JP4308940B2 - Fuel assembly - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly for a boiling water reactor, and more particularly, to a fuel assembly having a high burnup with improved cold shutdown at the initial stage of a cycle without deteriorating fuel economy and core characteristics.
[0002]
[Prior art]
The core of a nuclear reactor is made up of a number of fuel assemblies and control rods that can be inserted and removed between them. Generally, fuel assemblies used in boiling water reactors contain uranium oxide in the fuel cladding tube. It is composed of a plurality of fuel rods filled with many fuel pellets.
[0003]
An example of this fuel assembly is a longitudinal sectional view of FIG. 13 (a), a sectional view taken along line BB in (a) of (b), and a sectional view taken along line CC in (a) of (c). Shown in
In the fuel assembly 1, a plurality of long fuel rods 2, short fuel rods 3, and one or two water rods 4 are bundled in a square lattice pattern with a plurality of spacers 5 arranged in the axial direction. .
[0004]
Among the bundled fuel rods, the long fuel rod 2 is fixed to the upper tie plate 7 and the lower tie plate 8 together with the external spring 6, and the short fuel rod 3 is fixed to the lower tie plate 8. The bundle of fuel rods is configured by surrounding the periphery with a channel box 9.
[0005]
The length and number of the long fuel rods 2 and the short fuel rods 3 and the water rods 4 in the channel box 9 and the shape and arrangement thereof vary depending on the reactor, so that the fuel assembly shown in FIG. The body 1 is not limited.
[0006]
By the way, one of the design conditions in the design of nuclear reactor fuel is to ensure a cold reactor shutdown margin of 1.0% Δk or more throughout the operation cycle. This reactor shutdown margin is the control with the greatest reactivity control effect. This represents the subcriticality when one rod is pulled out and all other control rods are inserted, and a reactor shutdown margin must be secured for reactor safety.
[0007]
In general, the infinite multiplication factor of fuel assemblies that do not contain flammable poisons decreases linearly with the degree of burnup as the fissile material burns. Get smaller.
[0008]
In order to solve this problem, some of the fuel pellets in the fuel rod contain a flammable poison such as gadolinia, which has the ability to absorb neutrons. The infinite multiplication factor decreases, and the necessary furnace shutdown margin can be secured.
[0009]
However, as the combustion proceeds, the flammable poison is burned out, and the ability to absorb neutrons is lost, so the effect of reducing the infinite multiplication factor is lost. That is, the flammable poison plays a role in flattening the combustion change at an infinite multiplication factor during the combustion period.
[0010]
However, if the content of the flammable poison is excessively increased, the flammable poison remains at the end of the cycle, and the reactivity is lost, resulting in a decrease in fuel economy. It will be.
In addition, when the flammable poison is contained uniformly in the axial direction, the power of the nuclear reactor is low in the periphery and upper and lower parts of the core. The flammable poison remains at the end of the cycle and the reactivity is lost.
[0011]
In order to deal with such a problem, for example, in Japanese Unexamined Patent Publication No. 59-102188 “Fuel Assembly” and Japanese Unexamined Patent Publication No. 60-259988 “Fuel Rod for Reactor”, the upper half of the axial direction is used. 19/24 to 21/24 (referred to as the shutdown zone) from the lower end where the number of flammable poison-containing fuel rods is reduced or the concentration of the flammable poison is lowered and the neutron flux is highest in the axial direction when cold. The high combustible poison loading area or the high concentration area is formed only in a partial area in the upper half of the axial direction in the range up to.
[0012]
In this way, most of the upper half has a low content of flammable poisons, so it is possible to reduce the loss of reactivity at the end of the cycle, and the neutron flux increases during cold temperatures. Since the content of the flammable poison is large only in a partial region of the section, it is possible to efficiently suppress the excessive reactivity of the core, and it is possible to secure a necessary reactor shutdown margin.
[0013]
In addition, the reason why the neutron flux increases at the upper part in the axial direction when the temperature is low is that in a boiling water reactor, voids are formed in the coolant water in the core during power operation. Therefore, the output at the lower part tends to be relatively higher than the upper part due to the negative reactivity effect due to the formed void.
Therefore, the design has been optimized to optimize the axial concentration and the distribution of flammable poisons so as to flatten the axial output distribution during output operation. In this case, the output distribution is increased.
[0014]
In the replacement core, the power peak at the upper part is particularly noticeable when the temperature is low. This is because the power is low at the core periphery and the upper and lower parts where neutron leakage is large. Combustion of flammable poisons is delayed.
In particular, in the upper part of the core, the void ratio is large, so that the neutrons are slowed down, and the combustion delay of the fissile material and combustible poison is remarkable. In addition, the generation of plutonium is promoted in the upper part of the core because the void ratio is large, and the accumulation of plutonium proceeds at a higher rate.
[0015]
As described above, in the replacement core in which the fuel having advanced combustion exists due to the delay in the burning of the fissile material and the accumulation of plutonium, the reactivity at the cold temperature becomes the maximum in the upper part of the core.
[0016]
[Problems to be solved by the invention]
In recent years, in order to improve the operational economy of nuclear power generation, the degree of combustion of fuel has been increased. In this high burnup fuel assembly, the initial enrichment of the fissile material is increased in order to achieve a high burnup, and therefore the infinite multiplication factor at the cold temperature is increased, and the furnace shutdown margin at the cold temperature is increased. There is a problem that the tendency to get worse becomes stronger.
In particular, in the new fuel, the neutron-absorbing material Sm (samarium) is not generated, and the reactor shutdown margin tends to decrease at the beginning of the cycle. This trend is the first time that all fuel loaded in the core becomes the new fuel. This is more noticeable in the loading core.
[0017]
However, increasing the content of flammable poisons in order to ensure the furnace shutdown margin at the beginning of the cycle will leave the flammable poisons at the end of the cycle and reduce the reactivity at the end of the cycle, thus reducing the fuel economy. As a result, this tendency has become a major limitation in the core design when increasing the burnup.
[0018]
The object of the present invention is that there is no decrease in reactivity at the end of the cycle and deterioration of core characteristics during operation from the region containing the flammable poison at the axial position of the fuel rod, and only the reactivity at the initial stage of combustion An object of the present invention is to provide a fuel assembly that suppresses and improves the cold shutdown time at the beginning of the cycle.
[0019]
[Means for Solving the Problems]
  In order to achieve the above object, a fuel assembly according to the first aspect of the present invention is a fuel assembly for a boiling water reactor configured by bundling a plurality of fuel rods.A region containing a flammable poison with a concentration of 1.0% or less is formed immediately below the upper natural uranium blanket, and the length of the region is 13% or less of the effective fuel lengthA fuel rod is provided.
[0020]
At the beginning of the cycle, there is a fuel rod containing a flammable poison with a concentration of 1.0% or less in the upper half of the axial direction where the neutron flux increases at cold temperatures, effectively suppressing excess reactivity and increasing the reactor shutdown margin. Improve.
[0021]
In addition, the concentration of the flammable poison in the upper half region in the axial direction of the fuel rod is 1.0% or less, which is extremely thin enough to burn out by the middle of the cycle, so that the effect of the flammable poison by the middle of the cycle. After this, the reactivity will not decrease.
Thereby, since the reactivity at the end of the cycle is not lowered, only the reactivity at the beginning of the cycle is suppressed, and the cold shutdown furnace margin is improved.
[0025]
  Claim2The fuel assembly according to the described invention is characterized in that in the fuel assembly, the number of fuel rods containing a combustible poison having a concentration of 1.0% or less is one or two.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
  An embodiment of the present invention will be described with reference to the drawings. In addition, about the same component as the above-mentioned prior art, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted. First embodimentInFIG. 1A is a transverse sectional view showing the arrangement of fuel rods in a high burnup fuel assembly, and FIG. 1B is an axial distribution diagram of fuel in the fuel rods and contained combustible poisons.
[0034]
In the fuel assembly, the distribution of the fissile material concentration and the concentration of the flammable poison are given in the axial and radial directions, but the detailed distribution will be omitted here. .
Further, in the first embodiment, in the same manner as when applied to the fuel assembly 1 shown in FIG. 13, the fuel rod arrangement is set to the normal 8 in order to increase the burnup and improve the thermal margin. The number of fuel rods is increased from row 8 column to 9 row 9 column.
[0035]
Further, in order to alleviate the increase in pressure loss of the coolant flow due to the increase in the number of fuel rods, a short fuel rod 3 shorter than a general long fuel rod 2 is used in part by using a short fuel rod 3. In particular, the coolant flow path is enlarged at the upper portion of the fuel where the pressure loss increases because the coolant becomes a two-phase flow.
[0036]
  That is, the fuel assembly 10 shown in FIG. 1 (a) is arranged around a control rod 11 which can be inserted into and removed from the core and has a cross-shaped cross section, and a fuel rod for uranium fuel (hereinafter abbreviated as U) is provided. 9 rows and 9 columns square grid array, long fuel rods U as 66 long fuel rods₁, A long Gd fuel rod G containing gadolinia (hereinafter abbreviated as Gd) which is a flammable poison₁2 and long Gd fuel rod G₂There are 13 books. In addition, eight short fuel rods U₂And two thick water rods 4 (W).The
[0037]
In general, high burnup fuel assemblies are provided with natural uranium blanket portions at the upper and lower ends in the axial direction in order to reduce the leakage of neutrons in the axial direction and improve the economy. As shown in b), the long fuel rod U₁And long Gd fuel rod G₁, And long Gd fuel rod G₂The lower natural uranium blanket portion 12a is formed in the region of 1/24 of the effective fuel length at the lower end portion, and the upper natural uranium blanket portion 12b is formed in the region of 2/24 length at the upper end portion. is doing.
[0038]
In general, fuel pellets enclosed in fuel rods contain Gd, which is a flammable poison for controlling excess reactivity, and the amount of reactivity control at the initial stage of combustion is approximately proportional to the number of fuel rods containing flammable poisons. The duration of reactivity control is approximately proportional to the flammable poison concentration.
Therefore, the concentration of the flammable poison is such that the replacement fuel is burned out at the end of the cycle, and is about 3.0 to 5.0% in the case of normal 13 to 15 month operation.
[0039]
For the fuel assembly 10, an example in which the Gd concentration is 3.5% is shown.₂Is about 3.5% Gd in the whole area in the axial direction excluding the natural uranium blanket portions 12a and 12b.
[0040]
Also, this long Gd fuel rod G containing 3.5% Gd₂Is appropriately set in order to control the excess reactivity in the core. That is, long Gd fuel rod G₂If the number of the control rods 11 is too large, the excess reactivity becomes low, and the number of control rods 11 inserted into the core becomes small, or the insertion depth of the control rods 11 needs to be reduced.
As a result, the power distribution in the radial direction or the axial direction of the core is distorted and the core characteristics are deteriorated.
[0041]
Conversely, long Gd fuel rod G₂If the number is too small, the excess reactivity increases and the number of control rods 11 to be inserted increases or the insertion depth increases, and in this case, the core characteristics are also deteriorated.
Therefore, in the fuel assembly 10, the long Gd fuel rod G is used as the number of Gd fuel rods optimally set in this way.₂The number is 13.
[0042]
  On the other hand, long Gd fuel rod G₁For these two, Gd with a concentration of 1.0% is contained in the 20/24 to 22/24 region of the effective length which is a part of the upper half at the lower end of the fuel immediately below the upper natural uranium blanket part 12b. HaveThe
  In addition, the enrichment of the average fissionable material in the fuel assembly 10 is set to a high enrichment of 4.0%, which is about 3.0% by weight or more higher than that of normal fuel, in order to achieve a high burnup.The
[0043]
FIG. 2 shows a typical cold stop reactor surplus characteristic diagram of the cold core shutdown margin combustion change in the replacement core loaded with the fuel assembly 10 configured as described above.
Here, curve 13 (solid line) is a long Gd fuel rod G₁In the case of the fuel assembly 10 containing 1.0% flammable poison, the curve 14 (dotted line) is a long Gd fuel rod G₁When flammable poisons are not included, curve 15 (two-dot chain line) represents the case where a high-burning fuel assembly of a conventional design with an average enrichment of about 3.7% is loaded.
[0044]
As shown in Fig. 2, the design target of the reactor shutdown margin is generally the high burnup fuel assembly of the conventional design with the reactor shutdown margin (% Δk) at the cold temperature of 1.0% Δk or more and the average enrichment of about 3.7%. The design target is achieved as shown by the curve 15.
However, when the average enrichment is raised to about 4.0%, as shown by curve 14, it is understood that 1.0% Δk of the cold shutdown margin at the beginning of the cycle is interrupted and the design target cannot be achieved.
[0045]
On the other hand, in the curve 13 in the case of the fuel assembly 10, compared with the case where the flammable poison with the concentration of 1.0% is not included (curve 14), the cold reactor shutdown margin is increased by about 0.3% Δk at the beginning of the cycle, The design goal is fully achieved.
From this, the long Gd fuel rod G as in the fuel assembly 10₁By providing a flammable poison with a concentration of 1.0% in a partial area in the upper half of the axis, the flammable poison effectively absorbs neutrons and the cold shutdown margin is improved.
[0046]
Note that the long Gd fuel rod G is used to improve the furnace shutdown margin.₁In the fuel assembly 10, the cross section of the region containing the flammable poison having a concentration of 1.0% is necessary to make the concentration of the flammable poison particularly provided in a partial region of the upper half in the axial direction 1.0% or less. The change in combustion at infinite multiplication factor is shown in the infinite multiplication characteristic diagram of FIG.
[0047]
Here, the curve 16 (solid line) is a long Gd fuel rod G₁In the case of the fuel assembly 10 containing 1.0% flammable poison, the curve 17 (dotted line) shows the long Gd fuel rod G₁When no flammable poison is contained, curve 18 (two-dot chain line) represents a case where a flammable poison with a concentration of 3.0% is contained.
[0048]
As shown in FIG. 3, the long Gd fuel rod G in the fuel assembly 10₁Therefore, the curve 16 when a flammable poison with a concentration of 1.0% is contained in the upper region in the axial direction is more effective at the initial stage of combustion than the curve 17 when a flammable poison is not contained. The infinite multiplication factor decreases.
However, as the combustion progresses, the effect gradually diminishes, and it can be seen that when the burnup (GWd / t) is about 5 GWd / t, the effect of the flammable poison is almost completely lost.
[0049]
Meanwhile, the long Gd fuel rod G₁When the flammable poison concentration at 3.0 is 3.0%, the infinite multiplication factor is low even after 5 GWd / t as shown by the curve 18, and the effect is maintained up to about 10 GWd / t.
Therefore, when the fuel assembly 10 is loaded in the replacement core, the average burnup of the fuel in the first cycle at the end of the cycle in the cross section containing the flammable poison of 1.0% is about 5 GWd / t. It has become.
[0050]
From this, in order for the flammable poison not to burn at the end of the cycle, it is necessary to burn out by about 5 GWd / t. In this case, the concentration of the flammable poison may need to be 1.0% or less. I understand.
[0051]
The reactivity characteristic diagram of FIG. 4 shows the long Gd fuel rod G.₁Indicates the amount of decrease in reactivity at the end of the cycle when the concentration of flammable poisons contained in is changed. When the flammable poison concentration is 1.0% or less, the concentration is 0%, that is, includes flammable poisons. Compared with the case without it, the reactivity hardly decreases.
[0052]
However, when the flammable poison concentration is set to 3.0%, the reactivity decreases by about 0.1% Δk. Therefore, if the flammable poison concentration is 1.0% or less, the reactivity at the end of the cycle is reduced. It can be seen that there is almost no decrease in fuel consumption, and therefore does not reduce the fuel economy.
[0053]
  Also, the long Gd fuel rod G in the fuel assembly 10₁Is a core loaded with a fuel assembly that does not contain a flammable poison at a concentration of 1.0% or less, and is formed in a region that includes the position where the axial neutron flux distribution is greatest at cold temperatures. ConfigurationThe
[0054]
The longer the axial length of the fuel assembly 10 containing the flammable poison in the long Gd fuel rod, the greater the control amount of the reactivity and the greater the effect of improving the reactor shutdown margin. On the surface, it is not preferable because it leads to loss of reactivity at the end of the cycle.
Also, from the viewpoint of axial power distribution during operation and surplus reactivity, it is better not to lengthen the region containing a flammable poison with a concentration of 1.0% in the long Gd fuel rods longer than necessary. The effect on the characteristics is also small, which is preferable.
[0055]
For this purpose, the long Gd fuel rod G in the fuel assembly 10 shown in FIG.₁In this way, by providing a flammable poison with a concentration of 1.0% only in the core region where the power distribution becomes the largest at cold temperatures, the power peak can be effectively suppressed without deteriorating the core characteristics and more than necessary. By containing no flammable poison, it becomes possible to burn the flammable poison completely by the end of the cycle.
[0056]
  Further, in the fuel assembly 10, the long Gd fuel rod G₁The area containing 1.0% flammable poison is formed immediately below the upper natural uranium blanket part 12b.The
  Generally, in a high burnup fuel assembly, natural uranium blanket portions 12a and 12b are formed at the lower upper end portion in the axial direction in order to reduce axial neutron leakage and improve fuel economy.
[0057]
The neutron flux in the cold core has a local peak at the upper part in the axial direction, but when the upper natural uranium blanket part 12b is present at the upper end in the axial direction, the output from the upper natural uranium blanket part 12b is low. For this reason, the output increases in the vicinity immediately below the upper natural uranium blanket portion 12b.
[0058]
For this purpose, the long Gd fuel rod G₁As shown above, by setting an area containing flammable poisons with a concentration of 1.0% or less immediately below the upper natural uranium blanket 12b, it is possible to efficiently secure a furnace shutdown margin and be completely flammable by the end of the cycle. It becomes possible to burn poisons.
[0059]
As shown in the axial power distribution diagram of Fig. 5, curve 19 (solid line) is when the fuel assembly 10 is loaded into the replacement core, and curve 20 (dotted line) contains a flammable poison with a concentration of 1.0%. Represents the core average axial power distribution at the beginning of the cycle when the fuel assembly is loaded with cold fuel.
[0060]
As can be seen from the curve 19 and the curve 20 in FIG. 5, the core average axial power distribution with respect to the axial node has a relatively large power in the region immediately below the upper natural uranium blanket portion 12b.
Also, compared to the fuel assembly curve 20 that does not contain a flammable poison with a concentration of 1.0%, the curve 19 in the fuel assembly 10 reduces the maximum axial output and effectively suppresses the output peak. I understand that
[0061]
  Also, the long Gd fuel rod G of the fuel assembly 10₁In Fig. 1 (b), the length of the region containing the combustible poison with a concentration of 1.0% is the effective fuel length in the region of 19/24 to 22/24 from the lower end of the fuel. Configured to form below 13%The
[0062]
In fuel rods containing flammable poisons in the fuel assembly, the axial control region length containing flammable poisons increases the reactivity control amount as the length increases, so the complete effect of the furnace shutdown margin increases. .
Therefore, as shown in the furnace stop margin characteristic diagram of FIG. 6, the long Gd fuel rod G in the fuel assembly 10₁By changing the length of the area containing flammable poison with a concentration of 1.0%, the degree of improvement in the furnace shutdown margin will change.
[0063]
At this time, the area containing a flammable poison with a concentration of 1.0% is the effective length area from 21/24 to 22/24 (4% of the total length) and the 20/24 to 22/24 area (full length). 8%), and the length was changed from 19/24 to 22/24 (13% of the total length).
As shown in FIG. 6, as the length of the region containing a flammable poison with a concentration of 1.0% increases, the furnace shutdown margin increases and the furnace shutdown margin improves, but the effect per unit length is It turns out that it becomes small, so that it tends to saturate at about 13%.
[0064]
On the other hand, as the length of the area containing a flammable poison with a concentration of 1.0% increases, the reactivity control amount in the core increases. Therefore, making the area longer than necessary increases the axial power distribution during operation. In addition, the excess reactivity during operation is affected, which is not preferable.
[0065]
From this, in the fuel assembly 10, the length of the axial region containing the combustible poison having a concentration of 1.0% or less is set to 13% or less of the effective fuel length, so that the core characteristics during operation are reduced. In addition, the reactivity control amount at the beginning of the cycle can be appropriately set within a range that does not significantly affect the excess reactivity, and the furnace shutdown margin at the beginning of the cycle can be improved.
[0066]
  Next, in the fuel assembly 10, a long Gd fuel rod G containing a flammable poison having a concentration of 1.0% or less.₁Is configured as one or twoThe
  As the number of fuel rods containing the flammable poison increases, the reactivity control amount increases, so the effect of improving the furnace shutdown margin increases.
[0067]
Therefore, as shown in the reactor shutdown margin characteristic diagram of FIG. 7, in the fuel assembly 10, a long Gd fuel rod G containing a combustible poison with a concentration of 1.0% is used.₁By changing the number, the furnace shutdown margin is improved as the number increases.
However, it can be seen that the improvement effect per number decreases as the number increases, and tends to be saturated at about two.
[0068]
On the other hand, long Gd fuel rod G₁Since the reactivity control amount in the core increases as the number of the reactors increases, the influence of the axial power distribution and surplus reactivity during operation increases, which is not preferable.
[0069]
For these reasons, the fuel assembly 10 has a long Gd fuel rod G containing a combustible poison with a concentration of 1.0%.₁By setting the number of the reactors to one or two, the reactivity control amount at the beginning of the cycle can be appropriately set within a range that does not significantly affect the core characteristics and surplus reactivity during operation. The furnace shutdown margin at the beginning of the cycle can be improved.
[0070]
The fuel assembly 10 is an example of a high burnup fuel assembly designed to achieve a high burnup, and accordingly, the long Gd fuel rod G₁And the length of the water rod 4, the number, shape and arrangement position of the water rod 4 and the short fuel rod U₂And the presence or absence of natural uranium blanket parts 12a, 12b, and long Gd fuel rod G₂The concentration of flammable poisons depends on the design.
[0071]
Further, in the first embodiment, the fuel rod arrangement is 9 rows and 9 columns, and the short fuel rods U₂However, the above-mentioned characteristics are not limited to this, and the fuel rod arrangement is 8 rows by 8 columns, 10 rows by 10 columns, etc. Similar effects can be obtained in other designs.
[0072]
  Second embodimentWill be described with reference to FIGS. 8A and 8B.Detailed description of the same components as those in the first embodiment will be omitted, and only different portions will be described.
[0073]
  FIG. 8A is a transverse sectional view showing the arrangement of fuel rods in the high burnup fuel assembly, and FIG. 8B is an axial distribution diagram of the fuel in the fuel rods and the contained combustible poison. The fuel assembly 21 is an example applied to the high burnup fuel assembly 1 shown in FIG. 13, and the long fuel rod U is used as 66 long fuel rods.₁, Long Gd fuel rod G containing Gd, a flammable poison_Three2 and long Gd fuel rod G_FourThere are six.
  In addition, eight short fuel rods U₂And two thick water rods 4 are arranged.The
[0074]
In addition, as shown in FIG.₁, G_Three, G_FourThe lower natural uranium blanket portion 12a is in the region of 1/24 of the effective fuel length at the lower end portion, and the upper natural uranium blanket portion 12b is in the region of the fuel effective length of the upper end portion is 2/24. Is provided.
Furthermore, six long fuel rods G as the first group of fuel rods_FourIn the above, the entire inside of the axial direction excluding the natural uranium blanket portions 12a and 12b contains 7.0% Gd which is a highly concentrated flammable poison.
[0075]
Also, a long fuel rod G as the second group of fuel rods_ThreeFor these two, just below the upper natural uranium blanket part 12b, Gd of 1.0% concentration in the 20/24 to 22/24 region of effective length from the lower end of the fuel, and high concentration of 7.0 in the other regions % Gd.
[0076]
  Here, an example of the fuel used for the initial loading core is shown, and the combustible poison concentration of the fuel for the initial loading core is generally higher than that of the replacement fuel from the viewpoint of excess reactivity control. The long fuel rod G_ThreeThe concentration of Gd outside the region where the concentration is 1.0% is set to 3.0% or more.The
  Further, the average enrichment of the fissile material in the fuel assembly 21 is about 4.0%, which is higher than that of normal fuel, in order to achieve a high burnup.
[0077]
In general, in boiling water reactors, voids are formed during power operation, and the void ratio is higher in the upper part. Due to the negative reactivity effect of these voids, the output at the lower part of the core tends to be relatively higher than the upper part of the core. There is.
Especially in the initial loading core, all the fuel is new fuel, and there is no fuel in the second and subsequent cycles in which combustion in the lower part of the axial direction has progressed to some extent. It becomes large in highly enriched fuel that has achieved high burnup.
[0078]
Further, in the fuel assembly 21, short fuel rods U₂This further promotes the power peak at the bottom of the core.
In addition, there is a method of providing a fuel rod that does not contain a flammable poison at the upper part in the axial direction as a means for suppressing the tendency for the output at the lower part of the core to increase.
[0079]
As an example of this, in the fuel assembly 22 shown in the cross-sectional view of the fuel rod arrangement of FIG. 9A and the axial distribution of the fuel and contained combustible poison in the fuel rod of FIG. Long fuel rod U as a long fuel rod of the book_Three, Long Gd fuel rod G containing Gd, a flammable poison_Five2 and long Gd fuel rod G₆There are six.
In addition, eight short fuel rods U₂And two large-diameter water rods 4 are arranged.
[0080]
Also, as shown in FIG. 9B, the long fuel rod U_Three, G_Five, G₆The lower natural uranium blanket portion 12a is in the region of 1/24 of the effective fuel length at the lower end and the upper natural uranium blanket portion 12b is in the region of 1/24 of the effective fuel length of the upper end. Is provided.
In addition, six long fuel rods G₆, High concentration of Gd 7.0% is contained in the whole area in the axial direction excluding the natural uranium blanket portions 12a and 12b.
[0081]
However, the long fuel rod G_FiveFor these two, in the lower part of the upper natural uranium blanket part 12b, in the axially upper part of the effective length from 20/24 to 22/24 from the lower end of the fuel, it does not contain any flammable poisons, A high concentration of Gd 7.0% is contained.
As a result, the power at the upper part of the core can be increased and the output peak at the lower part of the core can be relaxed.
[0082]
However, in this fuel assembly 21, the long fuel rod G_ThreeThe long fuel rod G in the fuel assembly 22_FiveThe portion not containing flammable poisons in the upper part of the axial direction is formed as a region containing flammable poisons with a concentration of 1.0% or less in order to increase the cold shutdown margin at the beginning of the cycle.
From this result, in the initial loading core, the reactor shutdown margin tends to deteriorate, especially at the beginning of the cycle, but the reactor shutdown margin can be improved by putting a flammable poison with a concentration of 1.0% in the upper region of the fuel rod. .
[0083]
Meanwhile, the long fuel rod G_ThreeBy forming a new region containing flammable poisons with a concentration of 1.0% or less in the upper region of the catalyst, the lower peak is promoted again at the beginning of the cycle, but the flammable poison concentration is low at 1.0% or less. By minimizing the length of the area where 1.0% or less of the toxic substance is necessary, there is almost no effect on the core characteristics and excess reactivity during operation.
[0084]
Furthermore, especially in the initial loading core, since the start-up test is performed at a low output during a period of about 1 GWd / t before the operation at 100% output starts, the core is not affected even if the axial power distribution is somewhat deteriorated. There is no problem in characteristics, and the effect of containing a flammable poison with a concentration of 1.0% is reduced during this start-up test period, and the influence on the axial output distribution after the start of 100% output operation is small.
[0085]
In general, the initial loading core is composed of a plurality of fuels having different enrichments, for example, three kinds of fuels of high, medium and low enrichment, but the initial loading when the fuel assembly 21 is used as a highly enriched fuel. An example of the change in combustion of the core shutdown margin when the core is cold is shown in the cold shutdown margin characteristic diagram of FIG.
[0086]
Here, the curve 23 (solid line) shows the case where the fuel assembly 21 is loaded, and the curve 24 (dotted line) shows the case where the fuel assembly 22 is loaded instead of the fuel assembly 21. The fuel assemblies 21 and 22 both have an initial enrichment higher than that of a normal fuel assembly in order to achieve a high burnup.
For this reason, when the fuel assembly 22 does not contain a flammable poison having a concentration of 1.0%, as indicated by the curve 24, the furnace shutdown margin is particularly small at the beginning of the cycle, and the design target is not satisfied.
[0087]
On the other hand, in the case of the fuel assembly 21 in which the flammable poison having a concentration of 1.0% is provided in the upper region, the reactivity can be effectively suppressed as shown by the curve 23, and the reactor is stopped at the cold temperature particularly at the beginning of the cycle It can be seen that the margin has been improved and increased by about 0.3 to 0.4% Δk, which satisfies the design target.
As a result, in the fuel assembly 21, the lower peak of the output during 100% output operation can be relaxed, the reactivity at the initial stage of combustion can be suppressed, and the cold shutdown furnace margin at the initial stage of the cycle can be improved. .
[0088]
In addition, although the example loaded to the initial loading core was shown here, the application of the present invention is not limited to the initial loading core fuel, and even in the case of replacement fuel, in order to suppress the output peak at the bottom of the operating core. It can be applied when the concentration of the flammable poison at the top of the fuel rod containing the flammable poison is 1% or less.
Further, such a fuel assembly 21 is an example of a high burnup combustion assembly designed to achieve a high burnup, in which fuel rods are arranged in 9 rows and 9 columns and a natural uranium blanket portion is provided.
[0089]
Accordingly, the length and number of fuel rods and water rods 4, shape and arrangement position, short fuel rod U₂Or the presence or absence of natural uranium blanket parts 12a, 12b, long Gd fuel rod G_FourFlammable poison concentration and long Gd fuel rod G_ThreeThe concentration of the lower flammable poisons in the can vary depending on the design.
[0090]
  Third embodimentWill be described with reference to FIGS. 11 (a) and 11 (b).In addition, detailed description is abbreviate | omitted about the component similar to the said 1st Embodiment, and a different part is demonstrated.
  As shown in the cross-sectional view of the fuel rod arrangement in FIG. 11 (a) and the axial distribution diagram of the fuel in the fuel rod and the contained combustible poison in FIG. 11 (b), the fuel assembly 25 has 66 long fuels. Long fuel rod U as a rod_Three53 long Gd fuel rod G containing Gd, a flammable poison₇2 and long Gd fuel rod G₈There are 11 books. In addition, eight short fuel rods U₂And two large-diameter water rods 4 are arranged.
[0091]
  Long fuel rod U_Three, G₇, G₈Are both provided with natural uranium blanket portions 12a and 12b in the region of 1/24 of the effective fuel length at the lower end and the upper end, respectively.The
[0092]
Further, eleven long Gd fuel rods G which are the first group of fuel rods₈, The entire inner part in the axial direction excluding the upper and lower natural uranium blanket parts 12a and 12b contains Gd, which is a flammable poison with a concentration of 3.5%.
[0093]
On the other hand, two long Gd fuel rods G which are the fuel rods of the second group₇In the region of 1/24 to 8/24 of the effective length from the lower end of fuel and 20/24 to 23/24, Gd, which is a combustible poison with a concentration of 1.0%, is contained. Therefore, the other area is only U.
Here, the average enrichment of the fissile material in the fuel assembly 25 is set to a high enrichment of about 4.0%, which is higher than that of normal fuel, in order to achieve a high burnup.
[0094]
In the high burnup fuel assembly as described above, in order to mitigate the increase in the coolant flow pressure loss due to the increase in the number of fuel rods, the short fuel rods shortened by about 2/3 from the general long fuel rods. May be used.
However, when the flammable poison is contained in the entire length of the long fuel rod, the number of fuel rods containing the flammable poison is equal in the upper half and the lower half in the axial direction, and the reactivity control amount at the initial stage of combustion is The number of fuel rods per cross section is small, and the upper part where the amount of moderator is large is larger than the lower part.
[0095]
For this reason, as a result, there is a tendency that the output of the lower core becomes larger. Therefore, as a measure for suppressing the tendency for the output of the lower part of the core to increase, in Japanese Patent Laid-Open No. 6-118188 “Fuel Assembly and Boiling Water Reactor Core”, a part of the axial lower region where the short fuel rods are present A low-concentration combustible poison is formed, and the output peak in the lower axial direction at the beginning of the cycle is alleviated during the output operation.
[0096]
However, the long Gd fuel rod G of the fuel assembly 25₇Then, in the fuel in which a flammable poison is formed in a part of the lower axial region as described above, a flammable poison is formed in a part of the lower axial region in order to further increase the cold shutdown margin at the beginning of the cycle. An area containing a flammable poison with a concentration of 1.0% or less is formed in the upper area of the same fuel rod.
[0097]
Therefore, it is not necessary to increase the number of fuel rods in the fuel assembly when forming a 1.0% concentration of flammable poison on the upper part in the axial direction. The furnace shutdown margin can be improved.
[0098]
FIG. 12 shows an example of the change in combustion of the cold stop reactor margin in the cold reactor stop margin characteristic diagram of FIG. 12 in the replacement core with the fuel assembly 25 configured as described above. The curve 26 (solid line) is when the fuel assembly 25 is loaded, and the curve 27 (dotted line) is the long Gd fuel rod G which is the fuel rod of the second group in the fuel assembly 25.₇The case where no flammable poison with a concentration of 1.0% in the upper axial direction of is shown.
The fuel assembly 25 has an initial enrichment higher than that of the normal fuel assembly 1 in order to achieve a high burnup.
[0099]
Therefore, as shown by the curve 27, when the long Gd fuel rod does not contain a combustible poison with a concentration of 1.0% in the upper part, the furnace shutdown margin is particularly small at the beginning of the cycle, and the design target is not satisfied.
On the other hand, in the fuel assembly 25, as shown by the curve 26, by providing a combustible poison with a concentration of 1.0% in the upper region, the cold shutdown margin especially at the beginning of the cycle is improved, and about 0.2 to 0.3% Δk It is increasing and it can be seen that the design goal is satisfied.
[0100]
As a result, according to the third embodiment, the lower peak of the output at the initial stage of the cycle can be relaxed, the reactivity at the initial stage of combustion can be suppressed, and the cold shutdown furnace margin at the initial stage of the cycle can be improved.
These characteristics are not limited to the case where the fuel rod arrangement is 9 rows and 9 columns and the natural uranium blanket portions 12a and 12b are provided. The fuel rod arrangement is 8 rows and 8 columns or 10 rows and 10 columns. Similar effects can be obtained in other designs such as columns.
[0101]
【The invention's effect】
As described above, according to the present invention, in the fuel assembly loaded in the boiling water reactor, high fuel degree is achieved, and combustion is achieved without reducing the reactivity at the end of the cycle with the flammable poison contained in the fuel rod. Only the initial reactivity is suppressed, and the cold shutdown furnace margin at the beginning of the cycle is improved.
[0102]
In addition, the excess reactivity is efficiently suppressed to improve the furnace shutdown margin, and the axial area and length of the flammable poison contained in the fuel rod and the number of installations are appropriately selected, so that the complete cycle is achieved by the end of the cycle. Burning flammable poisons to improve fuel economy.
[Brief description of the drawings]
FIG. 1 is a fuel assembly according to a first embodiment of the present invention, in which (a) is a cross-sectional view of a fuel rod arrangement, and (b) is an axial distribution diagram of fuel in a fuel rod and a combustible poison.
FIG. 2 is a chart showing a cold furnace shutdown margin characteristic with respect to the burnup in the first embodiment according to the present invention.
FIG. 3 is an infinite multiplication factor characteristic diagram with respect to the burnup according to the first embodiment of the present invention.
FIG. 4 is a reactivity characteristic diagram at the end of a cycle with respect to a flammable poison concentration in the first embodiment according to the present invention.
FIG. 5 is an axial output distribution diagram of the first embodiment according to the present invention.
FIG. 6 is a furnace shutdown margin characteristic diagram for a flammable poison region having a concentration of 1.0% in the first embodiment according to the present invention.
FIG. 7 is a furnace shutdown margin characteristic diagram for the number of fuel rods containing a combustible poison having a concentration of 1.0% in the first embodiment according to the present invention.
FIGS. 8A and 8B are fuel assemblies according to a second embodiment of the present invention, in which FIG. 8A is a cross-sectional view of fuel rod arrangement, and FIG. 8B is an axial distribution diagram of fuel in the fuel rods and combustible poisons.
FIGS. 9A and 9B are fuel assemblies of a comparative example in the second embodiment, wherein FIG. 9A is a cross-sectional view of the arrangement of fuel rods, and FIG.
FIG. 10 is a graph showing a furnace shutdown margin characteristic with respect to the burnup in the second embodiment according to the present invention.
11A and 11B are fuel assemblies according to a third embodiment of the present invention, in which FIG. 11A is a cross-sectional view of the arrangement of fuel rods, and FIG. 11B is an axial distribution diagram of fuel in the fuel rods and combustible poisons.
FIG. 12 is a graph showing a furnace shutdown margin characteristic with respect to the burnup in the third embodiment according to the present invention.
13A is a longitudinal cross-sectional view of a fuel assembly having a high burnup, FIG. 13B is a cross-sectional view taken along line BB in FIG. 10A, and FIG. 13C is a cross-sectional view taken along line CC in FIG. Figure.
[Explanation of symbols]
1, 10, 21, 22, 25 ... Fuel assembly, 2, U₁~ U_Three... long fuel rods, 3, U₂... short fuel rod, 4 ... water rod (W), 5 ... spacer, 6 ... external spring, 7 ... upper tie plate, 8 ... lower tie plate, 9 ... channel box, G₁~ G₈... long Gd fuel rod, 11 ... control rod, 12a ... lower natural uranium blanket part, 12b ... upper natural uranium blanket part, 13, 16, 19, 23, 26 ... curve (solid line), 14, 17, 20, 24 , 27 ... Curve (dotted line), 15, 18 ... Curve (dashed line).

Claims (2)

In a fuel assembly for a boiling water reactor consisting of multiple fuel rods, a region containing a flammable poison with a concentration of 1.0% or less is formed immediately below the upper natural uranium blanket, and the length of that region is A fuel assembly comprising a fuel rod having a length of 13% or less of the effective fuel length . In the above fuel assembly, concentration claim 1 Symbol mounting assembly as characterized in that the one or two the number of fuel rods containing 1.0% or less of the burnable poison.

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