Measurement of neutron fluxes

5 7 ensnrement of Neutron Fluxes. The data presented below have been analyzed and compared with previous calculations. Neutron flux distributions were measured by determining the saturated activity (A ) induced in bare and cadmiUm-covered metal foils. The measurements can be divided into three series of experiments, based on the experimental conditions and the informstion sought in each series. 

In the first group of experiments the northwest thimble was left open. Fluxes were measured- in the fuel lattice., the, northwest quadrant of the graphite reflector, and at other uiscellaneous points in the structure. 

In the second series the northwest thimble hole was closed, as described later, and additional foil exposures were made at many of the locations mentioned above. Neutron attenuations in the water above the lattice up to the elevation of the shim-rod electromagnets were measured also. 

The third series included attenuation in.concrete blocks and in paraffin stacked behind a portion of the steel thermal shield outside- the. graphite s tack.  

Gold foils (1 cm square, 0.005 in. thick) were used for exposures in the graphite. The foils were spaced 6 in apui 1/16-in.-thick aluminum strips 

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The measurement of neutron fluxes by foil activation is more complicated because the neutrons are not monoenergetic and the monitor cross sections are energy dependent. The simplest case is monitoring slow neutron fluxes. Radiative capture (ivy) reactions have their largest cross sections at thermal energies and are thus used in slow neutron monitors. Typical slow neutron activation detectors are Mn, Co, Cu, Ag, In, Dy, and Au. Each of these elements has one or more odd A isotopes with a large thermal (n,y) cross section, 1-2000 barns. The (n,y)... 

Use Measurement of neutron flux, reactor fuels, fluorescence activator in phosphors. 

Fission counters are used extensively for both out-of-core and in-core measurements of neutron flux in nuclear flux in nuclear reactors. In out-of-core situations, they monitor the neutron population during the early stages of power ascension when the neutron flux level is very low. For in-core measurements, fission counters are used for flux mapping (and consequently, determination of the core power distribution). They are manufactured as long thin q lindrical probes that can be driven in and out of the core with the reactor in power. Typical commercial fission counters for in-core use have diameters of about 1.5 mm (0.06 in), use uranium enriched to at least 90 percent in as the sensitive material, and can be used to measure neutron fluxes up to 10 neutrons/(m s) [10 neutrons/(cm s)]. 

Boron Thermopiles. The boron thermopiles are used for an additional check on neutron flux level. They are located, in the graphite. (VG-27, 28, 42, 44, 56 see Fig.- 3.F) all at the same horizontal level. These instruments consist of a large number of thermocouple junctions in series, with alternate junctions coated with boron. The coated junctions become warm owing to absorption of neutrons, thus yielding a net voltage which is a measure of neutron flux. The instrument case is about H in. in diameter by 6 in. long. The approximate sensitivity is 6 mv in a flux of 10. Internal resistance is of the order of 3 to 4 ohms. By proper mounting and thermal insulation of the thermocouples the instruments may be made quite insensitive to ambient temperature (say, 40 /xv/°C). For transient conditions the time constant is about 1 sec. 

W. U. McELROY, "Measurement of Neutron Flux Spectra by a Multiple Fmi Activation Rerative Method and Comparison with Reactor Physics Calculations and Spectrometer Measurements," Ihicl. Sei. Eng.. 36, 109 (1969). 

S. The computer could be used for the automatic control of moderator boron concentration from measurements of neutron flux and moderator height. This would assist in overriding xenon transients and minimize the amount of boron addition and removal. 

The greater the reliability of the individual components within an I C system, the greater the rehabihty of the overall system. There are, however, practical limits to the levels of rehabihty of individual components. Higher reliability is achieved by the use of redundancy or diversity. For example, it may be possible to monitor reactor power with multiple channels or by diverse means such as measurement of neutron flux or temperature and fluid flow or pressure. The use of redundancy provides protection against random failures. Use of diversity provides protection against certain common cause failures. 

B. Measurements of Neutron Flux by Induced Activation Detector Activity... 

Whenever it is necessary to measure reactor neutron flux profiles, a section of wire or foil is inserted directly into the reactor core. The wire or foil remains in the core for the length of time required for activation to the desired level. The cross-section of the flux wire or foil must be known to obtain an accurate flux profile. After activation, the flux wire or foil is rapidly removed from the reactor core and the activity counted. 

The Ar-Ar technique is able to achieve higher levels of internal precision than K-Ar dating since it does not depend upon separate absolute measurements but instead requires only the ratios of Ar isotopes and can achieve precision of better than 0.25%. However, external precision and accuracy are affected by the uncertainty in the age of mineral standards, as we will see in the following section. In order to achieve optimum precision in the mass spectrometric measurements, the neutron flux (which affects the magnitude of the J value) must be carefully selected. The flux must be sufficient for precise... 

In the case of a spherical wave, that is, radiation emitted or scattered by a point source, it is more convenient to express the flux J by the amount of energy transmitted per second through a unit solid angle rather than a unit area. In this way the measure of the flux becomes independent of the distance R from the source to the point of observation. In terms of the particle stream, the flux is then given as the number of photons or neutrons transmitted per solid angle per second. It is also understood that the amplitude A of the spherical wave is so defined that its square still gives the flux J independent of R. 

The derivation of Eqs. 14.3 and 14.7 was based on the assumption that the neutron flux is uniform over the volume of the counter. A measure of the flux uniformity is the value of the factor exp (-2,/), where... 

The LSL-M2 program package determines the neutron energy spectrum based on information obtained from a combination of neutron flux calculations and threshold foil activation measurements. The results of LSL-M2 are used primarily for the determination of radiation damage to reactor components and... 

Figure 17.7 Calculated MDA of iodine as a function of sodium concentration in a sample by neutron activation analysis. Measurement condition neutron flux, 7 x 10 cm s sample weight, 0.1 g irradiation time, 30s counting time, 5min.

The electric conductivity of various samples of germanium and silicon doped with °B during the neutron irradiation was measured by Kervalishvili et al. (1993). In particular, measurements of neutron fields of various nuclear plants with sensors made of these materials have been carried out. The selection of the base material and the concentration of °B impurity allowed the efficiency of neutron detection and flux measurement to be varied. Such neutron dosimeters possess memory and can be used as witnesses of unusual situations in nuclear plants. 

Experiments at Los Alamos determined the critical masses of U235 and Pu239. Adding U235 cubes to a subcritical assembly within blocks of beryllium tamper measurably increased neutron flux. 66. The Los Alamos Tech Area. 

Measurements of neutron and gamma fluxes inside and above the fuel, in the graphite reflector outside the tank, and in the thermal shield. 

Figures A4-R and S show the data for the mid-plane and top plane of the active lattice. Typical curves showing vertical distribution of neutron fluxes in several holes in the graphite reflector are presented in Figs. A4.T. U, and V. Table A4.G lists the therma1-neutron flux values at various mid-plane positions in the graphite reflector with the cadmium ratio at the points where it was measured.

Two major groups of measurements were made at various points in the graphite reflector. In the first experiments the arrangement of the experimental holes and tank thimbles was that of the first series of neutron-flux measurements. The second series of ion chamber measurements corresponds to the second series of neutron experiments. Tables A4.K and L and Figs. A4.0C, DD, EE, and FF contain the results. These data,have been analyzed in ORNL CF-50-7-86,... 

In order to calculate P-T curves, values of toughness are needed at the 14-thickness and %-thickness locations in the RPV wall. Calculations are usually performed to determine the attenuation of neutron flux/fluence from the inside surface of the RPV into the wall. The best available method for making these projections is to use displacements per atom (dpa) as the measure of fluence change since dpa takes into account the change in neutron spectra that occurs as the neutrons are attenuated. Regulatory Guide 1.99, Revision 2 uses this approach and specifies a generic exponential decay function (f = where fx is fluence at depth x in RPV wall,... 

The nuclear instrumentation systems are designed to measure reactor neutron flux over the full operating range of the reactor. These measurements are required as inputs to the reactor regulating system and safety systems. The instrumentation for the safety systems is independent of that used by the reactor regulating system. 

NUclear Instrumentation measures the levels the distribution and the rate of change of neutron flux density in the reactor. Monitoring data and safety circuit trip signals are provided as appropriate over the full range of neutron flux levels from those existing during sub-critical shutdown conditions to those characteristic of full production level operation. 

Insertion of absorber rods and addition of unirradiated absorber elements ensured clearly subcritical conditions at any moment during unloading of the core, which was confirmed by the measured values of neutron flux density. 

We conclude the present chapter with a few remarks on the units of measure for neutron flux and collision density. It is customary to specify neutron densities in neutrons/cm thus, if the neutron speed is given in cm/sec, the flux has the units neutrons/cm /sec. Likewise if, as is the custom, the neutron microscopic cross sections are given in square centimeters and the nuclear densities in nuclei/cm , then the macroscopic cross sections Z are given in cm" and the collision densities in collisions/cmVs We can obtain some idea of the magnitudes of... 

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