Xenon poisoning

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As xenon poisoning is called a high concentration of a neutron absorber acting ( "neutron poison") xenon -135 in one with thermal neutrons operating nuclear reactor . It occurs when the power is throttled and when the reactor is switched off and prevents the power from starting up again for a certain period of time.

Explanation

During nuclear fission, iodine- 135 is produced in the nuclear fuel . 135 I decays with a half-life of 6.6 hours to form xenon -135, which is also radioactive , a noble gas. 135 Xe has a half-life of 9.2 hours.

The capture cross-section of 135 Xe for thermal neutrons is 2.65 million barn , which is several orders of magnitude more than the typical capture cross-sections of neighboring nuclides . By neutron capture is 135 Xe in the stable 136 Xe converted. In steady-state operation of the reactor, an equilibrium is established between the generation and degradation of 135 Xe.

After the shutdown, however, on the one hand (practically) no more neutrons are released, on the other hand, 135 Xe is reproduced for a longer period of time by the existing 135 I at approximately the same rate as before the power reduction. If the reactor was previously running at full rated power, the neutron absorption of the 135 Xe results in such a high negative reactivity that it can be started up even when all available positive reactivity reserves are used, i.e. H. with the control rods fully drawn , is no longer possible. Renewed criticality can only be achieved again after one or two days when the 135 Xe concentration has decreased sufficiently due to decay.

Impact of the Chernobyl disaster

135 Xe played an important role in the Chernobyl nuclear disaster . In preparation for an experiment, the reactor's output was greatly reduced for a long time, contrary to the instructions in the operating manual, which resulted in xenon poisoning. In order to increase the output again, the control rods were pulled out very far, but this initially had no effect due to the xenon poisoning. When the neutron flux increased for a short time after the steam supply to the turbine was closed due to the positive vapor bubble coefficient , 135 Xe was reduced, which increased the reactor output and thus the further reduction of 135 Xe. This rapid increase in power was one of the causes that led to the explosion of the reactor core.

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