A general survey is presented of gascentrifuge developments in Europe for the enrichment of U-235 and detailed reasons why the gascentrifuge approach has been adopted in Europe. Also, a full description is given of recent progress made in this domain and of the arrangements for international co-operation in Europe, especially between the Federal Republic of Germany, the Netherlands and the United Kingdom.

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The migration of single radionuclides disposed of underground have been studied extensively, and many results have been reported. The migration of multiple radionuclides under similar conditions, has, on the other hand, scarcely been studied, and yet it is very important for evaluating the safety of the ground disposal of radioactive wastes.
This paper bases its discussion on two theories (that of frontal analysis proposed by Sillen & Inoue's theory) on the behavior of multiple nuclides. It is shown that frontal analysis is applicable theoretically to the estimation of the migration of multiple radionuclides, but that some difficulties are encountered in its practical use in the field. The difficulties are, for instance, the estimation of the quantities of soil in contact with underground water, and the effect of the diffusion of the nuclides in the underground water. Inoue's theory, on the other hand, well depicts the migration of multiple radionuclides under the ground, but the theory requires field observation of the ground water movement.

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External exposure due to radioactive inert gases from a stack presents important problems to the environment near nuclear power stations, and the monitoring of these gaseous effluents has been undertaken by measuring the gross radioactivities of gaseous effluents in Ci/sec, using a stack monitor. But, for evaluating the external exposure from a gaseous mixture of Kr and Xe isotopes in the environment, the usual methods are not satisfactory and even meaningless when the fractions of radionuclides in the mixture gas change according to the storage or hold-up time.
In this paper, for estimating theγ-exposure from the mixture gas, a new concept of stack monitor to measure the photon energy dissipating rate Σ(QEF), the photon dissipating rate Σ(QF)and the average energy E is discussed, and it is shown that this method simplifies the evaluating procedures and improves the accuracy of γ-exposure evaluation. Further, the design of a new stack monitor is presented, which consists of a NaI (Tl) scintillation head with a specially designed Pb shield cap and an electronic circuit for pulse height weighting.

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