Xenon-135

As xenon poisoning is called a high concentration of the neutron poison xenon -135 in an operating nuclear reactor with thermal neutrons. It occurs in a power reduction, and the shutdown of the reactor and prevents for a certain time, a re- start-up performance.

Explanation

When nuclear fission in nuclear fuel produced, among other iodine -135 ( 135I ). This decays with a half-life of 6.6 hours also radioactive xenon -135 ( 135Xe ). Its capture cross section for thermal neutrons is 2.65 million barn, orders of magnitude more than typical capture cross sections of neighboring nuclides. Through the capture of neutrons the 135Xe is degraded to stable 136Xe.

In steady-state operation of the reactor is a balance between production and degradation of 135Xe sets. After the shutdown, however, ( virtually ) no more neutrons are released on the one hand, on the other hand 135Xe is simulated at the same rate through the existing 135I for some time as before the power reduction. Provided that, the reactor was previously at full rated power, the neutron absorption of the 135Xe gives such a high negative reactivity that a high driving even in all existing Reaktivitätsreserven positive, i.e., all control rods drawn, is no longer possible. Re- criticality is again only after one to two days reached when the 135Xe concentration has decreased sufficiently by the decay with its half-life of 9.2 hours.

Effects of the Chernobyl disaster

135Xe played an important role in the Chernobyl disaster. When preparing an experiment the performance of the reactor was contrary to the provisions of the Operations Manual strongly throttled a long time, which led to a xenon poisoning. To increase the power back on, the control rods have been pulled out far, but remained at first without effect due to the xenon poisoning. When after closing the steam supply to the turbine of the neutron flux for a short time increase due to the positive void coefficient, thereby 135Xe was reduced, which greatly increased the reactor power and thus the further reduction of 135Xe. This rapid increase in performance was no longer to be supported by the insertion of the control rods, and eventually led to an explosion of the reactor core.