Thermal neutron reactors, fission product

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

Nuclear and magneto-hydrodynamic electric power generation systems have been produced on a scale which could lead to industrial production, but to-date technical problems, mainly connected with corrosion of the containing materials, has hampered full-scale development. In the case of nuclear power, the proposed fast reactor, which uses fast neutron fission in a small nuclear fuel element, by comparison with fuel rods in thermal neutron reactors, requires a more rapid heat removal than is possible by water cooling, and a liquid sodium-potassium alloy has been used in the development of a near-industrial generator. The fuel container is a vanadium sheath with a niobium outer cladding, since this has a low fast neutron capture cross-section and a low rate of corrosion by the liquid metal coolant. The liquid metal coolant is transported from the fuel to the turbine generating the electric power in stainless steel... 

The isotope used to prepare labelled compounds is obtained by irradiation in a nuclear reactor, of solid targets containing atoms of nitrogen (aluminium or beryllium nitride), by neutrons of low energy, known as thermal neutrons, themselves the product of the controlled atomic fission of The radiocarbon formed is next isolated from the target sample by oxidation to Ba " 003, the variety in which it is delivered to chemists. From C02, it is possible to use a plethora of organic chemical reactions to synthesize different compounds in which the radio-isotope can be introduced to a specific position. 

In thermal-neutron reactors has an important advantage over or Pu in that the number of neutrons produced per thermal neutron absorbed, tj, is higher for than for the other fissile nuclides. Table 6.1 compares the 2200 m/s cross sections and neutron yields in fission of these three nuclides. Thorium has not heretofore been extensively used in nuclear reactors because of the ready avaUabihty of the U in natural or slightly enriched uranium. As natural uranium becomes scarcer and the conservation of neutrons and fissile material becomes more important, it is anticipated that production of U from thorium will become of greater significance. 

The program YCALC allows the calculation of the independent yields of any fission product for 12 different fission reactions (i.e., the spontaneous fission of Cf, the thermal-neutron-induced fission of h, Np, Pu, Pu, Am, and Cf, the reactor-neutron-... 

The net yield of thermal neutrons from the fission of is higher than from that of and, furthermore, Th is a more effective neutron absorber than As a result, the breeding of is feasible even in thermal reactors. Unfortunately the use of the Th/ U cycle has been inhibited by reprocessing problems caused by the very high energy y-radiation of some of the daughter products. 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

The radiation - induced changes noted are in weight loss, gas evolution, mechanical sensitivity, thermal sensitivity and stability, and ex pi performance. The effects will be described with the type of nuclear radiation used. The format describes the radiation effects on expls, propints and pyrots with the sequence of radiations utilized (when applicable) as follows, a - particles, neutrons, fission products, reactor radiation (fast and slo w neutrons plus gammas), gammas (7), underground testing (UGT), X-rays, electrons, and other nuclear radiations... 

Studies of the effect of neutron irradiation are divided into three groups slow or thermal neutrons, fission products and reactor neutrons. The slow neutrons are obtained from a radioactive source or high energy neutrons that are produced by deuterium bombardment of a beryllium target in a cyclotron and slowed down passing thru a thick paraffin wax block. The fission products in one case are produced when a desired sample is mixed or coated with uranium oxide and subsequently irradiated with slow neutrons. The capture of neutrons by U23S leads... 

In a reactor, the energy per fission, including the energy of the delayed neutrons and of the fission products, is 200 MeV. To produce 1 MW thermal energy, 3.1 x 1016 fissions per second are required. If the half-life of the fission product is short compared with the duration of operation of the reactor, its activity comes into equilibrium when creation by fission equals radioactive decay. Assuming a constant level of power for a duration of Tsecs, the activity is 3.1 x 104/(1 — exp—AT) TBq per MW. Some fission products themselves absorb neutrons (the socalled reactor poisons) and for them the calculation of activity is more complicated. Figure 2.2 shows the combined activity of 1 g of fission products formed in an instantaneous burst of fission and also from 1 g of fission products formed over a period of a year (Walton, 1961). The activity from a short burst decays approximately as t-1 2. 

While a reactor is working the uranium-235 or plutonium-239 is slowly converted into fission products. The materials are intensely radioactive. Some of them capture thermal neutrons and thus diminish the efficiency of the pile. It is therefore necessary to take out the fuel rods from time to time and remove the fission products. For this purpose they are kept about 100 days to allow the short-lived radioelements to decay and then dissolved in nitric acid. Nitrous acid ensures that all the plutonium is in the Pu form. The reactions, with 11+ written for H3O+ (p. 194), arc... 

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