FR2620558A1 - Minimisation or elimination of the reloading of a nuclear reactor - Google Patents

Abstract

A liquid-metal fast-neutron nuclear reactor 23 which has a reloading cycle of between 10 and 30 years is obtained by making the reactor 23 operate with a low linear power density, typically at 0.08 kW/cm of the fuel rods 77, instead of the 0.25 or 0.5 kW/cm of the existing technique. The height of the core 37 and its diameter are sized so that the ratio of the height to the diameter approaches 1 as far as possible, bearing in mind the control constraints and bearing in mind the fact that the pressure drop in the heat carrier must not be excessive. The surface area of a cylinder of a given volume is minimal when this height-to-diameter ratio is employed. When the surface area is minimised the neutron leakages are reduced by virtue of the reduction in the linear power density, of the increase in the core volume, of the reduction in the enrichment in fissile materials and in the optimisation of the core geometry, and the breeding of fissile fuel inside the core is substantially improved, and this makes it possible to take the reloading cycle to 30 years.

Description

MINIMIZATION OR ELIMINATION OF RECHARGING
OF A NUCLEAR REACTOR
This invention relates to the operation of power plants whose main source of energy is a fast liquid metal neutron reactor and the reactors for these plants. This invention relates in particular to the loading of reactors for these power stations.

Although this invention is only applicable to power plants whose reactors are of the fast neutron and liquid metal type, it is realized that, in its most general aspects, it can find an application in power plants of which the reactors are of the types different, such as pressurized water reactors, boiling water reactors or gas-cooled reactors. To the extent that this invention is applied, or finds its use, in power plants having reactors of different types from neutron reactors liquid metal rapids or in the reactors of these other plants, such an application and such use are included within the scope of the applicability of the equivalences of this invention
A liquid metal fast reutron nuclear reactor (LMFR) has a core of fissile fuel and fertile material. The fissile fuel is usually plutonium oxide. Plutonium in the
PuO2 contains a number of isotopes: Pu238, Pu239,
Pu240, Pu241, Pu242. Pu239 and Pu241 are fissile. The
Pu239 constitutes approximately 86 of Pu and Pu241 approximately 2%. The fissile fuel is in the form of fuel rods, each rod comprising a sheath of stainless steel or of zirconium alloy which contains pellets of plutonium oxides and mixed uranium designated as [(Pu-U) O2] and UO2 pellets. In both cases, U02 contains depleted uranium. U02 is used as fertile material; the proportion of U235 in this uranium is approximately 0.2. The proportion of PuO2 in the fuel rods is about 12 to 25; the rest is UO2. The core also has covers of depleted uranium used as fertile material, arranged circumferentially and above and below. Also, the fissile fuel can be UO2 enriched between 12 and 25% in U235 mixed with U02 with depleted uranium. Plutonium and uranium carbides can be used in place of the oxides.

 Typically, the core can be considered to have the overall shape of a circular cylinder, although, strictly speaking, its cross section is polygonal with a large number of sides. The fissile fuel rods are assembled in a generally cylindrical unit. The covers may include a ring encircling the exterior of the fissile cylinder and disks arranged on the bases of the fissile cylinder. The fissile fuel rods can also be mounted in a ring having a cover part in the center in addition to the external cover parts. The coolant for LMFR is a liquid metal, typically liquid sodium.

 The cores of fast liquid metal neutron reactors have a high capacity for linear power density. Linear power density is defined as the number of kilowatts per centimeter (kW / cm) of the fuel rod. According to the lessons of the existing technique, the LMFRs were designed to take advantage of their large capacity of power density.

The core volumes were kept to a minimum in order to obtain an average linear power density of approximately 0.246 kW / cm for the oxide fuel and 0.492 kW / cm for the carbide fuel. This minimization of core volume has resulted in fluency and exhaustion levels that limit the life of the fuel assemblies to a range of three to five years. In addition, the recharging interval was usually limited, by the possibilities of current control systems, to about one year due to the significant loss of reactivity due to exhaustion.

 The need for frequent recharging has serious drawbacks. Reloading is in itself a complex process including removing the head from a reactor and replacing radioactive fuel rods in a radioactive environment. It is also necessary to handle the highly reactive coolant which quickly transforms into a caustic base in air. The cost of labor is substantial and the reactor is shut down for relatively long intervals. Frequent refueling also causes initial fuel loss, as partially burned fuel assemblies are removed during the first few refueling cycles; in an annual refueling cycle, during which a quarter of the core is replaced each year, the first and second and third reloads remove fuel that is only quarter, half and three-quarters burned, respectively. In this case, the loss is three-eighths of a heart; in the case of recharging every five years, this loss is two-fifths of a heart. The loss can be offset by reprocessing, but reprocessing is not possible in the United States, and, if it becomes possible, it will be costly.

The construction of a power plant with an LMFR of the existing technique is complicated and the cost is materially increased by the need to provide the fuel handling apparatus, the buildings to contain this equipment and the apparatuses for moving this equipment between the buildings and the reactor. In addition, there must be fuel storage pools and cooling equipment to handle the annually removed fuel batches, which are still highly fissile and which are also highly radioactive. Another drawback is the deterioration of the fuel rod cladding and of the fuel by the disintegration resulting from operation with a high linear power density.

 The object of this invention is to remedy the drawbacks of the existing technique and to provide a method for using a power plant whose main source of energy is an LMFR, so that recharging is not necessary throughout the duration life of the LMFR, to provide an LMFR to implement this process, and to provide a process for using a power plant whose main source of energy is an LMFR so that infrequent recharging is necessary during the period of plant life, typically at intervals of at least ten years.

 According to this invention, the operation of an LMFR power station without recharging the reactor during the lifetime of the core is carried out based on the over-regeneration of the fissile fuel from the fertile material in order to extend the useful lifetime of the core while working at a low linear power density. Typically, the life of the heart is about 30 years. Over-generation of fissile fuel limits, or compensates for, the loss of reactivity. The over-generation of fissile fuel is increased by minimizing the escape 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 close as possible to 1. It can be shown that a circular cylinder has a minimum surface for a ratio of 1, so that , in such a cylinder, the neutron leakage from the surface is minimized. However, the ratio of 1 cannot, in practice, be achieved, since it is necessary to take into account the necessities of the control (long control rods and displacement of the control rods over substantial distances) and the fall of pressure in the coolant as it flows through the core.

To avoid excessively high control costs and too high a pressure drop, or an excessive increase in the cost of the heat transfer pumps, the ratio is usually less than 1. With regard to certain reactors, it has been found that the improvement obtained for an increase in the height-diameter ratio beyond around 39t did not guarantee additional costs. Typically, the linear power density is limited to about one third of the linear power density of the prior art. For oxide fuel, the linear power density is limited to approximately 0.082 kW / cm. To produce with an LMFR operating with a low linear power density an energy of the same value as that produced by an LMFR of the technique existing, which operates with high linear power densities, the core volume must be increased in order to adapt to the enlarged fuel rods and the increased fertile material which are necessary. The height of a core of the existing technique is typically 1.016 m. The core according to this invention has a height of between 1.52 and 1.83 m. The diameter of the heart is also increased, but the ideal of the ratio of 1 between the height and the diameter is approached but not reached.

Regarding the enrichment of fissile fuel (Pu239, Pu241, U235) for a long-lived core, it depends on the specific needs of the power plant, i.e. the power level, the lifetime of the heart (10 years, 15 years, 30 years), etc. In general, at any power level, the long-lived heart has a lower enrichment than a heart which is recharged annually due to the larger volume and reduced neutron leaks from the long-lived heart. Typically, a heterogeneous core of 1300 MW electrical which must be recharged annually requires an enrichment in Pu of approximately 30%, while a long life core of the same output power requires approximately 21%. long life has more fissile material at the start of operation, as illustrated in Tables IA, IB, IC which follow
TABLE IA
USE OF FISSILE MASS IN A HEART OF
1000 MW ELECTRIC FOR A DEPLETION CYCLE
(30 YEARS OF OPERATION)
Recharge interval Annual 15 years 30 years
Initial load in fissile plutonium (kg) 3,604 6,320 9,015
Additional plutonium required (kg) 20.903 * 6.320
Total plutonium required (kg) 24,507 12,640 9,015 * 29 annual reloads
TABLE IB
USE OF FISSILE MASS IN A HEART OF
330 MW ELECTRIC FOR A DEPLETION CYCLE
(30 YEARS OF OPERATION)
Recharge interval Annual 15 years 30 years
Initial load in fissile plutonium (kg) 1,644 2,400 3,390
Additional plutonium required (kg) 11,919 * 2,400
Total plutonium required (kg) 13,563 4,800 3,390 * 29 annual reloads
TABLE IC
USE OF FISSILE MASS IN A HEART OF
110 MW ELECTRIC FOR A DEPLETION CYCLE
(30 YEARS OF OPERATION)
Annual recharging interval 15 years
Initial loading in fissile plutonium (kg - 1 to 1/4 of the core) 845 1,865
Additional plutonium required (kg) 6.126 * 1.865
Total plutonium required (kg) 6,971 3,730 * 29 annual reloads
Practicing this invention produces the following main advantages
1.The construction of the nuclear power plant is simplified, with a concomitant reduction in cost, in particular thanks to the elimination of the recharging equipment and its buildings and of the transport equipment that this equipment requires, and, also, thanks to the elimination of fuel storage equipment which is withdrawn year after 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 little time in the case of infrequent reloads, is lost on reloading.

 3. The need to manufacture, transport and reprocess fuel is eliminated, except during the start-up and decommissioning of a power plant.

 4. The operational reliability of the power plant is improved because the reactor operates with a low linear power density and the possibility that the core or the coolant will overheat is reduced.

 5. Storage of nuclear waste with a high level of radioactivity is eliminated.

 6. The proliferation of plutonium which could accidentally fall into the hands of terrorists or dangerous fanatics is reduced.

 7. Since, in the case of a non-refillable reactor, the reactor closing head does not need to be removed until the plant is decommissioned, the structure of the closing head is simplified.

 8. The deterioration of the fuel rods is reduced and / or the fraction of the fuel volume can be increased.

9. It is estimated that the fuel combustion capacity is increased by about 30%. Technological advances have currently brought the combustion capacity from 170,000 megawatt-days per metric ton to approximately 220,000
MW-day / t approximately. With a non-rechargeable core over 30 years according to this invention, a combustion capacity of 290,000
MW-day / t approximately is possible.

 10. It is estimated that the cost of energy is reduced by around 20%.

 This invention favors the extension of the life of the fuel, to the detriment of the increase in power density. This assumes that the amount of heavy metals (fertile material) and the volume of the heart are increased, typically by a factor of between 2 and 3.5, depending on whether the fuel contains carbides or oxides of uranium or plutonium. This increase in volume produces a corresponding decrease in the level of flux, and a less aggressive environment which produces less deterioration due to the irradiation of the fuel in a given period of time. When this increase in volume is combined with the increased energy capacity of the fuel mentioned above, a lifetime of 30 years without recharging the assembly appears highly attractive from an economic point of view.

 For a non-refillable core with a fuel containing heavy metals and carbides, the internal over-regeneration characteristics within the core limit the losses of reactivity to an easily controlled level situated between 2 and 3 t AK / K. Oxide fuels produce less over-regeneration of fissile materials and therefore suffer from greater reactivity losses. Predictions for the operation of a long-lived oxide core are achievable by quantifying the sensitivity of nuclear reactivity to the height of the core. An oxide core with a height of the existing technique of 102 cm (40 inches) ), but with three times the volume of the existing technique, presents losses of reactivity greater than 10t AK / K. If this same core volume is kept while bringing the height to 183 cm (72 inches), the reactivity losses are reduced to 3t AK / K.

Thanks to an appropriate selection of the fuel parameters, for example the diameter of the spindles, the pitch-diameter ratio, the increase in core temperature, etc., the increase in the pressure drop of the coolant as it flows through the heart -st minimized or entirely eliminated.

 Changes in the heart, according to this invention, can also affect temperature-reactivity coefficients. The value of the Doppler constant increases by 17%, while the coefficient of radial expansion decreases by approximately 5%. The coefficient of radial expansion relates the change in the reactivity of the heart (AK / K) to the changes in the radius of the heart occurring due to thermal expansion of the support structures of the heart.

Increasing temperatures generally dilates the core, which keeps the fuel rods away, and therefore reduces reactivity. The Doppler effect consists of a decrease in reactivity when the temperature of the coolant increases during commissioning and an increase in reactivity when the temperature of the coolant decreases during shutdown. The Doppler effect and the radial expansion coefficient introduce negative feedback factors when starting and positive feedback factors when stopping. The changes in heart described above do not appear to have a significant effect on the characteristics of the effect.
Doppler and the coefficient of radial expansion.

 A non-rechargeable reactor, typically with a non-rechargeable core having a lifespan of 30 years, has a unique advantage. But we can have power plant installations in which a higher linear power density and a core with a smaller volume, requiring minimal recharging, are desirable. A power plant operation in which the recharging interval is at least 5 years and coincides with the other main maintenance interruptions of the power plant is within the scope of the applicability of the main aspects of this invention. In the event that interruptions at shorter intervals become necessary for maintenance purposes, limited recharges may occur during these interruptions. Limited recharging must be distinguished from the annual recharging which is necessary for LMFR of the existing technique in which a quarter or a fifth of the fuel rods are replaced annually. The recharging cycle of 5 years or more is within the scope of the applicability of this invention. It assumes a substantially lower linear power density and a substantial increase in the volume of the core compared to the LMFR of the existing technique. A recharging interval of 5 years or more will provide increased central availability of a core with 30 years of lifespan, while allowing a reduced core volume compared to the core with 30 years of lifespan. the invention will not offer the simplification of the closing head, nor the elimination of the fuel handling equipment and the reprocessing equipment presented by the core with 30 years of service life.

For a better understanding of this invention, as regards both its organization and its operating method, as well as additional objects and advantages, reference is made to the following description, taken in conjunction with the accompanying drawings , wherein
Figure 1 is a schematic view showing a power plant in the operation of which the method of this invention is practiced and also showing a fast liquid metal neutron reactor representing this invention;
Figure 2 is a cross-sectional view showing a homogeneous core included in the LMFR shown in
Figure 1;
Figure 3 is a cross-sectional view showing a heterogeneous core which could also be included in the reactor shown in Figure 1;
Figure 4 is a graph showing the effect on reactivity of different height to diameter ratios of hearts having the same volume;
Figure 5 is a graph showing the relationship between service life and excess reactivity for a reactor according to this invention, compared to a reactor according to the teachings of the existing technique, both being designed to deliver a power of same value
Figure 6 is a graph showing the effect of the volume fraction of fuel in the core on the excess reactivity;
Figures 7 and 9 are graphs similar to Figure 4 showing the effect on the lifetime of the height to diameter ratio of the core with the same volume for reactors of 330 MW electric and 110 MW electric operating at low density linear power, and Figure 8 is a graph comparing the effect of a heterogeneous core layout to a homogeneous core layout for a 330 MW electric reactor;
Figure 10 is a graph comparing reactivity and run time with low linear power density and high linear power density; ;
Figure 11 is a graph comparing excess reactivity versus time for long life bars containing uranium oxide fuel and mixed oxide fuel [(Pu-U) O2], all two used with a low linear power density;
FIG. 12 is a graph showing the unavailability of the power station as a function of the recharging interval, and
Figure 13 is a graph showing the excess reactivity as a function of time for cores designed to operate during different lifetimes.

 FIG. 1 shows a power station 21 comprising a reactor 23 for rapid neutrons with liquid metal of the swimming pool type, an external heat exchanger 25 and a turbine 27. FIG. 1 is a simplified presentation.

The reactor 23 is suspended on a support 29. Mainly, the components of the reactor include a reactor vessel 31 which is enclosed in a safety vessel 33. Inside the reactor vessel there is a pump 35, a core 37 , an intermediate heat exchanger 39, and upper internal equipment generally indicated at 41. These components are surrounded by a pool 42 of liquid sodium, typically enclosed inside the reactor vessel 31. It will be noted that this invention is equally applicable to a loop type reactor as to a pool type reactor.

 In its cross section, the heart 37 has the shape of a polygon. However, as the dimension of the cross section through the center of the heart is large and the number of sides of the polygon is high, for practical reasons, it can be considered that the cross section of the heart is circular, with a radius equal to the radius of the circle circumscribing the polygon. The heart can be considered to be circular and cylindrical.

 LMFRs are described as being homogeneous or heterogeneous depending on the structure of their heart.

Figure 2 shows the cross section of a homogeneous 37M core. Such a core comprises a central cylinder 51 of fissile fuel with low enrichment, characteristically between 17 and 19%, surrounded by a ring 53 of fissile fuel with higher enrichment, typically between 25 and 30%. The cylinder 51 and the ring 53 are composed of oxides or carbides of Pu and U mixed [(Pu-U) O2; (Pu-U) C] of fissile fuel, and depleted uranium oxide or carbide. Plutonium comprises all of its isotopes which are usually present, but contains large quantities of the fissile isotopes: Pu239 (approximately 86%) and Pu241 (approximately 2%). The proportion of plutonium in the fuel pellets is between 20 and 30% approximately. The uranium in the cylinder and the ring serves as fertile material and is converted into plutonium by the neutron reaction. The ring 53 is surrounded by a ring 55 of fertile material, typically oxide or depleted uranium carbide with about 0.2t of U235. There are also depleted uranium cover discs above and below the core 37.

 Referring to Figure 3, the heterogeneous core 37T comprises a central cylinder 57 of fertile material surrounded by rings 59, 61, 63, 65, 67, 69, alternately in fertile material and in fissile fuel.

 The core 37 is mounted on core supports 71 and is enclosed in a shielding barrel 73. The coolant, liquid sodium, is transmitted to the base of the core 37 by the pump 35 via the conductor 75 and is entrained by pump up through the heart. The control bars 77 are actuated by a mechanism (not shown) in the upper internal equipment 41 so as to enter and leave the core as a function of the demand for power or other indications.

 The fissile fuel is in the form of pellets coated 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 determined by an appropriate starting procedure by moving and positioning the control rods 77.

 According to this invention, the operation with a long lifespan of the reactor is obtained by reducing the linear power density to a fraction, typically about a third, of the power density which is used in the operation of the existing technique and by increasing the volume of the core, i.e. the number or length of the fuel rods, so that the necessary power is derived for the LMFR power plant. In addition, the core is dimensioned in such a way so that the ratio of its height to its diameter is as close to 1 as possible, taking into account the other constraints of the reactor and the operation of the power plant, and, in particular, the constraints of control and pressure drop in the coolant when driven through the core 37 by the pump 35. The fact is that there must be a compromise between the desire to achieve the ideal of equality between the height and the di and the reality of the entrainment of a coolant through the heart with reasonably available pumps and control rods of reasonable dimensions. When we arrived at this invention, we realized that in in certain situations, an increase in height beyond a height-to-diameter ratio of approximately 39 t only brought about a slight improvement in the operation or in the life of the reactor.

 The advantages of this invention and some of its ramifications will now be presented with reference to Figures 4 to 13.

 In Figure 4, the excess reactivity in percentage (% au) is plotted vertically as a function of the service life in years of the reactor without recharging, which is plotted horizontally. The graph is based on the data for a fast-neutron liquid metal breeder reactor delivering an electrical power of 1000 megawatts (2740 MW thermal) charged with plutonium oxides mixed with depleted uranium oxide and arranged as shown below. -above. All the curves are given for a reactor of the same volume and with the same proportion of heavy metals, but for different heights and different diameters, operating with a linear power density of 0.08 kW / cm (2.5 kW / foot) The curves are drawn for hearts, as shown, having heights of 240 cm (8 feet), 180 cm (6 feet) and 100 cm (40 inches). Table 1, below, shows the relevant data for hearts.

TABLE 1
Height ratio
Height in cm Diameter in cm on diameter
240,405 0.60
180 460 0.39
100,625 0.16
As indicated, the dashed lines 81 and 83 represent the life cycle for a reactor having a height of 100 centimeters (40 inches, 3 1/3 feet), with a surplus of initial reactivity (AK / K) by 2% and 8%, respectively, and the desired EK priming is obtained by varying the initial enrichment. The excess reactivity in each case decreases continuously at a reduced speed, the operation starting with a surplus of 2t AK / K having a lifespan of approximately 14 years and the other having a lifespan of approximately 21 years. The lifetime of the operating reactor 14 years can be extended by reloading as indicated by the line of dashes and sawtooth dots 85.

 The curve of lines and dots 87 shows the excess of reactivity as a function of the lifespan for a core having a height of 180 centimeters (6 feet) and the same volume as for curves 81 and 83. In the case of the heart of 180 centimeters (6 feet), the reactivity increases during the first fourteen years, approximately, then decreases gradually thereafter until the end of the lifespan, at 30 years. At maximum reactivity, the core delivered around 150,000 megawatt-days per metric ton, and, at the end of its service life, the core delivered around 300,000 MW-days / t. The increase in reactivity shown by curve 87 results from the fact that the outer surface of the core is reduced as a function of the increase in the height to diameter ratio (H / D) from 0.16 to 0.39. the reduced surface for the cylinder of 180 centimeters (6 feet), the neutron leaks are minimized and the over-regeneration of fissile fuel is maximized correspondingly.

 The solid lines curves 89 and 91 are the lifespan curves for the core having a height of 240 centimeters (8 feet), the curve 89 for an excess of reactivity at start-up of 2%, and the curve 91 for an excess 6%, and for H / D = 0.60. The life of this heart of 240 centimeters (8 feet) is only extended up to approximately 31 years for oX / K = 2t and up to approximately 32 years for ssK / K = 6%. This small increase in service life does not solve the increased control constraints and pressure drop problems that the 240 centimeter (8 foot) heart brings in comparison with a 180 cm (6 foot) heart. the reactor described, operating with a low linear power density, the H / D ratio = 0.39 is an unusually happy compromise.

Table 2 below shows the start-up overgeneration rates in the different regions of the heart:
TABLE 2
OVERGENERATION RATE BY REGION
ACCORDING TO HEIGHT OF HEART
(1000 MW electric, low power density)
Over-regeneration rate
Cover Height Heart Cover (cm) Total Active Heart Axial Radial
240 1.46 1.26 0.15 0.05
180 1.45 1.23 0.14 0.08
100 1.43 1.11 0.11 0.21
The over-regeneration rate is defined by
Fissile fuel produced
Fissile fuel consumes
The distribution of overregeneration in heart 37 shown in Table 2 is significant. An increased H / D ratio shifts the overgrowth of the covers (mainly axial) to the active region of the heart. The axial overgrowth is reduced from 0.21 in the axial cover of the heart from 100 centimeters (40 inches) to 0.08 in the axial heart coverage of 180 centimeters (6 feet), and is increased from 1.11 in the active region of the heart by 100 centimeters (40 inches) to 1.23 in the active region of the heart by 180 centimeters (6 feet).

In all the cases represented in FIG. 4, the fraction of the volume of the fuel is 46 t and the volume of the core is kept constant when the height varies. The
Figure 4 shows that, when the height is increased, the losses due to reactivity are reduced. The increase in height reduces the surface area, which reduces the neutron losses caused by the leaks.

 In Figure 5, the excess reactivity (% AK / K) is plotted vertically as a function of the service life in years, which is plotted horizontally. The graph shows the lifespan for a heart of 1000 MW electric having a height of 180 centimeters. The dashed curve 93 shows the operation with a linear power density of 0.25 kW / cm and the solid line curve 95 shows the operation at 0.08 kW / cm. The core operating at high linear power density contained one-third of the heavy metal contained in the core at low linear power density and was one-third the length of the fuel rods. The service life for operation with power density high linear power is about nine years, while the service life for operation with low linear power density is thirty years. Operation with a third of the power density extends the service life by a factor of more than 3. At 10 years and 30 years, respectively, the use of heavy metal is the same for operation at high power and at weak power. Fuel depletion is the same for operation with low power density and for three times low power density, and therefore the lifespan with low power density should, in theory, be equal to three times the lifespan with high power density.The difference can be attributed to increased leakage and reduced internal overgeneration in the smaller active region in the core with high linear power density.

Operation with a lower linear power density subjects the cladding to reduced neutron flux levels and decreases the expansion of the fuel pellets. An advantage of this operation is that the thickness of the cladding can be reduced and the fuel volume fraction correspondingly increased, or, also, that the cladding can be used for a longer interval. This advantage, like the advantage of the ratio
Higher H / D is subject to compromise because excessive reduction in sheath thickness could cause deterioration of the sheath even with low linear power densities.

Figure 6 shows the effect of the fuel volume fraction on the lifespan for hearts 180 centimeters (6 feet) high and 100 centimeters (40 inches) high operating at 0.25 kW / cm ( 7.5 kW / foot). In
Figure 6, the excess reactivity in percentage is plotted vertically and the lifetime is plotted horizontally.

The dashed lines 101 and 103 correspond to nominal fuel volume fractions of approximately 42% for the 100 cm (40 inch) core and for the 180 cm (6 foot) core. The solid lines 105 and 107 are the corresponding curves for the 46% fuel by volume fraction. In the case of the 100 cm core, the fraction of 46% of fuel by volume extends the lifespan from approximately three years to five years, and, in the case of the 180 cm (6 feet) core, the duration of life is extended from 7.5 years to around 8.75 years. The increase resulting from an increase of only 5% in the volume fraction of the fuel is substantial.

 The fact that an increase in the H / D ratio also increases the lifetime of the reactors delivering a power lower than 1000 MW electric is shown in Figures 7 and 9. These graphs are similar to that of Figure 4. Figure 7 presents the relation for a homogeneous reactor of 330 MW electric and Figure 9 compares the relation between the excess of reactivity and the lifespan of a heart of 100 MW electric for different H / D ratios. For the height of 180 centimeters (6 feet), H / D is greater than 1, which is not an optimal relationship.

 Figure 8 compares heterogeneous and homogeneous hearts of 330 MW electric.

The differences in the reactivity curves for the three power levels can be understood by comparing the over-regeneration rate data. The
Table 3 contains the relevant over-regeneration rate data for these power levels.

TABLE 3
OVERGENERATION RATE BY REGION
ACCORDING TO THE POWER OF THE HEART
Low power density with
height of 180 centimeters (6 feet)
Over-regeneration rate
Coverage Power Core Coverage (MWe) Total Active Axial Radial Core
1000 1.45 1.23 0.14 0.08
330 1.45 1.17 0.20 0.08
110 1.39 0.96 0.36 0.07
The over-regeneration rates for the 1000 and 330 MW electric cores are essentially the same, but the total over-regeneration rate drops substantially below 330 MW electric. The distribution of the fissile material overgrowth also has an important influence on the lifetime. The rate of over-regeneration of the axial cover is almost independent of the power of the reactor. This corresponds to the forecasts, because the hearts are all of the same height. But the reduction of the power of the heart, which is carried out by reducing the active diameter of the fuel, displaces the production of fissile material from the active heart to the radial covers (we can compare the third and fourth columns). Over-regeneration in the coverage region is symptomatic of the increased neutron leakage from the active core, which decreases internal over-regeneration and therefore increases the loss of reactivity. Decreasing the power level (and volume) reduces the lifespan that the core can achieve by increasing the combustion variation of the reactivity.

 In Figure 10, the estimated combustion in MW-days / t and the estimated lifetime are plotted vertically to the left and to the right, respectively as a function of the reactor power plotted horizontally for a reactor with a homogeneous mixed oxide core. Curve 113 shows operation with a high linear power density and curve 114 shows operation with a low linear power density.

 In small cores, the higher reactivity overload constraints for combustion limit the operational life during which criticality can be maintained, and therefore limit the maximum fuel combustion that can be achieved. For large cores, a smaller excess of reactivity is necessary and the fuel combustion capacity is limited by the forces on the cladding. The limits of sheath effort and excess reactivity are reached with higher combustions for hearts with a low linear power density than with a high power density. For heterogeneous core configurations, the maximum fuel combustion will generally be higher at each power level than what is shown in Figure 10.

 Table 4 below shows the maximum estimated combustion that can be obtained depending on the power level.

TABLE 4
MAXIMUM COMBUSTION FOR HEARTS
LINEAR COMBUSTION
(HOMOGENS)
Maximum linear power
(kW / cm) 0.5 0.16
Reactor power
(MW electric) Combustion (MW-days / t)
1300 200,000 290,000
330 180,000 250,000
110 90,000 180,000
In large cores (for example, 1300 MW electric), the full combustion capacity of the fuel with mixed oxides (fissile PuO2 and depleted U02) based on the stress limits of the claddings can be used.

The combustions that can be obtained are reduced when the size of the reactor decreases because of the limits of the excess reactivity. However, even in a small core, the maximum fuel combustion could be doubled by operating at a low power density thanks to the increased service life derived from the improved breeder characteristics.

 Table 5 below compares the use of fissile mass or the consumption of fissile mass for a core of the existing technique and a core with a long lifespan delivering 1000 MW of electricity in 30 years.

TABLE 5
USE OF FISSILE MASS
(1000 MW electric)
Heart of the Heart to
long technique
existing lifespan
Fissile mass burned in 30 years 27,000 27,000
Net fissile gain 7,600 4,200
Overall fissile production (kg) 34,600 31,200
The heart of the existing technique has a fuel autonomy of 5 years with a maximum combustion of 170,000 MW-days / t. One fifth of the heart is recharged each year. The long-life core requires a significantly higher initial loading than that of an existing technique core. However, the recharging requirements for a core with 30 years of operation of the existing technique with a full recharging cycle very much outweighs this significant initial fissile mass. Ultimately, the long-lived core requires significantly less plutonium loading . As the two concepts produce a power of the same value, it is essentially the same fissile mass which is burned. During this period, the core of the existing technique produces a net fissile gain of approximately 7,600 kg, while the long-lived core produces a gain of 4,200 kg. In this operating mode, the long-life core has reduced fuel cycle costs because less fissile fuel is required (by a factor close to 2; see Table IA). Most of the fissile mass burned during the lifetime of the core is produced in situ in the long-lived reactor core, instead of being added periodically through recharging.

 In Figure 11, the excess reactivity as a percentage is plotted vertically as a function of the service life in years plotted horizontally for a 1000 MW electric reactor charged with plutonium oxide mixed with depleted U02, and for a charged reactor enriched uranium oxide mixed with depleted UO2. The two cores have the same volume and operate with the same low power density. The mixed oxide [(Pu-U) 02) is present in the amounts described above, that is to say between 20% and 30% of plutonium oxide and the rest of depleted uranium oxide (0.2% enrichment). The U02 reactor includes uranium at about 17g enrichment. Curve 115 shows that the excess reactivity for the UO2 reactor decreases 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 4% approximately at approximately 14 years, then decreases until 0 to 30 years.

 Although a 30-year lifespan of the heart is more difficult to obtain with a U02 heart than with a mixed oxide heart, the lifespan of the U02 heart can be increased materially by adding provisions to the heart that facilitate over-regeneration. A 5-year improvement in lifespan can be obtained with the introduction of a heterogeneous core set model optimized to maximize the yield of the over-regeneration. An additional 5-year life extension can be achieved through an enlarged active heart region.

The calculations indicate that 20% more fuel (corresponding to an additional deviation of the characteristics of the core of 0.066 kW / cm) would be sufficient. These characteristics would produce a lifetime of 30 years from the heart to U02. A long-lived U02 reactor is within the scope of the equivalences of this invention.

 In Figure 12, the unavailability of the power plant in percentage is plotted vertically according to the recharging intervals in years, plotted horizontally. This curve 121 is based on the assumptions that recharging is carried out in one month and that the other stops are independent of recharging. Although these assumptions may vary, the curve provides a representative estimate for the design of large power plants. With an annual recharging approach, the plant is unavailable more than 8% of the time. A 30-year lifespan reduces downtime to less than 0.3%. Most of the gain in availability was obtained with a recharging interval of only 10 years.

 We will now take into account recharging after 15 years. Figure 13 is a graph of the reactivity surplus plotted vertically as a function of time in years plotted horizontally for a core of 30 years of lifespan and two hearts of 15 years of lifespan. Curve 131 is the plot for the core of 30 years of lifespan and curves 133 and 135 are the plots for hearts of 15 years of lifespan. The hearts shown in Figure 13 are hearts with PuO2 and depleted UQ.

 Table 6 below presents the parameters for the three cores.

TABLE 6
COMPARISON OF CONCEPTS OF HOMOGENEOUS HEARTS
1000 MW LONG LIFE ELECTRICS
30 years lifespan 15 years lifespan
Option 1 Option 2
Curve 131 Curve 133 Curve 135
Heart height (cm) 180 150 150
Heart radius (cm) 220 195 205
Relative volume 1.0 0.62 0.65
Average linear power (kW / cm) 0.08 0.13 0.07
Outer diameter of the pins (mm) 10 10 8
Fuel fraction by volume 0.46 0.42 0.38
Pressure drop in the heart (atmospheres) 4.5 7 7
Proportion of heavy metal 1.0 0.59 0.58
The important difference between hearts with 30 years of life and 1 5 years of life is due to the influence that the drop in pressure in the heart exerts on hearts. Ideally, a heart of 15 years of life of life should contain half as much heavy metal as a 30-year core life to keep the same combustion of fuel and therefore keep fuel cycle costs low. One approach would be to keep the fuel core dimensions and use only half as many fuel sets.

However, identical dimensions produce an increased specific power density which requires a higher flow rate and increases the pressure drop. Consequently, the 15-year-old heart must be opened (increase in the ratio not to the diameter of the pins) and the height of the heart must be reduced to reduce the pressure drop. These changes effectively reduce the volume fraction of the fuel and therefore reduce internal over-regeneration and shorten the life of the heart. A second alternative is to use smaller fuel pins; but smaller pins also have the effect of reducing the volume fraction of the fuel. According to Table 6, the two options at 15 years of life produce almost the same proportion of heavy metal and fissile mass. The two hearts require almost 60% of the heavy metal of the core of 30 years of life, which is 10% higher than the identical point of use of heavy metal. A 180-centimeter (6-foot) core should nominally improve over-regeneration and reduce heavy metal, but the pressure drop would be too high. The large spindle option has a larger volume fraction, and a slightly longer life with less supply of fissile material. Option 2 is interesting in that the reactivity specifications of the control bars are approximately 1.5tAK lower. The fuel cycle costs for these 15-year options are almost the same since they have almost the same proportions of fissile material and heavy metal.

 Tables 7 to 10 which follow present economic comparisons between the existing technique and this invention.

TABLE 7
TOTAL COSTS OF ELECTRICAL ENERGY
(MIXED OXIDES)
Thousands of dollars / kWh
LMFR of the
LMFR technique of
existing the invention
Capital 38.0 37.1
Fuel cycle 10.1 5.5
Operation Reduction and maintenance 6.1 6.1 of the cost of
the energy
Total cost of electric power 54.2 48.7 10%
Increased availability 43.7 19%
TABLE 8
USE OF FISSILE MASS *
(30 YEARS)
10 year old heart
lifetime
Existing technique of the invention
Initial fissile plutonium loading (kg) 5,830 8,430
Additional plutonium required for reloading (kg) 56,347 16,860
Total plutonium required (kg) 62,347 25,290 * Assuming there is no reprocessing
TABLE 9
COMPARISON OF INITIAL COSTS (MILLIONS OF DOLLARS)
10 year old heart
lifetime
Existing technique * of the invention
First production of the heart 150 188
First core and reload plutonium, at $ 10 / g ($ 25 / g) 89 (223) 84 (210)
Initial cost of the heart (s) 239 (373) 272 (398)
Reload interval (years) 1 10 * 1 to 2/3 of hearts
TABLE 10
FUEL CYCLE COSTS (THOUSANDS OF DOLLARS / KWH)
Disposal Reprocessing
Heart of Heart of
Components of Tech. the inv. Tech. the inv.

fuel cycle exist. (30 years) exist. (30 years)*
First manufacturing of the heart 1.0 3.3 1.0 3.3
Annual manufacturing 4.9 --- 4.9
Initial plutonium of the heart 0.6 1.2 0.6 1.2
Annual plutonium 2.6 -
Annual plutonium gain --- --- -0.6
Final plutonium of the heart --- --- -0.5 -1.4
Annual reprocessing --- --- 2.7
Final heart treatment --- --- 0.4 1.2
Fuel storage and transport costs 1.0 1.0 1.0 1.0
Total cost of the fuel cycle with a capacity factor = 0.65 10.1 5.5 9.5 5.3
= 0.725 --- 5.0 --- 4.9
Reduction of the cost of the fuel cycle with disposal M 5.1 thousandths of dollars / kWh * Reprocessing at the time of decommissioning
These tables speak for themselves.

 Although the preferred practice and a preferred embodiment of this invention have been described, many modifications thereof can be made. This invention should not be restricted except to the extent that this is made necessary by existing technology.

Claims (2)

Claims 1. Liquid metal fast neutron reactor (23) having a generally circular cylindrical core (37) with fissile and fertile materials placed inside thereof, a pump (35) for transmitting the coolant through said core (37), and control bars (77) connected to the core (37) to control the reactivity of the latter, characterized in that the height to diameter ratio of the core is generally between 0.39 and 1 and in this that said fuel rods (77) have fissile and fertile materials arranged inside them and a low linear power density, so as to form a reactor (23) with a fuel reloading cycle generally comprised between 10 and 30 years old.  2. Liquid metal fast neutron reactor (23) of claim 1, characterized in that the fuel rods have a linear power density generally in the range of 0.08 kilowatts per centimeter.