621.059.54 During the last few years there has been increased interest in employing thorium in nuclear power [1]. Specialists, in particular, have shown increased interest: they have seen new areas of research in nuclear power, connected with nuclear fuel supplies, the increased safety of reactors and improvements in the ecological acceptability of nuclear power plant. Many perceive the main advantage of the thorium cycle is its considerable potential ability to resist the leakage of nuclear materials. In particular, the proposal to use fuel based on thorium and plutonium in order to utilize plutonium in reactors is connected with this.This paper mainly reflects the work carried out at the Physico-Energy Institute (FI~I). It has been constantly supported in the institute, although on a small scale. Without pretending to give an exhaustive discussion of the problem, we indicate ways in which it can be solved.Work on the thorium cycle has been carried out at the V. G. Khlopin Radium Institute, the Kurchatov Institute of the RNTs NIIARe, MIFI and VNIINM. The thorium cycle has been actively promoted at the VNIIt~F. A considerable contribution to the experimental work was made by specialists at the PO nMayak," the Siberian Chemical Factory and the BN-350.The first experiments to investigate the accumulation of 233U in thorium, placed in a uranium-graphite reactor, showed that conversion with the liberation of 233U is not simple. The isotope 232U, which is accumulated with it during decay, is the source of daughter products with high-energy ~,-radiation [2]. It turned out that only when up to one gram of 233U has been accumulated per 1 kg of thorium can one calculate the possibility of long-term operation with 233U in compartments. The concentration of 232U is approximately 5 million-I. For a concentration of 232U of greater than 10 million-l it is necessary either to remove the decay products and rapidly carry out all technological operations (directed over a period of four months), or to produce acceptable sanitary conditions with highly automated shielded equipment.Recent investigations and experiments have confirmed that in thermal reactors the accumulation of 233U up to --1 g/kg leads to a concentration of 232U of 5 million -l . It is also known that a large amount of 233U of approximately the same purity with respect to 232U has been obtained in the USA [2].During the initial period investigating the 233U-Th thermal cycle it would be a considerable simplification to use acceptably pure 233U, in order to make samples and targets and experimental fuel rods, and to introduce technological processing. It is impossible to carry out calculations in the future only on the basis of such pure 233U, since, in a power reactor (for example, the PWR) with thorium in the active region during acceptable depletion, 233U will be accumulated with a concentration of 232U of 2000-3000 million -l. The large-scale use of 233U requires a thermal cycle that operates with 233U contaminated with 232U. However, without any decline in t...
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