A nuclear reactor system comprising: a pressure vessel housing a nuclear core and a volume of coolant within the nuclear core; plurality of fuel assemblies housed within the nuclear core with at least some of the fuel assemblies having one or more primarily gamma sensitive, prompt responding self-powered neutron detectors located therein, responsive to a fission reaction activity within the core to provide an output current indicative thereof, the fission reaction activity comprises a first fraction caused by gamma radiation released by fission products and a second fraction of the measured signal caused by fission events; characterised by: a subcritical reactivity monitor configured to receive the output from the one or more of the primarily gamma sensitive, self-powered neutron detectors and to responsively generate a core average value of a gamma radiation distribution within the core; a processing system configured to calculate a value of K eff in accordance with the following equation; K eff t = 1 − f t Ref I Ref f t I t 1 − K Ref; and to output the value of K eff; and a Chemical Volume Control System configured to receive the value of K eff and initiate a triggering signal in order to add Boron to the coolant in the event a predetermined, undesirable change in K eff is detected, wherein t Ref is the time after shutdown, t is the time after t Ref, wherein a ratio of f(t ref)/f(t) is a ratio of the first fraction at time t to the first fraction at time t ref based on a time dependent fission product gamma decay relation, I(t) is the output current of the self-powered neutron detector at time t, I Ref is the output current of the self-powered neutron detector at time t Ref, and K Ref is a neutron multiplication factor at time t Ref. a pressure vessel housing a nuclear core and a volume of coolant within the nuclear core; plurality of fuel assemblies housed within the nuclear core with at least some of the fuel assemblies having one or more primarily gamma sensitive, prompt responding self-powered neutron detectors located therein, responsive to a fission reaction activity within the core to provide an output current indicative thereof, the fission reaction activity comprises a first fraction caused by gamma radiation released by fission products and a second fraction of the measured signal caused by fission events; characterised by: a subcritical reactivity monitor configured to receive the output from the one or more of the primarily gamma sensitive, self-powered neutron detectors and to responsively generate a core average value of a gamma radiation distribution within the core; a processing system configured to calculate a value of K eff in accordance with the following equation; K eff t = 1 − f t Ref I Ref f t I t 1 − K Ref; and to output the value of K eff; and a Chemical Volume Control System configured to receive the value of K eff and initiate a triggering signal in order to add Boron to the coolant in the event a predetermined, undesirable change in K eff is detected, wherein t Ref is the time after shutdown, t is the time after t Ref, wherein a ratio of f(t ref)/f(t) is a ratio of the first fraction at time t to the first fraction at time t ref based on a time dependent fission product gamma decay relation, I(t) is the output current of the self-powered neutron detector at time t, I Ref is the output current of the self-powered neutron detector at time t Ref, and K Ref is a neutron multiplication factor at time t Ref. a subcritical reactivity monitor configured to receive the output from the one or more of the primarily gamma sensitive, self-powered neutron detectors and to responsively generate a core average value of a gamma radiation distribution within the core; a processing system configured to calculate a value of K eff in accordance with the following equation; K eff t = 1 − f t Ref I Ref f t I t 1 − K Ref; and to output the value of K eff; and a Chemical Volume Control System configured to receive the value of K eff and initiate a triggering signal in order to add Boron to the coolant in the event a predetermined, undesirable change in K eff is detected, wherein t Ref is the time after shutdown, t is the time after t Ref, wherein a ratio of f(t ref)/f(t) is a ratio of the first fraction at time t to the first fraction at time t ref based on a time dependent fission product gamma decay relation, I(t) is the output current of the self-powered neutron detector at time t, I Ref is the output current of the self-powered neutron detector at time t Ref, and K Ref is a neutron multiplication factor at time t Ref.