IL32076A - Nuclear reactor utilizing plutonium in peripheral fuel assemblies - Google Patents

Description

NUCLEAH REACTOR UTILIZING PLUTONIUM IN PERIPHERAL.FUEL ASSEMBLIES NUCLEAR REACTOR f TILIZING PLUTONIUM IN PERIPHERAL FUEL ASSEMBLIES Abstract of Disclosure A nuclear reactor core for effectively utilizing plutonium fuel. The plutonium fuel is placed in the fuel assemblies at the periphery of the core, in the low energy neutron spectrum region, to take advantage of the characteristic s of the plutonium fuel and to optimize the use of the peripheral portion of the core.

Disclosure The release of large amounts of energy through nuclear fission reactions is now well known. In general, a fissile (fis sionable) atom such as U -233, U-235, Pu-239, or Pu-241 absorbs a neutron in its nucleus and undergoes a nuclear disintegration or fission. This produce s, on the average, two fission products of lower atomic weight with great kinetic energy and also several fission neutrons of high energy.

The kinetic energy of the fission products is dissipated as heat in the fuel elements of the reactor. If there is at least one net neutron remaining on the average from each fission event and this neutron induces a subsequent fission event, the fission reaction becomes self- sustaining and thus the heat generation is continuous. The heat is removed to perform useful work by pas sing a working medium or coolant such as water in heat exchange relationship with the fuel elements.

A s the sustained reaction continue s the fissionable atoms are gradually consumed. Some of the fission products produced are neutron absorbers (fission product poisons) which by their capture of neutrons lower the number of neutrons available to cause fission. This decreases the fission reactivity and hence the heat produced.

In a known type of nuclear reactor, for example as used it^he Dresden Nuclear Power Station near Chicago, Illinois, the reactor core is of the heterogeneous type. That is, the nuclear fuel is in the form of elongated, cladded rods . These fuel rods or elements are grouped together and contained in open-ended tubular flow channels to form separately removable fuel as semblie s or bundles. A sufficient number of fuel assemblies are arranged in a matrix, approximately a right circular cylinder, to form the nuclear reactor core capable of the self- sustained fission reaction mentioned hereinbefore . The core is submer sed in a fluid, such as light water , which serves both as a coolant and as a neutron moderator. The water which surrounds the core also serve s as a neutron reflector . A plurality of control rods, containing neutron absorbing material, are selectively insertable among the fuel assemblies to control the reactivity of the core.

In commonly used nuclear power reactor fuel, fertile materials such as U -238 are included in addition to the above -noted fissionable atoms. For example, a commonly used fuel consists of uranium dioxide (UC^) in which approximately 2 percent of the uranium atoms are U -235 which are fis sionable in a thermal neutron flux, while the remaining 98 percent of the uranium atoms are the fertile isotope U -238 which are not significantl fissionable in a thermal neutron flux. In the course of operating the reactor the fissionable atoms (U -235) are gradually consumed and a part of the fertile atoms (U-238) are converted into a fissionable isotope (Pu-239). The concentration of Pu-239 gradually rises and approaches an equilibrium value. Since the Pu-239 atoms are fissionable by thermal neutrons, they contribute to the maintenance of the chain fission reaction.

Normally, in a thermal reactor (a reactor in which most of the fissions are caused by thermal neutrons) the rate of production of fissionable atoms is les s than the rate of fis sionable atom cons umpticSfe Also, s ome of the fission products produced are neutron absorbers or poisons . Thus the potential reactivity of the fuel charge decreases with exposure and if the design power level is to be maintained, the reactor eventually must be refueled by replacement of some or all of the irradiated fuel.

The spent or irradiated fuel removed from the reactor contains, in addition to a valuable quantity of the original fis s ionable material, a significant quantity of plutonium including fis sionable Pu- 239 and Pu-241 and fe rtile Pu-240. Such spent or irradiated fuel can be re proces sed to separate and recover the uranium and plutonium for reuse.

When the cost of the recovered plutonium fuel becomes comparable to the cost of uranium fuel it then becomes de sirable for economic reasons to utilize such plutonium fuel in refueling the reactor and/or in initial fueling of a reactor.

The plutonium recovered from spent or irradiated fuel from a thermal reactor is a mixture of several is otopes of plutonium including fissionable Pu-239 and Pu-241, fertile Pu- 240, and Pu-242 which in a thermal reactor is a parasite or poison. An is otopic composition of plutonium typical of that recovered from spent uranium fuel from a boiling water reactor is as follows: Isotope Atom Fraction Pu-239 0. 590 Pu-240 0. 257 Pu-241 0. 121 Pu-242 0. 032 The use of plutonium fuel in a reactor designed to use uranium fuel requires consideration of differences in reactor performance because of the differences in the nuclear characteristics of the two fuels. For example, the thermal neutron capture and fission cross sections of the fissile plutonium isotopes, Pu-239 and Pu-241 , are greater than those of the fissile uranium isotope U-235. Also, the isotope Pu-240 presents a large capture cross section for neutrons near one electron volt in energy.

The fissile nuclides Pu-239 and Pu-241 have large neutron cross section resonances at about 0. 3, electron volts (the upper end of the thermal neutron energy spectrum). The ratio of the probability of a neutron being parasitically captured in Pu-239 and Pu-241 to the probability of neutrons causing fission in Pu-239 and Pu-241 is considerably increased for neutrons s with energies near these resonance energies. At thermal neutron energies below the 0. 3 electron volt resonance energies, the capture-to-fission ratio decreases. Thus nuclear and economic efficiency is improved by locating plutonium fuel in regions of low thermal neutron energy.

Previously suggested arrangements for utilizing initial inventories of plutonium fuel involve the use of the plutonium fuel in the same fuel assemblies with enriched uranium fuel. This can have several adverse effects such as changing the power coefficient of reactivity and reducing the fraction of delayed neutrons produced per fission and other variables of reactor control requirements and dynamic response. Additionally!* economically optimum fuel assembly geometric variables such as fuel rod diameter and spacing, fuel density, moderator content and the like and optimum design performance variables, such as discharge exposure, are different for fuel containing both plutonium and uranium as opposed to fuel fabricated from uranium only.

For example, when fuel assemblies containing a combination of initial plutonium and uranium are distributed uniformly throughout the reactoij e core, the reactor control requirements are increased by the affect of the plutonium on the power coefficient. This problem can be solved by increasing the water-to-fuel volume ratio. However, if the water-to-fuel ratio is decreased through reduction of fuel rod diameter, the heat transfer surface decreases. Thus reduction of the fuel rod diameter usually requires an increase in the number of fuel rode per fuel assembly to maintain the, desired rod surface area. Also, a decrease in fuel rod diameter increases the specific power (unit power per unit of fuel) with the result that a larger refueling batch size is required to maintain a fixed refueling interval. Thus fuel cycle economics are penalized. An increase in the number of fuel rods per fuel assembly also increases fuel fabrication costs.

Furthermore when initial plutonium is mixed with U-238 in a fuel assembly, the large thermal neutron absorption cross section of plutonium competes with fertile captures in U-238 thus reducing the conversion ratio (the ratio of fissile atoms produced per fissile atom consumed). The fore-going problems are alleviated, costs are reduced and a net integrated improvement in conversion ratio is achieved by separation of the initial fissile plutonium and uranium fuel.

In a reactor core of finite size, the neutron flux varies both radially across the core and axially through the core because of neutron leakage from the core. For example, the thermal neutron flux decreases radially from the center toward the periphery of the core with a steep upward flux gradient near the periphery due to the action of the reflector. There are also local variations in the neutron density and energy because of the variations ill distribution and density of the moderator. For example in the region of the periphery of the core, the low energy neutron pupulation is relatively high because of the presence of the large and relatively cool mass of moderator-reflector surrounding the core. Since the local power density is directly related to the local neutron flux, the power distribution throughout the core also tends to be non-uniform. > The power distribution s important because the power level of operation of the reactor is generally limited by the temperature limits of the materials of the core in the region of the highest power density. Thus when the power distribution is not uniform, only the region which is operating at the highest permissible power density is producing power at its maximum rate with the result that the overall power output is less than is theoretically possible. The practical result of a non-uniform power distribution is the requirement of a larger more expensive core and containment and greater fuel inventory for a given reactor power output.

In general, power reactor operation is based on the concept of an operating cycle. That is, reactor operation is periodically interrupted for refueling to r estore the necessary reactivity. From the point of view of fueling or refueling the reactor core, the removable fuel assembly (or bundle of fuel rods) is the basic replaceable subdivision of the nuclear fuel. According to known refueling schemes, only a fraction, for example 20-3^^ percent, of the fuel assemblies are replaced at each refueling. The particular refueling pattern employed has an important effect on the power distribution throughout the reactor core. When such partial reloading schemes are employed it is evident that the local reactivity of the various fuel assemblies will differ because of the difference s in the exposure which they /have accumulated. The reactivity distribution affects the neutron flux distribution and hence the power distribution.

Various refueling patterns have been proposed including zone -in-out, zone -out-in, distributed or scatter, and combinations of zone and distributed. In the zone type of refueling pattern the fuel assemblie s are distributed in a plurality of concentric zones. In the zone -in-out pattern, the fuel assemblies in the outermost zone are discharged, the remaining fuel assemblies are shifted to the next outer zone and fresh fuel assemblie s are loaded into the central zone. This pattern generally aggravates the flux peaking in the centred portion of the core.

In the zone -out-in pattern, the fuel assemblies in the central zone are discharged, the assemblies of all other zones are permuted inward, and fresh fuel assemblies are loaded into the peripheral zone. This arrangement tends to flatten the power distribution because the fresh fuel increases the flux in the region of the periphery of the core while the fuel of greate st exposure depres ses the flux in the central zone. In both types of zone patterns all of the fuel assemblies are moved at each refueling and there is considerable flux variation and power mismatch from zone to zone.

In the distributed or scatter reloading pattern, fuel assemblie s are i removed and replaced from locations distributed as uniformly as possible throughout the core whereby the fuel as semblies need not be moved during their lifetime in the core. This arrangement minimizes downtime for refueling and, in a large loosely coupled reactor, can minimize the mismatch among the unequally depleted fuel assemblies. A disadvantage of the distributed refueling pattern is that the fuel in the low neutron flux regions near the periphery of the core accumulates less exposure for a given residence time than the fuel in the more central regions.

Some of the foregoing disadvantages can be overcome by the use of a combined zone-distributed pattern wherein fresh fuel assemblies are loaded into a zone at the periphery of the core, and the partly exposed fuel as semblies from this peripheral zone are moved into a distributed pattern throughout the central portion of the core. This pattern also Improves the radial power distribution by its concentration of the high reactivity, fresh fuel in the peripheral zone of the core.

An important disadvantage of both the zone and the zone-distributed loading patterns is that the fuel as semblies reside in the steep local neutron flux gradients near the neutron reflector for a large fraction (for example, 20 - 30 percent) of their irradiation history. These steep local gradients in neutron flux skew the exposure distribution within the fuel assemblies and cause increased local power peaking when the fuel is moved into the central portion of the core. The local powe r distribution from rod-to- rod within a fuel as sembly can be flattened by variation of relative fis sile or abs orber V' content of the rods . For example, a lower fis sile fuel content can be us ed in fuel rods which are exposed to a large thermal neutron flux. However, if the fuel assemblies are to be moved between locations having different thermal neutron spectra, the power distribution cannot be tailored effectively in this manner. Furthermore, the fuel experiences a cooler (lower energy) neutron flux spectrum during its residence in the peripheral zone of the core which tends , among other effects, to reduce the conversion of fertile fuel relative to fertile fuel conversion in the central portions of the core.

The peripheral zone of the core is thus a specialized region characterized by: steep local neutron flux gradients caused by leakage of fast neutrons out of the core and a return current of thermal neutrons from the reflector- moderator surrounding the core; a ratio of fast-to-thermal neutron flux which is smaller than in the central region of the core; a coole r (lower energy) thermal neutron spectrum; and a reduced magnitude of neutron flux relative to the central region of the core.

These and other objects of the invention are achieved by utilising plutonium fuel in fuel assemblies placed in the peripheral zone of the nuclear reactor core. In this manner these fuel assemblies can be designed to take advantage of the characteristics of plutonium fuel specifically with regard to the characteristics of the peripheral zone. The number of plutonium fueled assemblies in the peripheral zone is a relatively small percentage (for example, in the order of 15 percent) of the total fuel assemblies in the core. Thus the cost of fabricating the radioactive and toxic plutonium fuel is minimized. A distributed fuel reloading pattern can be used in the central and peripheral zones to reduce refueling time and fuel handling damage since none of the fuel assemblies need to be moved during their residence in the fuel core. .Local power shaping can be achieved by spatial variation of the fuel from fuel rod-to-fuel rod across the fuel assemblies. The reduced excess reactivity of plutonium fuel allows greater power density for a given strength of reactivity control. The improvement in power distribution makes possible a reduction in the size of the core, vessel and containment for a given reactor power output.

Further features and advantages of the invention are presented in the following more specific description with reference to the accompanying drawing wherein: Figure 1 is a schematic diagram of a typical nuclear reactor power plant; Figure 2 is an elevation view of a typical fuel assembly; Figure 3 (parts A and B taken together) is a schematic plan view of a nuclear reactor core as employed in the reactor of Fig. 1 ; Figure 4 illustrates the radial, thermal neutron flux distribution of a typical large, reflected power reactor core; Figure 5 illustrates typical radial neutron flux characteristics in the peripheral region of the core; Ψ : Figure illustrates the energy dependent microscopic cross sections of uranium and plutonium fuels ; Figure 7 is a plan view of a portion of a core illustrating an improved refueling pattern; and Figures 8, 9 and 10 are schematic plan views of peripheral fuel assemblies illustrating the spatial distribution of plutonium fuel therein.

While not limited thereto, the utilization of plutonium fuel in the peripheral zone of a fuel core in accordance with the invention is described herein in connection with a water cooled and moderated reactor, an example of which is illustrated schematically in Fig. 1. Such a reactor system includes a pres sure vessel 10 containing a nuclear chain reactor core 1 1 submersed in a coolant 12 such as light water. The core 1 1 include s a plurality of spaced fuel assemblies each of which comprises a plurality of elongated fuel elements or rods positioned in spaced relation within a coolant flow channel. A plurality of control rods 13 (shown in dashed lines) of cruciform shape and containing neutron absorbing material are selectively insertable into the space s among the fuel assemblies by drive means 14 for mechanical control of the reactivity of the nuclear core. A pump 1 circulates the coolant through the core. The coolant remove s heat produced in the fuel elements by the fission process whereby a part of the coolant water is converted to steam. The steam thus produced is utilized by some means such as a turbine 17. The exhaust steam is condensed by a condenser 18 and returned as feedwater to the vessel 10 by a pump 19.

A fuel assembly 20 is illustrated in elevation view in Fig. 2. The fuel assembly 20 comprise s a tubular flow channel 21 of square cros s section containing an array of elongated fuel elements or rods 22 ^ supported between Upper and lower tie plates 23 and 24.

A nose 26 is provided with openings 27 through which the coplant water is received to flow upward past the fuel elements . The fuel elements 22 may be formed of a tubular cladding containing a plurality of cylindrical pellets of fuel. The n clear core 11 is shown in schematic plan view in Figs. 3A and 3B. The core 11 is_ formed by a plurality of fuel as semblies 20 arranged in spaced groups of four surrounding each control rod 13. An interior or central zone 31 is outlined with a dashed line in Fig. 3 A while a peripheral zone is indicated as 32..

For simplicity only one quadrant or one-quarter of the core 11 is shown in greater detail in Fig. 3B. The peripheral zone 32 of the core includes a plurality of peripheral fuel assemblies 34, 35 and 36 each having an edge and/or one or more sides adjacent the moderator-reflector, surrounding the core. Each of the peripheral fuel assemblies is legended P in Fig. 3B. The peripheral zone may also include a plurality of intermediate fuel assemblies 37 (legended I) which, though separated from the edge of the core by the peripheral fuel as semblies , may be substantially within the zone of influence of the edge of the core.

Illustrated in Fig. 4 is a typical radial thermal neutron flux distribution along a radius of a relatively large, reflected power reactor core. The flux curve remains 32076 2 relatively flat over the central portion of the core, drops rapidly toward the periphery because of fast neutron leakage from the core, and increases sharply near the edge of the core due to the return of thermal neutrons from the ref ector—moderator surrounding the core .

The characteristics of the peripheral zone of the core are more completely sh own by Fig, 5 which illustrates typical radial distributions of resonance (opi-thermal ) neutron flux, thermal neutron flux, and the ratio of the therraal-to-resonance neutron flux in the peripheral region of the core. Also illustrated in ig. 5 is a typical radial distribution of the energy of the thermal neutron flux of thermal neutron spectrum in the peripheral zone.

Fig. 5 thus illustrates that, as compared to the interior fuel assemblies, the peripheral fuel assemblies, and to a lesser extent the intermediate fuel assemblies, are subjected to a reduced magnitude of neutron lux, steep local neutron flux gradients, an increased ratio of thermal—to—resonance flux, and to a co&ler (lower energy) thermal neutron flux s ectrum.

In accordance with the present invention the purposes of using plutonium fuel in a nuclear reactor and of optimizing the use of the peripheral zone of the core are both served by providing plutonium fueled fuel assemblies especially designed for use in the peripheral zone.

B "Epical" is meant that the flux distributions in the peripheral zone illustrated in Fig. 5 are those vhich are found in a reflected, non- lux- lattened core fueled with uniform or similar fuel assemblies or reloaded in an evenly distributed or scatter reloading pattern as shown, for example, by M.M. El.-Vakil in Figure 5 - 11 of Nuclear Power Engineering. McGraw-Hill Book Co. ,1962.

The properties of the plutonium fuel which are used to advantage in the peripheral region of the core are discussed hereinafter with reference to Fig. 6 which illustrates the variation in microscopic cross section of U-235 and of the isotopes of plutonium of interest over thermal and resonant energy ranges.

The capture-to-fisseon ratio of the fissile plutonium nuclides, Pu-239 and Pu-241 , decreases as the neutron energy decreases* This trend is caused by the large low energy Pu-239 and Pu-241 cross section resonances near 0.3 electron volts which have large capture-to- ission ratios For a thermal reactor this property thus favors location of the plutonium fuel in the low energy thermal neutron spectrum of the peripheral zone of the core* The mean fission cross section of plutonium in a thermal neutron flux is about 2. 7 time s greater than that of uranium. Since the power generation rate is directly proportional to the fis sion cros s section time s the neutro flux, the greater fis sion cross section of plutonium is an aid in increasing the relative power in the peripheral zone to thereby improve the radial power distribution of the core . The re sulting more rapid burnup of the plutonium fuel also aide in matching the refueling cycle periods of the peripheral and central zone s .

The plutonium isotope Pu-240, which is present in plutonium produced in a thermal reactor, has a very large resonance absorption cross section at 1. 0 electron volt. Neutron captures in the fertile Pu-240 produce fissile Pu-241 . However, capture s in Pu-240 reduce reactivity and, hence, increase the plutonium f«el inventory requirements. The plutonium fuel inventory requirements are reduced by placing the plutonium fuel in the low energy neutron spectrum of t e peripheral zone whereby neutron capture in Pu-240 is reduced.

Because of the pre sence of the fertile Pu-240 and its relatively high conver sion rate , plutonium fuel change s le ss in reactivity per unit of exposure than doe s uranium fuel. In a lar ge loosely coupled reactor , -the power distribution is strongly affected by the local infinite multiplication. Thus plutonium fuel in the peripheral zone aids in maintaining a flattened radial power distribution toward the end of the fuel cycle .

Because plutonium can be chemically separated from uranium, variations in plutonium concentration can simply be acnieved by blinding. This makes it economically feasible to provide the several variations in plutonium content needed to minimize local power peaking in the steep neutron flux gradients of the peripheral zone .

Further advantage s to the use of plutonium fuel in a separate J peripheral zone of the core include the following. Plutonium fuel is radioactive and biologically toxic; thus it is difficult and expensive to handle, The cost of plutonium fuel fabrication is minimized by concentrating it in a relatively small number of the fuel a ssemblies of the core.

The use of plutonium fuel in the fuel assemblie s of the peripheral zone make s feasible an improved refueling pattern. Though not a limitation of the invention, it is contemplated that separate distributed or scatter refueling patterns will be used for both the plutonium fueled peripheral zone and the uranium fueled central zone. Thus none of the fuel as semblie s are moved during their residence in the core with the result that refueling time and handling damage are minimized. A quadrant of a fuel core is shown in Fig. 7 illustrating such a refueling scheme. The scheme is based upon an approximately 25 percent batch reload, that is, each fuel as sembly resides in the core through four cycles of operation. The fuel assemblie s of the peripheral zone are marked "P" while the fuel a ssemblies of the central zone are marked "C" . The number s indicate the number of cycles the fuel as sembly has been in the core . The fuel assemblie s marked "4" are the next to be replaced and so forth. For this refueling scheme the enrichment of the fuel in the peripheral zone is selected to provide a specific power (kw/kg) level approximately equal to the specific power level in the central zone so that the core re sidence time is the same for both the peripheral zone and central zone fuel assemblies . However , it is poss ibl to de sign the peripher fuel for lower specific power and either discharge the peripheral fuel at a lower exposure or with a longer residence time in the core.

The use of plutonium fuel in special fuel as semblie s for use only in the peripheral zone of the core, and which do not require movement during For example, the average fissile plutonium content of the intermediate as semblies may be about 1. 2 atom percent as compared tw- the average fis sile plutonium content of about 1. 37 atom percent in the peripheral assemblies.

In the foregoing, examples of fuel assemblies with spatially varied fissile plutonium fuel content for matching the neutron flux gradients of the peripheral zone have been described. A similar re sult can be achieved by-using a uniform distribution of fissile plutonium fuel and by appropriate spatial variation of the content of other nuclear materials such as U -235, Pu-240, U -239, U-233 and Th-232. However , the chemical separability of plutonium favors the use of varying plutonium content.

Reactor control, as provided by movable control rods and other neutron absorbing arrangements such as soluble and burnable poisons , is an important consideration in the design of a reactor core because it is necessary to be able to shut down the core under all conditions . To maintain the desired high specific power in the peripheral zone of the core, it is nece ssary to provide both a fuel with a large fission cross section (such as plutonium fuel) and a high neutron flux level. The neutron flux level in the pei-ipheral zone is a function of both the neutron leakage from the central zone and the local exce ss multiplication of the fuel of the peripheral zone , k A s is well-inown, most thermal reactors are designed so that the neutron multiplication is greater in the cold condition than in the hot condition. Thus the cold shutdown condition requires the maximum control. The design of periphe ral zone fuel assemblie s for high specific power in the peripheral zo may lead to control problems in this zone. here are several appr oache s to the solution of this possible problem including the following.

Control rods of greater control strength may be used in the peripheral zone . Howeve r, this may result in excess control rod worth whereby control may be marginal in the case that one of the control rods becomes inoperative.

A control poison which is soluble in the coolant may be used for reactor control. Soluble poison may be restricted to use for hot-to-cold shutdown only, or in some reactor systems it also may be used for control at power. In both cases , soluble poison can provide adequate control to permit design for equal specific power in the peripheral and central zones. .

The thickness of the peripheral zone can be made small enough (for example , only the peripheral fuel assemblies) so that the neutron flux is more a function of the leakage from the central zone and le ss a function of the local excess multiplication of the peripheral zone fuel. The reduced excess reactivity of plutonium fuel contributes to the feasibility of this design. Because of the large capture cros s section of the fertile Pu-240, it is possible to provide a large fissile plutonium content and yet maintain a relatively small excess reactivity.

Since the fuel assemblies of the peripheral zone are specially designed, moderator -to-fuel ratio can be selected such that the local reactivity decreases, rather that* increases, from the hot to cold condition. This can be accomplished in several ways including variations in the size, number and spacing of the fuel rods and by introducing special moderator channels in the fuel assemblie s . Increasing the widths of the water gaps W and w (Figs. 8, 9 and 10) between the peripheral and central zone s can also aid in reducing the cold control requirements of the peripheral zone.

= Thus what has been described is a nuclear reactor core utilizing plutonium fuel in fuel assemblies for use in the peripheral zone of the core to optimize the utilization of plutonium and the use of the peripheral portion of the core. ' 32076/2 i

Claims (9)

1. A thermal neutron reactor having a fuel core comprising an array of spaced fuel assemblies radially surrounded by a moderator—re lector water layer, comprising: a second region of said core constituted by those fuel assemblies having at least one edge adjacent said layer of water, a first region of said core including all of the fuel assemblies of said core except those in said second region, a first initial fissile fuel in the fuel assemblies of said first region, and a second initial fissile fuel having a thermal neutron fission cross section greater than said first fissile fuel in said second region of said core.

2. The reacior of Claim 1, wherein said second fissile fuel includes an initial substantial quality of Plutonium and wherein said first fissile fuel includes uranium initially free of significant quantity of plutonium.

3. The reactor of Claim 1, wherein said second region includes a group of fuel assemblies between said first and second regions and wherein the fuel assemblies in said group contain a third initial fissile fuel having a thermal neutron fission cross section greater than said first fissile fuel.

4. The reactor of Claim 1, wherein the initial enrichment of said second fuel is graduated across said second region substantially in inverse proportion to a reflected unflattened radial thermal neutron flux distribution of said second region.

. A fuel assembly for use in the periphera

6. A method of fueling and refueling a thermal nuclear reactor as defined in Claims 1 to 5» comprising placing first and second initial fissile fuels respectively in the fuel assemblies in said first and second regions, the thermal neutron fission cross section of the first initial fissile fuel being greater than that of the second initial fissile fuel.

7. The method of Claim 6, therein said first fissile fuel includes an initial substantial quantity of plutonium and wherein said second fissile fuel includes uranium initially free of a significant quantity of plutonium.

8. The method of Claim 7, including the further steps of: determining a reflected unflattened radial thermal neutron flux distribution of said core, and varying the initial enrichment of said first fuel substantially in inverse proportion to said radial flux distribution across said first region. 32076/2

9. The method of Claim 8, wherein said second region includes a group of fuel assemblies located intermediate said first and second regions, including the futther steps oft placing a third initial fuel of thermal neutron fission cross section greater than said second fuel in the fuel assemblies of said group; and varying the initial enrichment of said third fuel in inverse proportion to said radical flux distribution in the region intermediate said first and second regions. AGENT FOR APPLICANT Tel Aviv, April 24, 1969