Movement of fuel tubes within an array

A method of operating a nuclear fission reactor, the reactor comprising a reactor core, and a coolant tank containing coolant, the reactor core comprising an array of fuel assemblies, each fuel assembly extending generally vertically and comprising one or more fuel tubes containing fissile fuel, the fuel tubes being immersed in the coolant, the method comprising:

• monitoring and/or modelling fuel concentrations and/or fission rates in each of the fuel assemblies;
• in dependence upon results of the monitoring and/or modelling, moving fuel assemblies horizontally within the array, without lifting any of the fuel assemblies from the array of fuel assemblies, in order to control fission rates in the reactor core;
• moving a spent fuel assembly to a horizontal periphery of the array without lifting the fuel assembly from the array by moving a row or part row of the array of fuel assemblies towards the periphery of the array;
• and
• removing the spent fuel assembly from the horizontal periphery of the array into a spent fuel storage area within the coolant tank without lifting the spent fuel assembly from the coolant.

A nuclear fission reactor, the reactor comprising a core, a coolant tank containing coolant, a fuel assembly moving unit, and a reactor core controller, wherein:

• the core comprises an array of fuel assemblies, each fuel assembly extending generally vertically and comprising one or more fuel tubes containing fissile fuel;
• the fuel tubes are immersed in the coolant;
• the fuel assembly moving unit is configured:
• to move fuel assemblies horizontally within the array without lifting any of the fuel assemblies from the array of fuel assemblies;
• to move a spent fuel assembly to a horizontal periphery of the array without lifting the fuel assembly from the array by moving a row or part row of the array of fuel assemblies towards the periphery of the array;
• and
• to remove the spent fuel assembly from the horizontal periphery of the array into a spent fuel storage area within the coolant tank without lifting the spent fuel assembly from the coolant and
• the reactor core controller is configured to determine a new configuration of the fuel assembly units, and to cause the fuel assembly moving unit to move fuel assemblies in order to achieve the new configuration.