"Reactivity" characterizes the deflection of reactor from the critical state.
Applicable units are those of quantitykind:Reactivity
http://en.wikipedia.org/wiki/Nuclear_chain_reaction
"Reactivity" characterizes the deflection of reactor from the critical state.
$\rho = \frac{k_{eff} - 1}{k_{eff}}$, where $k_{eff}$ is the effective multiplication factor.
http://www.iso.org/iso/catalogue_detail?csnumber=31895