Neutron absorption rate

Boron compounds have low solubility relative to some cadmium, gadolinium, or europium compounds in nitric acid solutions, and would be more likely to precipitate from solution if high concentrations are required. Cadmium is a desirable poison however, it may be volatile at temperatures encountered in the waste conversion proqess and would therefore require an additional process for recovery. Additional requirements for, soluble poisons Include chemical stability in the solution and processing behavior similar to the fissile material. Each poison s neutron absorption rate is uniquely dependent on the neutron energy spectra of the system hence, the relative neutronic effectiveness of each poison varies from one system to another. Consequently, a mixture of poisons in a solution may be more effective than dither poison alone. ... 

Numerical calculations have shown that the use of this relation in reactor calculations yields an overestimate of the critical mass. This may be seen from the fact that in estimating the removal rate (effectively the neutron absorption rate) we have ignored the contributions of the higher modes in (8.314). Thus the neutron production rate in the core has been underestimated, and the resulting computation of the critical mass will be larger than that required to maintain the system at steady state. For this reason we call (8.316) the upper approximation. 

For neutrons in equilibrium with moderator at room temperature (293 K), Vq = 2200 ms S corresponding to a kinetic energy for the neutron of 0.0253 eV. Thermal neutron cross sections are customarily quoted for neutrons of this energy. For example, the neutron absorption rate per unit volume of any constituent of the reactor will be given by... 

The method may be illustrated by considering the measurement of the capture-to-fission ratio of Pu. Since we have /c = 1 for the test region, the rate of neutron production within this region must be equal to the neutron absorption rate, i.e.. 

Discuss how the variation of temperature would be detected by use of a l/v and a black absorber, from the viewpoint of cross sections. Do the same from the viewpoint of neutron-absorption rate and reactivity. 

As given in Equation (5.7), the neutron production rate can be expressed as a function of U content. From the production rate, we can calculate the neutron flux (X) with the relation fn = v x n, where v denotes the mean velocity of neutrons and n is an equilibrium concentration of neutrons. The latter quantity is related to the neutron production rate (pn) as n = (pjt) where T denotes the time constant for the neutron absorption in the medium (-2500 s1). Andrew et al. (1986) estimated the average neutron flux in the Stripa granite to be 5.5 x 10 4 neutrons cm 2s, which is in good agreement with the measured flux of 4.7 x 10 4 neutrons cm 2s 1 in the borehole in the granite. 

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... 

Pu) of thermal absorption cross section a , and Ng atoms of fertile material ( U or Th) of thermal absorption cross section Og. For this model we shall develop expressions for the number of neutrons produced or absorbed at any point in the neutron cycle per unit volume per unit time. Assume that the fissionable material absorbs only thermal neutrons. The rate of... 

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. 

During thorium irradiation Pa may exist in sufficient concentration that its destruction by chain-branching neutron absorption can reduce the rate of formation of For this reason, thorium-uranium breeder reactors tend to optimize at lower neutron fluxes, and at lower specific power, than do uranium-plutonium breeders. 

To optimize isotopic purity when producing Cm it is desirable that the irradiation of the "Am target be carried out at low neutron flux. At higher fluxes the increased chain branching by fission of Am and the increased neutron capture in Cm result in greater contamination by Cm per unit amount of Cm produced. The actual production rate of Cm optimizes at a neutron flux of about 8 X 10 n/(cm -s). At higher fluxes the increased chain branching from Am fission more than offsets the increased rate of neutron absorption in Am [K2]. 

The thermal-neutron absorption cross section of natural boron, which contains 19.61 percent is 759 b, whereas that of separated B is 3837 b. Thus, enriched B is useful in applications where the highest volumetric rate of neutron absorption is wanted. Examples are compact shielding for thermal neutrons and control rods for fast reactors. 

The choice of cladding material for fast reactors is less dependent upon the neutron absorption cross section than for thermal reactors. The essential requirements for these materials are high melting point, retention of satisfactory physical and mechanical properties, a low swelling rate when irradiated by large fluences of fast neutrons, and good corrosion resistance, especially to molten sodium. At present, stainless steel is the preferred fuel cladding material for sodium-cooled fast breeder reactors (LMFBRs). For such reactors, the capture cross section is not as important as for thermal neutron reactors. 

The size of the system will vary, depending upon the on the rate of neutron absorption by U and is also pro-K factor of the system, and upon other things. If the portional to the rate at which fissions occur in U . This reproduction factor K is greater than unity, the number 15 in tui n is controlled by the thermal neutron density exist-... 

The rate of production of element 9423 will depend on the rate of neutron absorption by and is also pro- 75... 

A recent innovation imder study in the area of radiation therapy is boron—neutron capture therapy for brain tumor. This form of therapy uses an inactive tracer containing boron, an element with a high absorption rate of neutron radiation. When the radiopharmaceutical has accumulated in the tumor, the patient is exposed to neutron radiation, which is absorbed by boron. The energy of secondary low-range a-radiation, which is formed in the boron—neutron reactions, is then absorbed by the tumor cells that are to be destroyed. 

Criticality (nuclear)— A fission process where the neutron production rate equals the neutron loss rate to absorption or leakage. 

The control rod calibration problem under study in the present discussion is concerned with a special situation where it is desired to calibrate a control rod during a xenon transient. What is meant by a xenon transient is explained briefly in what follows. When a reactor is in operation, certain nuclei with large neutron absorption cross sections are produced, so that they act as poisons. Of these poisons, xenon-135 is the most troublesome. In a reactor operating at power a balance is eventually achieved between rates of formation and loss of the absorbing nuclei, so that an equilibrium concentration is attained in the reactor. However, when a reactor operating at power is shut down, the xenon continues to increase [1, p. 335] without a sufficient neutron flux available to hum out the xenon, so to speak. Thus, the xenon will eventually disappear by radioactive decay, but not before it builds up to a maximum of substantial proportions. The maximum concentration will occur at about 12 hours after shut-down, the magnitude of the peak concentration depending on the power level before shut-down. This explains why, whenever it is necessary to be able to restart a reactor at any time after shutdown (e.g., a submarine reactor), the reactor must be sufficiently fueled so that it is possible to override maximum xenon at any time. 

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