Reactivity (nuclear technology)

The reactivity , a quantity of the dimension number (formerly called dimensionless quantity ) in reactor physics and nuclear technology , is a measure of the deviation of the multiplication factor k from the value k  = 1. Like the multiplication factor, it describes the criticality of a fissile material arrangement, e.g. B. a nuclear reactor .

definition

The reactivity (Greek rho ) is defined as: ${\displaystyle \rho }$

${\displaystyle \rho ={\frac {k-1}{k}}}$

The following applies:

${\displaystyle \rho }$<0 corresponds to <1, the reactor is subcritical. ${\displaystyle k}$

${\displaystyle \rho }$= 0 corresponds to = 1, the reactor is critical. ${\displaystyle k}$

${\displaystyle \rho }$> 0 corresponds to > 1, the reactor is supercritical. ${\displaystyle k}$

Compared to the multiplication factor, reactivity has the advantage that it is approximately additive and therefore the more descriptive quantity. If, for example, two absorber rods with certain reactivity values ​​are inserted into the reactor core, the total reactivity is reduced by the sum of these values. Instead of a change in reactivity, one often speaks in practice of the supply of positive or negative reactivity to the reactor.

Reactivity measures

The reactivity is z. B. in percent or in dollars or cents (see criticality ). These are auxiliary units of measurement . The reactivity measures dollar and cents are particularly clear for the practitioner because 1 dollar = 100 cents means the distance between the states , which is important for safety, delayed critical and prompt critical .

Reactivity coefficient

→ Main article : reactivity coefficient

A reactivity coefficient indicates how reactivity is influenced by changing a certain other variable . Mathematically it is the differential quotient . Important examples: ${\displaystyle \rho }$ ${\displaystyle x}$ ${\displaystyle {\frac {d\rho }{dx}}}$

• the Doppler coefficient describes the influence of temperature,
• the coolant loss or vapor bubble coefficient describes the influence of the proportion of voids in the coolant.

Other reactivity coefficients are also defined in the literature as required.

Reactivity incident

Reactivity accident is a general term for reactor accidents , the triggering event of which is an unintentional or recklessly induced increase in reactivity. A reactivity incident known worldwide is the Chernobyl nuclear disaster on April 26, 1986.

A dangerous increase in reactivity can also occur as an indirect consequence of faults triggered differently (cascading faults); this is technically not counted as reactivity accidents.

Individual evidence

1. ↑ Dieter Smidt: Reaktortechnik , Vol. 1, Karlsruhe 1976, ISBN 3-7650-2018-4
2. ↑ Dieter Emendörfer, Karl-Heinz Höcker: Theory of Nuclear Reactors , Vol. 1, Mannheim / Vienna / Zurich 1982, ISBN 3-411-01599-3
3. ↑ Safety rules of the KTA. (PDF) Nuclear Technology Standards Committee, October 1979, archived from the original on March 3, 2012 ; accessed on December 5, 2018 .