A watt is the SI (standard scientific unit system) measure of power dissipation, that is, energy dissipated per unit time. 100kW is 100 kilowatts (100 000 watts) of power produced by the reactor, dissipated as heat.
I suspect the meaning intended was for 100kW-hr as a measure of quantity of heat generated naturally by the system.
Energy, quantity of heat, and Work are all expressed in units of Joules (N.m) in the SI system,
Power or Radiant Flux is measured in units of Watts (W) = J/s in the SI system.