Xenon poisoning (negative xenon load varies with core burnup)

[Sanjay94]

TL;DR Summary

Variation in negative xenon load w.r.t core burnup

I would like to know how the negative xenon load varies with core burnup. Does it increase or decrease as the core progresses from beginning of life to end of life? What is the reason for this change?

Any help will be much appreciated

 

[Astronuc]

Summary:: Variation in negative xenon load w.r.t core burnup

I would like to know how the negative xenon load varies with core burnup. Does it increase or decrease as the core progresses from beginning of life to end of life? What is the reason for this change?

I-135 and Xe-135 reach an equilibrium level during operation of a reactor. When power is decreased, e.g., stepwise, meaning that the neutron flux decreases, then more I-135 decays producing a rise in Xe-135. The effect is known as Xe-poisoning, and must be considered in power maneuvers in a reactor. The amount of Xe-135 depends on the power level in the reactor and the magnitude of power decrease. At the same time, with burnup, the fissile inventory gradually decreases (by depletion), and so for a given amount of Xe-135, the poisoning effect is more significant, until one gets to the end of a cycle, or operating period, and there is insufficient fissile inventory to continue operation. Note that with depletion of fissile species, there is also accumulation of other fission products that also absorb neutrons.

See - http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/xenon.html

One of the extraordinary sequences in the operation of a fission reaction is that of the production of iodine-135 as a fission product and its subsequent decay into xenon-135. Iodine-135 is a rather common fission product, reportedly amounting to up to 6% of the fission products. It has a rather small probability for absorbing a neutron, so it is not in itself a significant factor in the reaction rate control. But it has a half-life of about 6.7 hours and decays into xenon-135 (half-life 9.2 hours). The xenon-135 has a very large cross-section for neutron absorption, about 3 million barns under reactor conditions! This compares to 400-600 barns for the uranium fission event.

In the normal operation of a nuclear reactor, the presence of the xenon-135 is dealt with in the balancing of the reaction rate. Iodine-135 is produced, decays into xenon-135 which absorbs neutrons and is therby "burned away" in the established balance of the operating conditions. There is an equilibrium concentration of both iodine-135 and xenon-135.