Accumulation of radioactive nuclear wastes all over the world is among the most severe ecological problems that must be solved without delay. In this regard, of great interest is the neutron method of transmutation of nuclear wastes using neutron fluxes from reactors. The neutron method of fusion of new elements is based on the successive capture of neutrons by the initial cores. The cores that absorbed neutrons undergo β-decay. As a result, new cores with masses and charges exceeding the initial ones are created. Interest in the neutron method arose in 1966, when a group of American physicists from the Los-Alamos National Laboratory performed underground experiments on thermonuclear fusion of transuranium elements in Nevada [1]. U and Am targets were irradiated by neutron fluxes produced during explosions of thermonuclear charges. The heaviest transuranium element they obtained was Fm. They tried to detect Fm and Md elements among the irradiation products, but did not succeeded. This failure was explained in 1980, when American physicists from the Los-Alamos and Livermore National Laboratories succeeded in fusion of Fm in the reaction Fm(t, p) Fm. The Fm half-life for spontaneous fission was T = 1.5 s. This is almost 10 times less than Т = 131 year for Fm [2]. Analogous decrease in Т with increasing number of neutrons in a fissionable nucleus was revealed in fusion of Fm in the reaction Fm(d, p) Fm [3]. For Fm, Т = 380 ± 60 μs, which is almost 10 times less than Т = 160 min for Fm [4]. The authors of these experiments explained this sharp destabilization of heavy fermium isotopes undergoing spontaneous fission by the shell structure of fragments. The latter become more and more close to the doubly magic nucleus Sn with Z = 50 and N = 82 as the number of neutrons in the fissionable nucleus increases. It should be noted that fragments of the heavy Fm isotopes – magic and nearly margic Sn–Sn isotopes – are weakly active cores. After a series of β-decays with half-lives from 2.2 min to 77.4 h they are transformed into stable Te or Xe cores [5]. Thus, the magic character of fragments of heavy fermium isotopes was a reason for their rapid spontaneous decay. Therefore, new transuranium elements with Z > 100 were not fused by the neutron method. The mendelevium element with Z = 101 was first discovered in USA in 1955 in the course of the nuclear reaction Еs(α, n) Md. The next elements with Z > 101 were fused in nuclear reactions with accelerated heavy ions [6]. Unsuccessful attempts to fuse the far transuranium elements [1] were caused by the unique obstacle – the predisposition of heavy fermium isotopes to spontaneous fission. This property could be useful for transmutation of nuclear wastes. There are grounds to believe that cores of radioactive wastes, including actinides and fission fragments, will gradually approach the fermium precipice in the process of neutron absorption and will end their existence in the form of weakly active Sn isotopes. These isotopes must be removed from the mixture of cores. Otherwise, the Sn isotopes can also start to absorb neutrons, new radionuclides will be accumulated, and the process as a whole will be reminiscent of the Sisyphys work. This work can be useful for the experts evaluating perspectives for the neutron method of transmutation of radioactive wastes.
Read full abstract
Dec 1, 2004
S.E Enescu + 3
Open Access