Thorium fast fission

The geochemistry of uranium and thorium has excited considerable interest on accoimt of their strategic importance. Smales determined uranium in rocks by neutron activation followed by isolation of fission product Ba (81). Interference from the fast fission of any thorium present in the sample and from beta-emitting barium isotopes formed by (n,y) reaction is discussed and methods of overcoming the diflSculties are described. The uranium content of two iron meteorites was determined by... 

In early work gross counting of delayed neutrons was used to determine the abundance of a single fissionable nuclide known to be in the sample. Brownlee 101> has reported techniques by which two or more fissionable species may be determined at the submicrogram level in a single irradiated sample. Nuclides fissionable only with fast neutrons may also be determined by this technique. One of the more interesting applications of the method is in the non-destructive determination of uranium and thorium at trace levels in minerals, rocks, and stony meteorites 102,108). 

Uranium 235 and plutonium 239, which can be made from uranium 238, are capable of undergoing fission when exposed to slow neutrons. It was also, shown by the Japanese physicist Nishina in 1939 that the thorium isotope Th - undergoes fission under the influence of fast neutrons. It seemed likely that all of the elements with atomic number 90 or greater can be made to undergo this reaction. 

The final product is thorium D or thorium lead. Several isotopes of thorium are known. Fast neutrons can cause fission with Th(232) as with U(238) and Np(237)... 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

Both thorium and U238 could be expected on theoretical grounds to behave similarly, he pointed out to Rosenfeld to fission only with fast neutrons above 1 MeV. And it seemed that they did. That left U235. It followed as a matter of logic, Bohr said triumphantly, that U235 must be responsible for slow-neutron fission. Such was his essential insight. 

If the content rises much above. 01 percent, the loss 25 bringing the density back to its former level, more xenon by fission requires it to be removed. By continually stir- 135 is formed and the process is repeated until an equi-ring the pellets, the thorium can stay in the absorption librium condition is reached where the xenon 135 formed zone until all the pellets 60 reach a concentration is transmuted by neutron absorption and by decay into requiring removal. When that time comes, valve 64 is isotopes of lower capture cross-section as fast as it is operated to dump all or part of the pellet 60 charge into 30 being formed. In the meantime, the control rod 77 (or discharge pipe 62 and into a shielded coffin (not shown) equivalent) has to be withdrawn by an amount thereby for chemical removal of the removing from the reactor, neutron absorbers at least... 

In addition to the uranium, all samples will contain low concentrations of thorium. Both the thorium and will fission when irradiated with fast neutrons. Therefore, it is important that the samples are irradiated with well thermalized neutrons. This allows for the assumption that all of the fission events produced during irradiation are from the fission of... 

The Th—cycle is of interest due to that the abundance of thorium in the Earth s cmst is between three to five times that of uranium (OECD/NEA and IAEA, 2014). In addition, there are large thorium deposits in some countries such as India, Brazil, Australia, and the USA (WNA, 2015a). The Pu cycle is the most effective with fast neutrons. For Pu, the number of emitted neutrons in a fission reaction per absorbed neutrons is greater when fission is induced by fast neutrons rather than thermal neutrons. The additionally emitted neutrons can be utilized for transforming more nuclides to Pu. Hence, the FBRs are based on Pu in which... 

The important physical constants regarding the fission process for the uranium and thorium isotopes and by-products are given in Table I. As is indicated natural thorium ( Th) is only fissionable by fast neutrons with an energy greater than 2MeV (2 x 10 eV). Even then, however, the probability of capturing a neutron that will cause fission (uy) is very low. 

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