Contribution to the study of the UCsMoIO system

The reaction in the UCsMoIO system at 1073 K have been studied as a function of oxygen potential. The chemical constitution and the morphology of the phases formed were examined by electron-probe microanalyzer and X-ray diffractometer. The existence of a two-phase field UO_2+x + Cs₂UO₄ was confirmed in the UCsO system. The threshold oxygen potential was determined for the decomposition of CsI by UO_2+x to form Cs₂U₄O₁₂ and gaseous iodine. The effect of molybdenum on the UO_2+x−CsI reaction was also investigated. Predominance of Cs₂MoO₄ over Cs₂UO₄ was verified in a certain range of oxygen potentials. Some assessments and interpertations of the experimental results were made with the aid of thermodynamic calculations.     

Study on kinetics of subcritical system: Contribution of delayed neutrons to the transient after a reactivity insertion

It is generally difficult to employ three dimensional spaces and time dependent kinetics by using a Monte Carlo method for nuclear reactor kinetics, because the random work analysis of a neutron becomes extremely difficult when conditions for neutron transport change with time.

However, in most cases of kinetic analysis, in particular relevant to ADS systems, we meet with many cases, where the lifetime of prompt neutron is much shorter than that of the neutron transport condition. In calculating the kinetic evolution of a subcritical (SC) system, in this paper we propose a new calculation technique where a steady state equation for the neutron flux and a time-dependent one for the delayed neutron precursor density will be treated.

In this paper, the accuracy of this theory is investigated by using a simple point reactor equation for several cases typical for ADS system. We obtained strict solution Φ* as a reference solution, Φ1 as a solution by the present method, and Φ2 as a solution where both neutrons and delayed neutron precursors are treated by using static equations. The obtained results show a good agreement between Φ1 and Φ*, though the Φ2 agrees with Φ* poorly in all our cases. Finally, we can employ a conventional Monte Carlo code for neutron transport and analysis many kinetics problems of ADS systems with the help of delayed neutron precursor analysis.     

Contribution to the theory of annihilationγ-ray logging

Karelian Polytechnical Institute. Translated from Atomnaya Énergiya, Vol. 72, No. 6, pp. 619-622, June, 1992.     

Contribution to the problem of the form of uranium in phosphorites

Results of the investigation of apatite separated out of uranium-bearing phosphorites are presented. The entrance of uranium into the apatite (uranoapatite) crystal lattice is established, from which it follows that apatite is one of the bearers of uranium in phosphorites.     

Thermodynamic study of the U-Si system

The uranium-silicon phase diagram is a key system to predict the possible interaction between the fuel kernel (U, Pu)C and the inert matrix SiC considered for the gas-cooled fast reactor systems. The experimental data from the literature on the uranium-silicon system are critically reviewed. Differential Thermal Analysis experiments are carried out to measure the temperatures of the phase transitions in the composition range 6-46% at Si. The experimental results are compared to the available data of the literature. The microstructure of the samples has been analysed using scanning electron microscopy. In view of the analyses, some solidification paths are proposed. Finally, the present experimental results and the available data of the literature have been used to perform a thermodynamic modelling of the uranium-silicon system using the CALPHAD method.     

Thermodynamic study of the Np-Zr system

A thermodynamic model of the Np-Zr system is developed using the CALPHAD method, and a review of previous work performed on this system is presented here. In general, results obtained are in good agreement with those proposed from experimental observations. It is found that the nature of reactivity of Np with Zr, is different from that of U and Pu: an expected elevation of melting point of Np-Zr alloys was not seen and a miscibility gap existed between the high-temperature bcc phases of Np and Zr. Formation enthalpy of the bcc phase obtained from the model is compared with results from KKR-ASA-CPA calculations. Lattice stabilities of various phases in the system are compared to values obtained from first-principles LDA and GGA calculations. The δ-NpZr₂ phase is modeled as a non-stoichiometric phase with a C32 structure, similar to what has been determined for the δ-phase in the U-Zr system. This phase is analogous to ω-phase in pure Zr, which is stabilized at high pressures. Two different possibilities for stability of the δ and ω phases have been proposed in the present work. Finally, calculated changes in enthalpy versus temperature are plotted for two alloys to guide future experimental work in resolving important issues in this system.     

Contribution to DEMO reactor RH maintenance assessment

The scope of this paper is a preliminary assessment of the maintenance scheme in support of the European study for the next generation of fusion reactor: DEMO. Despite other fusion machine requiring remote handling maintenance operations, DEMO is supposed to work under steady state operational conditions. Therefore, requirement on the maintenance scheme is stronger. To target a good availability of the machine along machine operation plan, it is necessary to draw an adequate maintenance scheme. Indeed, due to the high fluxes generated by the plasma in the vacuum vessel, the first wall lifetime is limited, so the frequent replacement is necessary. On current fusion experimental machine, as first wall load conditions are less severe, DEMO condition implies high level of requirement on maintenance time. During DEMO lifetime, several full first wall replacements are expected. To provide access to the vacuum vessel machine for first wall removal, preparatory work is required to set the machine to adequate maintenance conditions and to open the machine properly, the same situation at the end of the maintenance period. Shutdown duration for first wall replacement should be as short as possible to reach the availability target of the machine. From this statement, the maintenance duration should not exceed 20% of the total lifetime of the reactor operation. First wall segmentation (i.e. total number of component to replace) has a high impact onto the replacement time. Considering the number of feasible designs for the first wall segmentation, we concentrate remote handling concept assessments one type of segmentation, the one minimizing the numbers of modules to replace [4], [5], [6]. Assumption on Divertor segmentation for these DEMO studies have similarities with Divertor ITER design; therefore ITER design output is relevant [1], [2]. We assume divertor removal performed in shadow time, while removing the other first wall modules.     

Contribution to the theory of radiation effects on some properties of graphite

A substantial change occurs in several of the properties of graphite subjected to in-pile irradiation. These changes are the result of damage to the crystal lattice structure of the graphite. The present article presents a calculation of the change in lattice constant and internal energy of a unit volume of a crystal of graphite containing Frenkel defect-type imperfections. The results of the calculation are compared with experimental data.In conclusion, the author would like to avail himself of this opportunity to express his profound gratitude to A. I. Leipunskii for his kind attention to the progress of the work and the numerous useful comments advanced in discussions of the results.