Fast fissions

Fast Reactor An advanced technology nuclear reactor that uses a fast fission process utilizing fast neutrons that would split some of the U-258 atoms as well as transuranic isotopes. The goal is to use nuclear material more efficiently and safely in the production of nuclear energy. 

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... 

Figure 3. Yield of major fission products for fast fission of U-235 and Pu-239. (After Ref. 2.)...

Mechanisms of formation of these defects in pure and damaged UO2 are different. In pure UO2, oxygen Frenkel pairs (OFP) defects appear spontaneously as thermally induced excitations of anionic crystalline sublattice, in damaged dioxide they normally produced by fast fission products. 

This short thermal shock , followed by fast quenching to relatively low temperature, like thermal spike, originated by a fast fission fragment, produces a lot of OFF defects. Below, after description of potential model applied, we present and discuss results of the above computer simulation experiment. 

Tabto 21.2 Delayed neutron parametera for fast fission ... 

Those fast neutrons that have energies greater than about 1 MeV may cause a limited amount of fission of fertile material. To account for this, the reactor designer usually specifies a quantity e, called the fast-fission factor, which is defined as the ratio of the net rate of production of fast neutrons to the rate of production of fast neutrons by thermal fission. The fraction e — 1 of the fast neutrons comes from fission of fertile material with fast neutrons e — 1 may be of the order of a few hundredths in a thermal power reactor. The net production rate of fast neutrons from fission is er N a 4>-... 

Deuterium oxide is available in ton quantities and is used as a moderator in nuclear reactors, both because it is effective in reducing the energies of fast fission neutrons to thermal energies and because deuterium has a much lower capture cross-section for neutrons than has hydrogen and hence does not... 

The n-capture in the r-process has been suggested to go up to Z about 100 and N 184. In the intense neutron field a considerable amount of (mainly fast) fission of the newly synthesized heavy elements probably also occurs. This partly explains the peaks at IV = 50 and 82 in Figure 17.7b, which also correspond to maximum yields at 4 = 95 and 140 in thermal hssion. Some stars are unique in that they have an imusually high abundance of fission products spectral lines from heavy actinides, like americium and curium, have also been observed in such stars. 

A rough estimate of the critical radius of a homogeneous unreflected reactor may be obtained simply by estimating the neutron mean free path according to (14.6). Assuming metal with a density of 19 g cm and a fast fission cross section of 2 X crn, one obtains = 10 cm. A sphere with this radius weighs 80 ks. For an unreflected metal sphere containing 93.5% the correct value is 52 kg. Pu has the smallest unreflected critical size for Pu (5-phase, density 15.8 g cm ) it is 15.7 kg ( 6 kg reflected), and for 16.2 kg ( 6 kg reflected). 

Calculate the number of collisions required to reduce a fast fission neutron ( = 2 MeV) to thermal energy ( 0.025 eV) in a light-water-moderated reactor, assuming that the data in Table 19.3 are valid. 

One of its attractive features of rhenium is that it is a spectral shift absorber (SSA), which means that it has a low relative absorption cross section for fast neutrons while in the thermal spectrum its absorption cross section increases dramatically. This has safety applications for the reactor design in accident scenarios. Rhenium has an absorption cross section of in the fast spectrum, however the magnitude of the difference between the absorption cross section and the fast fission cross section of is low compared to the difference at a thermal spectrum. It also provides a barrier that protects Niobium 1% Zirconium from nitrogen attack and damage caused by other fission products that outgas from the fuel. Most of the other SSA materials have a relatively low melting point, making them less attractive. 

Fermi would have known of the Met Lab discussions. His proposal to Oppenheimer at the April conference was difierent from those essentially defensive concerns, however, and clearly offensive in intent He may well have been motivated in part by his scientific conservatism may have asked himself what recourse was open to the United States if a fast-fission bomb proved impossible—it could not be demonstrated by experiment for at least two years—and have found the answer in the formidable neutron flux of CP-1 and its intended successors. Oppenheimer swore Fermi to intimate se-... 

Genia and Rudolf Peierls. While American efforts stalled, Peierls and Otto Frisch in England in 1940 worked out the essential theory of a fast-fission uranium bomb fueled with U235 and convinced his British colleagues that it was feasible. 

The Fast Effect. Though Szilard seems to have been the first to point out the importance of fast fission in uranium-238, Wigner and his group were the first to systematize the calculation, particularly the calculation of the first collision escape probability in various geometries. (Wigner made the latter calculation by first solving the diffusion equation, where the kernel is of the form e" /a , and then integrating with respect to a to get the transport kernel,. ... 

Szilard, also working at Columbia, became interested around this time in what is now called the fast effect. The fast effect, is the increase in the multiplication constant obtained by the emission of neutrons by which is induced to fission by the fission neutrons before they are slowed down. Szilard measured both the cross section of such fission neutrons to induce fast fission and also their inelastic cross section, i.e., the probability for their being slowed down below the fast fission threshold by an inelastic collision with uranium. He concluded on the basis of these measurements that one may obtain an increase of as much as 6-8% in the multiplication constant by using large and metallic lumps of uranium. Szilard was also somewhat discouraged by the low multiplication constant which Fermi s experiment gave but was far from giving up hope. 

There are two new ideas in the breeder program which are particularly worthy of attention. The first of these concerns the use of beryllium as moderator, as was first proposed, I believe, by Krasin et al. in Russia 11). It has been estimated that the (w,2n) reaction in Be may increase the total number of neutrons produced by fission by a factor as high as 1.12. A more realistic estimate is, perhaps, 1.07. The proposition has two drawbacks one economic and the other nuclear. The economic problem concerns the availability of beiyllium. This seems questionable. The nuclear problem centers about the facts that the (w,a) reaction, which leads to a neutron loss, can be induced by neutrons above 0.695-Mev energy but the (n,2w) reaction, which leads to the increase in the neutron number, only by neutrons with 1.85 Mev or more. However, the high-energy neutrons can induce also fast fi on in U and the question is, therefore, not really, Shall we make use of the n,2n) reaction in Be Instead, the proper question to be posed is, Does the (w,2w) reaction in Be or the fast fission reaction in U jdeld more neutrons ... 

Papers 29 and 30 served as the basis for the earhest calculations of the multiplication constant in an infinite heterogeneous reactor. By early 1942, the significance of fast fission was recognized, and all later calculations included estimates of e, the fast effect. This gave the four-factor formula, k = rjepf rj being the number of neutrons released per neutron absorbed in U. The quantity actually calculated in the following reports was rj = (1/ep/), k = r /rj ... 

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