A modular water purification system for a nuclear power plant, the system comprising: a plurality of modules (100, 250, 270, 300, 320, 340, 350) that may be selectively connected together in a plurality of arrangements, wherein the plurality of modules includes: a pump module that includes an inlet and an outlet, and is configured to provide a fluid flow, at least two purification modules configured to be interchangeably or simultaneously connected to the pump module, and a fuel assembly bottom nozzle cleaning module comprising: a receptacle configured to engage a lower nozzle of a fuel assembly of the plant; an outlet in fluid communication with an interior of the receptacle, and configured to connect to an inlet (100a, 250a, 270a, 300a, 320a, 340a, 350a) of at least one of the plurality of modules so that the pump module causes a water flow that flows sequentially through the fuel assembly in a backwashing direction, into the receptacle, through the outlet of the fuel assembly bottom nozzle cleaning module, and on to the at least one of the plurality of modules; and at least one ultrasonic transducer mounted to the receptacle and shaped and configured to direct ultrasonic energy toward a lower nozzle of a fuel assembly that is engaged with the receptacle. a plurality of modules (100, 250, 270, 300, 320, 340, 350) that may be selectively connected together in a plurality of arrangements, wherein the plurality of modules includes: a pump module that includes an inlet and an outlet, and is configured to provide a fluid flow, at least two purification modules configured to be interchangeably or simultaneously connected to the pump module, and a fuel assembly bottom nozzle cleaning module comprising: a receptacle configured to engage a lower nozzle of a fuel assembly of the plant; an outlet in fluid communication with an interior of the receptacle, and configured to connect to an inlet (100a, 250a, 270a, 300a, 320a, 340a, 350a) of at least one of the plurality of modules so that the pump module causes a water flow that flows sequentially through the fuel assembly in a backwashing direction, into the receptacle, through the outlet of the fuel assembly bottom nozzle cleaning module, and on to the at least one of the plurality of modules; and at least one ultrasonic transducer mounted to the receptacle and shaped and configured to direct ultrasonic energy toward a lower nozzle of a fuel assembly that is engaged with the receptacle. a pump module that includes an inlet and an outlet, and is configured to provide a fluid flow, at least two purification modules configured to be interchangeably or simultaneously connected to the pump module, and a fuel assembly bottom nozzle cleaning module comprising: a receptacle configured to engage a lower nozzle of a fuel assembly of the plant; an outlet in fluid communication with an interior of the receptacle, and configured to connect to an inlet (100a, 250a, 270a, 300a, 320a, 340a, 350a) of at least one of the plurality of modules so that the pump module causes a water flow that flows sequentially through the fuel assembly in a backwashing direction, into the receptacle, through the outlet of the fuel assembly bottom nozzle cleaning module, and on to the at least one of the plurality of modules; and at least one ultrasonic transducer mounted to the receptacle and shaped and configured to direct ultrasonic energy toward a lower nozzle of a fuel assembly that is engaged with the receptacle. a receptacle configured to engage a lower nozzle of a fuel assembly of the plant; an outlet in fluid communication with an interior of the receptacle, and configured to connect to an inlet (100a, 250a, 270a, 300a, 320a, 340a, 350a) of at least one of the plurality of modules so that the pump module causes a water flow that flows sequentially through the fuel assembly in a backwashing direction, into the receptacle, through the outlet of the fuel assembly bottom nozzle cleaning module, and on to the at least one of the plurality of modules; and at least one ultrasonic transducer mounted to the receptacle and shaped and configured to direct ultrasonic energy toward a lower nozzle of a fuel assembly that is engaged with the receptacle.