Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/02—Fuel elements
  • G21C3/04—Constructional details
  • G21C3/06—Casings; Jackets
  • G21C3/07—Casings; Jackets characterised by their material, e.g. alloys
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C1/00—Reactor types
  • G21C1/04—Thermal reactors ; Epithermal reactors
  • G21C1/06—Heterogeneous reactors, i.e. in which fuel and moderator are separated
  • G21C1/08—Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being highly pressurised, e.g. boiling water reactor, integral super-heat reactor, pressurised water reactor
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/42—Selection of substances for use as reactor fuel
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C5/00—Moderator or core structure; Selection of materials for use as moderator
  • G21C5/18—Moderator or core structure; Selection of materials for use as moderator characterised by the provision of more than one active zone
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E30/00—Energy generation of nuclear origin
  • Y02E30/30—Nuclear fission reactors

Definitions

• thorium is known to be at least three times as plentiful as uranium in the earth's core, no economic method of producing nuclear power from thorium, with or without proliferative fuels, has been found.
• economic is used herein to mean that most of the nuclear reactor energy is generated from thorium without the very expensive process of extracting the highly gamma-active U-233 and fabricating it into fuel elements.
• thorium contains no natural fissionable material. Thorium can be made to produce energy only by (1) an initial addition of fissionable material, as is described in the report entitled “Thorium Utilization in PWRS”, ' ⁇ Kernutzutz J ⁇ lich GmbH (1988) , or (2) providing a neutron current into the thorium regions of the core, using a “seed-blanket” arrangement, as described in the "CRC Handbook of Nuclear Reactor Calculations", 1986, Volume III, pp. 365-448.
• seed- blanket core arrangements have been used as described in the "CRC Handbook of Nuclear Reactor Calculations", supra.
• Such cores consist of seed regions which have multiplication (criticality) factors greater than one and blanket regions with multiplication factors less than one.
• the blanket regions have been constructed primarily of natural thorium and the seeds have contained either U-235 or U-233 of weapons grade quality.
• the cores have been controlled typically by upward motion of each seed region from a position well below the core. This method of control has resulted in severe mechanical problems because of the heavy weight of the seeds to be moved. Furthermore, heat removal is difficult because of great variations in the power levels throughout the length and width of the core.
• a nuclear reactor core having one or more seed regions containing seed fuel elements essentially comprising U-235 and U-238 in the maximum ratio which is nonproliferative; a blanket region surrounding the seed region(s) containing blanket fuel elements essentially comprising Th-232 with a small percentage of nonproliferative uranium; and a nonparasitic mechanically simplified control system, all of which are described in detail below.
• Seed Regions These regions contain fuel elements of U-235/U-238, preferably in the ratio of 20:80, in the shape of rods and/or plates consisting of uranium-zirconium alloy.
• the water to fuel element volume ratio is in the range of six to approximately ten, far above the accepted norms of approximately two to one in conventional reactors.
• the high water content results in a resonance escape probability of above 0.95 in the U-238.
• the reduction of plutonium output comes first of all from the change in enrichment.
• a change in enrichment from the conventional value of U-235/U-238 (3:97) to U-235/U-238 (20:80) reduces plutonium production by a factor of seven.
• the high value of the resonance escape probability of the seed fuel further reduces the rate of plutonium production by a factor of six.
• the high value of the resonance escape probability also results in a high value of the seed multiplication factor, which increases the proportion of energy obtained from the blanket to the range of seventy-five to eighty percent of the total core power. Taking into account that the seed regions produce only twenty to twenty-five percent of the core power, it is evident that the rate of production of plutonium in the seed regions is well below one percent of that in a conventional reactor.
• the seed regions also contain some blanket fuel elements and are referred to as "composite seed-blanket regions".
• the blanket region contains fuel elements of mixed thorium-uranium oxide rods and/or plates.
• the uranium oxide volume content in the thorium-uranium mixture is in the range of six to approximately ten percent.
• the uranium oxide is U-235/U-238 in the ratio of 20:80.
• the water to fuel volume ratio is in the range of .8 to 1.5.
• the blanket multiplication factor stays approximately constant during an irradiation of about 100,000 MWD/T. An irradiation of this magnitude has been shown to be feasible by experiments in Oak Ridge, Tennessee. See "Irradiation Behavior of Thorium-Uranium Alloys and Compounds" by A.R.
• the U-238 inserted in the thorium serves a further purpose by being mixed uniformly with and thus denaturing the U-233 remnant in, the thorium at the end of the blanket life.
• the plutonium production rate will be, at most, 0.6 percent of that of a conventional core (eight percent U-238 content times seventy-five percent blanket power share divided by ten to twelve years of the blanket residence in the core) .
• the blanket fuel elements may be of solid cylindrical shape or of annular shape with the center hole open to the water.
• the annular shape has superior nuclear and heat removal characteristics, but this shape requires internal as well as external cladding.
• the term "blanket” is also used to describe the regions in the reflector around the core which are utilized primarily to reduce neutron leakage from the core.
• Such blankets will have fuel compositions and fuel element shapes similar to those described above, except that depleted uranium would be used instead of the U-235/U-238 (20:80). The purpose of the depleted uranium is to ensure that any U-233 formed in these reflective blanket regions will be denatured by U-238.
• Nonparasitic Control System A nonparasitic control system is provided to increase safety and maximize the amount of core energy obtainable from the thorium. This control system ensures that all neutrons available from the seed are utilized usefully in the core blanket region, thus minimizing the number of fissions required in the seed regions. This is in contrast to conventional cores in which all excess neutrons are wasted by absorption in parasitic control materials.
• control system requires a uniform motion of the control rods of only approximately forty-five centimeters throughout the core length, as contrasted with the travel over the whole core length, typically about twelve feet, of conventional control rods.
• the operating principle of the control system according to the invention depends upon the fact that the seed regions have a high multiplication factor, with correspondingly high neutron leakage, such that the core reactivity is greatly affected by small changes in effective seed dimensions.
• Fig. 1 is a schematic diagram of a pressurized water reactor power system of the type to which the present invention relates.
• Fig. 2 is a schematic diagram of a seed/blanket core of a nuclear reactor of the type to which the present invention relates.
• Fig. 3 is a diagram showing the neutron absorption probability of U-238 over a spectrum of neutron energies.
• Fig. 4 is a diagram showing the multiplication factor of a natural thorium oxide blanket with respect to time as compared to that of a thorium oxide blanket having some initial fissile fuel.
• Fig. 5 is a diagram showing the blanket energy production of various thorium and uranium blankets for given inputs of seed neutrons.
• Fig. 6 is a diagram showing the wasted neutrons over time in a nuclear reactor core controlled by conventional means.
• Fig. 7, comprising Figs. 7a-7d are schematic diagrams of a single seed/control/blanket assembly illustrating the principle of the nonparasitic control system of the invention. These Figs, show the maximum and minimum reactivity positions, respectively, of the control system.
• Figs. 7a and 7b the control system depicted indicates the movement of both seed type fuel (20% Uranium- 235, 80% Uranium-238) and blanket fuel (thorium-uranium oxide) in the operation of the control system.
• seed type fuel elements only are moved in the operation of the control system.
• Figs. 8a and 8b are horizontal sections (plan views) of a portion of a nuclear reactor core according to the invention showing respectively two equally preferred embodiments, which will be referred to for convenience as first and second preferred embodiments.
• Figs. 9a and 9b are vertical sections (elevational views) of one-half a nuclear reactor core showing the first and second preferred embodiments of Figs. 8a and 8b, respectfully, for the first seed cycle and each subsequent odd numbered seed cycle. Similarly, Figs. 9c and 9d apply to the second cycle and each subsequent even numbered seed cycle.
• Figs. 10a and 10b corresponding to Figs. 9a and 9b, are representational elevational views showing a portion of the control regions in their maximum reactivity positions.
• Figs. 10c and lOd apply similarly to Figs. 9c and 9d.
• Figs. 11a to lid corresponding to Figs. 10a to lOd, are representational elevational views showing the control regions in their minimum reactivity positions.
• Figs. 9 to 11 apply except that movable blanket tyupe fuel elements are omitted. DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Fig. 1 schematically illustrates a pressurized light water nuclear reactor power system (pressurized water reactor or "PWR") of the type to which the present invention relates.
• PWR pressurized water reactor
• this system comprises two fluid circuits between the nuclear reactor, which is the heat source, and a steam turbine which drives an electric generator.
• the primary fluid circuit maintains ordinary (light) water under pressure to prevent the formation of steam.
• This water is heated in the nuclear reactor pressure vessel and supplied to a steam generator which transfers heat energy to ordinary (light) water of the secondary fluid circuit.
• the water in the secondary circuit is converted to steam which is used to drive the steam turbine.
• Systems of this type are well known and are described in detail, for example, in Nuclear Fuel Management, H.W. Graves, Jr. , John Wiley & Sons, New York (1980) .
• the present invention relates specifically to the nature of the nuclear reactor core.
• the reactor core is fueled by a fissionable (fissile) material such as the isotope uranium-235 (U-235) .
• a fissionable (fissile) material such as the isotope uranium-235 (U-235) .
• U-235 isotope uranium-235
• natural uranium contains only about 0.7 percent U-235, the rest being nonfi ⁇ sionable U-238, this natural uranium is "enriched" until the U-235 is about 3 to 4 percent of the total.
• a sufficient amount of such enriched uranium fuel can provide enough energy for a year to eighteen months of reactor operation.
• uranium oxide is used, usually in the form of 1 cm. diameter rods clad in zirconium, a metal which has good corrosion resistance and very little neutron absorption. It is also possible to use a metallic alloy of uranium and zirconium, either in the form of rods or plates.
• uranium oxide fuel elements in the nuclear reactor core there are two possible arrangements for the uranium oxide fuel elements in the nuclear reactor core. The most common arrangement is for all the uranium rods or plates to have the same enrichment. Another arrangement, which is illustrated in Fig. 2, includes a number of small islands of moderately enriched uranium, having a reactivity greater than one, surrounded by regions of fertile material which have a reactivity less than one: for example, natural uranium or thorium.
• seed- blanket core This type of arrangement has come to be called a "seed- blanket" core, the islands being called “seeds” and the surrounding region the “blankets". Since the blanket regions have a reactivity of less than one and the seed regions a reactivity greater than one, the seeds supply the neutrons needed to keep the blanket neutron population at a high enough level to generate the fissions necessary for the rated power. Seed-blanket cores have operated successfully for over 30 years at the world's first commercial nuclear power plant at Shippingport, Pennsylvania.
• the U-233 formed in the thorium is fissioned ("burnt") in place so that it is not necessary to fabricate U-233 fuel elements.
• the thorium in the form of oxide is retained in the core for its full metallurgical lifetime. If fissionable material were added to the thorium to make it critical (reactivity greater than one) for such a long lifetime, so much would be required that there would be no space for the thorium.
• the present invention therefore employs a seed-blanket core arrangement, as shown in Fig.
• the thorium in the form of oxide can be left as a blanket in the core for 10 or more years, and only the seed regions need be replaced at the end of a normal refueling period.
• the blanket is always subcritical with a reactivity of about 0.9, which is designed to be nearly constant during operation.
• the seed regions must therefore supply about 10% of the blanket neutron population.
• an objective of the present invention is to keep the plutonium production rate to a minimum: to about 1 to 2% of that of a conventional reactor core.
• the seed regions therefore utilize 20% enriched uranium, (20% U-235 and 80% U-238) ; that is, approximately the highest enrichment of uranium which is nonproliferative.
• Fig. 3 shows the neutron absorption of U-238 versus neutron energy, evidencing that U-238 has sharp lines, called resonances, at higher energies, where the absorption of neutrons, to make plutonium, is most intense.
• the high energy fission neutrons are reduced to low energies, bypassing the resonances.
• thorium has resonances similar to those of U-238, the low energy neutrons coming from the seed regions to the blanket regions bypass the blanket resonances and are thus used more efficiently. While the water to fuel volume ratio in the seed regions is higher than in a conventional core, that in the blanket regions is lower, so that over-all core volume is no greater than that of a conventional core of the same power output.
• two objectives are served by the relatively high (20%) enrichment of the seed fuel: (1) the reduction to a very low level of the amount of plutonium created in the seed regions, and (2) (for a given power generated in the seed regions) maximizing the number of neutrons into the blanket so as to increase the amount of energy generated from the thorium.
• the blanket design instead of using pure thorium oxide, a few percent of 20% enriched uranium oxide is initially added to the fuel elements. This again has two purposes. Without the uranium, the thorium would be "dead" at the beginning, since it contains no fissionable material. Consequently, all the power would have to be generated in the small seed regions, and overheating would result. By enriching the thorium, the blanket immediately starts to generate power and, as the U-233 content builds up, the blanket maintains an almost constant reactivity for very high burn-up, over a period of 10 to 12 years. This effect is illustrated by the two curves in Fig. 4. The blanket power is maintained by burning the U-233 as it is formed in place.
• a thorium blanket produces nearly twice as much energy as does a natural uranium blanket. Also, the thorium blanket with a small amount of U-235, as in the present case, starts much higher and remains higher in energy output than a natural thorium blanket.
• An important aspect of the present invention is the system of control which results in major gains in safety and in reduction of costs, as well as advancing the objective of nonproliferation.
• This control system actually overcomes a basic defect in the control method of conventional power reactors.
• the core initially must contain much more than the amount of enriched uranium needed to just sustain a chain reaction (reactivity of 1.0).
• "control" materials with high neutron absorption are inserted into the core.
• the control system according to the present invention is mechanically simple and ensures that all neutrons originating in the seed are absorbed usefully in the thorium to make U-233.
• the control system is entirely "nonparasitic"; i.e., nonwasteful of neutrons.
• control system may be visualized as a kind of "Venetian blind” in which each control element has to move only a small distance to go from “light to dark”, from high reactivity to shutdown.
• control rods in a conventional core are like a “window shade” in having to traverse the whole length of the core to go from maximum to minimum reactivity.
• Fig. 7 illustrates schematically the method of operation of the nonparasitic control system.
• the seed is divided into vertical layers each approximately 45 cm. long. If we number successive layers as #14 and #15, each #14 layer has higher fuel density in the seed fuel elements than in the #15 layer.
• Fig. 7a shows the position of maximum reactivity.
• Movable seed fuel elements in the center of the seed on the #14 layers are connected by zirconium extensions, located in the #15 layers.
• Movable blanket (mixed thorium uranium oxide) fuel elements in the center of the seed on the #15 layers are connected by zirconium extensions, located in the #14 layers.
• the movable blanket fuel elements are positioned on either side of the movable seed extensions.
• Fig. 7b shows the position of minimum reactivity (shutdown) .
• the movable seed elements are now located in the #15 layers, and the movable blanket elements are now located in the #14 layers between the stationary seed fuel.
• the reactivity of the core has been decreased because: (1) the movable high density seed fuel has moved to a volume of lower multiplication factor; and (2) The regions of stationary high density seed fuel elements are now separated by blanket fuel, causing these regions to have a lower effective thickness and thus much higher leakage of neutrons to blanket fuel.
• the control system according to the present invention is also much simpler mechanically than conventional control systems for nuclear reactor cores.
• the pressure vessel is one of the most expensive items in a nuclear power plant.
• the present control system enables the pressure vessel height to be reduced with consequent lower cost.
• the present control system both improves safety and reduces the initial construction cost.
• Fig. 8 shows two preferred- geometries for the composite seed-blanket regions according to the present invention: In Fig. 8a relatively small annuli and in Fig. 8b much larger and relatively narrower annuli. Seed fuel elements 11 are surrounded by blanket fuel elements 12. The control assemblies 13 are located in the center of the annuli.
• Figs. 9a and 9b show the vertical structures of the stationary portions of the composite seed-blanket assemblies of Figs. 8a and 8b, respectively. These assemblies are made up of alternating forty-five centimeter thick layers 14 and 15.
• Layer 14 consists primarily of seed fuel elements.
• Layer 15 consists of blanket fuel elements and seed fuel elements of reduced uranium content. Since it is necessary to refuel the seed at intervals of twelve to eighteen months while the blanket fuel remains in the core for ten to twelve years, the following construction is adopted to permit separate removal of the seed fuel. Advantage is taken of the large spacing of e seed fuel elements. As shown in Figs.
• stationary seed fuel elements 16 consist of a sequence of forty-five centimeter lengths of uranium- zirconium alloy 17 alternating with forty-five centimeter lengths of reduced content uranium-zirconium alloy 18 throughout the length of the core. Thus all the seed fuel elements 16 can be removed from the core and replaced by fresh fuel, while leaving all the blanket fuel elements in place.
• Figs. 10 and 11 also show the details of the nonparasitic control system.
• the movable seed fuel elements 19 of the control assembly 13 consist of a sequence of forty-five centimeter lengths 20 of uranium-zirconium alloy alternating with forty-five centimeter lengths 21 of pure zircalloy throughout the length of the core.
• the movable blanket fuel elements 22 of the control assembly 13 consist of a sequence of forty-five centimeter lengths 23 of thorium-uranium oxide alternating with forty-five centimeter lengths 24 of pure zircalloy throughout the length of the core. These blanket fuel elements 22 extend between the seed fuel elements 16 and 19.
• the spacing of uranium- zirconium lengths 20, when opposite the layers 14, takes into account the water displaced by zircalloy connectors 24.
• the seed fuel elements 19 of the control assembly are moved down forty-five centimeters from layers 14 to layers 15.
• the blanket fuel elements 22 move from layers 15 to 14. Just the opposite motion is used to increase reactivity.
• Both the blanket and seed fuel elements of the control system have yoked drives 25 and 26 (Fig. 9) , which move together while the reactor is in operation. During shutdown for seed refueling the drives can be unyoked and the seed fuel elements removed and replaced without disturbing the blanket fuel elements of the control system.
• An important feature of the invention is the provision of uniform axial depletion of the blanket fuel. It is evident that, since the seed fuel is of lower density in layer #15 than in layer #14, there will be lower seed power in layer #15 and hence fewer neutrons supplied to the blanket, resulting in lower blanket power on that level.
• the moving blanket fuel For the movable blanket fuel there is no problem. When a fresh seed is inserted (seed reactivity a maximum) , the moving blanket fuel will be located in layer #14. As the seed depletes, the moving blanket fuel will gradually descend to layer #15. Thus in the course of a seed lifetime, the moving blanket fuel will experience approximately equal exposure to the seed fuel on both layers.
• each successive seed has the relative positions of the #14 and #15 layers reversed, as shown in Figs. 9c and 9d, 10c and lOd, and lie and lid.
• a separate control drive 28 may be provided for each annulus, or a common control drive may be provided for two or more annuli.
• a number of separate control drives 28 may be provided as shown.
• each of the seed regions are set by a compromise between minimizing the number of seeds so as to simplify the core design, yet having enough seeds to provide as uniform a power distribution as possible within the blanket.
• the height of the axial layers which is also the length of the stroke of the control mechanism, is set by a compromise between making the control stroke as small as possible, yet not having the sensitivity (change of reactivity per unit length) so large as to cause problems in the control drive mechanism.
• Table II sets forth typical operating parameters for a 1300 megawatt electric pressurized water reactor employing the principles of the present invention.
• Rhoads et al., "DOT- Two Dimensional Discrete Ordinates Radiation Transport Code", ORNL CCC-276, Oak Ridge Laboratory, Oak Ridge, Tenn., (1976) and W.W. Engle, Jr., "ANISN - A One-dimensional Discrete Ordinates", Transport Code with Anisotropic Scattering, K-1699, Oak Ridge National Laboratory, Oak Ridge, Tenn., (1967).
• Seed Region a. The principal source of plutonium in the seed is the capture of neutrons by the resonances of the U-238, which forms eighty percent of the uranium fuel of the seed.
• the fraction of neutrons which escape such capture by U-238 may be denoted by p, the resonance escape probability.
• 1 - p is the fraction of neutrons captured by the U-238, resulting in the formation of plutonium.
• the water to fuel volume ratio in the blanket (in the range of 0.8 to 1.5) and the fraction (in the range of 6 to 10 percent) of uranium oxide (U-235/U-238 in the ratio of 20:80) are chosen so as to keep the blanket multiplication factor, k B , as high and as constant as possible over the entire blanket lifetime of 100,000 MWD/T.
• the blanket multiplication factor k B is defined as usual as the number of neutrons produced per neutron absorbed. Many complex factors are involved so that the optimum choices must be determined by computer calculations. Representative curves are given on pp 384-5 in "Seed-Blanket Reactors", CRC Handbook of Nuclear Reactor Calculations, Volume III, CRC Press, (1986) .
• P B is the power in the blanket
• P s is the power in the seed
• k ⁇ is the multiplication factor of the blanket
• k s is the multiplication factor of the seed
• *k BS is related to the current of thermal neutrons from the blanket to the seed.
• the sign of 5 k BS is negative; however, with the present invention, because of the very high water content of the seed, the sign of 5 k BS is positive.
• the magnitude of 5 k BS is about 0.25, but it strongly influences the ratio of blanket to seed power, as will be seen in the following numerical example.
• the lowest value of k s (when the seeds are about to be discharged) is about 1.4.
• the average value of P B is about 0.93. Due to the inclusion of the *k BS term, the ratio of P B to (P B + P s ) is over 0.8, so that more than eighty percent of the core power is derived from the blanket. c.
• the U-238 will absorb about as many neutrons as a similar amount of U-238 in a conventional uranium reactor core.
• the maximum amount of U-238 in the blanket is eight percent (taking the upper range of ten percent uranium content in the blanket) . Since the blanket will stay in the core at least ten years, the plutonium production rate per year will be 0.8 percent of that of a conventional core. The rate of production is actually about 0.6 percent of a conventional core (i.e., 0.8 x 0.75) since the blanket produces approximately seventy-five percent of the power of a conventional core.
• control system motion of approximately forty-five centimeters was calculated on the basis of highly accurate codes ANISN and DOT 4.2, utilizing fifteen energy groups.
• the neutrons in a reactor are distributed over a wide spectrum of energies ranging from over a million volts to a fraction of one electron volt. To make sure that all these neutron energies are properly treated, the spectrum of neutron energies is divided into a large number of groups. In the present calculations, it was found that increasing the number of groups above fifteen made no appreciable difference in the results. Thus, it was concluded that the use of fifteen neutron energy groups was adequate.
• the nuclear reactor core according to the present invention obtains about seventy-five percent of its power from thorium or Th-232. Therefore, some words of explanation about this fuel are appropriate.
• Thorium is quite widespread in nature.
• the ores of interest contain five to eight percent thorium, as contrasted with one to four percent for uranium ores.
• the thorium utilized in the present reactor core blanket is in the form of oxide, just as uranium oxide is utilized in conventional cores.
• the manufacturing processes for thorium oxide and uranium oxide are very similar. Thus no new techniques or tools are required for manufacturing thorium fuel elements.
• thorium differs from uranium
• Thorium is at least three times as abundant as uranium. There are major supplies in India and Brazil. Very little prospecting for thorium has been done since its market price is very low.
• Natural thorium contains absolutely no fissionable material.
• Thorium has about three times the neutron absorption probability of U-238.
• U-233 When thorium absorbs a neutron, after about one month it transmutes to U-233, a fissionable form of uranium.
• the U-233 can be used for weapons, just as U-235 and Pu-239.
• U-233 is superior since it emits about 10% more neutrons per neutron absorbed than either U-235 or Pu- 239.
• U-233 emits intense gamma radiation. For this reason, fabrication of U-233 into fuel elements must be done remotely, behind heavy shielding, a very expensive process. In contrast, U-235 can be handled without any special precautions. The handling of plutonium requires the use of face masks to prevent inhalation, so that plutonium fabrication is more expensive than for U-235, but much less expensive than for U-233.
• Thorium oxide has superior metallurgical properties to uranium oxide, in that thorium oxide can withstand 10% or more of the atoms fissioned, more than twice as much as for uranium oxide. This is because thorium oxide forms a perfect cubic lattice, which is very strong, while uranium oxide has a structure with many irregularities. The present invention takes advantage of this property of thorium.
• Thorium oxide has a higher melting temperature, as well as better thermal conductivity, than uranium oxide, which results in a greater resistance to meltdown in case of a loss of coolant accident.
• Nonproliferation The United States Department of Defense is understandably concerned about the tonnages of plutonium generated by today's reactors. An even greater danger is posed by countries like Japan, which are planning to build sodium cooled fast breeder reactors that will produce vast quantities of weapons grade plutonium, only few kilograms of which are needed for a nuclear bomb.
• the main item in the cost of operating a conventional nuclear reactor today is the uranium fuel.
• the cost of fueling a core constructed in accordance with the present invention will be reduced by at least 2/3 since only 20 to 25% of the useable energy will be obtained from uranium.
• the cost of fueling the core will also be reduced because 3/4 of the core (the thorium blanket region) will last for 10 to 12 years instead of the three years of a conventional core.
• Other substantial savings are also available in the initial cost of constructing the core.
• Nuclear Waste The nuclear reactor according to the present invention discharges less than half the high level nuclear waste than conventional reactors.
• the seed fuel employed in the reactor core according to the present invention is 20% U-235/80% U-238. This is the type of fuel which the U.S. Department ⁇ f Energy specifies for all research reactors, since even an infinite quantity of this fuel could not produce a nuclear-explosion. As this fuel burns, the ratio of U-235 to U-238 is reduced.
• the fuel discharged from the blanket cannot be used for nuclear bombs for two reasons: a.
• the only fissionable fuel created in the blanket is U-233, but it will be denatured by being uniformly mixed with relatively large amounts of nonfissionable isotopes which are: the U-238 that was included in the blanket at the start, and U-232 and U-234, which are created during operation.
• the U-233 discharged from the blanket will be accompanied by extremely intense gamma radiation. For this reason alone it would be impracticable to build a useful nuclear weapon from the U-233 because of the great weight of gamma shielding required for handling and personnel protection.
• a conventional light water nuclear electrical power plant spends about $90,000,000 a year to replace one third of the reactor core.
• the core according to the present invention where only the relatively small seed regions are replaced each 12 to 18 months, this amount is reduced by half.
• the blanket will be replaced once in ten to twelve years. Its cost will be much less than that of the seed regions and will be spread over many years.
• the basic reason for this economic advantage is that about seventy to eighty percent of the energy is obtained from the thorium which, at present, is essentially "free". This load factor is achieved by using a nonparasitic control system, which greatly reduces the number of neutrons required from the seed.
• the core design according to the present invention results in a saving of about fifteen to twenty percent of the total plant costs.
• This saving is attributed to (1) the elimination of the soluble boron system with thousands of feet of pipe, mixing tanks, filters, injectors, etc. ; (2) the reduction in the cost and complexity of the control rod drives; (3) the reduced height of the pressure vessel and (4) the resultant reduced size of the containment.
• the so-called "load following" in a conventional reactor core is both slow and cumbersome due to the soluble boron control system. This is particularly so at the beginning of an operating cycle when there is a lot of boron in the core.
• the so-called “throttle control” technique can be used. This means that if there is an increase in power demand, the throttle is opened allowing more cold water to flow into the core increasing the reactivity and then the power level. With a conventional core the cooling water increases the density and the concentration of the dissolved boron reducing the reactivity. To overcome this difficulty in conventional cores, additional special control rods ("half" rods and “gray” rods) are installed at considerable extra expense. Further, the slow response to power demand changes means that some power is wasted, increasing operating expenses. 3. Safety
• the reactor core concept according to the present invention is superior from the safety standpoint to conventional light water reactor cores in the following respects:
• control rods and drive mechanisms extend approximately three times the core height of about twelve feet (i.e., a total of thirty-six feet) for a 1000 MWe rating.
• Each typical rod terminates in twenty-seven absorbing pins, each twelve feet long and one centimeter in diameter, which must be inserted into holes in the fuel assemblies. It is evident that driving such thin pins from more than twenty-four feet away involves a risk that the pins will suffer some distortion which could prevent them from penetrating the core.
• LOFA loss of flow accident
• control system according to the present invention requires a movement of only about forty- five centimeters and therefore will shut down the core much more quickly.
• the present arrangement is also such that distortion is much less likely.
• the core according to the present invention has several points of superiority.
• the seed regions with their high neutron leakage will behave much like small cores.
• the water in the seeds will start to boil first, resulting in a quick reduction of reactivity.
• the fuel elements in the seed regions are preferably of metallic, uranium-zirconium alloy, which have much less stored heat than the ceramic, uranium oxide fuel elements of conventional reactors.
• Even the blanket region of the present core has an advantage over conventional cores, since thorium oxide has higher thermal conductivity than uranium oxide.
• Conventional light water reactors now utilize boric acid in the coolant to control the reactivity and power level of the core during operation.
• the reactor core according to the present invention has no advantage over conventional cores since the quantity of such waste depends only upon the total energy generated.
• the amount of radioactivity the present core will discharge will be less than half the amount from a conventional reactor core.
• the explanation is as follows:
• the seed regions which are refueled every twelve to eighteen months, will discharge high level waste at the same proportionate rate as a conventional reactor, but only twenty to twenty-five percent of the total energy is generated in the seeds.
• the blanket region which stays in the core for ten to twelve years, the radioactivity of the high level wastes will decrease by at least a factor of seven, simply because these wastes disintegrate rapidly and form residues with much smaller amounts of radioactivity. This process will be aided by neutron absorption in the high level waste while it is in the core, which also results in transmutation to nuclei which are less radioactive.
• the radioactivity of the high level waste discharged from the blanket will be less by at least a factor of seven than the proportionate amount discharged from a conventional reactor core. If the amount of radioactivity produced from both the seed and blanket regions is weighted by the amount of energy produced from each region (twenty to twenty-five % from the seed regions, eighty to seventy-five % from the blanket) , the total radioactive waste discharged can be shown to be well below half of the high level waste discharged from a conventional core.

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• 1993
  • 1993-02-04 CZ CZ941812A patent/CZ181294A3/cs unknown
  • 1993-02-04 AU AU36116/93A patent/AU3611693A/en not_active Abandoned
  • 1993-02-04 HU HU9402276A patent/HUT68211A/hu unknown
  • 1993-02-04 BR BR9305893A patent/BR9305893A/pt not_active Application Discontinuation
  • 1993-02-04 JP JP5514189A patent/JPH07503545A/ja active Pending
  • 1993-02-04 SK SK934-94A patent/SK93494A3/sk unknown
  • 1993-02-04 EP EP93904924A patent/EP0625279A4/en not_active Withdrawn
  • 1993-02-04 CA CA002128514A patent/CA2128514A1/en not_active Abandoned
  • 1993-02-04 WO PCT/US1993/001037 patent/WO1993016477A1/en not_active Application Discontinuation
• 1994
  • 1994-08-03 NO NO942877A patent/NO942877L/no unknown
  • 1994-08-03 BG BG98951A patent/BG98951A/xx unknown
  • 1994-08-03 FI FI943610A patent/FI943610A/sv not_active Application Discontinuation
  • 1994-08-04 KR KR1019940702679A patent/KR950700594A/ko not_active Application Discontinuation

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