135Xe

Verify the statement that the reactor poison 135Xe reaches a maximum 10h after shutdown of a high flux (> 1014 n/cm2s) reactor. 

Ans. 139Cs - p + 139Ba 93Rb + 93Sr 135I -> p + 135Xe 96Y - / + 96Zr... 

In neutron irradiation, the yield of 13<5Xe may be significantly enhanced due to neutron capture on 135Xe (isobaric yield 6.5%, half-life 9hr, thermal neutron cross section 3.6 x 106 bams). 

The sample should be processed quickly because of the short half-life of 135Xe (9.10 h) compared to133 Xe (5.234 d). The ARSA performs three sample analyses per day. The fair match of this frequency to the resolving time (6 h) of meteorological measurements made worldwide facilitates coordination with atmospheric transport models. The rapid analysis capability allowed the measurement of 135Xe in the Earth s atmosphere, see Fig. 15.9 (Bowyer et al., 1999). 

This system suppresses radon by several orders of magnitude, but the fact that every atom of radon is radioactive means that a specialized radiation coincidence detection apparatus is still required to resolve radioxenons from radon. Moreover, the radiations emitted by the two xenon radioisotopes must be resolved by spectral analysis. This approach yields an impressive sensitivity of about 100 pBq (0.006 dpm or 3,000 atoms of 135Xe) per m3 of air based on sampling of about 40 m3 of air sample and a 24 h radiometric measurement. 

The continuous removal of the noble gas xenon means that there is no reactor dead time after shutdown or a power decrease—a phenomenon that most solid-fueled reactors must deal with due to the production of 135Xe from the decay of 1351. As well, there needs to be no excess reactivity in place to deal with such situations. 

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