JP2610254B2 - Boiling water reactor - Google Patents

Description

The present invention relates to a high burn-up boiling water reactor loaded with plutonium fuel.

(Prior Art) An 8 × 8 fuel assembly conventionally used in a boiling water reactor will be described with reference to FIG. This fuel assembly 1
Is composed of a bundle of a number of elongated cylindrical fuel rods 2 bound together. In this bundle, the fuel rods 2 are held at equal intervals by spacers 5, and a water rod 6 is incorporated in the bundle in addition to the fuel rods 2. The outer periphery of this binding is surrounded by a channel box 7, which has an upper tie plate 3 at the top.
The lower part is joined to the lower tie plate 4.

The fuel rod 2 has a large number of cylindrical UO ₂ fuel pellets (not shown) loaded in a cladding tube, and the upper and lower ends of the cladding tube are sealed by an upper end plug 8 and a lower end plug 9. The upper end plug 8 has an extension that can be inserted into the support cavity in the upper tie plate 3, and the lower end plug 9 has a fitting part that fits into the support cavity in the lower tie plate 4. Have.

The water rod 6 is provided with a cooling water inlet hole 10 at a lower portion and a cooling water outlet hole 11 at an upper portion. Then, the cooling water flows in the water rod 6 from below to above.

FIG. 11 shows a horizontal section II of the fuel assembly 1 in which the fuel rods 2 and the water rods 6 are regularly arranged in a grid of 8 rows and 8 columns. Reference numeral 12 denotes a control rod for controlling a nuclear reaction of the nuclear reactor.

In the above configuration, the conventional fuel assembly must keep the output per unit length (linear output density) below the limit value so as not to impair the fuel integrity. Has a complex design like.

That is, in the case of a boiling water reactor, the thermal neutron flux distribution in the cross section of the fuel assembly is highest in the outermost fuel rods facing the water gap and lower in the inner fuel rods. Therefore, in order to reduce output peaking, four or more types of enrichment of nuclear fuel rods are usually used, and the enrichment of outer fuel rods having a high thermal neutron flux is reduced.

In recent years, for uranium fuel, fuel design with increased enrichment has been promoted in order to increase the extraction burnup and improve fuel economy.

In general, the thermal neutron absorption of ²³⁵ U increases with higher enrichment and the neutron flux spectrum becomes harder, but the neutron moderating effect decreases with this, and the absolute value of the void coefficient increases and the control rod Reactor core characteristics are degraded due to a decrease in reactor shutdown margin due to a decrease in value. In order to improve these characteristics, it is necessary to improve the hardware to increase the water-to-fuel ratio (ratio of the number of atoms of neutron moderator hydrogen and nuclear fuel material (heavy metal) such as uranium and plutonium) in order to improve these characteristics. Good. For example, in a conventional fuel grid as shown in FIG. 11, the nuclear characteristics are improved to the extent that the fuel rods at the center are replaced with water rods. For example, a fuel assembly in which a large-diameter water rod 13 is disposed at the center of a fuel assembly as shown in FIG. 12 has been proposed.

By the way, from the viewpoint of effective utilization of uranium resources, a plutonium thermal plan for recycling plutonium in spent uranium fuel extracted from a light water reactor to a light water reactor again is being advanced. This is because uranium fuel assemblies are replaced with plutonium-enriched mixed oxide (MOX) fuel rods in which some or most of the uranium fuel rods in the uranium fuel assemblies are replaced. It is used by loading it into a light water reactor together with the body, but its characteristics must be as close as possible to uranium fuel. As described above, the design of uranium fuel is in the direction of high enrichment, and accordingly, it is necessary to design the MOX fuel to have high enrichment.

The major difference between MOX fuel and uranium fuel in fuel handling is that plutonium is harmful to the human body and requires special consideration in fuel production. It is intended to be stored temporarily for a certain period without being reprocessed. Considering these, it is desirable to increase the plutonium loading amount per unit as much as possible so as to reduce the number of MOX fuel assemblies. However, when the plutonium loading ratio is increased due to the difference in nuclear characteristics between uranium and plutonium, the difference in characteristics from the uranium fuel assembly opens, deteriorating the core characteristics. This is fissile material ²³⁹ P
u and ²⁴¹ Pu have a thermal neutron absorption cross section larger than ²³⁵ U,
^{This is} caused by the fact that the neutron flux spectrum of MOX fuel becomes harder than that of uranium fuel and the neutron moderating effect decreases due to the large resonance neutron absorption by ²⁴⁰ Pu. As a result, similarly to the phenomenon caused by the high enrichment of ²³⁵ U, the margin of the transient characteristics decreases due to the increase in the absolute value of the void coefficient and the strain in the axial power distribution increases or the moderator reactivity coefficient increases. This may cause a decrease in the furnace shutdown margin.

Deterioration of these characteristics is acceptable when the fissile plutonium loading is about 1/3 of the total fisile, and it is known that it can be used as it is without changing the uranium fuel hardware. If it exceeds this, it is necessary to increase the water to fuel ratio of the fuel grid to increase the neutron moderating effect.

In addition, the BWR has a characteristic that power peaking easily occurs in the lower part of the core, but the uranium fuel easily cancels the void reactivity difference by making the reactivity difference by enrichment and gadolinia in the vertical direction, and the axial power distribution easily It has been found to be flattened and has already been put to practical use.

On the other hand, if this method is applied to a MOX fuel assembly having a large plutonium loading and a large number of MOX fuel rods, a plutonium enrichment difference, a gadolinia concentration difference, etc. will be provided in the vertical direction of the MOX fuel rod. However, since all MOX fuel rod production is performed remotely, the design needs to be as simple as possible, and it has been desired that other methods be developed independently of this method.

(Problems to be Solved by the Invention) An object of the present invention is to prevent deterioration of core characteristics in a high burnup boiling water reactor loaded with MOX fuel.
Another object of the present invention is to flatten the axial distribution without complicating the enrichment and gadolinia distribution in the MOX fuel rod.

[Constitution of the Invention] (Means and Actions for Solving the Problems) In order to achieve the above object, the present invention provides a mixed fuel assembly composed of plutonium fuel and uranium fuel, and a fuel assembly composed of uranium fuel. , The weight of fissile plutonium contained in a mixed fuel assembly occupies more than 1/2 of the total weight of fissile material in the fuel assembly, and hydrogen vs. heavy metal atoms during power operation The number of water rods or the diameter of the water rod is increased so that the number ratio is 5% to 15% higher than that of a uranium fuel assembly having the same reactivity life, and the fuel rod is disposed at the center of the horizontal section of the fuel assembly. It is characterized by the following.

According to the present invention, the neutron spectrum can be suppressed from being hardened, and the nuclear properties can be brought close to those of the uranium fuel assembly. In addition, the burnable poison is arranged in the lower portion of the inside of the water rod having the largest outer diameter, and thereby the power distribution in the axial direction of the core can be flattened by using the highly enriched MOX fuel.

In this specification, the term “heavy metal having a hydrogen to heavy metal atom ratio” means a heavy metal of a nuclear fuel material.

(Example) An example of the present invention will be described with reference to the drawings.

First, the change in moderator reactivity when the water-to-fuel ratio (H / HM) and the fissile plutonium loading ratio (Puf weight ratio) with respect to all fissiles in FIG. 4 are changed will be described. FIG. 4 shows the fuel assembly of FIG. 12 which has already been described.
FIG. 8 shows the change in moderator reactivity in the three MOX fuel assemblies that make the reactivity life (removal burnup) almost equal when the thickness is changed to 0.8 mm and 0.8 mm in wall thickness.

As the moderator reactivity on the vertical axis, the difference 無限 k ^CH between the infinite multiplication factor at low temperature and during output operation is taken, and the horizontal axis is the average hydrogen to heavy metal atom ratio H / HM during output operation. It is shown. The weight ratio of Puf in the figure means the weight ratio of fissile plutonium (Puf) in the MOX fuel to the total fissail (Puf + ²³⁵ U). This figure shows that as the Puf weight ratio increases, the △ k ^CH increases and the reactivity change with the moderator density change increases.However, the 水 k ^CH can be reduced by increasing the water to fuel ratio. I understand.

In general, MOX fuel is inferior in neutron moderating effect to uranium fuel, but at a Puf weight ratio of about 1/3, for example, the same hardware as uranium fuel as shown in Fig. 12 (H / HM is about 4.
It has been known from previous studies that 7) can be used. Therefore, Puf weight ratio exceeds 1/3
Regarding the hardware design of MOX fuel, P shown by the dotted line in the figure
H / H to be equal to △ k ^CH of MOX fuel with uf weight ratio of 33 w / o
You can choose M.

For example, a MOX fuel with a Puf weight ratio of 33 w / o (H / HM = 4.7) indicated by a circle in the figure shows a H / HM with a Puf weight ratio of 60 w / o.
If HM is about 5.0 (6% increase) and 85 w / o is about 5.3 (13% increase), Δk ^CH can be the same. Therefore, for MOX fuel with a Puf weight ratio of about 1/2 or more, if the H / HM is increased by 5% to 15% in proportion to the weight ratio, the MOX fuel with a Puf weight ratio of 1/3 The fuel and neutron moderating effects can be the same. Therefore, as a specific means for increasing the H / HM, it is effective to replace the fuel rod at the center of the fuel assembly having a small thermal neutron flux with a water rod or to increase the diameter of the water rod.

Next, an embodiment of the present invention when the Puf weight ratio is set to 33 w / o, 60 w / o and 85 w / o will be described with reference to FIGS. 1, 2 and 3. In the figure, symbol U is enriched uranium rod, P is MOX fuel rod based on natural uranium, G is enriched uranium or MOX
The fuel rod with gadolinia added to the fuel rod, W represents a water rod.

FIG. 1 shows a first embodiment of the present invention, which uses the same hardened MOX fuel as the uranium fuel assembly shown in FIG. Gadolinia is added to 10 enriched uranium rods, and 20 MOX fuel rods are about 1/3 of the total 60 fuel rods.
Occupy.

FIG. 2 shows a second embodiment of the present invention. In FIG. 1, two fuel rods are replaced by water rods at the center. The number of MOX rods is 36, about 3 out of 58 fuel rods.
/ 5. Gadolinia is added to ten enriched uranium rods.

FIG. 3 shows a third embodiment of the present invention. In FIG. 1, four fuel rods are replaced with water rods at the center. All fuel rods are composed of MOX fuel rods and use 14 gadolinia fuel rods.

FIG. 5 shows a fourth embodiment of the present invention, in which the MOX fuel assembly shown in FIG. 3 has a fuel rod arrangement of 9 rows and 9 columns as shown in FIG. Here, a large-diameter water rod having an outer diameter of 42 mm and a wall thickness of 1.4 mm is used in the center.

Next, it is shown that the power distribution in the axial direction of the core is flattened by arranging gadolinia rods in the lower part of the largest diameter water rod in the MOX fuel assembly without complicating the design of the MOX fuel rod.

The present invention is particularly effective when the number of uranium fuel rods is zero or small as shown in FIG. That is,
In this case, by adding gadolinia to the lower part of the MOX rod, the void reactivity difference between the upper and lower fuels is canceled out,
It is possible to flatten the axial power distribution. However, plutonium has a higher neutron absorption than uranium, so neutron absorption of gadolinia is small, and a large number of MOX rods with gadolinia distributed in the axial direction are required, which makes MOX fuel design and production complicated, which increases cost. Had become.

FIG. 6 is a cross-sectional view of the above-described embodiment of the present invention,
As shown in the figure, when a large-diameter water rod on which the partial-length gadolinia rod 14 is disposed is used, the neutron deceleration in the large-diameter water rod is good, and this gadolinia rod 1
The thermal neutron absorption effect of the book is much larger than that of the 151 gadolinia-containing fuel rods in the figure, and the reactivity of the upper and lower voids can be sufficiently canceled.

FIG. 7 schematically shows an example of the arrangement of the gadolinia bars in the height direction. This is composed of a non-fissionable substance (for example, alumina or zirconia) having a small neutron absorption as a base material at the lower part of the water rod and stored in a zirconium cladding tube 22 in a pellet form.

The gadolinia pellet has a cylindrical shape 21 as shown in FIG. (A-1) or (b-1) and (b-2) in FIG.
As shown in FIG. 5, there is an annular structure 23 in which water passes through the central axis. FIG. (A-2) is a diagram (a-1)
FIG. 2 is a cross-sectional view taken along line II-II of FIG.
Is provided, thereby maintaining a space between the water rod 25 and the tube wall.

FIGS. 8 (a) and (b) show FIGS. 10 (a-1) and (b-
It is a detailed view of the gadolinia bar shown in 1). Zircaloy 33, which covers gadolinia pellets 31 and 32, has a structure integrated with water rod. The projection 35 in FIG. 7A is for keeping the gadolinia bar at a fixed position in the horizontal direction. In FIG. 1B, an annular pellet 32 is used.

Next, the effect of flattening the axial power distribution by the partial length gadolinia rod described above will be described. Here, the outer diameter of the water rod is 34 mm, the wall thickness is 0.7 mm, and the shape of the gadolinia rod is that shown in FIG. 8 (a). The base material of gadolinia is alumina, to which gadolinia having a diameter of 10 mm and a concentration of 2 w / o is added at the center. FIG. 9 shows the effect of flattening the axial power distribution by such a gadolinia rod.

Here, FIGS. 9 (a) and 9 (b) show the power distribution in the axial direction of the core in the state where the control rods are completely withdrawn at the beginning of the cycle and at the end of the cycle, respectively. In the figure, the dotted line A indicates the case of the conventional type, and the solid line B indicates the case of the present invention. From this, it is understood that the present invention is superior in flattening the axial power distribution. In this example, the gadolinia rod tip position is 10/24 of the entire length as viewed from the lower end of the core. In the case of the conventional core shown in curve a, the power peaking is 1 /
4 or less, the tip position is about 1/3 to 1 /
The object of the present invention is attained when it is within the range of 2.

[Effects of the Invention] As described above, according to the present invention, the neutron spectrum is increased by increasing the number and diameter of water rods and changing the water to fuel ratio according to the fissile plutonium loading ratio in the MOX fuel assembly. Can be suppressed, and deterioration of core characteristics can be prevented. In addition, without complicating the enrichment and gadolinia distribution in the MOX fuel rod, the axial power distribution of the fuel assembly is flattened by simply changing the partial length gadolinia rod in the large-diameter water rod. Can be. Further, the plutonium can be effectively used and the uranium resource can be saved without changing the furnace internal structure and operation method in the existing plant. Furthermore,
Spent MOX as the amount of loaded boron in MOX fuel increases.
The number of fuels is reduced, which is advantageous for intermediate storage.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a first embodiment of the present invention, FIG. 2 is a cross-sectional view of a second embodiment of the present invention, and FIG.
FIG. 4 is a diagram showing a change in moderator reactivity when the fissile plutonium loading ratio (Puf weight ratio) with respect to the water-to-fuel ratio and the total fissail is changed, and FIG. In the fuel assembly shown in FIG.
FIG. 6 is a cross-sectional view of a fourth embodiment of the present invention in a row, and FIG. 6 is a fifth embodiment of the present invention in which a partial length gadolinia rod is disposed below a large diameter water rod in the fuel assembly of FIG. Example cross-sectional views, FIGS. 7 (a-1), (b-1) and (a)
-2) and (b-2) are schematic views of a longitudinal sectional view and a transverse sectional view of a partial length gadolinia rod according to the present invention, and FIGS.
(B) is a detailed sectional view of the partial length gadolinia rod of FIG. 7;
FIG. 9 shows a comparison of the axial power distribution of the core of FIG. 3 with the partial length gadolinia rod loaded and the core of the core not loaded, and FIG. 9 (a) shows the initial cycle, and FIG. FIG. 10 is a diagram showing a core axial power distribution in a state where control rods are completely withdrawn at the end of a cycle, FIG. 10 is a longitudinal sectional view of a conventional fuel assembly,
FIG. 11 is a cross-sectional view taken along line II of FIG. 10, and FIG. 12 is a cross-sectional view of a uranium fuel assembly corresponding to high enrichment. 14 …………………………………………………………………………………………………………………………………………………………………………………………….

Claims (2)

(57) [Claims] In a boiling water reactor loaded with a mixed fuel assembly composed of plutonium fuel and uranium fuel and a fuel assembly composed of uranium fuel, the weight of fissile plutonium contained in the mixed fuel assembly is reduced. It accounts for more than 1/2 of the total fissile material weight of the fuel assembly, and its hydrogen to heavy metal atomic ratio during power operation increases by 5% to 15% over uranium fuel assemblies having the same reactivity life. The number of water rods or the diameter of the water rods is increased, and the fuel rods are arranged at the center of the horizontal section of the fuel assembly. 2. The boiling water reactor according to claim 1, wherein a burnable poison having a non-fissionable substance as a base material is disposed in a lower portion of a water rod having the largest outer diameter.