Classifications

• • 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
  • G21C5/20—Moderator or core structure; Selection of materials for use as moderator characterised by the provision of more than one active zone wherein one zone contains fissile material and another zone contains breeder material
• • 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

• the present invention relates to nuclear reactors, and particularly to nuclear reactors of the seed and blanket type.
• nuclear reactors are the chief factors in limiting the future use of nuclear reactors for generating power.
• a seed-blanket reactor includes a seed region of fissile material, and a blanket region of fertile material capable of being converted into fissile material by neutron capture.
• a fissile material is one which will undergo fissions with neutrons below 1 MED in energy, the only naturally-occurring fissile material being U-235 constituting 0.7% of naturally-occurring uranium, the remainder being U-238.
• Fertile materials which can be converted into fissile material through neutron capture, include thorium-232 and uranium-238 which are converted, respectively, to uranium-233 and plutonium-239 fissile material.
• the seed is of small volume but is sufficiently enriched to approach criticality.
• the first two seed-blanket cores were at Shippingport, Pa., USA, and consisted of highlyenriched uranium fuel for the seeds, and natural uranium for the blankets; but studies were also made, as reported in the above publications, with blankets of natural thorium and of thorium with a small amount of natural uranium.
• the initial amounts of uranium mined were much higher, the power densities were much lower, and the fuel utilization was about the same.
• the possibilities of nuclear proliferation were present because of the use of highly enriched uranium in the seeds, and the presence of U-233 in the depleted blankets.
• An object of the present invention is to provide a new seed-blanket core design for a nuclear reactor having advantages in the above respects.
• an object of the invention is to provide a nuclear reactor having a novel seed-blanket core which lends itself well to the use of nonproliferative fuel, permits excellent uranium utilization, provides normal overall power densities, and requires low initial amounts of mined uranium.
• a nuclear reactor having an . active core comprising a seed region of fissile material and a blanket region of fertile material capable of being converted into fissile material by neutron capture; characterized in that the seed and blanket regions are such as to produce a net reverse flow of neutron current, wherein the net fast neutron flow is from the blanket region to the seed region and the net thermal and epithermal neutron flow is from the seed region to the blanket region.
• this use of reverse neutron current lends itself well to the use of nonproliferative fuel, excellent uranium utilization, normal overall power densities, low initial requirements of mined uranium, and particularly to producing seed-blanket type corres in which the blanket regions contain thorium.
• the invention is described below, particularly with respect to light-water reactors, wherein light water is used as the coolant and moderator in the seed and blanket regions, the reverse neutron currents being produced by providing a seed water-to-fuel volume ratio within the range of 3.0:1 to 9.0:1, and a blanket water-to-fuel volume ratio within the range of 0.8:1 to 2.0:1.
• the seed waterto-fuel volume ratio is 9:1, and the blanket water-tofuel volume ratio is 1:1.
• the seed macroscopic thermal absorption is kept low by using up to 20% enriched uranium (this being the maximum for nonproliferation) sufficient for the usual refueling period of about one year to 18 months.
• the thermalization is sufficiently great in the seed to keep the resonance escape probability (P B ) near unity so that the magnitude of the thermal leakage current ratio is enhanced and very little plutonium is generated in the seed as will be described below.
• the fuel in the seed is intended to be reprocessed and reused, and there is insufficient plutonium formed in the seed to present a proliferation problem as a result of reprocessing.
• the seed volume is selected to be within the range of 15% to 25% of the core volume in a geometry which corresponds to a seed infinite multiplication factor (k S ) of about 1.4-1.8.
• the seed regions may be stationary, and control may be effected by burnable poison and control rods in the seeds. This arrangement results in a lower power density in the seeds, so that the volume of fuel is adequate for heat transfer despite the large fraction of water in the seed region.
• the blanket In order to obtain large negative thermal currents, the blanket should have a much larger ratio of macroscopic absorption to slowing down cross-sections than the seed.
• a thorium blanket lends itself well to such a situation without going to extremely tight lattices. This is because the effective thorium crosssection is much larger than that of U-238.
• the U-233 builds up to have a macroscopic cross-section approximately equal to that of the thorium, just as in a uranium lattice the plutonium builds up to have a macroscopic absorption cross-section approximately matching that of U-238.
• the .preferred embodiment uses about 10% uranium enriched to about 10% U-235, thereby providing a concentration of about 2% U-235 in the thorium.
• This ratio is within the conventional range and does not impose any penalties in construction or thermal performance, as was the case for the very tight lattices employed in the LWBR and which would be required to create reverse thermal neutron flows with a uranium blanket.
• the 8% initial U-238 content in the thorium blanket ensures that the U-233 formed in the blanket will be heavily denatured and could not be used for weapons without isotope separation.
• Enlightenment is provided by the 2-Group formula which is: (neglecting leakages out of the core which will be small for a large core) .
• k ⁇ BS represents the leakage of thermal (slow) neutrons from the blanket to the seed and is usually positive. It is given by:
• ⁇ is the blanket fast effect.
• Superscripts S and B refer to seed and blanket respectively.
• k is the value of k ⁇ in a region.
• the seed/ blanket assembly is assumed to be critical.
• ⁇ S is the macroscopic thermal absorption crosssection.
• D s is the thermal diffusion constant.
• /P B is the resonance escape probability in the blanket.
• /P S is the resonance escape probability in the seed.
• v S is the number of neutrons emitted per fission in the seed.
• v B is the number of neutrons emitted per fission in the blanket.
• ⁇ k BS S has always been considered sccething of a nuisance since it was positive and therefore admonished the power in the blanket.
• an objective of the core design usually was to lengthen the endurance of the seed , which involved putting more U-235 fuel into the seed and making it more highly absorbing, so that the magnitude of ⁇ k BS S increased , further decreasing the blanket power fraction .
• the U-233 resonance absorption integral is more than twice that of U-235 and the U-233 resonance fission integral is also very high.
• the seed may be considered as a multiplying reflector for the blanket, which provides an increase in the number of thermal and epithermal neutrons in the blanket without having to run the gauntlet of blanket resonance capture.
• ⁇ 1 and ⁇ 2 represent fast and thermal, fluxes respectively.
• V is the volume of the region.
• ⁇ R is the slowing down cross-section.
• ⁇ a is the macroscopic absorption cross-section.
• the loss of neutrons to the control system can be minimized with a seed blanket core. Also, the unique properties of a thorium blanket can be utilized to raise the total fraction of energy generated in the blanket regions and hence improve the uranium utilization.
• Fig. 1 is a cross-section of a light-water reactor constructed in accordance with the invention
• Fig. 2 illustrates one seed-blanket module in the reactor core of Fig. 1;
• Fig. 3 illustrates a seed-fuel cluster in the module of Fig. 2;
• Fig. 4 illustrates a blanket bundle in the core of Fig. 2;
• Fig. 5 illustrates the build-up of the blanket multiplication factor with blanket irradiation for various blanket water-to-fuel volume ratios
• Fig. 6 illustrates the build-up of U-236 and U-234 in the thorium blanket
• Fig. 7 illustrates another seed and blanket cluster arrangement that may be used for the seed-blanket module
• Fig. 8 and 9 illustrate two arrangements that may be used in a CAN-DU type heavy-water reactor constructed in accordance with the present invention; and Fig. 10 illustrates a module for a highlyenriched light-water burner reactor. DESCRIPTION OF PREFERRED EMBODIMENTS Light-Water Reactor of Figs. 1-8
• the reactor comprises an active core, generally designated 2, within a pressure vessel 4, which core is enclosed by a thermal shield 6 and a core baffle 8.
• the active core 2 comprises a plurality of fuel modules 10, each including a seed region of fissile material, and a blanket region of fertile material capable of being converted into fissile material by neutron capture, as will be described more particularly below with respect to Figs. 2-4.
• the outer region 9 of core 2 is a power flattening region and is occupied by, e.g., elements having the same composition as in the blanket regions of the modules 10, but .including less enriched uranium, e.g., about one-half the enrichment as in the blanket regions of the modules.
• Fig. 2 more particularly illustrates the construction of each of the seed-blanket modules 10 in the active core 2 illustrated in Fig. 1.
• each of these modules includs a central blanket region 12 enclosed by an annular seed region 14, which in turn is enclosed by an annular blanket region 16.
• the seed region 14 includes fissile material
• the two blanket regions 12 and 16 each include fertile material capable of being converted into fissile material by neutron capture.
• An example of the construction of the fuel elements in the seed region 14 is illustrated in Fig. 3, and an example of the construction of the fuel elements in the blanket regipns 12 and 16 is illustrated in Fig. 4.
• the seed fuel elements are in the form of plates 20.
• the cross-sectional area of the seed fuel plates 20, and of the water channels between them, determines the seed water-to-fuel volume ratio. As indicated earlier, this ratio is substantially higher in the seed region 14 than in the blanket regions 12 and 16, it being preferably within the range of 3.0:1 to 9:1. in the seed region, and frofii 0.8:1 to 2.0:1 in the blanket regions.
• the seed region 14 illustrated in Fig. 3 further includes spacer elements 24 between the seed subassemblies 22 to define channels 26 for the control rods.
• the fertile elements in the blanket region 12 (and also in the blanket region 16, Fig. 2) are in the form of rods 30 mounted between a pair of end plates 32, 34.
• the blanket fuel rods 30 are mounted in spaced relationship so as to define the spaces or channels for the water coolant-moderator.
• the water-to-fuel volume ratio may be 1:1.
• the fuel elements in these blanket regions may consist of thorium oxide rods with about 10% of uranium oxide enriched to 20%.
• These blanket regions are designed to provide an average k B of about 0.9 for 100,000 MWD/T (megawattdays/ton). The blankets are not intended to be reprocessed, but to be merely thrown away or otherwise disposed of.
• the seed fuel elements 20 are in the form of plates of 20% enriched uranium clad in zirconium alloy.
• the seed fuel loading is de ⁇ signed to provide an infinite multiplication factor (k S ) of 1.5 for about one year at the customary 70% load factox. Because of the highly thermal spectrum, and the small U-238 content of the seed, the seed fuel will contain very little plutonium, and therefore may be reprocessed without presenting a proliferation problem.
• each unit module 10 having an inner blanket region 14 of 28 cm. radius, and an outer blanket region of 36 cm. radius, whereby the inner and outer blanket regions would each constitute about 40%, of module volume, and the seed region 14 would constitute about 20% of the module volume.
• Fig. 5 illustrates the characteristics of a thorium oxide blanket region in which the thorium contains 10% uranium enriched to 20% in U-235.
• k ⁇ infinite multiplication factor
• increasing the MW/T causes a reduction in the infinite multiplication factor (k ⁇ ). This is due to the increased protactinium.
• k ⁇ infinite multiplication factor
• the values of k ⁇ are lower but by less than that due to the increased absorption of the water. This substantiates the above statement that the reverse thermal currents, which make the blanket more thermal without adding more water to the blanket, will not decrease the blanket k ⁇ .
• Fig. 6 illustrates the build-up'of U-236 and U-234 in the thorium blanket regions for a water-to-fuel volume ratio of about 1.5 in these blanket regions. Since in our above-described example, this ratio is to be 1:1, this will increase the U-234 ratio. Also, there will be more U-238 and U-235 initially, which will result in more non-fissile material, U-238, U-236, and U-234 to mix with the U-233. Thus, if the final mixture could be exploded at all, it would have to be very large. But then, the extreme shielding required because of the high gamma activity of the U-233 would.make a bomb impracticable. Fig.
• FIG. 7 illustrates another proposed module construction, including two inner blanket regions 40, 42, a seed region 44, and two outer blanket regions 46 and 48.
• This module construction may be called the rectangular analog of the hexagonal module illustrated in Fig. 2.
• blanket region 40 inwardly of the seed region 44, blanket region 40 includes 21 assemblies, and blanket region 42 includes 24 assemblies; and outwardly of seed region 44, blanket region 46 includes 40 assemblies and blanket region 48 includes 28 assemblies.
• Heavy-Water Reactor (Figs. 8 and 9)
• the heavy-water reactors illustrated in Figs. 8 and 9 generally follow the design of the CAN-DU Plant at Douglas Point in Canada, or of its predecessor, the NPD-2 (Canadian Nuclear Power Demonstration) reactor completed in 1962.
• Both reactors are of the pressure-tube type utilizing heavy water at a pressure of about 1150 pounds per square inch as the moderator and coolant.
• the fuel is normal uranium dioxide jacketed in zirconium alloy supported in horizontal tubes of the same alloy.
• the coolant leaves the reactor at 277° C (530° F) and produces steam at about 230° C (446° F) in a heat exchanger.
• the NPD-2 reactor produced a gross electrical power output of 22 megawatts with a thermal efficiency of about 25%; whereas the
• CAN-DU plant produces over 100 megawatts of electrical power at a thermal efficiency of about 29%.
• Fig. 8 schematically illustrates the core in such a reactor. It includes a large tank or vessel, generally designated 102, pierced with a number of double-jacketed tubes 104, called calandria tubes. Each tube 104 includes a cluster of fuel rods 106 of fissilematerial-containing elements, usually natural uranium, or in some instances, very slightly enriched uranium.
• the tank is filled with heavy water at ordinary pressure which fills the space between the tubes 104 and thereby serves as the moderator, this water remaining essentially at ordinary temperature.
• the tubes 104, enclosing the fuel rods 106, are filled with the coolant, also heavy water, under a pressure of 500 to 1500 pounds per square inch, the coolant flowing in the annular channels between the fuel rods 106 and the inner walls of the double-jacketed tubes 104. Further details of the construction and operation of such reactors are readily available in the published literature, and therefore are not set forth herein.
• each calandria tube 104 except the boundary ones, is surrounded by eight calandria tubes in a rectangular pattern. Any non-boundary calandria tube 104 may be selected to serve as the "seed," and its eight surrounding calandria tubes may then be used as the "blanket.”
• the center in the group of nine-calandria tubes 104 illustrated in Fig. 8, the center.
• each calandria tube 104s may be used as the seed, and the surrounding eight calandria tubes 104b may serve as its blanket region.
• each calandria tube there would be 37 12-segmented fuel elements, with a composition of each fuel element depending on whether it serves as a seed or blanket.
• the fuel in the seed calandria tube 104s may be 4 to 15 volume per cent
• v/o of uranium enriched to about 20% in zirconium clad in zirconium alloy; and the fuel in the blanket calandria tubes 104b may be thorium oxide with about 10 v/o of uranium oxide enriched to about 10-15% .
• On-line refueling will be retained as in the present CAN-DU, except that all the seed fuel elements will be replaced each year and reprocessed, while the blanket fuel elements will be replaced only at 10-year intervals corresponding to about 100,000 MWD/T burn-up, and then discarded.
• Fig. 9 illustrates an alternative modular arrangement that may be used in a heavy-water reactor in accordance with the present invention.
• this modular arrangement there are 25 calandria tubes 204, constituted of an inner seed tube 204s, enclosed by eight further seed tubes 204s, the latter being enclosed by 16 blanket tubes 204b.
• the blanket water-to-fuel volume ratio may be approximately 0.3.
• the blanket compositions may be as follows:
• Double-pellet blanket having the above U-233 content in the thorium part and the plutonium in the uranium part.
• the seed water-to-fuel volume ratio may be 3:1 to 9:1.
• the seed fuel loading may be the same as described above with respect to the light water reactor illustrated in Fig. 1-5. Other alternatives would be to use seed fuel loadings of U-233 or of plutonium to yield the same reactivity.
• the aim of using the reverse neutron current in accordance with the invention could be to permit control of the core from a relatively small portion of the core volume, namely, from the seed region, while the bulk of the power is produced from the blanket regions.
• Fig. 10 illustrates one form of module which may be used in such a reactor.
• This module includes a seed region 304s which is a small fraction, e.g., from 5-10%, of the total volume of the module including the surround blanket region 304b, and which provides 2-5% of the total power.
• the seed region 304s could be cylindrical having a radius of 9 cm.
• the blanket region 304b could also be cylindrical having a radius of 36 mm., whereupon the seed volume would be 6.25% of the total volume including the blanket region.
• the water-to-fuel volume ratio in the seed region 304s may be about 4:1; and the fuel loading may be 3 v /o (volume percentage) of highlyenriched uranium (93% U-235) in zirconium having a clodding of zirconium alloy.
• the fuel in the blanket region may be 25 v/o of highly-enriched uranium oxide in an alloy of 25 v/o hafnium, and 50 v/ ⁇ zirconium.
• the water-to-fuel volume ratio in the blanket may be about 0.9:1.
• Burnable poisons may be utilized in order to maintain the operating infinite multiplication factor (k ⁇ ) of the blanket at about 0.94, and to maintain a flat power distribution throughout the blanket.
• k ⁇ operating infinite multiplication factor
• Preferably about 2% of the core power would come from the seed region, which region would still control the core since the remainder of the core would be subcritical.

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