Xenon override

The internal recirculation pumps are provided with more than 10% excess, flow rate capacity, which allows xenon override, and the fine motion control rod drives and the grey-tipped control blades allow control rod movements at full power. The excess pump capacity is utilized for hydraulic spectral shift operation the core coolant flow is increased towards the end of the operating cycle. The built-in "redundanc> " also implies that the reactor can be operated at full power even if one pump should fail. 

Xenon override capability Because the steam void reactivity effect is large compared with xenon reactivity, the BWR core has the excellent capability of overriding the xenon effect, thereby increasing power after a power decrease. 

Ultimately, bundles can be removed from the channel during on-power refueling and reshuffled, and reinserted in any order. This axial shuffling provides nearly unlimited capability for shaping the axial power distribution, if necessary. Adjuster rods are located interstitially between fuel channels, in the low-pressure moderator. They flatten the power distribution with NUE fuel, a function not required with enriched fuel, and provide xenon-override capability. With an enriched fuel, the adjuster rods can be easily replaced, if desired, or even eliminated, providing further flexibility in accommodating advanced fuel cycles. 

Although the postshutdown concentration of samarium can be a serious consideration in some reactor situations, the control of the xenon concentration is a problem of more general importance in all thermal reactors. This problem is especially serious when the I concentration at shutdown is relatively large. Under these circumstances the Xe concentration increases rapidly after shutdown and can reach appreciable magnitudes before decaying away. This buildup is important from the standpoint of control, since a large positive reactivity change must be available if it is to be required that there be a restart of the reactor around the time the maximum occurs. Thus special xenon-override control rods would be required in a mobile nuclear power plant if short-time startups were to be an essential operational feature. 

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. 

If this explanation made sense, then an inspection of the chart of nuclei showed that the mother had to be 6.68 hr [iodine] and the daughter 9.13 hr [xenon]Within an hour Fermi arrived with detailed reactivity data which checked this assignment. Within three hours two additional conclusions were clear, (a) The cross section for absorption of thermal neutrons by Xe was roughly 150 times that of the most absorptive nucleus previously known, [cadmium], (b) Almost every Xe nucleus formed in a high flux reactor would take a neutron out of circulation. Xenon had thrust itself in as an unexpected and unwanted extra control rod. To override this poison more reactivity was needed. 

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. 

Ideal extremal path if sufficient built-in reactivity ( 62.5) is available to override Xenon ot onset of first optimal control pulse... 

Constraint if built-in eoctivity of 375 is available to override Xenon... 

Because ot the large negative moderator density (void) coetficient ot reactivity, the BWR has a number ot inherent advantages. These are the use ot coolant flow as opposed to control rods tor load following, the inherent selt-flattening ot the radial power distribution, the ease ot control, the spatial xenon stability, and the ability to override xenon to follow load. 

Answer After a pile is started and its power level is held at a constant level, its potential reactivity (excess in rods) w ill increase for a period of several hours until the xenon formation rate from iodine decay balances, and then overrides, xenon burnout and decay plus graphite heating effects. This point of maxim jm reactivity" is called "t ornarc und. ... 

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. 

The steam blower is therefore used for the reactor start-up. Start-up with reduced coolant density also helps in overriding the xenon peak, at least to a certain extent. 

The mass-133 chain represents a fission reactor problem. Iodine-133 with a 21-hour half-life decays to 5-day xenon-133, which has a high cross section for thermal neutrons. This was discovered when the first Hanford reactors unexpectedly shut down after a few hours operation. Fortunately, these reactors were built with provision for the extra fuel that could override the xenon effect. However, when a reactor is shut down, the xenon-133 grows in from its 1-133 parent. Unless it is started up again in a few hours, several days must pass before the Xe-133 has decayed enough to allow restart. 

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