Reactivity coefficient

In nuclear engineering, a reactivity coefficient describes the change in the reactivity of a reactor caused by a change in another size . So it is a differential quotient ${\displaystyle \xi }$ ${\displaystyle \rho }$ ${\displaystyle x}$

${\displaystyle \xi ={\frac {d\rho }{dx}}}$.

If the volume in the coolant is missing , this is referred to as the vapor bubble coefficient (more generally: coolant loss coefficient or void coefficient ). If the temperature is called the temperature coefficient ; this is usually Doppler coefficient named because the effect mainly due to the Doppler broadening of resonances in the cross section of neutron capture in ²³⁸ U comes about. Further reactivity coefficients can be defined in this way. ${\displaystyle x}$ ${\displaystyle x}$ ${\displaystyle \xi }$

A reactivity coefficient is generally not a constant because the function is usually not linear. Rather, the reactivity coefficient itself depends on the value of the respective influencing variable (and usually on other parameters as well). If only a single value of the respective coefficient is given for a reactor, it usually refers to the normal operating condition. Often the sign of the reactivity coefficient is of particular interest, i.e. H. whether an increase in the influencing variable reduces or increases reactivity. In terms of reactor safety, the aim or requirement of the two reactivity coefficients described above is that they are negative in all operating states of the reactor . ${\displaystyle \rho (x)}$

With a sufficiently large negative temperature coefficient one provides z. B. ensure that the (possibly unintentional) rise in temperature, the reactivity drops and the possibly supercritical reactor thereby returns to criticality.

Some pulsed research reactors such as the TRIGA reactor make use of this. They are the only reactors that can even be brought to immediate supercriticality , since their large negative temperature coefficient reliably brings about a return to subcriticality after milliseconds. In the case of the Haigerloch research reactor , which had practically no control options, one relied on the reactivity limitation by the nuclear Doppler effect in the event that it had reached criticality.

Individual evidence

1. ↑ Safety rules of the KTA. (PDF) Nuclear Technology Standards Committee, October 1979, archived from the original on March 3, 2012 ; accessed on February 21, 2016 . 
2. ↑ Uwe Paul: The super-GAU. A critical examination of the possible consequences. March 15, 2011, accessed January 2, 2015 . 
3. ↑ Physical explanation of the TRIGA principle ( Memento from January 10, 2015 in the Internet Archive )
4. ↑ W. Heisenberg, K. Wirtz: Large tests to prepare the construction of a uranium burner. In: Naturforschung und Medizin in Deutschland 1939 - 1946. Edition of the FIAT Review of German Science intended for Germany , Vol. 14 Part II (Eds. W. Bothe and S. Flügge), Wiesbaden: Dieterich. Also printed in: Stadt Haigerloch (ed.): Atommuseum Haigerloch , Eigenverlag, 1982, pages 43–65, here p. 63