Abstract:
A method for reducing thermal striping in liquid metal fast breeder reactors by reducing temperature gradients between adjacent fuel and blanket assemblies by shuffling blanket assemblies at each refueling outage so as to progressively shuffle the blanket assemblies to the core periphery through multiple moves and to generally locate fresh blanket assemblies adjacent to exposed fuel assemblies and exposed blanket assemblies adjacent to fresh fuel. Additionally, assembly orificing is altered to provide less flow to blanket assemblies needing less flow due to an otherwise decreased temperature gradient and providing additional flow to fuel assemblies which need more flow to sufficiently reduce temperature gradients to prevent thermal striping.

Description:
This application is a continuation of application Ser. No. 371,332, filed Apr. 23, 1982 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a blanket shuffling method for a liquid metal fast breeder reactor (LMFBR). 
     LMFBR&#39;s, especially the heterogeneous core variety, have a design problem, explained below, which is termed &#34;thermal striping&#34;. The basic source of this problem is the inherent difference between the power generation in fuel and blanket assemblies in a heterogeneous LMFBR core. A heterogeneous core is a core having a plurality of fuel and blanket zones interspersed throughout the core causing a multitude of blanket to fuel interfaces. The power generated in a fertile material fueled blanket assembly increases continuously with the breeding of fissile fuel, while the power generated in a fissile material containing fuel assembly decreases continuously during burnup. During its lifetime, a blanket assembly increases its power output by a factor of 2 to 5 before it reaches its design limits and must be removed from the reactor and replaced. The coolant flow rate through a blanket assembly is controlled by a fixed inlet orifice, the design of which is dictated by those limits which are approached at the end of life. Thus, a blanket assembly is overcooled over most of its lifetime, which for internal blanket assemblies in a heterogeneous core, is on the order of 2 to 3 years (same as fuel assembly lifetime). However, the overcooling in radial blanket assemblies is even more pronounced because of longer lifetimes (4 to 5 years) and higher power gradients across the assembly. That is, the coolant flow rate is set by the rod with the maximum power which may be as much as 5 times higher than that in the minimum power rod. 
     The effect of blanket overcooling is that at beginning of life, the coolant from a blanket assembly may be as much as 350° F. cooler than that from an adjacent fuel assembly. If this &#34;maximum potential&#34; temperature difference were completely mitigated by coolant mixing, conduction and entrainment, there would be no thermal striping problem. However, flow testing of reactor models has demonstrated that large differences in assembly outlet temperatures result in hot and cold coolant streams impinging on surrounding structures. Temperature differences from 30 to 60 percent of the maximum potential were observed in flow patterns away from the outlet nozzles, in the Upper Internals Structure (UIS) and as much as 60 to 80 percent of the maximum potential was observed near assembly outlet nozzles. When the hot and cold flow streams impinge upon adjacent structures, thermal stresses, due to differential thermal expansion, are induced in these structures. If the stresses exceed the fatigue strength of the material, crack initiation and, if stresses are severe enough, crack propagation can occur. This is the problem called &#34;thermal striping&#34;. For Type 316  stainless steel the limits on maximum fluid temperature difference are 80-120° F. for permanent structures and 120-160° F. for replaceable structures. As can be seen, large temperature differences on the order of 350° F. violate these limits even with partial mitigation by mixing and conduction. Inconel 718 can be used to solve the problem because its design limits are approximately twice those for type 316 stainless steel, but its cost is higher. Thermal striping problems are especially severe in heterogeneous cores because of the high number of blanket fuel interfaces where the temperature differences occur. Consequently, it is desired to provide a method to mitigate thermal striping to such a degree that 316 stainless steel can be used for replaceable and permanent reactor structures, in an LMFBR having fuel and blanket regions comprising a heterogeneous core. 
     SUMMARY OF THE INVENTION 
     A new fuel and blanket management and core orificing method has been developed for large LMFBR heterogeneous cores. The method comprises multiple shuffling of blanket assemblies into other blanket assembly positions throughout the core with a controlled residence time in each position. In general, the shuffling trend is from an inner core blanket region to outer core radial blanket positions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan schematic of an LMFBR core (core I), showing the gradient reduction capability of this invention; 
     FIG. 2 is a schematic of the blanket shuffling method; 
     FIG. 3 is a plan schematic of an LMFBR core I, illustrating orificing zones arrangement; 
     FIG. 4 is a plan schematic of another LMFBR core (core II) illustrating sequence A of a first alternate blanket shuffling method; 
     FIG. 5 is a plan schematic of LMFBR core II illustrating sequence B of a first alternate blanket shuffling method; 
     FIG. 6 is a schematic of the first alternate shuffling method; 
     FIG. 7 is a plan schematic of LMFBR core II illustrating a second alternate blanket shuffling scheme; and 
     FIG. 8 is a schematic of the second alternate shuffling method. 
    
    
     DETAILED DESCRIPTION 
     The invention is a blanket shuffling scheme which greatly reduces the temperature gradients between blanket assemblies and adjacent fuel assemblies. The method described herein reduces maximum gradients by 150° F. or more, mitigating thermal striping to the point where the use of Inconel for core components is unnecessary. 
     The size of the temperature gradient between a fuel assembly and an adjacent blanket assembly can be reduced by increasing the outlet temperature of the coolant exiting from the blanket assembly (since the coolant flow exiting from the fuel is always hotter than that from blanket assemblies). Such blanket outlet temperature increase can be achieved by reducing coolant flow through the blanket assembly or by increasing blanket assembly power. The basic invention is to shuffle blanket assemblies between core locations having fixed orifices which therefore appropriately alter coolant flow through the blanket assembly while also arranging that radiation exposed blanket assemblies, which are higher power producing than fresh assemblies due to the build-up of fissile elements, are located adjacent to fuel assemblies of high power. In general terms, fresh blanket assemblies are to be adjacent to exposed or &#34;used&#34; fuel assemblies while used blanket assemblies are to be adjacent to fresh fuel assemblies. 
     The invented blanket shuffle will be illustrated by a specific application. 
     The reference core used for this shuffling description is a heterogeneous oxide core with thorium blanket capability. This core features scattered refueling, three-year residence fuel assemblies, three-year residence inner blanket assemblies, (except positions in the sixth row labeled 11, 13, 14 and 16 which have a two year lifetime), and six-year residence radial blanket assemblies. This core was selected because a very detailed orificing and thermal-hydraulic analysis existed for use as an analysis base. The assembly number, and maximum assembly-to-assembly temperature gradients for this reference and for the invented shuffle scheme are shown in FIG. 1, which is illustrative of only a 60° portion of the otherwise symmetric core. 
     The invented blanket management scheme is shown in FIG. 2. One-half (60) of the Inner Blanket (IB-1) assemblies are shuffled after one year (N) to another Inner Blanket Position, (IB-2) where they reside in the second year (equilibrium year N+1, where N is the year the assemblies are fresh), then they spend their third year (N+2) in the radial blanket first row, and the final year (N+3) in the radial blanket second row. At the end of the fourth year, they are discarded. The remaining one-half of the inner blanket follows the same steps, staggered by one year. Thus, there are two sequences, called A and B, which completely represent the blanket assembly shuffling. A few assemblies do not participate in the shuffling; they are the inner blanket assembly at the core center and 18 first row radial blanket assemblies. In the case of the central assembly, since this is the only assembly of its type in the reactor, obviously it cannot be shuffled to any other position because of symmetry. This assembly has low power, low flow, and is flanked by other inner blanket assemblies. It can be left in place until limiting constraints are reached. The reason 18 radial blanket assemblies do not participate is simply that in this particular core, there are 78 radial blanket first row positions available, while only 60 inner blanket assemblies are shuffled to the radial blanket first row on a yearly basis. Unless a very complicated shuffling scheme is devised, the most obvious choice is to simply replace assemblies in these 18 positions at regular intervals. 
     Table 1 shows the moves of the various assemblies being shuffled. In selecting the shuffling of a given assembly from one position to another, care was exercised with respect to the implicit exclusions built in the two sequences. For example, &#34;donor&#34; inner blanket assemblies in sequence A, must be &#34;receiving&#34; assemblies in sequence B and vice versa. Likewise, radial blanket positions must accept one sequence A and one sequence B assembly, not two from the same sequence. 
     
                       TABLE 1______________________________________Summary of Blanket Assemblies Shuffling Moves IB-1  →          IB-2  →                   RB1  →                          RB2  →                                 DiscardedYear  N (fresh)          N + 1    N + 2  N + 3  N______________________________________Sequence A (all assemblies belonging to this sequence aremoved the same year)35         39       212      217 6         13       209      222 1         16       213      22127         34       211      22014         41       202      21628         32       210      21433         26       201      21540         31       205      223 2         11       204      21930         25       207      218Sequence B (all assemblies belonging to this sequence aremoved the same year, which is the precedingand following year to moves of sequence A)25         28       204      21841         40       205      21516         35       212      22234          2       209      22113         30       211      21739         33       207      21411         27       202      21632         14       201      21926          6       210      22331          1       213      220______________________________________ Notes: IB #64 is never moved. Leave in position as long as compatible with constraints, then discard. RB1 #203, 206 and 208 are &#34;oneyear&#34; assemblies, loaded and discarded ever year. 
    
     Flow orificing is of course fixed in the core position, with all the blanket assemblies being physically identical to allow shuffling. The philosophy followed in defining the flow allocation was to design the flow as much as possible compatible with continued satisfaction of flow constraints on blanket lifetime and transient accommodation. The specific details as to how this was achieved varied depending upon whether the considered location was in the inner blanket, radial blanket first row or radial blanket second row. Generally, a good indication of the new flow requirements was given by the ratio of the peak rod linear power rating at the end of the residence time for the shuffled core to the power rating at end of life for the reference core. An estimate of the change in the peak rod linear power rating following shuffling is obtained using simple algorithms. Basically, when assembly Y is shuffled to position Z, the fraction of power generated by neutron fission depends on Y, the assembly being shuffled (since it is related to the amount of fissile material in the assembly). The power fraction due to gamma heating depends on the environment, i.e., the location Z to which the assembly is shuffled. It is further assumed that the relative power change during one year residence after shuffling in a given location is equal to the relative change during a corresponding year in the lifetime of the assembly in the reference core location. The peak rod (and in some instances total assembly) power histories for all possible shuffling combinations were calculated in this study. This allowed the optimum shuffling sequence to be determined for each assembly. 
     In calculating the new flow requirements for each location following shuffling, the parameter used was the required flow (minimum flow necessary to satisfy the most limiting constraint) for each location. This is important because while the orificed flow is the same for all assemblies in a given orificing zone, the required flow is the same as the orificed flow for only one worst assembly. All the other assemblies in the orifice zone have an orificed flow greater than the required flow. Since the required flow following shuffling is equal to the reference core required flow multiplied by a power depending flow factor (i.e. dependent upon which assembly is shuffled into what location), it follows that the designer had an additional degree of freedom in optimizing the orificing. However, the choice of the assembly to be shuffled was somewhat tempered by the constraint of minimizing the gradient. Through judicious selection of the shuffling moves and exploitation of the differences between required and orificed flow in the reference core, it was possible to optimize the orificing such that the variation between required and orificed flow was substantially lower than for the reference core. This resulted in both flow savings and lower temperature gradients, since an orificed flow higher than the required flow not only is flow &#34;wasted&#34; but also yields a lower coolant exit temperature from the blanket assembly. 
     A comparison of the flow orificing in the shuffled and the reference core is reported in Table 2. The flow is significantly reduced in the inner blanket and radial blanket first row and is increased in the radial blanket second row to follow cladding damage accumulation in shuffled assemblies. It is believed that the flow estimate for the radial blanket second row is pessimistic (i.e., lifetime of these assemblies could be achieved with less flow than now estimated). In fact, the highest degree of uncertainty exists in the second row since this is the last of the three moves and all the approximations and assumptions inherent in this study finally accumulate to a maximum in the last year. However, it must be emphasized that any uncertainty on the flow in the second row of radial blankets does not affect the conclusions of this study. This is because inter-assembly gradients are of no concern in this region and the only consequence will be an adjustment in the flow required. 
     
                                           TABLE 2__________________________________________________________________________Orificing Requirements for Reference and &#34;Shuffled&#34; Core             Reference Core Shuffled CoreAssembly Type     Orificing Zone             # Assys/Zone                     Flow (lb/hr)                            # Assys/Zone                                    Flow (lb/hr)__________________________________________________________________________1B         6      36      156,200                            24      121,0001B         7      24      129,600                            36      104,0001B         8      54      112,700                            54      85,0001B         9       7       41,000                             7      42,000Partial Total             15,106,400     11,022,000RB1       10      18      138,800                            18      93,700RB1       11      36       85,200                            42      66,600RB1       12      24       59,300                            18      23,400Partial Total             6,988,800      4,905,000RB2       13      60       27,500                            30      59,400RB2       14      --      --     30      49,000Partial Total             1,650,000      3,252,000Total Blanket Flow        23,745,200     19,179,000__________________________________________________________________________ 
    
     As indicated by Table 2, there is a substantial net gain projected in blanket assembly flow (over 4.5×10 6  lb/hr or ˜4.5% of the total reactor flow). The gain is more than enough to offset any &#34;surprises&#34; which may occur when moving from a conceptual to an actual design. Also this available flow could allow preferential flow allocation to the fuel assemblies, which could reduce the interassembly gradients even below the levels estimated herein. 
     FIG. 3 shows the new orificing zones for the shuffled cores. A comparison of the distribution of maximum inter-assembly gradients in the reference and shuffled cores was reported earlier in FIG. 1. For each inner blanket and radial blanket first row assembly the maximum gradient is the difference between the blanket exit temperature and the highest exit temperature of the adjacent fuel assemblies. As it can be seen, the maximum gradient in the inner blanket is reduced from 284° F. to 150° F. and in the radial blanket first row from 307° F. to 147° F. Even allowing for inevitable uncertainties associated with the assumptions used in this study (but keeping in mind that extra flow exists) it can be safely concluded that blanket shuffling indeed has the potential to reduce inter-assembly gradients by one half. The highest gradient between adjacent fuel and blanket assemblies occurs when both the fuel and the blanket are fresh (fuel at highest power, blanket at lowest); the lowest gradient occurs when both assemblies are at end of life (fuel power minimum by depletion effects, blanket power maximum by plutonium generation). It follows that a very efficient method to assure low inter-assembly gradients is to locate fresh fuel assemblies next to burnt (shuffled) blanket assemblies and vice versa. This requires that fuel and inner blanket assemblies have the same lifetime as well as an &#34;out of synchronization&#34; fuel management scheme. Gradients at the fuel/inner blanket interface reported in this study used this new/old configuration, (i.e., gradients calculated were for fresh fuel/shuffled blanket and burnt fuel/fresh blanket). The higher of the two gradients (generally the first one) was consistently reported in FIG. 1. It has been seen before that by shuffling, in this scheme, one-half of the inner blanket assemblies each year, the blanket assemblies which are fresh are limited each year to those replacing the shuffled assemblies, i.e., one-half of the inner blanket. The positions of the fresh inner blanket assemblies alternate every other year. Flow cannot be reduced in the fresh assemblies, since the position orificing is dictated by the cooling requirement of the assembly being shuffled in the second year. The solution therefore to the high gradient which will occur between adjacent fuel and blanket assemblies when both are fresh is through appropriate fuel management. Since the exit temperature of the fuel assemblies is maximum at beginning-of-life and minimum at end-of-life, while the opposite is true for the blanket assemblies, it is clear that significant reduction in inter-assembly gradient is attained when a fresh fuel is next to a shuffled blanket (both at their higher temperature) and when a burnt fuel is next to a fresh blanket (both at their lower temperature). In order to achieve  this, fuel and inner blanket must have the same lifetime (2 years in this example) and the core configuration must be such that fuel and blanket residence times are indeed out of synchronism. The scheme elaborated in this invention, by shuffling one-half of the inner blanket each year, requires that one-half of the fuel assemblies also be loaded each year. 
     Complete implementation of the &#34;out of sync&#34; loading of fuel and blanket assemblies may not be advisable, since this will effectively decouple (in a nuclear physics sense) the central part of the core with consequent excessive linear power rating in the fuel assemblies, when fresh. However, a first alternate assembly management scheme can be devised in which the fuel limiting power ratings are not exceeded and the number of interfaces where the &#34;out-of-sync&#34; arrangement does not hold is limited to only a few positions. FIGS. 4, 5, and 6 report such a scheme for a proposed core configuration currently studied (different from the one considered in the previous conceptual study). As shown by FIGS. 4 and 5, the not &#34;out-of-sync&#34; interfaces are only 12 out of a possible total of 65 . The gradients in these locations will be controlled by allocation of part of the existing excess flow to the affected fuel assemblies. Note that in order to provide the &#34;out-of-sync&#34; arrangement, inner blanket assemblies are moved to the radial blanket third row and subsequently in the third year to the shuffled position in the inner blanket (see Table 3). This &#34;third row packing&#34; has the effect of shifting by one year the loading sequences in the fuel and inner blanket, thus providing the &#34;out-of-sync&#34; effect. 
     A second alternate shuffling scheme is proposed, (see FIGS. 7, 8) in which the &#34;out-of-sync&#34; concept is replaced by maximization of the flow reduction in all the inner blanket assemblies. In the first alternate scheme each inner blanket position will alternatively host a fresh and a shuffled assembly (sequences &#34;A&#34; and &#34;B&#34;); therefore, at the beginning of the year when the assembly is fresh the assembly outlet temerature is minimum, since the power production is at its lowest while the flow is allocated to accommodate the (maximum) power production at the end of the second year in the shuffled assembly. Of course, the gradient in this scheme is controlled through the &#34;out-of-sync&#34; concept. 
     In the second alternate scheme, instead, the gradient is controlled through flow management of the inner blanket assemblies. Each inner blanket position will accept at every year (refueling interval) either a fresh or a shuffled assembly; therefore the power swing from beginning to end of life of the assembly in any position is limited to one year instead of two years as in the first alternate scheme. The flow allocation will be tailored, depending whether the position accepts fresh or shuffled assemblies, being significantly lower in the former case. FIG. 7 shows the inner blanket assemblies arrangement for this scheme. After the second year the assemblies are successively shuffled to the radial blanket first, second and third row where they reside for one year in each position, for a total lifetime of 5 years. The characteristic feature of this scheme is that fresh and shuffled inner blanket assemblies alternate as &#34;necklace beads&#34;, as shown in FIG. 7; fuel loading can be separately optimized to minimize linear power rating and thus increase the margin to fuel melting. 
     Tables 3 and 4 illustrate first and second alternate shuffling schemes. 
     
                       TABLE 3______________________________________SHUFFLING SEQUENCE - FIRST ALTERNATE______________________________________Sequence A - Starts Years 2,4,6 . . .Years    Years    Years      Years  Years2,4,6 . . .    3,5,7 . . .             4,6,8 . . .                        5,7,9 . . .                               6,8,10 . . .IB Fresh RB3      IB         RB1    RB2______________________________________ 3       211      36         202    228 4       224      39         204    22717       229      15         207    21222       333      13         214    22118       330      45         208    22634       331      10         209    22532       332      47         213    22233       223      56         206    21035       334      48         215    220______________________________________Sequence B - Starts Years 1,3,5 . . .Years    Years    Years      Years  Years1,3,5 . . .    2,4,6 . . .             3,5,7 . . .                        4,6,8 . . .                               5,7,9 . . .IB Fresh RB3      IB         RB1    RB2______________________________________10       211      32         202    22847       333      33         214    22613       334      22         208    22715       229      18         215    22145       330       4         204    22056       224      34         207    21248       331      35         209    22536       332      17         213    22239       223       3         206    210______________________________________ RBI 205 stays 2 years  moved to RB2 203 for 2 more years, then out RBI 216 stays 2 years  moved to RB2 213 for 2 more years, then out RBI 217 stays 2 years  moved to RB2 218 for 2 more years, then out 
    
     
                       TABLE 4______________________________________SECOND ALTERNATEFLOW MANAGEMENT SHUFFLING SEQUENCESIB positions refueled fresh every year:2,3,15,13,17,56,36,34,32,48Shuffling Sequence    2nd Year  3rd Year  4th Year                                5th YearIB Fresh IB        RB1       RB2     RB3______________________________________ 2       45        216       220     211 3        35       202       212     22315        47       208       203     22913        22       207       226     33417        10       214       222     33156        39       215       221     333**36      4        213*34      201 (RB1) 210 (RB2) 224 (RB3)32        18       209       225     33248        33       204       228     330______________________________________ *shorter sequence (4 yrs.) IB directly moved to RB1 **only 3 years sequence, from 213 out ***RB1 205 stays two yrs  moved to RB2 227 for two more yrs then out ***RB1 206 stays two yrs  moved to RB2 218 for two more yrs then out ***RB1 217 stays two yrs  moved to RB2 219 for two more yrs then out