Installed Neutron Sources

Which of the following is not a purpose of an installed neutron source in a reactor core ... 

Which ONE of the statements below describes why installed neutron sources are used in reactor cores ... 

A reactor is source critical If neutrons contributed by an installed neutron source are necessary to make K f = 1 and to maintain criticality. A neutron source contributes fast neutrons to the generation cycle, If the source is removed from a source critical reactor, the reactor v/ill become subcritical and shutdown. 

Another way (Anadrill) is to use removable sources. Figure 4-285 shows the sources being installed in the tool at surface. The two sources, neutrons and gammas, are mounted on the same flexible shaft. They are moved from the shield to the sub without being exposed. Furthermore, if the BHA becomes stuck, they can be fished out with an overshot that connects to the fishing head on top of the neutron source. 

Spontaneously fissioning radionuclides may be applied as neutron sources in those cases in which irradiation in a nuclear reactor is not possible, for example if manganese nodules at the bottom of the sea are to be analysed. For that purpose, Cf is a suitable neutron source. It has a half-life of 2.645 y and decays in 96.9% by emission of a particles and in 3.1% by spontaneous fission. It may be installed together with a shielded y-ray detector in the form of a mobile unit. The neutron production of Cf is 2.34 lO s g. The neutron flux density is only of the order of 10 cm s , but this is sufficient for applications in which high sensitivity is not needed. 

The height of filling in technical installations, e.g. in vessels, containers or tubes, can easily be controlled from outside by means of radionuclides as radiation sources. The radionuclides are selected according to the most appropriate y energy for the task, in particular the diameters of the equipment and the thickness of the walls. The phase boundary between two liquid phases dilfering markedly in their properties as neutron moderators can be located from outside by use of a neutron source and a detector for low-energy neutrons. 

Neutron activation is a well-established method of elemental analysis capable of attaining very low limits of detection for most elements. It is not as widely used as other methods because large installations or complex machines are needed to achieve low limits of detection, although this is now changing with the advent of the californium-252 isotopic neutron sources. 

An antimony beryllium neutron source is required in the initial core and all subsequent reloads. Because this needs to be activated by exposure to neutrons, the initial core also requires a californium (plutonium-beryllium is a possible alternate) primary source, which spontaneously emits neutrons during initial core loading, reactor start-up and initial operation of the first core however, the primary source becomes depleted as it becomes irradiated, so it is not a long-term solution. Neutron source assemblies are positioned at opposite sides of the core. Four source assemblies are typically installed in the initial load of the reactor core two primary source assemblies and two secondary source assemblies. Each primary source assembly contains one primary source rod and a number of burnable absorber rods each secondary source assembly contains a symmetrical grouping of secondary source rodlets. Figure 4.2-14 of Reference 6.1 shows the primary source assembly, and Figure 4.2-15 shows the secondary source assembly. 

The PRIMARY reason that a neutron source is installed in the reactor is to ... 

Figure 4-285. Radioactive sources being installed in the neutron-density sub. (Courtesy Anadrill [113].)...

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Mass balance measurements for 41 elements have been made around the Thomas A. Allen Steam Plant in Memphis, Tenn. For one of the three independent cyclone boilers at the plant, the concentration and flow rates of each element were determined for coal, slag tank effluent, fly ash in the precipitator inlet and outlet (collected isokinetically), and fly ash in the stack gases (collected isokinetically). Measurements by neutron activation analysis, spark source mass spectroscopy (with isotope dilution for some elements), and atomic adsorption spectroscopy yielded an approximate balance (closure to within 30% or less) for many elements. Exceptions were those elements such as mercury, which form volatile compounds. For most elements in the fly ash, the newly installed electrostatic precipitator was extremely efficient. 

The first source is installed in the secondary beam lines (H6) from the Super Proton Synchrotron (SPS). A proton beam is stopped in a copper target, 7 cm in diameter and 50 cm in lei jh. These roof-shields produce almost uniform radiation fields over two areas of 2 x 2 m, each divided into 16 squares of 50x50 cm. Each element of these grids represents a reference exposure location. The intensity of the primary beam is monitored by an air-filled, precision ionisation chamber (PIC) at atmospheric pressure. One PIC-count corresponds to 2.2 x 10 particles (error 10%) impii ng on the target. Typical values of dose equivalent rates are 1-2 nSv per PIC-count on top of die 40 cm iron roof-shield and 0.3 nSv per PIC-count outside the 80 cm concrete shields (roof and side). Behind the 80 cm concrete shield, the neutron spectrum has a second pronounced maximum at about 70 MeV and resembles the high-energy component of the radiation field created by cosmic rays at commercial flight altitude. ... 

TA-V installations that could potentially affect or be affected by the HCF include the Annular Core Research Reactor (ACRR), Gamma Irradiation Facility (GIF), Auxiliary Hot Cell Facility (AHCF), Radiation Metrology Laboratory (RML), and the Sandia Pulse Reactor III (SPR III). The GIF provides two cobalt cells for total dose irradiation environments. A new GIF is under construction in the northeast quadrant of TA-V. SPR III provides intense neutron bursts for effects testing of materials and electronics. The RML provides radiation measurement services to Sandia s reactors, isotopic sources, and accelerator facilities. The AHCF provides a capability to handle limited quantities of radioactive material in a shielded cell. These facilities have separate SARs that describe potential accidents. The most severe accidents for all of these facilities involve the release of radiological materials which could necessitate a site evacuation. No physical damage to the HCF could be induced by any of the postulated accidents, nor could any of the HCF accidents physically affect any of the other facilities. 

The major sources contributing to Tc in the environment are fallout from atmospheric nuclear weapons tests and releases from the nuclear fuel cycle, i.e., authorized or accidental releases from nuclear installations (e.g., reprocessing or enrichment plants, nuclear reactors), releases from waste disposal sites, and from dumping of nuclear materials. Contributions from natural processes, i.e., spontaneous fission of in mineral ores such as pitchblende or nuclear reactions in molybdenum ores irradiated with cosmic-ray neutrons are negligible. 

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