XENON-135 and SAMARIUM-149 POISONING ???

Yes my fellow friends.,.Xenon-135 and Samarium-149 was the fission products poisons !!

Believe it or not Xenon-135 has a cross-section of approximately 3,000,000 barns, over 4000 times

that of U-235. That is, each atom of xenon-135 absorbs as many neutrons as 4000 U-235 atoms. About 6.6% of all fissions produce a nuclide of Xe-135, either directly as a fission product or indirectly as a

fission product daughter. Xenon is a major problem in our reactors because of its remarkable neutron absorption and high yield.

Xenon Production:

Xe-135 is produced directly in only 0.3% of all U-235 fissions. The

following example is typical:

Did you know that Xenon-135 is mainly produced as a fission product daughter, by iodine

decay as follows:

Iodine-135 = 6.3% of U-235 fissions.

xenon production =95% (6.3/6.6 = 0.95).  (banyak racun !!)

You should know that Iodine-135 does not absorb neutrons.,.,BECAUSE  many fission products absorb neutrons. Most absorption cross-sections are small and are not important in short-term operation....=)

Xenon Loss:

Xenon is removed from the reactor by decay as follows:

or by neutron absorption (radiative capture):

The rate of burnout depends on the neutron flux. For a CANDU at full\power, neutron absorption accounts for about 90% of Xe-135 loss, decay for only 10%. Cesium-135 and xenon-136 do not absorb neutrons.

Equilibrium Xenon Load:

Guys,you need also to know that Xenon slowly builds to an equilibrium level after the reactor is started because there is no xenon in the fuel of a reactor that has been shut down for a
long time (or has never been operated)..

The equilibrium level depends on the steady state reactor power.

ILLUSTRATION OF XENON TRANSIENT TERMINOLOGY

 
· reactor start-up at time = 0 after a shutdown of one month
· reactor trip at t = 50 hours !!!!

see this graph to beleive: 

 

Samarium-149  and Other  Fission  Product Poisons:

 WHAT IS SAMARIUM-149 ?? Samarium-149 is produced directly from fission and from the decay of promethium-149 during reactor operation.   Samarium-149 is removed from the core by neutron absorption. 

Production  and  Removal  of  Samarium-149:

  

For your information,Samarium-149 is the second most important fission-product poison because of its high thermal neutron absorption cross section of 4.1 x 10⁴ barns.  Samarium-149 is produced from the decay of the neodymium-149 fission fragment as shown in the decay chain below. 

So guys,Why does this matter?? 

Because if we had it our way, we wouldn’t want any Xe-135 or Sm-149 gobbling up neutrons in our reactor. Neutrons they eat are neutrons that we can’t use to make energy by splitting fissile nuclides like Pu-239 or U-233.  

Xe-135 has two major differences from Sm-149. The first is that it is radioactive. It goes away if you leave it for awhile. It has a half-life of about nine hours.

While Sm-149 doesn’t go away. It is not radioactive. It is stable.