EP0160702A4

EP0160702A4 - Nuclear reactor of the seed and blanket type.

Info

Publication number: EP0160702A4
Application number: EP19850900512
Authority: EP
Inventors: Alvin Radkowsky
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-10-21
Filing date: 1984-10-16
Publication date: 1986-02-20
Also published as: WO1985001826A1; IL70026A0; EP0160702A1; JPS61500380A

Abstract

A nuclear reactor of the seed-blanket type having an active core (2) comprising seed regions (14) of fissile material and blanket regions (12 and 16) of fertile material capable of being converted into fissile material by neutron capture, the seed (14) and blanket regions (12, 16) being such as to produce a net reverse flow of neutron current as compared to conventional seed-blanket reactors, i.e. in the present reactor, the net fast neutron flow is from the blanket regions (12, 16) to the seed region (14) and the net thermal and epithermal neutron flow is from the seed region (14) to the blanket regions (12, 16). Such a reactor lends itself to the use of nonproliferative fuel, provides normal overall power densities, and requires low initial amounts of mined uranium.

Description

NUCLEAR REACTOR OF THE SEED AND BLANKET TYPE BACKGROUND OF THE INVENTION

The present invention relates to nuclear reactors, and particularly to nuclear reactors of the seed and blanket type. Among the chief factors in limiting the future use of nuclear reactors for generating power is the wasteful manner in which the reactors utilize uranium since they burn only a very small percentage, atid the limited supply of uranium available in natural form. Many attempts, largely unsuccessful, have been made ever since the beginning of the Manhattan Project to find an. attractive way of utilizing thorium as a substitute for uranium, since thorium is about three times as plentiful as uranium; a summary of this work may be found in Assessment of Thorium Fuel Cycles in

Pressurized Water Reactors, EPRI NP 359 Feb. 1977. However, since thorium has no natural fissile content, highly enriched uranium (essentially U-235) must Be added to the thorium oxide when it is substituted for uranium oxide in a conventional reactor design. It turns out that the amount of uranium to be mined to start off is considerably greater than in a conventional uranium-fuel reactor core. Moreover, after the core is depleted, there is some U-233 created in the thorium, which must be extracted and used for a second core loading. After about 30 years, the total amount of uranium mined will be down to the same level as for a conventional uranium-fueled core, but the amount of separative work will still be greater. Moreover, the fuel required is definitely proliferative in the use of highly-enriched uranium, and also in the extraction and reprocessing of the U-233. It should also be noted that there is no commercial method of reprocessing U-233; and if and when such a commercial process is developed, it is expected to be highly expensive because of the very energetic gamma rays associated with U-233, therefore requiring all work to be done by remote control behind heavy sheilding. The possibility of using less than highly-enriched uranium was also considered for supplying the initial fissile content, but this was found to have an even worse "uranium utilization," resulting in even a greater need for mined uranium.

Attempts have also been made to utilize the seed-blanket concept for thorium fuel, as described, for example, in report Large Power Reactor (LPR) Program, WAPD LPR 256 September 1964, in USA Patent 3,154,471 granted to Alvin Radkowsky October 27, 1964, and in the article Seed and Blanket Reactors, Geneva III P/208 by A. Radkowsky, G.W. Hardigg and R.E. Luce. Briefly, 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. In a seed-blanket type reactor, 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. However, as compared with conventional uranium cores, in these cores the initial amounts of uranium mined were much higher, the power densities were much lower, and the fuel utilization was about the same. Moreover, 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. Another seed-blanket core design, the Light

Water Breeder Reactor LWBR, described in the report Shippingport Atomic Power Station Safety Analysis Report for the Light Water Breeder Reactor, 1975, and in the Final Environmental Statement Light Water Breeder Reactor Program, ERDA 1541 June 1976, required such large initial amounts of mined uranium that it would take at least 80 years of running the core, and continual reprocessing, to bring uranium utilization back to that of a conventional core; moreover, the core power densities were relatively low, and the fuel was proliferative at every stage. OBJECTS OF THE INVENTION 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. More particularly, 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.

BRIEF SUMMARY OF THE INVENTION According to a broad aspect of the present invention, there is provided 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.

As will be described below, particularly in the section entitled "Theoretical Considerations," in the conventional seed-blanket reactor, there is a positive current of thermal neutrons from the blanket to the seed; that is, the net fast neutron flow is from the seed region to the blanket region, and the net thermal and epithermal neutron flow is from the blanket region to the seed region. In the reactor of the present invention, the reverse is true; that is, 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. As will also be described below, 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. In the preferred embodiment described below, 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. Thus, 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.

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.

In order to have initially a good power share in the thorium blanket., it is necessary to enrich the thorium to approximately the value of the multiplication factor it will build up to when the U-233 approaches its asymptotic concentration of about 1.5% to 2%. This value of multiplication factor is approximately 0.9. 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. With such blanket fuel, a strong reverse thermal neutron current flow (i.e., from seed to blanket) can be obtained with a blanket water-to-fuel volume ratio of 1:1. 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. While the use of reverse thermal current reduces the amount of U-233 created in the blanket, it does not reduce the blanket multiplication factor, since this is enhanced by the higher value of "η" (the average number of neutrons produced by fission for each neutron absorbed by the uranium, U-233), and the lower protactinium capture resulting from the greater thermalization.

Several preferred embodiments of the invention will be described below, but it is believed that a discussion of a number of theoretical considerations at this point will enable a better understanding of the invention and of the advantages

THEORETICAL CONSIDERATIONS Power Sharing Between Seed and Blanket

From the standpoint of uranium utilization, it is very important to maximize the total fraction of the core power generated in the blanket since the blanket produces power with much less expenditure of uranium than does the seed. The conventional 1-Group formula used to determine the split in power between seed and blanket regions is :

(1) where: is the ratio of blanket to seed power. k^S is the value of k_∞ (infinite multiplication factor) in the seed and k^B in the blanket. The core is assumed to be critical. This formula indicates that the main factor in getting a high blanket power fraction is primarily a high blanket k_∞ and secondarily a high seed k_∞. However, there is much more to the story than this.

Enlightenment is provided by the 2-Group formula which is: (neglecting leakages out of the core which will be small for a large core) .

(2) k δ BS represents the leakage of thermal (slow) neutrons from the blanket to the seed and is usually positive. It is given by:

(integral taken over blanket seed interface) (3)

(integral taken over blanket volume) (4)

Here: ε 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. Σ _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.

The latter value "v" is substantially the same for U-233 and U-235, and there, in our case, v^S/v^B may be cancelled, whereby the 2-Group formula becomes:

(5)

The term ^δkBS^S has always been considered sccething of a nuisance since it was positive and therefore admonished the power in the blanket. In the past, 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 δk_BS ^S increased , further decreasing the blanket power fraction .

Now, with a nearly all thorium blanket , which has much greater macroscopic absorption than a natural uranium blanket, we have an opportunity to change the sign of δk_BS ^S, and thus this term will increase the blanket power fraction rather than to decrease it. With (1 - k^B) in the denominator of the power sharing formula equal to 0.1 (k_B approximately = 0.9, as mentioned above), a negative value of fairly small magnitude (~.03-.05) for δk_BS ^S can result in a large increase in the blanket power fraction, i.e.

By use of a seed which has very low macroscopic absorption and high water to fuel ratio, we can reverse the direction of the lower energy current to be from the seed to the blanket. This is especially important at epithermal energies when the U-233 builds up. 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. Thus, there will be a large reverse current from the seed to the blanket which will considerably improve the blanket power fraction. 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. Thermal Current Between Seed and Blanket

Assume a constant geometry and the thermal current, A, is from the seed to blanket. Then in each region using subscripts B and S for Blanket and Seed respectively: .(Slowing down source)_S - A = (Thermal Absorption

Rate)_S (6)

(Slowing down, source)_B + A = (Thermal Absorption

Rate)_B (7)

(Slowing down source)_S - (8) (Thermal, Absorption Rate)b_c = (9) and similar expressions for the blanket.

Here Φ₁ and Φ₂ represent fast and thermal, fluxes respectively.

V is the volume of the region. Σ_R is the slowing down cross-section.

Σ_a is the macroscopic absorption cross-section.

(10)

(11)

(12) where d is a constant. Hence

or

Host likely if we use narrow strips of seed . Therefore the right hand side becomes

or (15)

Note that if V_S is small, A will also be small.

Of course what we are really interested in is is small compared with

Then (16)

Thus the left side will be positive if the bracket in the right side is positive. Using Thorium as the Blanket

There is one basic difficulty in the use of a thorium blanket. This is that a thorium blanket has initially nearly zero power output, until the U-233 content of the thorium is built up. Thus, the total power output of the core would have to be very low for several years, until the blanket U-233 built up since the small seed regions could not possibly supply all the core power. However, it turns out also that a great many neutrons are wasted in building up the thorium to close to its equilibrium U!-'233. concentration. A study of the above-cited 1964 Geneva Paper (particularly Fig. 2 thereof), shows that the number of neutrons fed in from the seed to bring the thorium blanket up to an output of 40,000 MWD/T is several times greater than should be necessary, based upon the amount of U-233 depleted and that remaining in the thorium. The reason for this turns out to be that, when the U-233 concentration is very low, the thermal flux in the thorium must be very high and, hence, a great number of neutrons are absorbed in the fission products, cladding, ahd water.

The two difficulties mentioned above can both be avoided by providing a small amount of fissile fuel in the thorium at the beginning of the blanket life. It is necessary to keep the amount of uranium added to a minimum for two reasons: 1) With 10% uranium content or less irradiation tests show that thorium oxide can withstand burnups of over a 100,000 MWD/T; 2) the smaller the amount of uranium, the higher will be the k_∞ of a thorium blanket at high burnups. It is preferable, therefore, to use the highest enrichment (20%) permissible from consideration of nonweapons proliferation. Infinite medium calculations show that about 10% of uranium oxide (enriched to about 10% in U-235, uniformly distributed in the thorium oxide, will maintain the k_∞ of the blanket at approximately 0.9 throughout very high burnups. The high burnup of the blanket is accomplished primarly through burning in place the U-233 which is created. The U-238 from the initial enrichment of the blanket will be uniformly mixed with the residual U-233 at the end of blanket, so that the U-233 will be "denatured" and useless for weapons.

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.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described below, by way of example only, with reference to the accompanying drawings, wherein:

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

As shown particularly in Fig. 1, 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. Thus, as shown in Fig. 2, 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. Each of the foregoing regions, as well as the complete modules, is of hexagonal external configuration. As indicated earlier, the seed region 14 includes fissile material, and 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, as illustrated in Fig. 3, are in the form of plates 20. There are a plurality of such plates mounted in each of a plurality of seed subassemblies 22 in spaced relationship to each other to define, between each pair of adjacent plates, a space or channel for the water coolant-moderator. 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. As illustrated in Fig. 4, 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.

In each of the two blanket regions 12 and 16, 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.

In the seed region 14, 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.

As one example, for a 1000 MWe core, there may be 46 unit modules 10, each 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. The previously described embodiment utilizes a water-to-volume ratio in the blanket of 1:1, and a MW/T (megawatt per tonne) of thorium and uranium = 20. It will be noted that increasing the MW/T causes a reduction in the infinite multiplication factor (k_∞). This is due to the increased protactinium. It will also be noted that for a water-to-fuel volume ratio of 3:1, where the blanket is much more thermal, 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. 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. In Fig. 7, 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) For purposes of example, 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.

As one application of the present invention, it is proposed to preserve the present CAN-DU dimensions and simply replace the fuel elements. This may be done by using a quasi-seed-blanket arrangement, taking advantage of the face that in the CAN-DU array, 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." Thus, in the group of nine-calandria tubes 104 illustrated in Fig. 8, the center. tube 104s may be used as the seed, and the surrounding eight calandria tubes 104b may serve as its blanket region. In 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. Thus, 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. Thus, in 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.

OTHER VARIATIONS AND APPLICATIONS Another possible application of the invention would be in aqueous breeders. This application could use the hexagon module construction illustrated in Fig. 2, but it would be necessary to make the blanket a very close-packed lattice. The seed could be designed to provide reverse neutron current. The advantage would be an increase in breeding (since most of the breeding occurs in the blanket in any case), and a decrease in the initial fissile fuel requirements since the seed fuel loading will necesarily be low.

A number of variations are contemplated, as follows:

1. A light-water thorium blanket.

2. A light-water uranium blanket.

3. 80% heavy-water, plus 20% light-water thorium blanket. 4. 80% heavy-water plus 20% light-water uranium blanket.

5. Either coolant with a double-pellet blanket.

In all cases, there can be utilized unit modules with 36 cm. outer radius, the inner seed radius being 24 cm. and the annular seed extending from 24 to 27 cm. The blanket water-to-fuel volume ratio may be approximately 0.3. The blanket compositions may be as follows:

1. Thorium oxide 2% U-233. 2. Depleted uranium oxide plus 4.5% plutonium oxide; the plutonium may be obtained by discharge from a conventional power station.

3. 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.

Another possible application of the invention is in highly-enriched light-water burner reactors. In this application, 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. As one example, the seed region 304s could be cylindrical having a radius of 9 cm., and 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. In this application, 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/σ zirconium. The water-to-fuel volume ratio in the blanket may be about 0.9:1. The aim would be to provide such a high resonance capture in the blanket that the reverse neutron thermal currents would be very effective. 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. 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.

Many other variations, modifications, and applications of the invention will be apparent.

Claims

WHAT IS CLAIMED IS:

1. A thermal neutron nuclear reactor with a core comprising a seed region and a blanket region, said region comprising fissile material and said blanket region comprising fertile material, characterized in that said seed and blanket regions are such as to produce a net flow of thermal neutrons from the seed region to the blanket region.

2. The reactor of Claim 1. wherein the blanket region has a larger ratio of "macroscopic thermal absorption cross section" than the seed region.

3. The reactor of Claim 2, wherein light water is used as a coolant and moderator in the seed region.

4. The reactor of Claim 3, wherein light water is used as a coolant and moderator in the blanket region.

5. The reactor of Claim 4, wherein the water-to-fuel volume ratio in the seed is in the range from 10: 1 to 5: 1.

6. The reactor of Claim 4, wherein the water-to-fuel volume ratio in the. blanket is in the range from 0.8:1 to 2.0:1.

7. The reactor of Claim 4, wherein the water-to-fuel volume ratio in the seed is substantially 9:1 and the water-to-fuel volume ratio in the blanket is substantially 1:1.

8. The reactor of Claim 1, wherein the seed macroscopic thermal absorption is kept low by using 20% enriched uranium whereby the reactor is non-proliferative.

9. The reactor of Claim 1, wherein the. infinite multiplication factor of the seed (k_S) is

1.3-1.5, and the infinite multiplication factor of the blanket ( k_B ) is substantially 0.9.

10. The reactor according to Claim 1, wherein the seed volume is 15-25% of the core volume.

11. The reactor according to Claim 1, wherein said blanket region comprises thorium.

12. The reactor of Claim 11, wherein the thorium is enriched with fissile material.

13. The reactor of Claim 12, wherein the blanket comprises thorium containing up to 12 v/o uranium enriched up to 20 v/o uranium 235.

14. The reactor of Claim 1, wherein the reactor is a heavy water reactor comprising a large tank pierced with a plurality of calandria tubes, each including fuel rods, the tank being filled with heavy water substantially at atmospheric pressure and serving as the moderator, the calandria tubes being filled with pressurized heavy water and serving as the coolant, the fuel in at least one of said calandria tubes including fissile material and defining said seed region, the fuel in at least another one of said calandria tubes being of fertile material and defining said blanket region.

15. The reactor of Claim 14, wherein the heavy water moderator is replaced by beryllium, graphite, or combinations thereof.

16. The reactor according to Claim 1, wherein the seed region includes highly enriched uranium and occupies a volume of 5-10% of the total core volume so as to permit control of the core from a relatively small portion of the core volume.

17. The reactor according to Claim 1, wherein the blanket region comprises a closely packed lattice so as to provide breeding capability.