FR2525800A1 - PROCESS FOR OPERATING FERTILIZED LAYERS OF RAPID NEUTRON REGENERATORS AND COOLING BY LIQUID METAL - Google Patents

Abstract

THIS PROCESS ALLOWS TO REDUCE THERMAL CRACKING IN RAPID NEUTRON REGENERATING REACTORS AND LIQUID METAL COOLING, BY REDUCING THE TEMPERATURE GRADIENTS BETWEEN THE COMBUSTIBLE ASSEMBLIES AND THE CONTIGUOUS FERTILE LAYER ASSEMBLIES. TO OBTAIN THIS RESULT, THE FERTILE LAYER ASSEMBLIES ARE MOVED AT EACH CHARGING INTERVAL SO AS TO BRING THEM CLOSER TO THE HEART PERIPHERY BY MEANS OF SEVERAL MOVEMENTS. FRESH ASSEMBLIES OF FERTILE LAYERS ARE GENERALLY PLACED NEAR IRRADIATED FUEL ASSEMBLIES AND IRRADIATED ASSEMBLIES OF FERTILE LAYERS NEAR FRESH FUEL ASSEMBLIES. FURTHERMORE, THE ASSEMBLY ORIFICES ARE MODIFIED TO REDUCE FLOW IN FERTILE LAYER ASSEMBLIES WHICH ARE NEEDED LESS THAN A REDUCED TEMPERATURE GRADIENT, AND TO PROVIDE ADDITIONAL FLOW TO MORE FUEL ASSEMBLIES. IN ORDER TO SUFFICIENTLY REDUCE TEMPERATURE GRADIENTS AND PREVENT THERMAL CRACKING.

Description

I Process for exploiting fertile layers of regenerative reactors

  fast neutrons and liquid metal cooling.

  The present invention relates to a method of moving the fertile layers of a fast neutron regenerating reactor and

liquid metal cooling.

  We meet in fast neutron regenerating reactors

  and cooling by liquid metal, especially when they are

  The fundamental cause of this defect is the inherent difference between the power generated in the fuel assemblies and in the fertile layer assemblies of a heterogeneous core, a structural defect which is described below and which is called thermal cracking.

  Heterogeneous core of regenerator reactor with fast and cool neutrons

  A heterogeneous core is a heart with

  interfering regions of fuel and fertile layers, which create a multitude of fertile fuel-layer interfaces The power generated in an assembly of layers containing fertile substances, increases continuously with the regeneration of fissile fuel, while the power generated in a fuel assembly containing fissile material, decreases continuously during nuclear combustion During its lifetime, an assembly of fertile layers sees its output power multiplied by a coefficient of 2 to 5 before

  that it does not reach its theoretical limits and must not be removed from

  replaced and replaced the flow rate of the coolant in an assembly

  of fertile layers is regulated by a given input orifice whose study is made according to the limits which are approached at the end of

  assembly life A combination of fertile layers is thus cooled

  more than it is necessary during the greater part of its lifetime which is, in the case of assemblages of inner fertile layers of a heterogeneous core, of the order of 2 to 3 years (similar to the lifetime O of fuel assemblies) However, the cooling abnormally

  high radial fertile layer assemblages is even more pronounced

  This is due to longer service lives (4 to 5 years) and higher power gradients through the assembly. This means that the coolant flow, which is set according to the maximum power bar can be up to 5 times higher than the flow rate

in the minimum power bar.

  Abnormally high cooling of fertile layers has a

  such as, at the beginning of the life of this layer, the temperature of the

  of cooling coming from an assembly of fertile layers can be

  177 C lower than that of the coolant coming from a

  contiguous fuel appearance If this temperature difference of "po-

  maximum "was completely attenuated by the mixing of the

  cooling, conduction and entrainment, there would be no

  need for thermal cracking However, flow tests in

  model reactors have shown that significant differences in

  at the outlet of the assemblies result in current shocks

  hot and cold coolant on the surrounding structures

  Temperature differences of 30 to 60 per cent of the maximum potential were observed in flow diagrams away from the outlet sleeves, in the upper internal structure, and from 60 to 80 per cent

  maximum potential in the vicinity of the outlet sleeves of the assemblies.

  When warm currents and cold currents strike structures,

  contiguous, they create thermal stresses in these structures

  due to differential thermal expansion.

  fatigue resistance of the material, cracks may occur

  and, if the constraints are large enough, spread.

  This defect is called "thermal cracking". In the case of stainless steel

  316 type, the limits of the maximum fluid temperature differences range from 26.7 to 49 i C for permanent structures and from 49 C to 71 C for interchangeable structures. As can be seen, significant differences in the order of 177 C exceed these lixites, mxae when there is attenuation by mixing and

  Conduction Inconel 718 "alloy can be used to solve the problem.

  ficulty because its theoretical limits are approximately double the

  those of type 316 stainless steel, but its cost is higher.

  Thermal cracking defects are particularly important in

  heterogeneous cores because of the large number of

  fertile layer in regions where temperature differences occur.

  temperature. Accordingly, the main object of the present invention is to provide a method for mitigating thermal cracking to a degree that

  that type 316 stainless steel can be used in

  maneuvers and interchangeable reactors of a fast neutron regeneration reactor and liquid metal cooling, including the fuel regions and

  fertile layers constitute a heterogeneous heart.

  In order to achieve this object, the present invention consists of a

  process for reducing the temperature gradient between the assemblies

  fuels containing fissile material and the contiguous

  layers containing fertile substances in a cooling reactor.

  liquid-metal core containing fertile layers of assemblies dispersed within the core to form a region of inner fertile layers, and assemblies of fertile layers surrounding the core to form a region of radial fertile layers comprising an inner row and an outer row of fertile layer assemblies This process is characterized in that about one half

  assemblages of fertile layers in the region of fertile layers

  is replaced by fresh assemblages of fertile layers -at

  each reloading interval, most of these cou-

  fertile areas each belonging to one of the two groups replaced by fresh combinations of fertile beds at alternating intervals of recharge; at the same time, the assemblages of fertile layers of the first group, replaced by fresh assemblages, are brought to

  locations in the region of fertile interior layers and pre-

  ally occupied by the second group; the layers of iron

  the second group, replaced by those in the first group, are

  born at locations in the inner row of radial fertile layers; the fertile layer assemblies of this inner row

  of radial fertile layers, replaced by the iron

  tiles of the second group are brought to locations in the

  outer row of radial fertile layers; and neck assemblies

  fertile areas moved from this outer row of fertile layers

diales, are removed from the reactor.

  This process makes it possible to reduce the temperature gradients between

  fuel assemblies and contiguous assemblies of iron

  It is such that the use of Inconel is not necessary.

  to constitute the elements of the heart.

  The present invention will be well understood on reading the

  following description made in connection with the attached drawings, in the-

  which: Figure 1 is a schematic plan view of a core (core I) of fast neutron regeneration reactor and liquid metal cooling illustrating the possibility of reducing gradients according to the present invention; Figure 2 is a diagram illustrating the method of moving fertile layers; FIG. 3 is a schematic plan view of the heart I of a

  fast neutron regenerator reactor and free metal cooling

  quide, illustrating the arrangement of the flow zones; FIG. 4 is a diagrammatic plan view of another core (core II) of a fast neutron regeneration reactor and cooling with a liquid metal, illustrating the sequence A of a first variant of the process for moving the fertile layers; FIG. 5 is a schematic plan view of the core II of

  fast neutron regenerator reactor and free metal cooling

  quide, illustrating the sequence B of this first variant of method of displacement of fertile layers; Figure 6 is a diagram illustrating the first variant method of moving the fertile layers; FIG. 7 is a schematic plan view of the core II of

  fast neutron regenerator reactor and free metal cooling

  quide, illustrating a second variant of the method of moving

  fertile waters; and FIG. 8 is a diagram illustrating the second variant of

  method of moving fertile layers.

  The reference core used for the description of this process

  of displacement of the fertile layers, is a heterogeneous oxide core with possibility of fertile layers of thorium The characteristics of this core are a scattered reloading of the fuel assemblies whose

  residence time is three years, assemblages of fertile layers

  whose residence time is three years (with the exception of

  placements 11, 13, 14 and 16 in the sixth row with a lifespan of two years) and radial fertile

  residence time is six years We chose this heart because of the existence

  a very detailed analysis of the flow and temperature

  This analysis can be used as a basis for the study. The reference number of the assembly and the maximum temperature gradients

  between assembly for the reference assembly and for the displacement diagram.

  which is the object of the present invention, are indicated in the figure

  I which only illustrates a sector of 600 of the otherwise symmetrical heart.

  The scheme of exploitation of the fertile layers which is the object of the present invention is illustrated in FIG. 2. One half (60) of the inner fertile layer assemblies (IB-1) are fed after one year (N) & one another location in the inner fertile layer region (IB-2), where they stay in the second year (year of equilibrium N + 1, N being the year in which the assemblages are fresh). These assemblages then pass their third year (N + 2) in the first row 6 th of radial fertile layers and their last year (N + 3) in the second row of radial fertile layers At the end of the fourth year, they were discarded The other half Inner fertile layer assemblies follow the same stages of displacement but staggered by one year. So there are two sequences, called A and B, which represent the entire displacement of the fertile layer assemblies. These are the assembly of inner fertile layers located in the center of the core and 18 assemblages of radial fertile layers of the first row. In the case of the central assembly, since it is the only assembly of this type in the reactor, it is obvious that it can not be brought to another location for reasons of symmetry This assembly has a

  low power, limited flow and is flanked by other

  Inner fertile layers can be left in place until the limiting stresses are reached. The reason why

  18 assemblies of radial fertile layers do not participate in these de-

  Placement is simple: in this particular heart, there are 78 locations available in the first row of radial fertile layers, whereas only 60 assemblages of inner fertile layers are brought each year into the first row of radial fertile layers unless you imagine a very complicated diagram of moving assemblies the most obvious choice is simply to replace at intervals

  regular assemblies that occupy these 18 locations.

  Table I illustrates the displacements of the various assemblies.

  By choosing to move a given assembly from one location to another

  the other, care was taken to respect the implicit incompatibilities

  For example, assemblies of inner fertile layers of the type N in sequence A must be

  "receiver" type assemblies in sequence B and vice versa.

  From m 6 mel the locations of the radial fertile layers must receive an assembly of sequence A and an assembly of sequence B, and not

  not two assemblies of the mid Ae sequence.

TA & E & U 1

  Summary of displacements of fertile layer assemblies IB-1 IB-2 1 81 RB 2 Scrapped Year (expenses) I + 1 1 + 2 X + 3 X Sequence A (all the assemblies belonging to this sequence are moved the first year )

39,212,217

6 13 209 222

1 16 213 221

27 34 211 220

14 41 202 216

28 32 210 214

33 26 201 215

31,205,223

2 11 204 219

25,207,218

  Sequence B O (all assemblies belonging to this sequence are displace-

  in the year that is the year before and the year in which

  follows the movements of the assemblages belonging to the

this A)

28,204,218

41 40 205 215

16 35 to 12 222

34 2 209 221

13 30 211 217

39 33 207 214

11 27 202 216

32 14 201 219

26 6 210 223

31 I 213 220

  Notes: The assembly of inner fertile layers IB has never been moved. It is left in place as long as it is compatible with the stresses, then discarded. Radial fertile layer assemblies. and 208 are assemblies whose residence time is one

  They are loaded and discarded each year.

  The flow is, of course, fixed in the location of the core, all the assemblies of fertile layers being physically identical to allow displacements. The guiding principle in the determination

  of the flow distribution was to conceive a flow of

  to meet the requirements as far as possible and continuously

  flow during the service life and transitional housing of

  fertile fields The particular details of this realization vary according to whether the location considered is in the fertile inner layers or in the first or second rows of radial fertile layers. In general, a good indication

  new exidencies of flow is given by the report of the

  Rated Linear / Peak Ratio at the end of the dwell time for the displacement heart, at the power at the end of the duration

  of life for the reference heart We obtain an evaluation of the

nominal

  the linear / peak power of the bars as a result of

  Placement, using simple algorithms Basically, when assembly Y is brought to location Z, the fraction of power created by the fission caused by the neutrons depends on Y, the assembly being displaced (since it is a function of the amount of fissile material contained in the assembly) The power fraction due to gamma heating depends on the environment, i.e. the Z location at which the assembly is brought. It is further assumed that the power variation

  during a residence period of one year after the assembly

  age has been brought to a given location, is equal to the variation of

  during a corresponding year of the duration of via the assembly located at this location in the reference core The data relating to the peak power of the bars (and in some cases assemblies

  whole) were calculated in this study for all the combinations

  possible movements, which made it possible to determine

  that assembly the optimum sequence of displacement.

  In the calculations of the new flow requirements for each

  that location as a result of a move, we used as parameter

  the required flow (the minimum flow required to meet the

  most importantly) for each location This is important because, although the flow is the same for all assemblies in a given flow area, the flow required is the same as the flow of the flow. most disadvantaged assembly only All other assemblies of

  the flow area have a flow greater than the required flow.

  Because the flow required as a result of displacement is equal to the required flow in the reference core multiplied by a factor

  of power-dependent flow (that is, dependent on the

  and the location to which it is brought), an additional degree of freedom has been placed in order to optimize the flow. However, the choice of the assembly to be moved has been somewhat limited by the requirement.

  to reduce the gradient to a minimum.

  and the exploitation of the differences between the flow required and the

  flow in the reference core, it was possible to optimize the

  in such a way that the difference between the required flow and the flow

  The result has been significantly lower than in the case of the reference core. This has resulted in savings in flow.

  and lower temperature gradients since a higher flow

  In addition to the required flow, not only is wasteful flow but also produces a lower temperature of the cooling fluid at the outlet of the urethane layer assembly.

  heart that has been displaced and in the heart of reference.

  is significantly reduced in the fertile layers

  and in the first row of radial fertile layers, and is increased in the second row of radial fertile layers in order to track the accumulation of damage to sheaths of displaced assemblies. the second row of radial fertile layers is forced (that is, the lifetime of these assemblages could be ensured with a flow rate lower than that estimated at the time) * In fact, the degree of uncertainty is the highest for the second row since it is the culmination of the last of the three trips and all the approximations and assumptions inherent in this study are cumulative at most during the last year. fact that any uncertainty regarding the flow in the second row of radial fertile layers, does not affect the conclusions of this

  study This is from milk that the temperature gradients between assem-

  are not of concern in this region and the only consequence

  will be a setting of the required flow.

TABLE 2

  Type Flow connection area B l B l B l B 7 'RBI RBI RBI il RB 2 Flows Exigences and flow for the reference heart and for the heart that has undergone re placenation Heart of reference No cueetkgh ent Assets / zouie Ecele 1 (k /) Subtotal Subtotal Total subtotal in fertile layers

* 70,851

58,785

51 120

18,597

6,852,142

62,959

38,646

3,170,063

12,474

748,426

770,631

  Displaced heart No Flow (kcg / h) of asset / area

54,884

47,173

38,555

19,051

4,999,491

42,502

209,614

2,224,869

26,943

22,226

I 475 081

8,699,441

  Coe As illustrated in Table 2, there is a predicted significant net gain of flow in the fertile layer assemblies (greater than 2 106 kg / h or about 4.5% of the total flow in the reactor ) This gain is

  more than enough to compensate for any 'surprise' that may occur.

  When we move from concept to realization In addition, -this flow

  available could allow a preferential allocation of the

  fuel assemblies, which would reduce the

  dients between assemblies, mae ean below evaluated levels.

  Figure 3 illustrates the new flow zones in the

  heart that has been displaced and in the heart of reference

  parison of the distributions of the maximum gradients between assemblies in the reference core and in the displaced heart, has been illustrated previously in Figure 1 For each assembly of the layers

  fertile inner and first row fertile radial layers

  the, the maximum gradient is equal to the difference between the output temperature of the fertile layers and the highest exit temperature of the contiguous fuel assemblies As can be seen, the maximum gradient is reduced from 140, CA 66 e C in the layers fertile interiewures

  and 15 ° C to 64 ° C in the first row of the radial fertile layers.

  Even taking into account the inevitable uncertainties related to the assumptions used in this study (but without losing sight of the existence

  additional flow of flow) it can safely be concluded that the

  fertile layers allows, in fact, to halve

  The highest gradient between fuel assemblies and contiguous assemblies of fertile layers, occurs when fuel assemblies and fertile layer assemblies are fresh (fuel assemblies are at their maximum power, assemblies of fertile their minimum power); the lowest gradient appears when both types of assemblies reach the end of their service life (minimum fuel power

  due to depletion effects, maximum power of the

  due to the production of plutonium). It follows that a very efficient process

  ficace to ensure low gradients between assemblies,

  It is a question of placing fresh fuel assemblies at the ends of displaced fertile layer assemblies, and vice versa. This requires fuel assemblies and internally fertile layer assemblies that have the long service life, as well as a desynchronized operating plan. of the

  fuel assemblies For gradients at the fuel-to-fuel interface

  Inner fertile layer, mentioned in the present study, new and old configurations were used (that is, calculated gradients relate to fresh fuel interfaces displaced fertile layer and spent fuel-fresh fertile layer) The highest of the two gradients (usually the first) has been reported regularly in Figure 1. It has been seen previously that by moving, in the present process, one-half of the inner fertile layer assemblages each year,

  the assemblages of fertile layers that are fresh are limited each year

  born to those who replace displaced assemblages, that is, half

  Inner fertile layer assemblages are alternated an-

  born on two sites fresh assemblies of inner fertile layers Flow can not be reduced in fresh assemblies

  because the flow & location is dictated by the cooling requirements

  In this case, the solution to the problem of the high gradient that will occur between the fuel assemblies and the contiguous fertile layer assemblies

  when both are fresh,

  suitably fueled Because the outlet temperature of fuel assemblies is maximum at the beginning of their life and minimum at the end of their life, while the opposite is true with respect to fertile is clear we get

  a significant decrease in the gradient between assembly when a

  The fresh fuel is located at a distance of a displaced assembly of

  fertile areas (the two assemblages being at their highest temperatures

  high) and when an impoverished fuel assembly adjoins an assembly

  of fertile layers (the two assemblages then being at their

  lowest temperatures) In order to achieve this combination,

  Combustible assemblies and

  have the same lifetime (2 years in the present example), and that the 3 C core configuration is such that the residence times of fuel assemblies and assemblies of fertile layers are actually

  The system developed in the present invention, according to the

  which one-half of the inner fertile layer assemblies is moved

  annually, requires that one-half of the fuel assemblies be

also loaded every year.

  The complete realization of the desynchronised loading of the assemblies

  Combustible materials and fertile layer assemblages may not be advisable since this would effectively decouple (in the "nuclear physics" sense of the term) the central part of the heart with, as a consequence,

  excessive linear power rating in fuel assemblies

  when they are cool However, one can imagine a first

  of the assembly operating system, in which the

* limits of the fuel are not exceeded and the number of interfaces, in which the disposition "desynchronized" is not maintained,

  limited to only a few places Figures 4, 5 and 6

  trent this system for a proposed core configuration and typically

  studied (different from the configuration considered in the pre-study

  As shown in Figures 4 and 5, the non-desynchronized interfaces are only 12 out of a total

  The gradients at these locations can be adjusted

  part of the excess flow existing between the assemblies

  fuels concerned It should be noted that, in order to ensure the

  "desynchronized", the assemblages of fertile layers

  in the third row of the fertile radial layers and then, in the third year, in the position of displacement in the

  inner fertile layer (see Table 3) This "trimming of the third

  row "results in a one-year shift of

  fuel assemblies and fertile layer assemblies

  which provides the effect of "desynchronization".

  A second variant of a displacement system is proposed to

  Figures 7 and 8 In this system, the idea of "desynchronization" is replaced by

  This is achieved by minimizing the flow in all interior fertile layer assemblies.

  displacement, each assembly site fertile layers inte-

  will be alternately occupied by a fresh assembly and a

  moved (sequences A and B) - therefore at the beginning of the year

  during which the assembly is fresh, the outlet temperature of the

  appearance is minimal since the power produced is the lowest, while the flow is adapted to the (maximum) production of power in the displaced assembly, at the end of the second year.

  this system, the gradient is adjusted by means of the concept of "desynchronization".

  On the other hand, in the second variant of the system, the gradient

  is regulated by the flow distribution in the pipe assemblies.

  Inner fertile areas Each indoor fertile layer assembly site will be occupied each year (recharge interval) either by a fresh assembly or a displaced assembly; Therefore,

  the power variation from the beginning to the end of the service life of the

  Any flow in any location, is limited to one year, instead of two as in the first variant of the system. The flow will be adapted according to whether the location is occupied by a fresh assembly or a displaced assembly, this flow being appreciably lower. in the first

  Figure 7 shows the arrangement of the iron

  In the second year, the assemblages are successively brought into the first, second and third rows of radial fertile layers where they remain at each

  one year, with a total life of five years

  The characteristic of this system is that the

  fresh assemblies and displaced assemblages of inner Lertile layers

  the next are the "necklace pearls", as illustrated in Figure 7; the fuel loading can be optimized separately so as to reduce to minimin the linear power rating and increase

  thus the margin of safety with respect to the fuel melting.

  Tables 3 and 4 respectively illustrate the first and

  the second variants of assembly displacement system.

TABLZ & U 3

  If Q 1 JEE OF DISPLACEMENT MFI Rm RE VAJIAMT Sequence A Coimeelesaimées 2, 4,, 6

  Armies 2,4,6 Years 3,, 5,, 7 Aimed 4,6,, 8, Armed 5,7,9 g Armed 6,8,10 o.

IB Fresh

18

IB I'El

I 1 82

  Sequence B Coxme the armies 1, 3, 5 Years 1,3,5 Years 2,4,6 Years 3,5 ,, 7 Armies 4,6,8, Years 5,7,99 IB Fees

13

36

  RB 3 21.1 IB IBI IB 2 MB 205 remains in place for 2 years

2 years later, then cut off.

  IBI 216 remains in place for 2 armies 2 years later, then discarded RB 1 217 remains in place for 2 years

2 years later, then put off.

  moved in 192 203 during the moved in XB 2,213 during the moved in 192 218 during the

TABLE 4

  DEU Xi IHE VARIAM SEWUE 1 S OF DI PLACEMNT WITH LVECO DISTRIBUTION Uc LEMENT Location IB receiving fresh assemblies each year: 2,3,15, 13,17, 56,36,34 t 32,48 Sequence of movement 2nd year 3 rd year 4 th year 5 th year l B Fees IB RB 1 1 B 2 RB 3 o O 2 45 216 220 211

3 35 202 212 223

47,208,203,229

13 22 207 226 334

17 10 214 222 331

56 39 215 221 333

** 36 4,213

  * 34 201 (BR 1) 210 (BR 2) 224 (BR 3)

32 18 209 225 332

48 33 204 228 330

  * shorter sequence (4 years) IB brought directly into RB 1

  ** sequence of 3 years only; after stay in 213 rejected -

  ** IBI 205 remains in place for 2 years; moved in RB 2,227 during the

2 years later, then put off.

  RBI 206 remains in place for 2 years; moved in RB 2 218 during

  the next 2 years, then put off.

  RB 1,217 remains in place for 2 years; moved in RB 2 219 during

  the next 2 years, then put off.

  For the appreciation of some of the indicated measurement values

  above, should take into account that they come from the conver-

  of imperial units in metric munitions.

  The present invention is not limited to the exemplary embodiments

  tion which is obviously to be described, it is on the contrary

  variants and modifications that will occur to those skilled in the art.

- -

REVEDICATIONS

  1 Method for reducing the temperature gradient between

  fuel assemblies containing fissile material and assemblies

  contiguous layers containing fertile substances in a

  liquid metal-cooled actor whose core contains assemblages of fertile layers, dispersed internally to form a

  region of fertile inner layers and iron

  tiles that surround the heart to form a region of fertile layers

  radial members comprising an inner row and an outer row of

  fertile layers, this method being characterized in that

  about half of the fertile layer assemblages in the region of cou-

  Inner fertile areas are replaced by fresh

  fertile areas at each recharge interval, most of these

  appearances of fertile layers each belonging to one of the two

  groups replaced by fresh assemblages of fertile layers

  alternating periods of reloading; at the same time, the assemblages of fertile layers of the first group, replaced by fresh assemblages, are brought to locations in the region of inner fertile layers and previously occupied by the second group; the assemblages of fertile layers of the second group, replaced by

  those of the first group are brought to locations in the

  inner layer of fertile layers; the assemblages of fertile layers of this inner row of radial fertile layers, replaced by the fertile layer assemblies of the second group, are brought to

  locations in the outer row of radiant fertile layers

  the; and the assemblages of fertile layers displaced from this ex-

  of radial fertile layers are removed from the reactor.

  Process according to Claim 1, characterized in that

  several fuel assemblies at the joint assembly locations

  fertile areas that would normally have a low temperature

  contiguous fuel assemblies, have

  relatively small in order to reduce the flow of coolant

drainage in the assemblies.

  Process according to one of claims 1 or 2, characterized

  in that several of these fuel assemblies are placed in such

  so that irradiated fuel assemblies are generally

  tigus to fresh assemblages of fertile layers; and that the

  Fresh fuels appear to be contiguous to assemblies

irradiated with fertile layers.

  4 Process according to one of claims 1, 2 or 3, characterized

  rised by the fact that a small number of fertile layers of

  inside the heart are an exception to the general system according to these claims.

  dications, these exceptional assemblies of fertile layers being replaced in synchronization with contiguous fuel assemblies for the purpose of coupling the core nuclear sense, these exceptional assemblages of fertile layers receiving an additional flow to compensate

this synchronization.

  Process according to Claim 49, characterized in that these exceptional assemblages of fertile layers are used according to

  a system called "third-row packing", which includes moving

  Exceptional assemblages of fertile layers, after irradiation during a recharge interval in a region of fertile layers

  interior, in the third outer row of fertile layers

  which is usually located on the outer perimeter of the fertile layer, during a recharge interval, after which these exceptional assemblages of fertile layers are brought to a location

  of the inner fertile layer region during a re-

  loading, these locations having an interface with a fuel assembly which is relatively fresh, this interface is therefore not mésésynchrouisée M, this location receiving an additional flow

  compensation of the coolant, and subsequently these assemblies

  exceptional wounds of fertile beds are brought to a similar location

  killed in the inner radial fertile layer during a re-

  loading, and then brought to a second location in the

  Radial fertile during a recharge interval, then removed from the reactor.

  Process according to one of claims 1, 2 or 3, characterized

  that the locations of the fertile layer assemblies in-

  are designated to alternately receive fresh assemblies

  and impoverished assemblages, this designation once fixed for one-

  location remaining unchanged, so that the distribution of

  between the locations of fertile layer assemblies

  are well suited to meet the requirements.