Thorium-Fueled Reactors

Figure 3.2 Principal nuclear reactions in thorium-fueled reactors.

A second type of GCR used the pebble bed concept with helium as a coolant. The uranium and thorium fuel was imbedded in graphite spheres and cooled with helium. The high temperature thorium fueled reactor (THTR) operated between 1985 and 1989 in Germany. It produced 760 MWt and 307 MWe. The thorium in the fuel pellets was used to breed Two GCR power plants have been operated in the United States. The first was Peach Bottom Unit 1, which provided 40 MWe. The second was the Fort St. Vrain reactor, which provided 330 MWe. 

XXX-4] FURUKAWA, K., KATO, Y, OHMICHI, T., OHNO, H., The combined system of accelerator molten-salt breeder (AMSB) and molten-salt converter reactor (MSCR), Japan-US Seminar on Thorium Fuel Reactors (October 1982, Nara, Japan), Thorium Fuel Reactors, Atomic Energy Society of Japan (1985) p. 271-281. [Russian Translation Atomnaja Technika za Rubezhom , 1983 [6], p. 23-29] (1983). 

A variation of the classical fuel cycle is the breeder cycle. Special breeder reactors are used to convert fertile isotopes iato fissile isotopes, which creates more fuel than is burned (see Nuclear reactors, reactor types). There are two viable breeder cycles U/ Pu, and Th/ U. The thorium fuels were, however, not ia use as of 1995. A breeder economy implies the existence of both breeder reactors that generate and nonbreeder reactors that consume the fissile material. The breeder reactor fuel cycle has been partially implemented ia France and the U.K. 

AECL has evaluated some of the basic information and development requirements in some detail (24, 25) and has outlined the type of fuel recycle development program which would be required. It would involve research and development of thorium fuels and fuel fabrication methods, reprocessing, demonstration of fuel management techniques and physics characteristics in existing CANDU reactors and demonstration of technology in health, safety, environmental, security and economics aspects of fuel recycle. 

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... 

Having now determined to total amount of nuclear electricity required, the thorium fuel input to the energy amplifiers can be calculated from the design data of Rubbia and Rubio (1996). The thermal output from the prototype design reactor is 1500 MW, with a fuel amount of 27.6 t in the reactor (Fig. 5.42). The fuel will sit in the reactor heat-generating unit for 5 years, after which the "spent" fuel will be reprocessed to allow for manufacture of a new fuel load with only 2.9 t of fresh thorium oxide supply. This means that 2.6/5 t y of thorium fuel is required for delivery of 5 x 1500 MWy of thermal power over 5 years, or 675 MWy of electric power, of which the 75 MWy is used for powering the accelerator and other in-plant loads. The bottom line is that 1 kg of thorium fuel produces very close to 1 MWy of electric power, and 1 kt thorium produces close to 1 TWh. ... 

The mean thermal expansion coefficient is given in Table 6.5. Because thorium crystallizes in the cubic system, it expands equally in all directions and is not subject to as much distortion on thermal cycling as uranium. For this reason, and because the a- transition temperature in thorium is much hi er than in uranium, thorium metal reactor fuel has much better limensional stability than uranium metal. 

Thorium makeup requirements for one reactor system, the HTGR (high-temperature gas-cooled reactor), may be estimated from Fig. 3.33. A 1000-MWe HTGR requires 7.4 MT of thorium as feed pet year. Reprocessing recovers 6.8 MT, which can be recycled after storage for 20 to 30 years to permit excess Th to decay. The net thorium consumption of a 1000-MWe reactor then is 0.6 MT/year. Thus, the 441,000 MT of U.S. ThOa thorium reserves listed in Table 6.14 would provide thorium fuel for... 

Because of the relatively small amount of high-mass plutonium nuclides produced in uranium-thorium fueling, the amounts of americium and curium produced are about two orders of magnitude less than in a uranium-fueled reactor with plutonium recycle. 

The calculated elemental composition, radioactivity, and decay-heat rate for discharge fuel are shown in Table 8.7 for the uranium-fueled PWR (cf. Fig. 3.31), in Table 8.8 for the liquid-metal fast-breeder reactor (LMFBR) (cf. Fig. 3.34), and in Table 8.9 for the uranium-thorium-fueled HTGR (cf. Fig. 3.33). These quantities, expressed per unit mass of discharge fuel, are useful in the design of reprocessing operations. For the purpose of comparison, all quantities are calculated for 150 days of postirradiation cooling. 

Table 9.13 lists the isotopes of plutonium important in nuclear technology and some of their important nuclear properties. Plutonium isotopes are produced in reactors by the nuclide chains shown in Fig. 8.5. Typical quantities and isotopic compositions of plutonium in various reactor fuel cycles are listed in Tables 8.4, 8.5,8.6, and 8.7. In reactors fueled with uranium and plutonium, Pu is the principal isotopic constituent, but Pu contributes the greatest amount of alpha activity. With U-thorium fueling, Pu is the principal isotopic constituent. 

Nuclear energy cannot be produced by a self-sustained chain reaction in thorium alone because natural thorium contains no Bssile isotopes. Hoice the thorium-uranium cycle must be started by using enriched uranium, by irradiation of thorium in a uranium- or plutonium-fueled reactor or by using a strong external neutron source, e.g. an accelerator driven spallation source. 

The sole reason for using thorium in nuclear reactors is the fact that thorium ( Th) is not fissile, but can be converted to uranium-233 (fissile) via neutron capture. Uranium-233 is an isotope of uranium that does not occur in nature. When a thermal neutron is absorbed by this isotope, the number of neutrons produced is sufficiently larger than two, which permits breeding in a thermal nuclear reactor. No other fuel can be used for thermal breeding applications. It has the superior nuclear properties of the thorium fuel cycle when applied in thermal reactors that motivated the development of thorium-based fuels. The development of the uranium fuel cycle preceded that of thorium because of the natural occurrence of a fissile isotope in natural uranium, uranium-235, which was capable of sustaining a nuclear chain reaction. Once the utilization of uranium dioxide nuclear fuels had been established, development of the compound thorium dioxide logically followed. 

Third, Pa (protactinium), which occurs in the transmutation chain for the conversion of thorium to acts as a power history dependent neutron poison in a thorium-fueled nuclear reactor. There is no isotope with comparable properties present in a fuel stem. 

Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient) than will a U-fueled reactor. 

A thorium fuel cycle is also possible for use in nuclear power reactors. This involves using thorium-232 to generate uranium-233, which is capable of undergoing fission processes to generate energy in the form of heat. 

A Thorium-Uranium Exponential Experltnenti C. If. Skeen and W. W. Broum(AI). Because of uncertainties in the knowledge of t nuclear properties of thorium fuel and lattices containing this fuel, an experimental study was made of a thorium based fuel that is to be loaded into the Sodium Reactor Experiment (8RB) in the near future. An exponential experiment was performed with a square-celled lattice of 7-rod elements (l-in. diameter rods) spaced 9.5 in. apart in praphite. The fuel Is a Th-U-23S alloy containing 7.6% uranium by weight which is 93.13 atomic per cent U-235. The feel elements were 5 ft. long. The subcrltical lattice was placed On thd thermal column of a water boiler reactor which served as the source of neutrons for the assembly. 

Furukawa, K. 1992. The Combined System of Accelerator Molten-Salt Breeder (AMSB) and Molten-Salt Converter Reactor (MSCR). Japan-US Seminar on Th Fuel Reactors, Nara, Japan. Furukawa, K. et al. 1990. Summary Report Thorium Molten-Salt Nuclear Energy Synergetics./. Nucl. Sci. Technol. 27,1155-1178. 

Ottewitte, E.H., 1982. Gonfigurations of a Molten Chloride Fast Reactor on a Thorium Fuel Cycle. PhD Thesis, UCLA, Los Angeles, CA. 

The abimdance of thorium in the earth s crust is about three times that of uranium. Hence, the thoriiun fuel cycle ensures a long-term nuclear fuel supply. For countries with abundant thorium reserves, the thorium fuel cycle in HWRs would enhance the sustainability of nuclear power and the degree of energy independence using a single reactor type. 

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