Nuclear reactors characteristics

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

Graphite is chosen for use in nuclear reactors because it is the most readily available material with good moderating properties and a low neutron capture cross section. Other features that make its use widespread are its low cost, stabiHty at elevated temperatures in atmospheres free of oxygen and water vapor, good heat transfer characteristics, good mechanical and stmctural properties, and exceUent machinabUity. 

It may seem unlikely that all these conditions could have been met, but at least one deposit of uranium ore has characteristics indicating that, long ago, it operated as a natural nuclear reactor. At Oklo in the Gabon Republic near the western coast of equatorial Africa (see photo), there are uranium deposits of high purity... 

What convinces scientists that sustained fission once occurred at Oklo is the presence of characteristic fission products in the ore. Elements of mass numbers between 75 and 160 occur in the ore in larger amounts than elsewhere. Furthermore, mass analysis of the elements in Oklo ore shows that they are distributed in the characteristic pattern shown in Figure 22-12. This isotopic signature, which is not found in any other naturally occurring materials, is so characteristic that it has convinced most scientists that the ore deposits at Oklo once formed a huge nuclear reactor. 

Flow instabilities are undesirable in boiling, condensing, and other two-phase flow processes for several reasons. Sustained flow oscillations may cause forced mechanical vibration of components or system control problems. Flow oscillations affect the local heat transfer characteristics and may induce boiling crisis (see Sec. 5.4.8). Flow stability becomes of particular importance in water-cooled and watermoderated nuclear reactors and steam generators. It can disturb control systems, or cause mechanical damage. Thus, the designer of such equipment must be able to predict the threshold of flow instability in order to design around it or compensate for it. 

Another characteristic that makes zirconium useful is the production of zircaloy, which does not absorb neutrons as does stainless steel in nuclear reactors. Thus, it is ideal to make nuclear fuel tubes and reactor containers. Zircaloy is the blend (alloy) of zirconium and any of several corrosion resistant metals. 

Curtis, D., Benjamin, T. et al. 1989. The Oklo nuclear reactors cumulative fission yields and nuclear characteristic of reactor 9. Earth and Planetary Science Letters, 89, 193-206. 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

A problem of obtaining zirconium with lowest possible contents of hafnium comes from construction requirements when using zirconium and its alloys in building nuclear reactors. The construction material must have good mechanical properties and must be resistant to corrosion in contact with heat carriers. Since reactor power is proportional to the quantity of neutrons, their absorption into construction materials should be as small as possible. Zirconium and its alloys are unique materials that satisfy these requirements. However, hafnium has approximately the same chemical characteristics as zirconium but it absorbs neutrons strongly. 

General Atomics (GA) and the Commissariat a Ytnergie Atomique (CEA) have been working on sulphur-iodine cycle flow-sheeting for several years, leading to sometimes differing efficiency estimates. They have undertaken to understand and reconcile these differences, and have come to consider in more detail the effect of the VHTR characteristics on the optimisation of the sulphur-iodine flow sheet. This paper will present the outcome of these studies, and stress the interplay between nuclear reactor and chemical process. 

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

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