Iodine neutron absorption

The neutron absorptions of tellurium, iodine, caesium and barium are relatively unimportant, but the neutron capture cross-section of radioactive xenon 135 has been measured to be about 2,500,000 X 10 < cm., many times larger than that of stable gadolinium, for example, the cross-section of which is about 30,000 X cm.. Upon... 

The most important fission product poison is xenon-135. Xenon-135 is formed directly as a fission product and by decay of iodine-135. Xenon-135 is lost by radioactive decay and by neutron absorption to become xenon-136 which is called burnout. 

Infra-red Gas chroma- tographic Ultra- violet Iodine titration lodo- metric 3- Alkaline Naphtho-titration quinoline Neutron absorption Freezing point mole%... 

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. 

In an attempt to determine how the actual number of resonance neutrons inside the furnace relative to those outside varied with temperature, caused by the different temperature-dependence of absorption by the carbon and other materials, two types of check experiments were made. At each temperature several bombardments were made of the cyclotron monitor, the 10-cm monitor, and the hot-dish monitor, but with no uranium present, leaving a hole in the graphite where its uranium oxide sphere was otherwise placed. These experiments were not very accurate, but they show that for a given bombardment of the furnace the number of neutrons absorbed by a hot iodine or Ga sample inside the hot furnace is the same within 8 percent as when the furnace is cold. 

The principal components of the stored activity for an 1100-MWe PWR are given in Table 12.7, which is taken from the APS study. The top line applies to the reactor immediately after shutdown following a long period of operation at full power, while the remainder of the table illustrates the rate of decay of activity, as a function of time after shutdown. The activities of the iodine and bromine isotopes, and the noble gases, whose behavior is particularly important in the event of a major dispersal event, are shown separately, and the activity induced by neutron irradiation of the reactor structure and coolant is also listed. The absorption of jS particles and y rays in the fuel and cladding leads to a considerable generation of heat in the core the last column of the table gives the total thermal power produced in this way as a function of time after shutdown. 

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