In the same period of time a series of experiments has been performed by General Atomic. The models to calculate thermalization in graphite have been tested measuring the neutron spectrum in a block of graphite with the time-of-flight technique’6’ with various temperatures and poisoning. The Peach Bottom HTGR critical experiment which ran from 1959 to 1962 included measurements of temperature coefficient, flux distributions, control-rod worth, Th resonance integral and reactivity coefficients of various materials in a rather simple and clean system.<7) The initial criticality of the Peach Bottom reactor in 1966 provided also a good test of the calculational methods.

Criticality was achieved when 682 fuel elements were loaded in the core, the predicted value being 689 ±20 elements.<8)

AVR

In Germany the commissioning of the pebble-bed AVR reactor gave also the opportunity of testing calculations of criticality, temperature coefficient and control rod worth.<U) The agreement between experiments and calculations was good, but one must notice that pebble-bed systems are not very suited to an accurate test of calculational methods, especially because of the difficulty of exactly locating the position of the fuel elements in the core.

From the time of these older experiments the calculational methods have evolved somewhat, besides, in the case of the HTRs with prismatic fuel the core geometry changed considerably so that it was felt that new experiments were needed.