Water-moderated reactors

Tritium is produced in heavy-water-moderated reactors and sometimes must be separated isotopicaHy from hydrogen and deuterium for disposal. Ultimately, the tritium could be used as fuel in thermonuclear reactors (see Fusionenergy). Nuclear fusion reactions that involve tritium occur at the lowest known temperatures for such reactions. One possible reaction using deuterium produces neutrons that can be used to react with a lithium blanket to breed more tritium. 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

The phrase "nuclear power" covers a number of technologies for producing electric power other than by burning a fossil fuel. Nuclear fission in pressurized water-moderated reactors—light water reactors— represents the enrrent teehnology for nuclear power. Down the line are fast breeder reactors. On the distant horizon is nnclear fusion. 

Redfield, J. A., 1965, CHIC-KIN, A Fortran Program for Intermediate and Fast Transients in a Water Moderated Reactor, USAEC Rep. WAPD TM-479, Westinghouse Electric Corp., Pittsburgh, PA. (5)... 

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... 

The Swedish nuelear program was initiated aheady at the end of the 1940 s and had - at that time - both a defenee and a eivilian side. The original Swedish Line for nuclear reactors was to use Swedish natural uranimn, existing in low-grade minerals in the middle of Sweden, in heavy water moderated reactors. 

Productive use of neutrons. In thermal reactors, the number of neutrons produced per neutron absorbed in fissile material (tj) is of the order of 2.0. One of these neutrons is needed to keep the fission reaction going, but the second neutron, in theory, is available to produce valuable by-products of nuclear power. In practice, of course, some of these extra neutrons are necessarily lost through leakage and absorption in reactor materials, but around 0.6 neutron is available in water-moderated reactors for productive use. Examples of productive uses of... 

This figure illustrates immediately one of the disadvantages of batch fuel management. The power density, which is proportional to the product of the neutron flux and the fissile material concentration, is just as nonuniform as the neutron flux. If the local power density must be kept below some safe upper limit, to keep from overheating the fuel or cladding, only the fuel at the center of the reactor can be allowed to reach this power density, and fuel at all other points wiU be operating at much lower output. In a typical uniformly fueled and poisoned water-moderated reactor, the ratio of peak to average power density is over 3, so that the reactor puts out only one-third as much heat as it could if the power density were uniform. 

Calculate the number of collisions required to reduce a fast fission neutron ( = 2 MeV) to thermal energy ( 0.025 eV) in a light-water-moderated reactor, assuming that the data in Table 19.3 are valid. 

Oxide fuels have demonstrated very satisfactory high-temperature, dimensional, and radiation stability and chemical compatibility with cladding metals and coolant in light-water reactor service. Under the much more severe conditions in a fast reactor, however, even inert UO2 begins to respond to its environment in a manner that is often detrimental to fuel performance. Uranium dioxide is almost exclusively used in light-water-moderated reactors (LWR). Mixed oxides of uranium and plutonium are used in liquid-metal fast breeder reactors (LMFBR). 

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178 the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. 

The control effectiveness of such alloys in water-moderated reactors can approach that of hafnium and is the control material commonly used in pressurized-water reactors. The alloys (generally 80% silver, 15% indium, 5% cadmium) can be readily fabricated and have adequate strength at water-reactor temperatures. The control material is enclosed in a stainless steel tube to protect it from corrosion by the high-temperature water. 

Advantages Excellent control for water-cooled, water-moderated reactors due to neutronic, mechanical, and physical properties. 

Advantages Highly effective neutron absorber. Control effectiveness in water-moderated reactors is close to hafnium. Used in pressurized-water reactors. Easily fabricated and adequate strength... 

The NSSS is equipped with an integral water-cooled water-moderated reactor with inherent safety and the following unique features ... 

Steels used for in-vessel devices (IVD) of water cooled water moderated reactors (VVR) ... 

The old proposed heavy water—moderated reactor had a core volume almost ten times greater than the present one, while the total power output, 3 10 kw, and virgin—neutron mean free path was the same. Its average virgin flux was thus smaller by a factor of 10. A comparison of the slow, resonance, and fast flux in these light water and heavy water reactors is given in Table 4. 2.A. 

A second desirable characteristic of the small-core light water—moderated reactor is that the thermal-neutron flux distribution is essentially flat in the core and for a short distance into the beryllium reflector is equal to or greater than the average core flux. The holdup of thermal neutrons in the... 

Papers 12 and 13 record Wigner s contributions to the instruction manual prepared by the Metallurgical Laboratory for the du Pont Company which had just assumed reponsibility for the Plutonium Project. Even at this early time Wigner had analyzed various combinations of coolant and moderator of particular note is his analysis of the engineering problems associated with a heavy-water moderated reactor. 

This order applies to all varieties of reactors including, but not limited to light water moderated reactors, heavy water moderated reactors, liquid metal cooled reactors, gas cooled reactors and short-pulse transient reactors. Space reactor power and propulsion systems and critical facilities require special design criteria. Attachment 4 is reserved for Nuclear Safety Design for critical facilities and space reactors. 

For the slowing down region of water moderated reactors, codes such as MUFT and SLAG are available to provide the infinite medium flux distributions [20]. For Hanford the Monte Carlo approach is being developed in the RBU code to obtain detailed flux distributions in a complicated geometry [21]. Cross sections are then averaged over these distributions and used in simpler diffusion calculations to calculate the detailed life history of a reactor. 

E. M. Gelbard, G. J. Habetler and R. Ehrlich, The role of digital computers in the design of water moderated reactors, P/1843, Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958. 

F. Schroeder, S. G. Forbes, W. E. Nyer, F. L. Bentzen, and G. O. Bright, Experimental study of transient behavior in a subcooled water moderated reactor. Nuclear Science and Engineering vol. 2 (1957) pp. 96-115. 

Neutrons with energies greater than 0.1 MeV are called fast neutrons. The fission spectrum of a light-water-moderated reactor provides as many fast neutrons as thermal neutrons. Therefore, fast neutron activation of certain elements via (n,p) reactions is a very selective technique, complementary to thermal and epithermal NAA. A typical example is Fe, for which the Fe(n,p) Mn activation reaction produces a better gamma-ray emitter than the thermal capture reaction. 

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