Moderators, nuclear system

Critoph, E. "The Thorium Fuel Cycle in Water-Moderated Reactor Systems" Paper IAEA-CN-36/177 at the IAEA International Conference on Nuclear Power and its Fuel Cycle, Salzburg. AECL-2705, 1977... 

These three forms of nuclear systems make respective use of hydrogen as neutron moderator, coolant and nuclear fuel. 

The first large-scale privately-financed nuclear power plant is being built by Consolidated Edison Company of New York. It is also the first thorium power plant in the world. The site of this 55,000,000 station, with a capacity of 200,000 kilowatts, is Indian Point, New York, on the Hudson River about 40 mil s north of New York City. This water-moderated breeder type of nuclear system will supply electricity to about one million New Yorkers starting in 1960. 

AECL is developing a supercritical heavy water moderated nuclear reactor (SCWR) [2] based on its successful CANDU reactor system currently deployed around the world. Since the Mark 2 [2] version of the heavy water moderated SCWR can satisfy the temperature requirements of the hybrid Cu-Cl cycle, AECL is collaborating with ANL in the development of this cycle. Also, AECL is particularly interested in this process since some of its hydrogen-economy related technologies are a good match for the developmental needs of this process, in particular for the development of the electrochemical step involved. 

Table II. Calculated Results for Tbermal Rate Constant Ratios Determined by the Moderated Nuclear Recoil Method in Model Systems...

VII.11] PARKS, C.V., WRIGHT, R.W., JORDAN, W.C., Adequacy of the 123-Group Cross-Section Library for Criticality Analyses of Water-moderated Uranium Systems, Rep. NUREG/CR-6328 (ORNL/TM-12970), US Nuclear Regulatory Commission, Washington, DC (1995). 

Criticality Precautions. The presence of a critical mass of Pu ia a container can result ia a fission chain reaction. Lethal amounts of gamma and neutron radiation are emitted, and a large amount of heat is produced. The assembly can simmer near critical or can make repeated critical excursions. The generation of heat results eventually ia an explosion which destroys the assembly. The quantity of Pu required for a critical mass depends on several factors the form and concentration of the Pu, the geometry of the system, the presence of moderators (water, hydrogen-rich compounds such as polyethylene, cadmium, etc), the proximity of neutron reflectors, the presence of nuclear poisons, and the potential iateraction with neighboring fissile systems (188). As Httle as 509 g of Pu(N02)4 solution at a concentration Pu of 33 g/L ia a spherical container, reflected by an infinite amount of water, is a critical mass (189,190). Evaluation of criticaUty controls is available (32,190). 

Heavy water [11105-15-0] 1 2 produced by a combination of electrolysis and catalytic exchange reactions. Some nuclear reactors (qv) require heavy water as a moderator of neutrons. Plants for the production of heavy water were built by the U.S. government during World War II. These plants, located at Trad, British Columbia, Morgantown, West Virginia, and Savaimah River, South Carolina, have been shut down except for a portion of the Savaimah River plant, which produces heavy water by a three-stage process (see Deuterium and tritium) an H2S/H2O exchange process produces 15% D2O a vacuum distillation increases the concentration to 90% D2O an electrolysis system produces 99.75% D2O (58). 

The DOE N-Reactor is one of the plutonium production reactors located on the Hanford Reservation near Richland, Washington. It is graphite moderated, pressurized water reactors that in addition to production of special nuclear materials also provided steam to turbine generators owned by the Washington Public Power Supply System for electric power production. It began op ition in 1 is put into standby status in 1988 and closed because of similarities to Chernobyl. 

To slow down and control the rate of reaction, a moderator is also required. Typically, the moderator is boric acid, graphite, or heavy water (D20) and is present in the high-purity water, which also serves as a primary coolant for the fuel and the reactor vessel. The tremendous heat generated by nuclear fission is transferred to this closed-loop coolant, which is contained within a reactor primary-coolant circulation system. The high-purity water coolant also contains a suitable pH buffer such as lithium hydroxide, which has the additional effect of limiting the corrosion of fuel-cladding and other components. 

Boric acid [B(OH)3] is employed in primary coolant systems as a soluble, core reactivity controlling agent (moderator). It has a high capture cross-section for neutrons and is typically present to the extent of perhaps 300 to 1,000 ppm (down from perhaps 500 to 2,500 ppm 25 years ago), depending on nuclear reactor plant design and the equilibrium concentration reached with lithium hydroxide. However, boric acid may be present to a maximum extent of 1,200 ppm product in hot power nuclear operations. 

Let us now examine these electronic-nuclear coupling effects in more detail. The moderating exchange of electrons between the molecule and its hypothetical electron reservoir determines the effects of the electronic-nuclear coupling in the open molecular systems. Let us assume the initial electronic and geometric equilibria in such an initially open system p° = p.rej and F° = 0. The LeChatelier stability criteria of these two (decoupled) facets of the molecular structure requires that the conjugate forces A/jl(AN) or AFS(AQS) created by the primary electronic (AN> 0) or nuclear AQs > 0 displacements,... 

The essential ingredients for producing heat in a thermal fission nuclear reactor are the fuel and a moderator. A heat transport system with its coolant is necessary to convey the heat from the reactor to boilers where steam is produced to drive the turbogenerator. The natural materials available for fuel and moderator are uranium ore and water natural uranium extracted from the ore comprises the fissionable isotope uranium-235 and water contains hydrogen which is a good moderator. (Table I)... 

Because early Canadian reactors used heavy water, and because it is also fundamentally the most efficient moderator, Canada naturally adopted the heavy water reactor for the development of a nuclear power system. By using heavy water both as moderator and as coolant, and by refuelling with the reactor at power, it was possible to develop the CANDU system to operate efficiently and economically with natural uranium fuel. This in turn resulted in the simplest possible fuel cycle. 

As the nuclear age progresses, man will be continually exposed to radiation from the radionuclides that are produced in the nuclear events and eventually localize in him. Accordingly, one of the most critical questions is that of assessing the effects upon man of low doses of radiation delivered at low rates or moderate doses delivered at low rates. It is well-known that exposure to 25 to 50 rads may cause biological harm. But what about lower doses Is all radiation harmful Should the extrapolation to a zero-rad dose be linear or curvilinear These questions are the subject for active experimentation in this Laboratory and in others. If the correct extrapolation is linear, it will be crucial to measure accurately the amount of radiation to which a person has been exposed. Such data on gamma-emitting radionuclides can be obtained with a system of the resolution and sensitivity we have described. 

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