Poisoning with xenon

In a thermal Nuclear reactor, the fission produced, with a delay of a few hours, Xenon 135 (¹³⁵Xe). This one is quickly degraded by absorbing Neutron S of fission. In normal weather, the production and degradation balance.

In the event of sharp decline of the quantity of fission, the produced neutrons are not enough any more with degradation of the ¹³⁵Xe, and this one continues to accumulate, fission product of the previous hours. Thereafter, if one tries to increase the quantity of fission (by withdrawing the graphite bars), rise to power does not follow, because the neutrons are absorbed by the ¹³⁵Xe which plays the moderating part of then in the place of graphite: it is the xenon poisening . It takes many hours to burn the ¹³⁵Xe in excess and to return to a normal neutron balance.

This phenomenon in particular occurred in engine 4 of the Nuclear plant of Tchernobyl. A bad knowledge of this phenomenon as well as a non-observance of the security instructions on the extraction of the control rods neutron caused the fusion of the engine the April 26th 1986.

The chain Xe

Xénon¹³⁵ is a Radionucléide (half-life: 9,17h). Xénon¹³⁵ with largest the cross Section known in the field of the thermal neutrons: 3 million Barn S.

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Starting, balance and reactor shut down

With the starting of a thermal nuclear reactor, the concentration in ¹³⁵Xe is null. The Xenon concentration gradually will increase (if the neutron flux is constant) until reaching a value of balance at the end of one day or two. Fact, of its important cross section, the ¹³⁵Xe absorbs neutrons by capture, from where the term “xenon poisening”. Under stable operation, this poisoning is proportional to the neutron flux of the heart. This poisoning under operation is about 3000 Pcm for an engine REFERENCE MARK of EDF. With the reactor shut down, the d'¹³⁵I production also stops, as well as the consumption of the ¹³⁵Xe by the reaction σ. The “tank d'¹³⁵I” is transformed then still into ¹³⁵Xe. Consequently the Xénon level increases (as well as associated negative reactivity). The level of the Xénon tank thus passes by a maximum (when the ¹³⁵I completely changed), then tends towards 0 by decrease (t_1/2=9 H). When the negative reactivity brought by the ¹³⁵Xe to the “peak Xénon” is too important and that the built-in reactivity are insufficient (for example at the end of the lifetime engine) the engine cannot any more rediverger during a few hours.

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