The results of calculations of a high-capacity subcritical molten-salt reactor burning transplutonium elements and operating with a fast-intermediate neutron spectrum and the salt LiF-NaF-KF are presented.It is shown that this reactor makes it possible to burn ~300 kg/GW of americium per year, i.e., long-lived wastes from approximately 30 VVER-1000.To develop nuclear power and ensure its competitiveness with conventional and new power directions it is necessary to solve the problem of handling spent nuclear fuel, first and foremost, the long-lived radioactive wastes, specifically, the transplutonium actinides americium and curium. The two main concepts for utilizing them -storage or burning together with other actinides Pu, Np, and U in recycled fuel for fast reactors -have not yet reached technological maturity [1]. The OMEGA program [2] presumes that they are utilized in specialized reactors, preferably with minimal use of fissile materials, specifically, plutonium [3]. The choice of a specialized reactor remains a subject of discussion, and the molten salt reactor is a candidate. The advantage of a fast neutron spectrum for such a reactor has now been recognized [3]. Attempts to secure a fast neutron spectrum by using chloride salts have been unsuccessful because of their high corrosiveness, so that modern designs of molten salt reactors are based mainly on fluoride salts.Fluoride salts, unlike chloride salts, are compatible with construction materials, but to increase the fast-neutron fraction in their spectrum the solubility of actinides must be increased. Fuel salt with high solubility of plutonium and americium fluorides was unknown until recently, and for this reason data showing their high solubility in the eutectic LiF-NaF-KF [4] are critical (Fig. 1). These results were placed at the basis of the concept of subcritical molten-salt reactor for incinerating transuranium elements. The subcritical operating regime is due to the low fraction of delayed neutrons in the fission of 239 Pu (β = 0.22%), 238 Pu (β = 0.14%), 241 Am (β = 0.14%), and 245 Cm (β = 0.18%).The basic scheme of a conventional reactor for subcritical systems includes a target assembly, consisting of a channel for introducing a proton beam and neutron-generating target and surrounded by a subcritical molten-salt blanket where transplutonium elements are burned (Figs. 2, 3 and Table 1). The transmutation zone is surrounded by the first cooling loop, which includes the circulation pumps and heat exchanger, in which the same salt as in the reaction zone LiF-NaF-KF is used

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A cascade subcritical liquid-salt reactor designed for burning long-lived components of the radioactive wastes of the nuclear fuel cycle is examined. The cascade scheme of the reactor makes it possible to decrease by a factor of three the power of the driving accelerator as compared with conventional accelerator-blanket systems of equal power. The fuel composition of the reactor consists of 20% Np, Am, Cm, and other transplutonium elements and 80% plutonium, which are dissolved in a salt melt NaF(50%)-ZrF 4 (50%). For a 10 MW proton accelerator, 1 GeV proton energy (10 mA current) and subcriticality depth 0.05, the thermal power of the reactor is 800 MW, which permits burning ~70 kg/yr Np, Am, Cm, and other transplutonium actinides, i.e., service five VVÉR type reactors of equal power.The problem of handling radioactive nuclear wastes from nuclear power generation remains unsolved even at the strategic level. The only solution which has reached commercial implementation -burial in deep geological formationsis being challenged today [1]. Another (strategically more natural) solution to this problem -burning (transmutation) in fast reactors -can be implemented only after most of the thermal reactors are replaced by fast reactors, and it requires a long period of time [2]. The third route -the development of special burner reactors -has been under intense discussion in recent years but is still at the conceptual stage. The main difficulty of developing such reactors is the low fraction of delayed neutrons (β = 0.17%) with fissioning of Np, Am, Cm, and other transplutonium actinides, which makes it impossible to construct high-capacity and safe critical burner reactors, since the admissible fraction of the actinides in the fuel of such reactors does not exceed 3-5% [3,4]. Switching to subcritical systems with k eff < 1 eliminates this difficulty but requires an intense source of neutrons. In accelerator-blanket systems, the neutrons produced by a beam of accelerated charged particles (electrons, protons, deuterons) in a heavy target is such a source. Specifically, each proton with energy E ≈ 1 GeV creates ~20 neutrons in a lead target. The thermal power of the blanket (W b ) and the power of the driving accelerator (W a ) are related by the relation [5]

Large-scale development of nuclear power is pointless if there is no long-term future for it and if it cannot meet the requirements that are predicted for the future. There is still no serious competitor for nuclear power in the long-term future, besides thermonuclear power, which must still prove to be economical and efficient, not only in the variant of purely thermonuclear power but also in a hybrid variant with subcritical blankets. This has not yet been demonstrated, and the possibility of nuclear power, whose realizability and cost-effectiveness have already been proven, must be analyzed seriously.In order to be realized on large scales in the long-term future nuclear power must meet the following requirements: it must be cost-effective, the necessary resources must be available, it must produce an excess of energy, it must be safe, and it must be ecologically acceptable. The first three requirements can be met by using a two-component structure consisting of solid-fuel thermal and fast reactors. In this structure natural resources (uranium and/or thorium) can be used much more efficiently, the production of uranium can be decreased, and therefore the amount of radon entering the biosphere can be decreased. The ways to achieve the required level of expanded production of fuel, to increase safety, and to decrease capital costs for both types of reactors are now well known, and only time and the means are required for implementing them. By the time society realizes the need for developing nuclear power, the technology for a two-component structure will actually be ready, though much will still have to be done in order to optimize nuclear power as well as the structure of the industry, including enterprises in the nuclear fuel cycle.Difficulties could arise with public acceptance of the level of the ecological effect. This will in many ways be determined by the quantity of radionuclides both in the fuel cycle (uranium, plutonium) and in the burial sites (Np, Am, Cm, other actinides, and fission products) as well as globally disseminated radionuclides (14C, T, 85Kr). 'It is now already possible to show that the risk from short-lived radionuclides (for example, 131I) as well as radionuclides of the type 9~ and 137Cs can be lowered to an acceptable level by meeting the required safety standards for nuclear power plants, burial sites, and enterprises in the fuel cycle. Ways to decrease carbon, tritium, and krypton emissions into the atmosphere are now known. The acceptability of such a risk can be demonstrated in practice or indirectly (for example, by showing that different barriers remain functional for hundreds and thousands of years). However, it is difficult to prove and impossible to demonstrate the reliability of the burial of long-lived actimdes and fission products for millions of years.Undoubtedly, the search for ways to bury relial~ly radioactive wastes must continue, but the possibility of using actinides to produce energy (rather than storing them), i.e., of closing the fuel cycle not only with respec...

Specialists are now convinced that reliable storage of radioactive wastes with half-lives of T1/2 -30 yr, such as, 137Cs and 9~ produced in the nuclear power production is possible. However, it is much more difficult to prove the safety of the storage of longer-lived radiologically toxic nuclei, for example, 129I (T1/2 -1.5.107 yr) and 237Np (T1/2 -2.106 yr). This is why specialists return again and again to the idea of transmutation of such nuclei into short-lived nuclei. This problem will become more acute in the future, since the amount of wastes produced in nuclear power production worldwide is still far from equilibrium. It is also obvious that until comparative investigations of these approaches are made, it Will be impossible to make a final choice of the best approach or to determine whether or not any advantage is gained by combining different approaches.There are many technological and economic limitations that make it impossible to burn up in standard reactors with a solid-fuel core all nuclear wastes that cannot be stored. For this reason, a new type of reactor is required to switch to a closed fuel cycle: a reactor whose main properties are the ability to burn upall wastes, the possibility of flexible control of the nuclide composition, and enhanced safety. The existence of such a reactor will give the closed fuel cycle the required efficiency and flexibility.From our point of view, one such reactor is a subcritical liquid-salt reactor that is replenished by an external neutron source [1-10]. Its positive qualities are the possibility of obtaining a tow quantity of fission fragments in the core, which makes it possible to decrease substantially the residual heat release, and the possibility of removing heat by natural circulation of fused salt and a high negative temperature coefficient of the reactivity. These and omer qualities of the reactor eliminate accidents associated with the disruption of heat removal.The motives for studying a subcritical reactor are obvious: adequate subcriticality eliminates reactivityaccident and after the external neutron source is switched off the reactor stops within -10 -3 sec, i.e. the external source is an additional instantaneous means of control. Systems based on magnetic confinement of plasma (tokamaks) [11,12], laser thermonuclear fusion [13], muon catalysis [14,15], gas-dynamic plasma traps [16], and so on have been proposed as external sources of neutrons. At the present time the possibility of using as such sources accelerators which accelerate protons up to -1 GeV are being actively discussed [17][18][19].Our objectives in the present paper, which is a continuation of previous investigations [6][7][8][9][10], are as follows: selection of an optimal scheme for a safe subcritical reactor, which would make it possible to achieve high neutron multiplication while at the same time eliminating reactivity accidents, to decrease substantially the intensity of the external neutron source and thereby to eliminate the main drawbacks of the standard proton-beam schemes...