (e.g., a submarine reactor), the reactor must be sufficiently fueled so that it is possible to override maximum xenon at any time. 

The present study represents an attempt to study the problem of control rod calibration during a xenon transient from a purely analytic point of view and then correlate theory with experiment to obtain the desired results in the best possible approximations. Particulars of the problem are described as follows. Suppose that a thermal reactor, fueled homogeneously with uranium-235, is maintained at criticality during the rising phase of a xenon transient by the continuous motion of a control rod. Then a control rod (or set of control rods) is suddenly pulled at = o- The approximate subsequent behavior of the flux has already been described. It is desired to find the reactivity thus introduced by pulling the rod x inches. 

Suppose now that a rod calibration experiment is initiated at = 0 when the reactor is just critical during the rising phase of a xenon transient. The realistic approximation is then made that the xenon increases in a linear fashion for the next few minutes following the initiation of the experiment. Thus,... 

Figure 4. Flux during a xenon transient with instantaneous rod pull.

The principal difficulty associated with the method outlined above, for a precise calibration of a control rod during a xenon transient, is of course the problem of the simultaneous solution of the two very complicated transcendental equations involved. However, this problem could be greatly simplified with the aid of a large scale digital computing device. 

To illustrate the concept of the target manifold, in this example a target curve, it is convenient to picture the behavior in the xenon-iodine phase plane (Fig. 1). For specified fluxes, trajectories in the phase plane trace the state of the system from given initial conditions. In particular, if the flux is put to zero at the end of the control period, a series of shutdown curves can be traced, covering different terminal points, to represent the xenon transient or shutdown peaking. Each such curve in Fig. 1 has a... 

Figure 6 3 2 1 Is a plat of the xenon transient and is identical to Figure C-8 of HJf-71 8 V0L2.

Answer (1) To control the approach to critical, (2) to adjust and control rate of rise, (3) for minute-to-minute adjustment of multiplication. factor during equilibrium operation, (4) compensates for reactivity transients that is, xenon transients, tesqperature effects, turnaround, etc., (5) to control heat distribution, and (6) to shut down the reactor quickly under controlled conditions. 

Using Xenon Transient Table (2), or carves in Figure 11, give the foll /. lnc ... 

How does control rod movement affect the xenon transient ... 

Answer Adding or subtracting an increment of poison by control rod adjustment results in absorbing more or less neutrons and a decrease cr increase In the local power or fl ux In one of the small pile" regions. Xenon transients resulting from an Increased rate of b ornout when flux is increased, or Increased rate of buildup when flux is decreased, v lll Induce f urther local fl ox changes unless the transient effect is controlled locally. 

Why are significant xenon transients hard to demonstrate in the UWNR (1.0)... 

A period of up to three days must elapse before the reactivity returns to the value it had before the shutdown. If it is a requirement that it be possible to start the reactor up again at any time during this period, a high percentage of excess reactivity has to be built into it in order to overcome the xenon transient. This is known as xenon override capacity. In normal operation this built-in reactivity excess has to be held down by a large equivalent negative reactivity in the form of control rods or some other mechanism. 

For a natural uranium reactor, where the possible excess reactivity is limited by the low value, it may not be possible to overcome the peak in the transient, and it will then be necessary to wait for some time after the peak is reached before the reactor can be started up again. One way of avoiding such a situation, employed in the CANDU heavy water reactor, is the use of booster rods of enriched uranium which can be inserted into the core to provide a temporary increase in reactivity. For any reactor with a limited capacity for xenon override, it is desirable to restart after an unscheduled reactor trip as soon as possible, before the xenon transient has had a chance to build up. The re-establishment of the xenon burn-out due to restoring the operational flux level then allows the equilibrium xenon concentration to be regained without any large change in the reactivity having taken place. 

An additional facility, used both for adjusting the flux shape for optimum peak-to-average ratio and to overcome the xenon transient following a trip from power, consists of 18 adjuster rods which may be inserted vertically between the calandria tubes. Cobalt is used as the absorber in the adjuster rods because of the commercial value of the radioisotope produced by irradiation. An alternative method of overcoming the xenon transient, which has been adopted for the subsequent Bruce generating station, is the use of booster rods containing enriched fuel. 

S. The computer could be used for the automatic control of moderator boron concentration from measurements of neutron flux and moderator height. This would assist in overriding xenon transients and minimize the amount of boron addition and removal. 

Velocities of the order of ten pcm/day with overall reactivity variations of some thousand pcm are needed to compensate fuel burn-up. A few pcm/s in the same range of reactivity variations are required by large load variations and xenon transients. 

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