RATHER less than a year has passed since the Atomic Energy Research Establishment at Harwell put into operation ‘Gleep', the first atomic pile to be built in Great Britain. Now a second and more powerful pile has been completed and put into operation. Its rated output is 6,000 kW., and it has been designed primarily as an experimental tool, to provide as many facilities as possible without unduly complicating the engineering of the structure. In addition, when operating at rated output, the pile should be able to produce, by transmutation of inactive elements, all the artificial radioactive isotopes required in Britain by medical and other research Workers. Like ‘Gleep', the new pile is a graphite-moderated air-cooled pile. The uranium rods are enclosed in aluminium cases which lie in channels in the graphite. Cooling air is drawn through the channels by several large electrically driven exhausters. The whole pile is surrounded by a concrete shield several feet thick, to protect Workers from radiations, and the cooling air is discharged up a chimney stack 200 ft. high. There are about forty holes in the shield, giving access to the strong fluxes of neutrons in the interior of the pile. These neutron fluxes, many times more intense than can be obtained in any other way, are required for many experiments in nuclear science, both in fundamental and in applied research. Thus, for example, the applied research programme will include an investigation of the effect of irradiation by neutrons on the properties of' materials, to provide information about materials used in the construction of piles. The experimental holes are also the means by which inactive elements* are placed in the pile for transmutation into radio-' active isotopes. Surrounding the pile structure are the control rooms and laboratories used by the scientific and operating staff. The operation of the pile is controlled by two sets of neutron-absorbing rods ; the first set of rods is adjusted to keep the pile operating at a constant-power level, and the second set is available to shut the pile down in any emergency. The basic calculations for the Harwell pile were begun in 1945 by a team working under Sir John Cockcroft in the laboratories of the National Research Council of Canada, and were continued when the team moved to the newly formed Atomic Energy Research Establishment at Harwell. It was the first task of the Department of Atomic Energy at Risley, under the direction of Mr. C. Hinton, to undertake the design of the pile, based on the information provided by the Harwell team. This work began in April 1946. The actual construction of the pile was made the responsibility of the Ministry of Works, assisted by their contractors, Messrs. Chivers.