The "Reactor Time Constant", also called the reactor period, is the time during which the neutron flux density in a reactor changes by the factor e = 2.718 (e: basis of natural logarithms), when the neutron flux density increases or decreases exponentially.
Applicable units are those of quantitykind:Time
https://www.euronuclear.org/glossary/reactor-time-constant/
The "Reactor Time Constant", also called the reactor period, is the time during which the neutron flux density in a reactor changes by the factor e = 2.718 (e: basis of natural logarithms), when the neutron flux density increases or decreases exponentially.
http://www.iso.org/iso/catalogue_detail?csnumber=31895