Fission products neutron absorption

Only the coohmt has to be circulate, and the same tope free from fission fragments. A schedule of replace-circulation cools the reaction and absorption zones. No 65 ment can readily be worked out to keep the reactor gas is evolved from the reaction zone and poisoning fac- operating with minimum shutdown times, in accordance tors, due to retention of fission product neutron absorb- with the power output and resultant use rate of the ers in the reaction zone, do not become critical because plutonium. 

Non-productive neutron absorption in the absorption zone, however, has a more serious effect. For example, protactinium is an intermediate stage between the thorium and the uranium isotope desired, and as far as is presently known is not fissionable. This element can cause a neutron loss in two ways. First, a neutron loss by 0Q the neutrons which Pa absorbs, and second, by formation of an element decaying into instead of into a known fissionable isotope. This effect however can be kept to a minimum by extracting the Pa from the slurry at sufficiently frequent intervals to reduce the ab-55 sorption by the Pa to about. 5 percent of the absorption by the thorium. 

The first two equations represent the fact that the D-D reaction can follow either of two paths, producing tritium and one proton or hehum-3 and one neutron, with equal probability. The products of the first two reactions form the fuel for the third and fourth reactions and are burned with additional deuterium. The net reaction consists of the conversion of six deuterium nuclei lnlo two helium nuclei, two hydrogen nuclei, and two neutrons along with a net energy release of 43.1 MeV. The reaction products—helium, hydrogen, and neutrons—are harmless as contrasted with the myriad fission products obtained in a fission reactor. The neutrons produced may be absorbed in sodium to produce an additional 0.25 MeV per cycle. Therefore, the D-D reaction produces at least 7 MeV per deuterium atom (deuteron) and, with absorption in sodium, more than 10 MeV per fuel atom. 

Fm, which undergoes spontaneous fission (ti = 0.38 ms). This point can be passed in two ways. One is to utilize a more intense neutron flux than can be obtained in a reactor, in the form of a thermonuclear explosion, so that a product such as undergoes further neutron absorption before fission can occur. Here, in the synthesis of Fmin Tvy Mike , the world s first thermonuclear test, atEniwetok atoll on 1st November 1952, the initial product of multiple neutron capture, underwent a whole series of rapid decays, yielding Fm. 

The term eff — 1 is called excess reactivity, and /eff — l)/ eff is called reactivity. Because the fissile material is continuously used up by fission and because the fission products absorb neutrons, a certain excess reactivity is necessary to operate a nuclear reactor. This excess reactivity is compensated by control rods that absorb the excess neutrons. These control rods contain materials of high neutron absorption cross section, such as boron, cadmium or rare-earth elements. The excess reactivity can also be balanced by addition to the coolant of neutron-absorbing substances such as boric acid. 

In order to prevent corrosion of the fuel and escape of fission products, the fuel is tightly enclosed in fuel rods. Good heat transfer and low neutron absorption are important properties of the cladding. Generally, the fuel rods are assembled to fuel elements to make their exchange easier. 

The fission products that are mainly responsible for neutron absorption are listed in Table 11.9. With respect to the mass the lanthanides represent the greatest fraction, but with regard to neutron absorption the noble gases are most important due to the high value of for Xe (2.65 10 b). 

More than 300 different nuclides have been observed as the primary products of fission. The term fission products usually refers to the primary fission products, i.e., the fission fragments and their daughters resulting from radioactive decay and neutron absorption. Only a few of the primary fission products are stable, the rest being beta-emitting radionuclides. As a fission-product radionuclide undergoes beta decay, its atomic number increases whereas its mass number remains constant. The direct yield of a fission-product nuclide is the fraction of the total fissions that yield this nuclide, essentially as a direct-fission fragment. The cumulative... 

For irradiation time of lO s. Calculated for no neutron absorption in fission products. 

The total decay-heat power Pa(T, t) for fission products from a reactor operating at constant total thermal power Pf, and neglecting neutron absorption in fission products, is given by the following simplified method, from the ANS Standard ... 

Neutron absorption in fission products has a small effect on decay-heat power for r < 10 s and is treated by a correction factor G. The corrected total decay-heat power is given by the ANS Standard, in terms of thermal-neutron flux (in neutrons/cm s), reactor operating time T (in s), and cooling time t (in s) as... 

To predict the decay-heat rate from fission products after cooling times of several years, additional corrections must be made for absorption of neutrons in long-lived fission products, particularly the absorption of neutrons in stable Cs to form 2.05-year Cs. Computer codes such as ORIGEN [B2] and CINDER [El] are particularly useful for this purpose. 

Estimated maximum values of the ratio G of fission-product decay-heat rate, with neutron absorption in fission products considered, to the decay-heat rate in the absence of neutron absorption in fission products are given in Table 2.13 [A2]. The data are calculated for U- U fuel irradiated for 4 years in a light-water reactor. For cooling times of <10 s, the... 

The fission product Xe has the largest absorption cross section of all the nuclides in a thermal-neutron flux, and its buildup is especially important in affecting the neutron balance in a thermal reactor. The fission-product decay chain involving the production and decay of Xe is... 

The equations of Sec. 6.2 give the number of atoms of each fission product after a reactor has been run at stated conditions for a specified time. If the reactor is then shut down, the fission products build up and decay in accordance with the laws of simple radioactive decay, which were outlined in Sec. 3. If the nuclides in the decay chain are removed orJy by radioactive decay during reactor operations, the equations of Sec. 3 describe the changes with time of the number of atoms of any nuclide in the decay chain. If a member of a fission-product decay chain or its precursors in the decay chain are removed by neutron absorption, equations for the amount of each nuclide present at time t after shutdown may be obtained by applying the equations of radioactive decay to the amount present at shutdown. 

We shall assume that a representative unit volume of this design contains atoms of a single fissile species (e.g., U) with absorption cross section ajjf and Ng atoms of a single fertile material (e.g., with absorption cross section Og. The unit volume is assumed also to contain steady-state amounts of Xe, Sm, and other fission products with cross sections above 10,000 b, which build up to equilibrium concentration in a few days at the neutron fluxes typical of power reactors. It is assumed that no other fission products are present to an extent sufficient to affect the neutron balance. The items that affect the thermal-neutron... 

Nuclide Subscript Absorption cross section, Ufl, b Neutrons produced Ratio of capture to fission cross section, a Poisoning ratio of high-cross section fission products, Q... 

Fission products. Burnout of fission products by neutron absorption will be neglected. The rate of formation of fission-product pairs from U is... 

For irradiation in a constant neutron flux, the activity of any fission-product nuclide can be evaluated from the equations in Chap. 2. When fissions occur at a constant rate and when neutron-absorption reactions in the fission product and its precursors can be neglected, the activity of a nuclide with relatively short-lived precursors can be evaluated by applying Eq. 

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