Minimizing or eliminating refueling of nuclear reactor
Demand for refueling of a liquid metal fast nuclear reactor having a life of 30 years is eliminated or reduced to intervals of at least 10 years by operating the reactor at a low linear-power density, typically 2.5 kw/ft of fuel rod, rather than 7.5 or 15 kw/ft, which is the prior art practice. So that power of the same magnitude as for prior art reactors is produced, the volume of the core is increased. In addition, the height of the core and it diameter are dimensioned so that the ratio of the height to the diameter approximates 1 to the extent practicable considering the requirement of control and that the pressure drop in the coolant shall not be excessive. The surface area of a cylinder of given volume is a minimum if the ratio of the height to the diameter is 1. By minimizing the surface area, the leakage of neutrons is reduced. By reducing the linear-power density, increasing core volume, reducing fissile enrichment and optimizing core geometry, internal-core breeding of fissionable fuel is substantially enhanced. As a result, core operational life, limited by control worth requirements and fuel burnup capability, is extended up to 30 years of continuous power operation.
Latest The Unites States of America as represented by the United States Department of Energy Patents:
This invention relates to the operation of power plants whose primary source of power is a liquid metal fast reactor and to reactors for such plants. This invention has particular relationship to the fueling of the reactors for such plants. While this invention is uniquely applicable to plants whose reactors are of the liquid metal fast type, it is realized that in its broader aspects, it may find application to plants whose reactors are of other types such as pressurized water reactors or boiling water reactors or gas-cooled reactors. To the extent that this invention is applied to, or find use in, plants having reactors of other types than liquid metal fast reactors or to the reactors of such other plants, such application and use is within the scope of equivalents of this invention.
A liquid metal fast nuclear reactor (LMFR) includes a core of fissile fuel and fertile material. The fissile fuel is usually plutonium oxide. The plutonium in the PuO.sub.2 includes a number of isotopes, Pu238, Pu239, Pu240, Pu241, Pu242. The Pu239 and Pu241 are fissionable. The Pu239 constitutes about 86% of the Pu and the Pu241 about 2%. The fissile fuel is in fuel rods, each rod including cladding of stainless steel or zirconium alloy which contains pellets of mixed plutonium and uranium oxides referred to as [(Pu-U)O.sub.2 ] and pellets of UO.sub.2. In both cases, the UO.sub.2 contains depleted uranium. The UO.sub.2 serves as fertile material; the content of U235 in this uranium is about 0.2%. The content of PuO.sub.2 in the fuel rods is about 12 to 25%; the remainder is UO.sub.2. The core also includes blankets of depleted uranium as fertile material circumferentially and on the top and bottom. Alternatively, the fissionable fuel may be UO.sub.2 enriched between 12 and 25% in U235 mixed with UO.sub.2 with depleted uranium. Plutonium and uranium carbide may be used in place of the oxides.
Typically, the core may be regarded as having the shape generally of a circular cylinder, although strictly in transverse section it is polygonal with a large number of sides. The fissile fuel rods are assembled in a generally cylindrical unit. The blankets may include a ring encircling the fissile cylinder externally and discs disposed on the bases of the fissile cylinder. The fissile fuel rods may also be mounted in a ring including a blanket section in the center in addition to the external blanket sections. The coolant for the LMFR is a liquid metal, typically liquid sodium.
Liquid metal fast reactor cores have a high linearpower density capability. Linear-power density is defined as the number of kilowatts per foot (kw/ft) of fuel rod. In accordance with the teachings of the prior art, LMFRs were designed to take advantage of their high power density capability. Core volumes were kept to a minimum to obtain an average linear-power density of about 7.5 kw/ft for oxide fuel and 15 kw/ft for carbide fuel. This minimization of core volume has resulted in fluence and burnup levels which limit the fuel assembly lifetimes to the range of three to five years. In addition, the refueling interval has usually been limited, by the available control system worth, to about one year because of the high reactivity loss with burnup.
The demand for frequent refueling has serious disadvantages. The refueling is itself a complex process involving removal of the head of a reactor and replacement of radioactive fuel rods in a radioactive environment. It is also necessary to handle the highly reactive coolant which rapidly turns into caustic alkali in air. The labor cost is substantial and the reactor is shut down for relatively long intervals. Frequent refueling also incurs front-end fuel losses since partially burned fuel assemblies are removed during the first few refueling cycles. In an annular refueling cycle in which one fourth of the core is replaced each year, the first, second and third refuelings remove fuel that is only one-quarter, one-half, and three-quarters burned respectively. In this case, the loss is three-eighths of a core; in case of refueling over five years, this loss is two-fifths of a core. The loss may be recovered by reprocessing, but reprocessing is not available in the United States, and if it becomes available, it would be costly. The construction of a prior art power plant with an LMFR is complicated and the cost is materially increased by the necessity of providing the fuel handling machinery, the buildings to contain this equipment and the facilities for moving this equipment back and forth between the building and the reactor. In addition, there must be fuel storage pools and cooling equipment to handle the annually removed batches of fuel which are still highly fissionable and are also highly radioactive. Another disadvantage arises from the deterioration of the cladding of the fuel rods and of the fuel by crumbling which results from operation at the high linear power density.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a method of operating a power plant whose primary power source is an LMFR in such a way that refueling shall not be required over the life of the LMFR. It is also an object of this invention to provide an LMFR for practicing this method. An ancillary object of this invention is to provide a method of operating a power plant whose primary source is an LMFR in such a way that infrequent refueling over the life of the plant, typically at intervals of at least ten years, shall be required.
SUMMARY OF THE INVENTIONIn accordance with this invention, operation of an LMFR plant without refueling the reactor over the life of the core is achieved by relying on the breeding of fissile fuel from the fertile material to extend the useful life of the core while operating at a low linear power density. Typically, the life of the core is about 30 years. The breeding of fissile fuel limits, or compensates for, the reactivity loss. The breeding of fissile fuel is augmented by minimizing the leakage of neutrons from the core. This object is accomplished by dimensioning the core so that the ratio of its height to its diameter is as near to 1 as practicable. It can be shown that a circular cylinder has minimum surface area for a ratio of 1 so that in such a cylinder the escape of neutrons through the surface is minimized. However, the ratio of 1 may not, in practice, be achieved because consideration must be given to the control demands (long control rods and movement of control rods over substantial distance) and to pressure drop in the coolant as it flows through the core. To preclude control costs and too great a pressure drop or excessive increase in the cost of coolant pumps, the ratio is usually less than 1. With respect to certain reactors, it has been found that the improvement achieved for an increase in the height-diameter ratio beyond about 39% does not warrant the additional cost. Typically, the linear-power density is limited to about one-third of the prior art linear power density. For oxide fuel, the linear-power density is limited to about 2.5 kw/ft. To produce power of the same magnitude with an LMFR operating at low linear power density as is produced by a prior art LMFR which operated at high linear-power densities, the volume of the core must be increased to accommodate the required increased fuel rods and fertile material. The height of a prior art core is typicaly about 40 inches. The core according to this invention has a height of 5 to 6 feet. The diameter of the core is also increased, but the ideal of the ratio of 1 for height to diameter is approached and not reached.
As far as enrichment in fissionable fuel (Pu239, Pu241, U235) for a long-life core is concerned, it depends on the specific needs of the power plant, i.e., power level, life of core (10 years, 15 years, 30 years) etc. In general, at any power level, the long-life core has a lower enrichment than a core which is refueled annually because of the greater volume and reduced neutron leakage of the long-life core. Typically, a heterogeneous 1300 MWe core to be refueled annually requires Pu enrichment of about 30%, while a long-life core of the same power output requires about 21%. The longlife core has more fissile material at the start of operation as illustrated in the following tables IA, IB, IC:
TABLE IA
______________________________________
1000 MWe CORE
FISSILE MASS UTILIZATION
FOR THROWAWAY CYCLE
(30 YEARS OF OPERATION)
Refueling Interval
Annual 15 Year 30 Year
______________________________________
Initial Fissile Plutonium
3,604 6,320 9,015
Loading, kg
Additional Plutonium
20,903.sup.(1)
6,320 --
Required, kg
Total Plutonium Required, kg
24,507 12,640 9,015
______________________________________
.sup.(1) 29 annual reloads.
TABLE IB
______________________________________
1000 MWe CORE
FISSILE MASS UTILIZATION
FOR THROWAWAY CYCLE
(30 YEARS OF OPERATION)
Refueling Interval
Annual 15 Year 30 Year
______________________________________
Initial Fissile Plutonium
1,644 2,400 3,390
Loading, kg
Additional Plutonium
11,919.sup.(1)
2,400 --
Required, kg
Total Plutonium Required, kg
13,563 4,800 3,390
______________________________________
.sup.(1) 29 annual reloads.
TABLE IC
______________________________________
110 MWe CORE
FISSILE MASS UTILIZATION
FOR THROWAWAY CYCLE
(30 YEARS OF OPERATION)
Annual
15 Year
______________________________________
Initial Fissile Plutonium
845 1,865
Loading, kg (11/4 Cores)
Additional Plutonium 6,126.sup.(1)
1,865
Required, kg
Total Plutonium Required, kg
6,971 3,730
______________________________________
.sup.(1) 29 annual reloads.
The practice of this invention results in the following principal advantages:
1. The construction of the reactor power plant is simplified with attendant reduction in cost, particularly by elimination of the refueling machinery and its buildings and the moving equipment which this machinery requires and, also, by the elimination of storage facilities for the fuel which is removed year by year, i.e., fuel storage pools, cooling systems, etc.
2. There is a significant increase in the time during which the LMFR is available because no time, or in the case of infrequent refueling, little time is taken out for refueling.
3. The necessity for fuel fabrication, shipment, and reprocessing are eliminated, except at the start and decommissioning of a power plant.
4. Safety in the operation of the power plant is improved because the reactor is operating at a low linear-power density and the possibility that the core or the coolant will be overheated is reduced.
5. The storage of high radioactive level nuclear waste is eliminated.
6. The proliferation of plutonium which might accidentally fall into the hands of terrorists or mindless zealots is reduced.
7. Since, in case of a non-refuelable reactor, the reactor closure head need not be removed until the plant is decommissioned, the structure of the closure head is simplified.
8. The deterioration of the fuel rods is reduced and/or the fraction of the fuel volume may be increased.
9. It is estimated that the fuel burnup capability is increased by about 30%. Technological advances have currently increased the burnup capability from about 170,000 megawatt days per metric ton to about 220,000 MWD/MT. With a 30-year non-refueling core in accordance with this invention, a burnup of about 290,000 MWD/MT is feasible.
10. It is estimated that the cost of power is reduced by about 20%.
This invention emphasizes extended fuel life, rather than high power density. This requires that heavy metal (fertile material) content and the volume of the core must be increased, typically by a factor of 2 to 3.5, depending on whether the fuel is carbides or oxides of uranium or plutonium. This volume increase produces a corresponding decrease in flux level, and a more benign environment which results in less fuel irradiation damage over a given period of time. When this volume increase is combined with the extended fuel burnup capability mentioned earlier, a 30-year assembly life without refueling appears highly attractive from an economic standpoint.
For a non-refuelable core with heavy metal and carbide fuel, the in-core breeding characteristics limit the reactivity loss to a readily controlled level of 2 to 3% .DELTA.K/K. Oxide fuels yield lower fissile breeding and therefore suffer larger burnup reactivity losses. The prospects for long-life oxide core operation is achievable by quantifying the sensitivity of burnup reactivity to core height. An oxide core at a prior-art height of 40 inches (102 cm), but at three times the prior-art volume, experiences a reactivity loss in excess of 10% .DELTA.K/K. If this same core volume is maintained while increasing the height to 72 inches (183 cm), the reactivity loss is reduced to 3% .DELTA.K/K. By proper selection of the fuel parameters; e.g., pin diameter, pitch-todiameter ratio, core temperature rise, etc., the increase in pressure drop in the coolant as it flows through the core is minimized or eliminated entirely.
The core changes, in accordance with this invention, may also impact the temperature-reactivity coefficients. The magnitude of the Doppler constant increases by 17%, while the radial-expansion coefficient decreases approximately 5%. The radial-expansion coefficient relates the change in core reactivity (.DELTA.K/K) to changes in the core radius arising from thermal expansion of core support structures. Increasing temperatures generally expand the core spreading the fuel rods and thus reduce reactivity. The Doppler effect is a decrease in reactivity as the temperature of the coolant increases during startup and an increase in reactivity as the temperature of the coolant decreases during shutdown. The Doppler effect and radial-expansion coefficient introduce negative feed-back factors during startup and positive feed-back factors during shutdown. The above-described core changes appear to have an inconsequential effect on performance as regards the Doppler effect and the radial-expansion coefficient.
A non-refuelable reactor, typically with a non-refuelable core having a life of 30 years, has marked unique advantage. But there may be power plant installations in which higher linear power density and a smaller volume core requiring minimal refueling is desirable. Power-plant operation in which the refueling interval is at least 5 years and coincides with other major plant maintenance shutdowns is within the scope of the broader aspects of this invention. In the event that shutdowns at shorter intervals becomes necessary for maintenance purposes, limited refueling can take place during such shutdowns. Limited refueling is to be distinguished from the annual refueling which is required for prior art LMFRs in whose practice one quarter or one fifth of the fuel rods are replaced annually. The 5-year or longer refueling cycle is within the scope of this invention. It involves substantially lower linear power-density and a substantial increase in core volume as compared to prior art LMFRs. A 5-year or longer refuel interval would provide the increased plant availability of a 30-year core, while allowing a smaller core volume relative to the 30-year core. This aspect of the invention would not offer the simplification of the closure head and the elimination of fuel handling equipment and reprocessing facilities aforded by the 30-year core.
BRIEF DESCRIPTION OF THE DRAWINGSFor a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection with the accompanying drawings, in which:
FIG. 1 is a diagrammatic view showing a power plant in whose operation the method of this invention is practiced and also showing a liquid metal fast reactor embodying this invention;
FIG. 2 is a view in transverse section showing a homogeneous core included in the LMFR shown in FIG. 1;
FIG. 3 is a view in transverse section showing a heterogeneous core which in the alternative might be included in the reactor shown in FIG. 1;
FIG. 4 is a graph showing the effect on reactivity of different ratios of height to diameter of cores having the same volume;
FIG. 5 is a graph showing the relationship, with respect to excess reactivity, for a reactor in accordance with this invention as compared to a reactor in accordance with the teachings of the prior art with both designed to deliver power of the same magnitude;
FIG. 6 is a graph showing the effect of fuel volume fraction in the core on excess reactivity;
FIGS. 7 and 9 are graphs similar to FIG. 4 showing the effect on life of the ratio of height-to-diameter of core at the same volume for 330 MWe and 110 MWe reactors operating at a low linear-power density, and FIG. 8 is a graph comparing the effect of a heterogeneous core arrangement with a homogeneous core arrangement for a 330 MWe reactor;
FIG. 10 is a graph comparing the burnup and life of operation at low linear power density and high linear-power density;
FIG. 11 is a graph compring excess reactivity as a function of time for long-life cores containing uranium oxide fuel and a mixed oxide [(Pu--U)O.sub.2 ] fuel, both operated at a low linear-power density;
FIG. 12 is a graph showing the power plant unavailability as a function of the refueling interval, and
FIG. 13 is a graph showing excess reactivity as a function of time of cores designed to operate over different long lives.
DETAILED DESCRIPTION OF EMBODIMENT OF INVENTION AND PRACTICE OF INVENTIONFIG. 1 shows a power plant 21 including a pool-type liquid metal fast reactor 23, an external heat exchanger 25 and a turbine 27. FIG. 1 is a simplified presentation. The reactor 23 is suspended on a support 29. Principally, the reactor components include a reactor vessel 31 which is enclosed in a guard vessel 33. Within the reactor vessel, there is a pump 35, a core 37, an intermediate heat exchanger 39, and upper internals generally shown at 41. These components are surrounded by a pool 42 of liquid sodium, typically contained within the reactor vessel 31. It should be noted that this invention is equally applicable to a loop-type as well as a pool-type reactor.
In transverse cross section, the core 37 has the shape of a polygon. However, since the cross-dimension through the center of the core is large and the number of sides of the polygon is high, for practical purposes, the transverse cross-section of the core may be regarded as circular with a radius equal to the radius of the circle circumscribing the polygon. The core may be regarded as circularly cylindrical.
LMFRs are described as homogeneous and heterogeneous based on their core structure. FIG. 2 shows the transverse cross-section of a homogeneous core 37M. Such a core includes a central cylinder 51 of fissile fuel of lower enrichment, typically 17 to 19%, encircled by an annulus 53 of fissile fuel of higher enrichment, typically 25 to 30%. The cylinder 51 and the annulus 53 are composed of mixed Pu and U oxides or carbides [(Pu--U)O.sub.2 ; (Pu--U)C], the fissile fuel, and depleted uranium oxide or carbide. The plutonium includes all of its isotopes which are usually present but has major quantities of the fissionable isotopes--Pu239 (about 86%) and Pu241 (about 2%). The fraction of the plutonium in the fuel pellets is about 20 to 30%. The uranium in the cylinder and ring serves as a fertile material and is converted into plutonium by neutron reaction. The annulus 53 is encircled by an annulus 55 of fertile material, typically depleted uranium oxide or carbide with about 0.2% U235. There are also blanket discs of depleted uranium at the top and at the bottom of the core 37.
With reference to FIG. 3, the heterogeneous core 37T includes a central cylinder 57 of fertile material encircled by annuli 59, 61, 63, 65, 67, 69 alternately of fertile material and fissile fuel.
The core 37 is mounted on core supports 71 and is enclosed in a shield barrel 73. The coolant, liquid sodium, is transmitted to the bottom of the core 37 by the pump 35 through conductor 75 and is driven by the pump upwardly through the core. Control rods 77 are actuated by mechanism (not shown) in the upper internals 41 to move in and out of the core in dependence upon power demands or other requirements.
The fissile fuel is in the form of pellets clad in cylinders of stainless steel or zirconium alloy. The cylinders containing the pellets are described as fuel rods. The linear-power density or reactive power level is set by appropriate startup procedure through control rod 77 motion and positioning.
In accordance with this invention, long life operation of the reactor is achieved by reducing the linear-power density to a fraction, typically about one third, of the power density which is used in prior art operation and increasing the volume of the core, i.e., the number or length of fuel rods, so that the required power is derived for the LMFR power plant. In addition, the core is dimensioned so that the ratio of its height to its diameter is as near to 1 as is practicable, taking into consideration the other demands of the reactor and power plant operation and, particularly, control requirements and pressure drop in the coolant as it is driven through the core 37 by the pump 35. The fact is that there must be a compromise between the desirability of achieving the ideal of equality of height and diameter and the reality of driving a coolant through the core with reasonably available pumps and control rods of reasonable dimensions. In arriving at this invention, it has been discovered that in some situations, increase of height beyond a ratio of height to diameter of about 39% buys little in improvement in the operation or life of the reactor.
The advantages of this invention and some of its ramifications will now be discussed with reference to FIGS. 4 through 13.
In FIG. 4, excess reactivity in percent (%.DELTA.K) is plotted vertically as a function of life in years of the reactor without refueling, which is plotted horizontally. The graph is based on data for a liquid metal fast breeder reactor (LMFBR) delivering 1000 megawatts electrical power (2740 MW thermal) fueled by plutonium oxides mixed with depleted uranium oxide and related as disclosed above. All curves are for a reactor of the same volume and heavy metal content, but for different heights and diameters, operating at linear-power density of 2.5 kw/ft. Curves are plotted for cores, as indicated, having heights of 8 feet, 6 feet and 40 inches. Table I below shows the pertinent data for the cores.
TABLE I
______________________________________
Height to Diameter
Height ft. Diameter ft.
Ratio
______________________________________
8 13.3 .60
6 15.2 .39
40 in. 20.5 .16
______________________________________
As indicated, the broken-line heavy graphs 81 and 83 plot the life cycle for a reactor having a height of 40 inches (31/3 ft.) at an initial excess reactivity (.DELTA.K/K) of 2% and 8% respectively. The desired start-of-life (SOL) .DELTA.K is obtained by varying the initial enrichment. The excess reactivity in each case decreases continuously at a reduced rate, the operation starting with a 2% .DELTA.K/K excess and having a life of about 14 years and the other a life of about 21 years. The life of the 14-year reactor may be prolonged by refueling as indicated by the light broken-line saw tooth curve 85.
The dash-dot curve 87 presents the life-time excess reactivity for a core having a height of six feet and the same volume as for the curves 81 and 83. In the case of the six-foot core, the reactivity increases during approximately the first fourteen years and then decreases gradually to end of life at 30 years. At the peak reactivity, the core has delivered about 150,000 megawatt days per metric ton and at the end of life, the core has delivered about 300,000 MWD/MT. The increase in reactivity shown by the curve 87 results from the fact that the external surface area of the core is reduced by reason of the increase in the height-to-diameter (H/D) ratio from 0.16 to 0.39. Because of the reduced surface area for the 6-foot cylinder, the neutron leakage is minimized and the breeding of fissionable fuel is correspondingly maximized.
The full line curves 89 and 91 are the life curves for the core having a height of 8 feet, curve 89 for a start of life excess reactivity of 2% and curve 91 for an excess of 6%, and H/D=0.60. The life of this 8-foot core is prolonged only to about 31 years for .DELTA.K/K=2% and to about 32 years for .DELTA.K/K=6%. This small increase in life does not warrant the increased control demands and the pressure-drop problems which the 8-foot core would involve as compared to a 6-foot core. For the reactor described, operated at low linear-power density, the ratio H/D=0.39 is an unusually happy compromise.
The following table II shows the start-of-life breeding ratios in the different regions of the core:
TABLE II
______________________________________
REGIONWISE BREEDING RATIOS
AS A FUNCTION OF CORE HEIGHT
(1000 MWe, Low Power Density)
Breeding Ratios
Core Radial Axial
Height (ft.)
Total Active Core
Blanket Blanket
______________________________________
8.0 1.46 1.26 .15 .05
6.0 1.45 1.23 .14 .08
3.3 1.43 1.11 .11 .21
______________________________________
The breeding ratio is defined as
##STR1##
? The distribution of breeding in the core 37 which Table II shows is
significant. Increased H/D ratio shifts the breeding from the blankets
(primarily axial) to the active core region. The axial breeding is reduced
from 0.21 in the axial blanket of the 40-inch core to 0.08 in the axial
blanket of the 6-foot core and is increased from 1.11 in the active region
of the 40-inch core to 1.23 in the active region of the 6-foot core.
In all cases represented in FIG. 4, the fuel volume fraction is 46% and the volume of the core is maintained constant as the height is varied. FIG. 4 shows that as the height is increased, the reactivity loss is reduced. The increase in height reduces the surface area, reducing the loss of neutrons by reason of leakage.
In FIG. 5, excess reactivity (%.DELTA.K/K) is plotted vertically as a function of lifetime in years which is plotted horizontally. The graph shows the lifetime for a 1000 MWe core having a height of 6 feet. The broken-line curve 93 presents the operation at a linear-power density of 7.5 kw/ft and the full-line curve 95 presents the operation at 2.5 kw/ft. The core operated at a high linear-power density contained one-third of the heavy metal contained in the lowlinear-power-density core and one-third the length of fuel rods. The lifetime at the high-linear-power-density operation is about nine years, while the lifetime for the low-linear-power-density operation is thirty years. Operation at one-third the power density extends the lifetime by a factor of more than 3. At 10 years and 30 years respectively, the heavy-metal utilization is the same for the high power and for the low power operation. The burnup of the fuel is the same for operation at the low power density and at three times the low power density and, therefore, the lifetime duration at low power density should, theoretically, be three times the lifetime at the high power density. The difference is attributable to the increased leakage and reduced internal breeding in the smaller active region in the high-linear-power-density core.
Operation at lower linear power density subjects the cladding to reduced neutron flux levels and reduced fuel pellet swelling. An advantage of such operation is that the thickness of the cladding can be reduced and the fuel volume fraction correspondingly increased, or alternatively, the cladding can be operated for a longer interval. This advantage, like the higher H/D advantage, is subject to compromise since excessive reduction in thickness of the cladding would lead to deterioration of the cladding even at low linear power densities.
FIG. 6 shows the effect of fuel-volume fraction on life for cores of 6-feet height and 40-inch height operating at 7.5 kw/ft. In FIG. 6, excess reactivity in percent is plotted vertically and lifetime is plotted horizontally. The broken-line curves 101 and 103 are for nominal fuelvolume fraction of about 44% for the 40-inch core and the 6-foot core. The full-line curves 105 and 107 are the corresponding curves for fuel-volume fraction of 46%. In case of the 40-inch core, the 46% fuel-volume fraction extends the life from about three years to five years, and in case of the 6-foot core, the life is extended from 7.5 years to about 8.75 years. The increase resulting from only a 5% increase in fuel-volume fraction is substantial.
That an increase in the H/D ratio also increases the lifetime for reactors delivering lower power than 1000 MWe is shown in FIGS. 7 and 9. These graphs are similar to FIG. 4. FIG. 7 presents the relationship for a 330 MWe homogeneous reactor and FIG. 9 compares the relationship between excess reactivity and life of a 110 MWe core for different ratios of H/D. For the 6-foot height, H/D is greater than 1, which is not an optimum relationship.
FIG. 8 compares heterogeneous and homogeneous cores at 330 MWe.
The differences in the reactivity profiles for the three power levels can be understood by comparing the breeding ratio data. Table III contains pertinent breeding ratio data for these power levels.
TABLE III
______________________________________
REGIONWISE BREEDING RATIOS
AS A FUNCTION OF CORE POWER
(Low Power Density at 6-Foot Height)
Breeding Ratios (BOL)
Core Radial Axial
Power (MWe)
Total Active Core
Blanket Blanket
______________________________________
1000 1.45 1.23 .14 .08
330 1.45 1.17 .20 .08
110 1.39 0.96 .36 .07
______________________________________
The breeding ratios for the 1000 and 330 MWe cores are essentially the same, but the total breeding ratio falls off substantially below 330 MWe. The distribution of fissile breeding is also a major influence on lifetime. The axial blanket breeding ratio is nearly independent of reactor power. This is expected since the cores are all of the same height. But reducing the core power, which is effected by reducing the active fuel diameter, shifts fissile production from the active core to the radial blankets. (Compare the third and fourth columns.) Breeding in the blanket region is symptomatic of increasing neutron leakage from the active core which decreases the internal breeding and therefore increases the reactivity loss. Decreasing the power level (and the volume) reduces the achievable core lifetime by increasing the reactivity burnup swing.
In FIG. 10, the estimated burnup in MWD/MT and the estimated lifetime are plotted vertically on the left and right respectively as functions of the reactor power plotted horizontally for a reactor with a homogeneous mixed-oxide core. Curve 113 presents the high linear-power density operation and curve 114 the low linear-power density operation.
In small cores, the larger excess reactivity requirement for burnup limits the operational lifetime during which criticality can be sustained and therefore limits the maximum fuel burnup which can be achieved. For large cores, less excess reactivity is required and fuel burnup capability is limited by cladding strain. Both cladding strain and excess reactivity limits are reached at higher burnups for low-linear-power-density cores than for high power density cases. For heterogeneous core configurations, the maximum fuel burnup should be generally larger at each power level than shown in FIG. 10.
Table IV below shows the estimated peak burnup achievable as a function of power level.
TABLE IV
______________________________________
PEAK BURNUP FOR
STRAIGHT BURN CORES
(HOMOGENEOUS)
Peak Linear Power
(kw/ft)
Reactor Power 15 5
(MWe) Burnup (MWD/MT)
______________________________________
1300 220,000 290,000
330 180,000 250,000
110 90,000 180,000
______________________________________
In large cores (e.g., 1300 MWe), the full burnup capability of mixed oxide fuel (fissile PuO.sub.2 and depleted UO.sub.2) based on cladding strain limits can be utilized. The achievable burnups are reduced as the reactor size decreases because of excess reactivity limitations. However, even in a small core, the peak fuel burnup could be doubled by operating at low power density because of the enhanced lifetime derived through improved breeding performance.
Table V below compares the fissile mass utilization or fissile mass flow for a prior art and a long-life core delivering 1000 MWe over 30 years.
TABLE V
______________________________________
FISSILE MASS UTILIZATION
(1000 MWe)
Prior-Art
Long-Life
Core Core
______________________________________
Fissile Mass Burned
Over 30 Years 27,000 27,000
Net Fissile Gain 7,600 4,200
Gross Fissile Production (Kg)
34,600 31,200
______________________________________
The prior-art core has a 5-year fuel life with a peak burnup of 170,000 (MWD/MT). One-fifth of the core is refueled each year. The long-life core requires an initial loading significantly greater than that of a prior-art core. But the refueling requirement for a 30-year prior-art core with a throwaway cycle more than offsets this large initial fissile mass. Overall, the long-life core requires significantly less plutonium loading. Since both concepts produce power of the same magnitude, essentially the same fissile mass is burned. Over this period, the prior-art core results in a net fissile gain of approximately 7,600 Kg, while the long-life core produces an additional 4,200 Kg. In this mode of operation, the long-life core exhibits reduced fuel cycle costs since less fissile fuel is required (by a factor of nearly 2, Table IA). Most of the fissile mass burned over the core life is produced in situ in the long-life reactor core, rather than added periodically through refueling.
In FIG. 11, excess reactivity in percent is plotted vertically as a function of life in years plotted horizontally for a 1000 MWe reactor fueled by plutonium oxide mixed with depleted UO.sub.2, and for a reactor fueled by enriched uranium oxide mixed with depleted UO.sub.2. Both cores have the same volume and are operated at the same low power density. The mixed oxide [(Pu--U)O.sub.2 ] is present in the quantities described above, i.e., 20 to 30% of plutonium oxide and the remainder depleted uranium oxide (enrichment 0.2%). The UO.sub.2 reactor included uranium at about 17% enrichment. Curve 115 shows that the excess reactivity for the UO.sub.2 reactor decays progressively from 4% to 0 in about 20 years. The excess reactivity of the mixed oxide reactor as shown by curve 117 increases from 2% to about 4% at about 14 years and then decays to 0 at 30 years.
Although 30-year core life is more difficult to achieve with a UO.sub.2 core than with a mixed-oxide core, the life of the UO.sub.2 core can be increased materially by adding breeding facilities to the core. A lifetime improvement of 5 years can be obtained with the introduction of an optimized heterogeneous core assembly pattern to maximize breeding efficiency. An additional 5-year life extension can be achieved through an enlarged active core region. Calculations indicate that 20% more fuel (corresponding to further derating the core to 2.0 kw/ft) would be sufficient. These features would result in a 30-year UO.sub.2 core lifetime. A long-life UO.sub.2 reactor is within the scope of equivalents of this invention.
In FIG. 12, power plant unavailability in percent is plotted vertically as a function of refueling intervals in years plotted horizontally. This curve 121 is based on the assumptions that refueling is accomplished in one month and that other outages are independent of refueling. Although these assumptions may vary, the curve provides a representative evaluation for large plant design. With an annual refueling approach, the plant is unavailable more than 8% of the time. Life of 30 years reduces the unavailability to less than 0.3%. The major part of the availability gain has been obtained at a refueling interval of only 10 years.
Consideration will now be given to refueling after 15 years. FIG. 13 is a graph of the excess reactivity plotted vertically as a function of time in years plotted horizontally for a 30-year life core and two 15-year life cores. Curve 131 is the graph for the 30-year core and curves 133 and 135 for the 15-year cores. The cores with which FIG. 13 deals are cores including PuO.sub.2 and depleted UO.sub.2.
The following Table VI presents the parameters for the three cores.
TABLE VI
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COMPARISON OF 1000 MWe LONG LIFE HOMOGENEOUS
CORE CONCEPTS
30-Yr 15-Yr Long Life
Long Life
Option 1 Option 2
Curve 131
Curve 133 Curve 135
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Core Height (ft)
6.0 5.0 5.0
Core Radius (ft)
7.6 6.5 6.8
Relative Volume
1.0 .62 .65
Avg. Linear Power (kw/ft)
2.4 3.9 2.1
Pin O.D. (in) .31 .31 .23
Fuel Volume Fraction
.46 .42 .38
Core Pressure Drop (psia)
.about.65 .about.100
.about.100
Relative Heavy Metal
1.0 .59 .58
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The important difference between the 30-year and 15-year cores is the influence that core pressure drop exerts on the cores. Ideally, a 15-year core should contain half the heavy metal of a 30-year core to retain the same fuel burnup and thus maintain low fuel cycle costs. One approach would be to retain the fuel core dimensions and use only one-half as many fuel assemblies. However, the same dimensions results in an increased specific power density which requires more flow and increases the pressure drop. Consequently, the 15-year core must be opened (increase pin pitch to diameter ratio) and the core height reduced to lower the pressure drop. These changes effectively reduce the fuel volume fraction and thereby reduce internal breeding and shorten the core lifetime. A second alternative is to utilize smaller fuel pins; but the effect of smaller pins is also to reduce the fuel volume fraction. Based on Table VI, the two 15-year options result in nearly the same heavy metal and fissile mass content. Both cores require nearly 60% of the 30-year heavy metal which is 10% higher than the point of equal heavy metal utilization. A 6-foot core would nominally improve breeding and reduce heavy metal, but the pressure drop would be too high. The large pin option has a higher volume fraction, and a slightly longer life with less fissile inventory. Option 2 is attractive in that the control rod reactivity requirement is lower by approximately 1.5%.DELTA.K. The fuel cycle costs for these 15-year options are nearly the same since they have nearly the same fissile and heavy metal content.
The following Table VII through X present economic comparisons between the prior art and this invention.
TABLE VII
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TOTAL POWER COSTS
(MIXED OXIDE)
Mills/kWh
Prior Art
Invention
LMFR LMFR
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Capital 38.0 37.1
Fuel cycle 10.1 5.5 Power cost
Operation & maintenance
6.1 6.1 reduction
Total power cost
54.2 48.7 10%
Increased availability 43.7 19%
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TABLE VIII
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FISSILE MASS UTILIZATION*
(30 YEARS)
Invention 10-Yr
Prior Art
Long-Life Core
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Initial fissile plutonium
loading, kg 5,830 8,430
Additional plutonium required
for refueling, kg 56,347 16,860
Total plutonium required, kg
62,347 25,290
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*Assuming no reprocessing
TABLE IX
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FRONT END COST COMPARISON (M$)
Invention 10-Yr
Prior Art*
Long-Life Core
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First core fabrication
150 188
First core & reload plutonium,
$10/g(25/g) 89 (223) 84 (210)
Initial cost of core(s)
239 (373) 272 (398)
Refueling interval, yrs.
1 10
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*12/3 cores
TABLE X
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FUEL CYCLE COSTS (MILLS/KWH)
Throwaway Reprocessing
Invention Inven-
Pr. 30-Yr Pr. tion (1)
Fuel Cycle Components
Art Core Art 30-Yr Core
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First core fabrication
1.0 3.3 1.0 3.3
Annual fabrication
4.9 -- 4.9 --
Initial core plutonium
0.6 1.2 0.6 1.2
Annual plutonium
2.6 -- -- --
Plutonium yearly gain
-- -- -0.6 --
Final core plutonium
-- -- -0.5 -1.4
Annual reprocessing
-- -- 2.7 --
Final core processing
-- -- 0.4 1.2
Spent fuel storage
and shipment 1.0 1.0 1.0 1.0
Total fuel cycle cost
(capacity factor = 0.65)
10.1 5.5 9.5 5.3
(capacity factor = 0.725)
-- 5.0 -- 4.9
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Reduction in throwaway fuel cycle cost .apprxeq.5.1 mills/kWh
(1)Reprocessing at decommissioning.
These tables are self explanatory.
While preferred practice and a preferred embodiment of this invention has been disclosed, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the prior art.
Claims
1. A fast breeder reactor comprising:
- a core having particular amounts of fertile material and fissile fuel so arranged within said core that fissile fuel will be bred from said fertile material and consumed during operation of the core, said particular amounts and breeding being sufficient to maintain a predetermined designed power output level for five years or longer without replacement of any portion of said core;
- control means connected to the core which is capable of regulating the power level of the core so that power is maintained at the predetermined designed power output level;
- cooling means for removing heat from the core; and
- confinement means that supports and confines the core, control means and cooling means.
2. The fast breeder reactor of claim 1, wherein said core includes rods of fissile fuel and fertile material which form a many sided polygon approximating a circular cylinder with fertile material forming blankets on the axial and radial surfaces of said core so that a smaller active core region cylinder is formed within said blankets, the ratio of the height to the diameter of said active core region cylinder being less than 1.0 and greater than 0.35.
3. The fast breeder reactor of claim 2, wherein said active core region cylinder includes a central cylinder of fissile fuel enriched 17% to 19% and a surrounding annulus of fissile fuel of higher enrichment than said central cylinder, and said blankets of fertile material surrounding said active core region cylinder are in the form of discs located axially adjacent to said active core region cylinder and in the shape of an annulus radially surrounding said active core region cylinder and said disc shaped blankets.
4. The fast breeder reactor of claim 3, wherein the fissile fuel is an oxide fuel and the power density of the core at the designed power outut level is 2.5 kw/ft. of fuel rod.
5. The fast breeder reactor of claim 3, wherein the fissile fuel is a carbide fuel and the power density of the core at the designed power output level is 5.0 kw/ft. of fuel rod.
6. The fast breeder reactor of claim 2 wherein said active core region cylinder includes a central core cylinder of fertile material and a plurality of surrounding annuli of alternating fissile fuel and fertile material with the outer annulus being fissile fuel, and said blankets of fertile material surrounding said active core region cylinder are in the shape of discs axially adjacent to said active core region cylinder and in the shape of an annulus radially surrounding said active core region and said disc shaped blankets.
7. The fast breeder reactor of claim 6, wherein the fissile fuel is an oxide fuel and the power density of the core at the designed power output level is 2.5 kw/ft. of fuel rod.
8. The fast breeder reactor of claim 6, wherein the fissile fuel is a carbide fuel and the power density of the core at the designed power output level is 5.0 kw/ft. of fuel rod.
Type: Grant
Filed: Sep 16, 1987
Date of Patent: Jul 25, 1989
Assignee: The Unites States of America as represented by the United States Department of Energy (Washington, DC)
Inventors: Richard A. Doncals (Washington, PA), Nam-Chin Paik (Pittsburgh, PA), Sandra V. Andre (Hempfield Township, Westmoreland County, PA), Charles A. Porter (Rostraver Township, Westmoreland County, PA), Roy W. Rathbun (Greensburg, PA), Ambrose L. Schwallie (Greensburg, PA), Diane S. Petras (Penn Township, Westmoreland County, PA)
Primary Examiner: Donald P. Walsh
Attorneys: Paul A. Gottlieb, William R. Moser
Application Number: 7/97,239
International Classification: G21C 1900;
