[en] The specific activity limits for beta-gamma emitters toward the decontamination of steel used in G2 and G3 Uranium Natural Graphie Gas reactors are: 1 Bq/g and Bq/cm2. The principle radionuclides are Co 60, its deposit accounts for 95% of deposits and encrustations on the steel pipes. About 90% of the steel from the dismantling can be reused. A brief history of the reactors and the dismantling methods are provided in the paper

No abstract available

[en] In situ laser extraction techniques were applied for the study of heavy noble gases in a polished section of Zone 13 from the natural nuclear reactor in Okelobondo. Three main mineral phases were identified in this polished section using SEM-EDX. The Xe and Kr isotopic structures were determined by multiple measurements in each of these phases. Twenty-four isotopic analyses of the gases extracted from two different U-rich phases revealed nearly normal fission spectra. All 9 analyses of a U-free phase, consisting mainly of alumophosphates, demonstrated an unusual isotopic composition (¹³⁶Xe/¹³⁴Xe/¹³²Xe/¹³¹Xe/¹³⁰Xe¹²⁹Xe/¹²⁸Xe = 1/1.25/1.73/0.89/0.0045/0.274/0) with concentrations ranging up to 10^-2 cm³ STP/g. This is the highest Xe concentration ever measured in a natural material. Kr was also anomalous, although to a lesser extent. These results confirm the presence of Chemical Fractionation of Fission Xe (CFF-Xe) in the Okelobondo alumophosphates. CFF-Xe is a decay product of intermediate fission fragments that have migrated out of the U-rich host phases into adjacent U-free minerals. The CFF-Xe spectra in the alumophosphates are also accompanied by ¹³⁰Xe excesses, which are attributed to neutron capture on fissiogenic ¹²⁹I that apparently migrated out of the nearby U-rich minerals. The ¹³⁰Xe/¹²⁹Xe ratio allows one to estimate the thermal equivalent neutron dose of 1.1 x 10²¹ n/cm². The presence of an unknown fission component remarkably similar in composition to CFF-Xe can be inferred from the atmospheric and terrestrial data. This leads one to the hypothesis that the CFF process has operated on a global scale on the Earth

[en] The moderator integral and differential worth as a function of moderator level in the core of NRX/CIR type reactor has been determined using standard computer codes WIMSD/4 and CITATION. The core reactivity without any moderator was calculated to be about -1096 mk whereas the moderator level of 306 cm adds a total of about 1140 mk reactivity in the core. (author)

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No abstract available

[en] Starting with the known reactor constants for a heavy water moderated reactor with reflector and a given number of control rods of a certain size, the reactivity equivalence of the control rods is calculated. The calculation is given in detail. The number, length and diameter of the control rods is then varied and the effect of these parameters on the reactivity is shown graphically. Flux plots are also given for the reactor with and without control rods

[en] Fuel management schemes and fueling patterns chosen for CANDU-type reactors are presented. The problems encountered during refueling are underlined

[en] CANDU reactors are studied with methods of integrated analysis using normative econometric models. (authors)

[en] The impact of neutron leakage form a core on core burnup and reactivity consumption rate (mk/EFPD) was analyzed. The two cores with different diffusion characteristics but having the same specific power (MWth/kg-U) were chosen and analyzed. It was found that the core with higher diffusion characteristics has higher reactivity consumption rate and smaller cycle length. (author)