[en] This report presents the algebraic equation written in spherical geometry with an infinite reflector for the calculation and comparison of critical masses of a pile as a function of the vessel nature and thickness, and of radius values. It presents the iterative process adopted for the calculation, and discusses some peculiarities of this calculation. It reports matrix critical calculations, and calculations performed by using the Hassit method

[en] Program described in this document describes in detail the following experiments: critical experiments with two reactor core lattices with 38 and 44 fuel channels, initial heavy water level being 1300 mm, criticality is achieved by adding heavy water; preliminary analysis of heavy water quality and verification of the fuel isotopic contents; experiment with the initial core which contains 56 fuel channels with maximum heavy water level according to the Russian proposal; measurement of neutron flux by Dy and In foils; measurement of reactivity excess dependent on the heavy water level and number of fuel rods; measurement of reactor period for determined reactivity change; measurement of moderator temperature coefficient; measurement of absolute flux

[en] The buckling in the bare heavy water natural uranium critical assembly was determined by measuring the thermal neutron flux distribution. The obtained value for the critical buckling at the temperature of 20 deg C is: B² = (8.516 ± 0.02) m^-2. The above error is a statistical one, obtained from several series of measurements. The possible systematic error was estimated as 0.1 m^-2. (author)

[en] An important part of the experiments carried out on the reactor G1 during a period of shut-down has consisted in determinations of critical sizes, and measurements of flux distribution by irradiations of detectors. This report deals with the following points: 1- Critical sizes of the flat pile, the long pile and the uranium-thorium pile. 2- Flux charts of the same piles, and study of an exponential experiment. 3- Determination of the slit effect. 4- Calculation of the anisotropy of the lattice. 5- Description of the experimental apparatus of the irradiation measurements. (author)

[en] The loading to criticality of the first core of the zero energy reactor ZENITH, and the results of the subsequent experimental programme are described. The composition of the reactor core corresponds to graphite: U235 and Th232: U235 atomic ratios of 3843 and 9.51 respectively. (author)

[en] In this report are described and interpreted some experiments, carried out in the pile G1 during a period of shut-down, which have made it possible to measure the variation of the material Laplacian of the lattice with the radius of the uranium bar. The variation of the reactivity of the pile is measured when an increasing number of fuel elements are progressively replaced in the central region by fuel elements of greater diameter; it is shown that, starting from measurements based on less than ten per cent of the total number of elements, the variation of reactivity corresponding to the replacement of all the elements can be determined; it is then easy to deduce the variations of the Laplacian. Results: the variations of the Laplacian with the uranium rod diameter are 0 (d. 26 mm), +0.065 ± 0.004 m^-2 (d. 28 mm) and +0.080 ± 0.008 m^-2 (d. 32 mm). (author)

[en] A resistor network is described which can be used to solve the partial differential equations for the scalar potential and for the only component of the vector potential in problems with cylindrical symmetry. To calculate the values of the resistors a general method is presented valid for any equation which can be solved by the resistor network analogy. (Author) 2 refs

[en] The various methods used during the physical study of the reactor G1 to determine the variations of the effective multiplication factor consecutive to a given change in the geometry of the multiplying medium, are presented and discussed. The comparison of the results obtained by these various methods has allowed their validity to be tested and precise conditions of use to be given. In the first part are presented the principles used and their ranges of validity. In the second part the experimental results are given, together with some indications on their comparison with theoretical estimations. (author)

[en] The results of two months' experiments, carried out at low power following the first divergence (4. July 1957) are presented. These experiments dealt with the following points: comparison of experimental and calculated values of the critical size; effects due to the introduction of absorbing materials into the principal experimental facilities, flux distribution measurements especially in these facilities, calibration of safety, control and compensating rods, determination of the average life of the neutrons. (author)

[en] During an experiment on the zero power heavy water reactor with natural uranium fuel in the Boris Kidric Institute of Nuclear Sciences, the reactor escaped control. Six staff members in the immediate surrounding of the bare assembly were exposed to high neutron and ionising irradiation. Other two employees who were at some bigger distance were exposed to doses higher than permitted. This paper deals with the circumstances that caused the accident, status of the dosimetry, control and alarm systems. Individual exposure doses were estimated according to the calculated neutron flux values obtained from measuring the activities of personal belongings made of gold and copper as well as radioactive phosphorous from urine