A nuclear reactor system having a reactor vessel that houses a nuclear core in which fissile reactions take place within a pressure boundary of the reactor vessel and a control rod system for controlling the fissile reactions, the control rod system comprising: a control rod configured to move, within a sonically conductive coolant, into and out of the nuclear core , to control the fissile reactions; a drive system for driving the control rod into and out of the core ; and a control rod position monitoring system comprising: a sonar pulse generator configured to generate a series of sonar pulses; an antenna fixedly positioned within the pressure boundary, above an upper limit of travel of the control rod and configured to receive the sonar pulses generated by the sonar pulse generator and transmit the sonar pulses to the upper portion of the control rod and receive a reflected sonar signal from the control rod and communicate the reflected sonar signal to a selected location a spaced distance from the antenna outside of the pressure boundary; and a receiver positioned at the spaced distance from the antenna and configured to receive the reflected sonar signal and communicate the reflected sonar signal to a processing unit that is configured to analyze the reflected sonar signal to determine a control rod position, wherein the sonar pulse generator is supported outside of the coolant and transmits the series of sonar pulses wirelessly to the antenna . a control rod configured to move, within a sonically conductive coolant, into and out of the nuclear core , to control the fissile reactions; a drive system for driving the control rod into and out of the core ; and a control rod position monitoring system comprising: a sonar pulse generator configured to generate a series of sonar pulses; an antenna fixedly positioned within the pressure boundary, above an upper limit of travel of the control rod and configured to receive the sonar pulses generated by the sonar pulse generator and transmit the sonar pulses to the upper portion of the control rod and receive a reflected sonar signal from the control rod and communicate the reflected sonar signal to a selected location a spaced distance from the antenna outside of the pressure boundary; and a receiver positioned at the spaced distance from the antenna and configured to receive the reflected sonar signal and communicate the reflected sonar signal to a processing unit that is configured to analyze the reflected sonar signal to determine a control rod position, wherein the sonar pulse generator is supported outside of the coolant and transmits the series of sonar pulses wirelessly to the antenna . a sonar pulse generator configured to generate a series of sonar pulses; an antenna fixedly positioned within the pressure boundary, above an upper limit of travel of the control rod and configured to receive the sonar pulses generated by the sonar pulse generator and transmit the sonar pulses to the upper portion of the control rod and receive a reflected sonar signal from the control rod and communicate the reflected sonar signal to a selected location a spaced distance from the antenna outside of the pressure boundary; and a receiver positioned at the spaced distance from the antenna and configured to receive the reflected sonar signal and communicate the reflected sonar signal to a processing unit that is configured to analyze the reflected sonar signal to determine a control rod position, wherein the sonar pulse generator is supported outside of the coolant and transmits the series of sonar pulses wirelessly to the antenna . <EOS>