JPH0636048B2 - Fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a fuel assembly having improved fuel economy.

[Technical background of the invention and its problems]

In a fuel assembly loaded in the core of a boiling water reactor, in recent years, in order to improve fuel economy, uranium enrichment distribution and burnable poisons (gadolinia) in fuel rods have been improved.
Studies are underway to optimize the distribution.

For example, in the fuel assembly shown in FIG. 6 of JP-A-58-196483 (known example 1) and FIG. 1 of JP-A-59-84184 (known example 2), the uranium enrichment is defined as the fuel assembly. It is high in the upper area and low in the lower area. Also, the number of fuel rods containing burnable poison (gadolinia) is large in the upper region of the fuel assembly and small in the lower region,
This is improving fuel economy.

However, of the electric power demand, it is expected that the dependence on nuclear power generation will increase more and more in the future, and therefore further improvement of fuel economy is demanded.

Further, in the fuel assemblies shown in the known examples 1 and 2, the uranium enrichment is high in the upper region of the fuel assembly where the neutron importance at the time of cold temperature is high, and thus the reactivity suppression effect by the burnable poison is reduced in the cycle. At the end of the period, the reactor shutdown margin tends to be slightly smaller, and from the standpoint of improving reactor safety, improvements are also required to increase the reactor shutdown margin.

By the way, as a proposal for increasing the reactor shutdown margin, Fig. 1 of JP-A-58-179391 is used. 5A, 5
B and FIG. 4C (known example 3), and JP-A-58-1
Fig. 79392. 5A-5H and FIG. 6A ~
6H (known example 4).

These known examples 3 and 4 disclose that the uranium enrichment in the upper region of the fuel assembly, which has a high neutron importance at the time of cold temperature, is made low, thereby improving the reactor shutdown margin.

That is, in Known Examples 3 and 4, the uranium enrichment is low in the upper region of the fuel assembly and high in the lower region, and the concentration of burnable poison (gadolinia) is low in the upper region of the fuel assembly.
It is higher in the lower area.

However, in the proposals for improving the reactor shutdown margin shown in the publicly known examples 3 and 4, both the uranium enrichment distribution and the burnable poison distribution are different from those of the publicly known examples 1 and 2 in which the fuel economy is improved. The conditions are opposite. As is clear from this, it has been extremely difficult to simultaneously improve the fuel economy and the reactor shutdown margin.

[Object of the Invention]

It is an object of the first invention to further improve the fuel economy of a fuel assembly loaded in the core of a boiling water reactor.

Another object of the present invention is to provide a fuel assembly capable of simultaneously improving fuel economy and ensuring a reactor shutdown margin for improving reactor safety.

[Summary of the Invention] In the first invention, at least one of the upper and lower ends of the fuel assembly is filled with any one of natural uranium, slightly enriched uranium, and depleted uranium, and the other regions are defined as upper and lower regions. The uranium enrichment averaged over the fuel assemblies in the upper region is divided into higher than the uranium enrichment averaged over the fuel assemblies in the lower region, and some fuel rods are burnable with burnable poisons over their entire length. A fuel rod containing poisonous substances, and the number of fuel rods less than the fuel rods containing full-length combustible poisons is a fuel containing a partial length of combustible poisons in which only a lower concentration of the combustible poison is contained in the upper region. It is characterized as a stick.

Further, in the second invention, at least one of the upper and lower ends of the fuel assembly is filled with any one of natural uranium, slightly enriched uranium and depleted uranium, and the other region is divided into an upper region, a central region and a lower region. The average uranium enrichment of the fuel assemblies in the central region was set higher than the average uranium enrichment of the fuel assemblies in the upper region and the lower region, and some fuel rods contained burnable poisons over their entire length. A fuel rod containing full-length combustible poison, and a smaller number of fuel rods than the fuel rod containing full-length combustible poison contains a lower concentration of combustible poison than the fuel rod containing full-length combustible poison in the central region only. It is characterized in that it is a fuel rod containing a poisonous substance.

Example of Invention

 First, an embodiment of the first invention is shown in FIG.

The figure schematically shows one of a large number of fuel assemblies loaded vertically in the core of a boiling water reactor, which is 8 × 8 fuel with uranium fuel. The bars are arranged vertically.

The fuel assembly has natural uranium (enrichment of 0.7
1%) and the other regions are divided into upper region I and lower region II, and the uranium enrichment in upper region I is 3.52%.
And the uranium enrichment of the lower region II is 3.33%.
Instead of natural uranium at the upper end, finely enriched uranium or depleted uranium may be used.

In addition, 8 fuel rods (fuel rods containing burnable poison with full length) were filled with burnable poison (cadrinia) having a concentration of 3.4% over the entire length, and another 2 fuel rods (fuel containing burnable poison with partial length) A burnable poison (cadrinia) having a concentration of 2.0% is put only in the upper region I of the bar).

With the above-described configuration, the reactivity in the upper region I is suppressed by gadolinia in the first half of the cycle, so that the axial power distribution of the core has a lower peak, the core average void fraction increases, and the accumulation of plutonium is promoted. To be done. on the other hand,
At the end of the cycle, since gadolinia is almost burned out, the axial power distribution of the core shifts to a peak in the upper region I where the uranium enrichment is high, which causes the core average void fraction to decrease.
The loss of reactivity due to voids decreases, and the plutonium accumulated in the first half of the cycle also burns. Thus, during the cycle, the neutron spectrum is changed by changing the core average void fraction, and the spectrum shift effect is generated, whereby the core reactivity at the end of the cycle is increased and the fuel economy is improved. Of.

In addition, it was decided to put any one of natural uranium, slightly enriched uranium or depleted uranium in the upper end part because the neutron importance is low in this part, so fuel efficiency will be further improved by using uranium of low enrichment. This is to increase.

Here, in order to maximize the spectrum shift effect,
It is more effective to lower the gadolinia concentration of the two fuel rods containing the partial length combustible poison in the upper region I. This point will be described with reference to FIG. The vertical axis of FIG. 2 represents infinite multiplication factor (K∞), and the horizontal axis represents burnup (Gwd / t).

The alternate long and short dash line (symbol A) in FIG. 2 indicates a uranium enrichment of 3.52.
%, There are eight fuel rods with gadolinia having a concentration of 3.4%.

The solid line (symbol B) in FIG. 2 has the same uranium enrichment of 3.52%, eight gadolinia-containing fuel rods with a concentration of 3.4%, and the concentration of 2.0% as in the upper region I of FIG. It shows the case where there are two fuel rods with gadolinia.

Furthermore, the chain double-dashed line (symbol C) in FIG. 2 shows the case where there are 10 fuel rods with gadolinia having a uranium enrichment of 3.52% and a concentration of 3.4%.

The symbol D in FIG. 2 indicates the range of the end-cycle burnup in the region II of FIG.

Therefore, as shown by the solid line (symbol B), the characteristics of the upper region I have a large reactivity suppressing effect in the first half of the cycle, and at the end of the cycle, two fuels containing gadolinia with a concentration of 2.0% are included. One-dot chain line (symbol A) in the figure without bars
Will be the same as

This is suitable for the axial power distribution to peak in the lower region II in the first half of the cycle and peak in the upper region I at the end of the cycle.

If the concentration of gadolinia in two fuel rods containing 2.0% gadolinia is increased to 3.4%, the chain double-dashed line in FIG.
As shown in (1), the amount of unburned gadolinia remains large even at the end of the cycle, the reactivity decreases, and the spectrum shift effect also decreases, so the fuel economy decreases.

Thus, it can be seen that the fuel economy is improved by using the two fuel rods as the fuel rods containing the partial length combustible poison in which the gadolinia of a low concentration is placed only in the upper region I.

The first invention is not limited to the above embodiment. For example, as shown in FIG. 3, any one of natural uranium, slightly enriched uranium and depleted uranium may be placed at the lower end portion, or as shown in FIG. 4, it may be placed at the upper end portion and the lower end portion.

Next, an embodiment of the second invention is shown in FIG.

The figure schematically shows a fuel assembly, and it is the same as the embodiment of FIG. 1 that any one of natural uranium, slightly enriched uranium and depleted uranium is put in the upper end portion, but in this embodiment, The other region is divided into three regions, an upper region Ia, a central region IIa, and a lower region IIIa, and the uranium enrichment of each of the upper region Ia and the lower region IIIa is 3.33%.
And only the uranium enrichment in the central area IIa is 3.52%.

In addition, 8 fuel rods (full length burnable poison-containing fuel rods) were filled with 3.4% concentration of burnable poison (gadolinia) over the entire length, and another 2 fuel rods (partial length burnable poison containing fuel) A burnable poison (gadolinia) having a concentration of 2.0% is put only in the central region IIa of the bar).

Also in the above configuration, the fuel economy can be improved by using natural uranium in the upper end portion where the neutron importance is low. Also, the central region IIa and the lower region III
In a, the spectrum shift effect can be produced by making the uranium enrichment and the number of gadolinia-containing fuel rods different. That is, the uranium enrichment is higher in the central region IIa than in the lower region IIIa, and the gadolinia concentration is higher in the central region IIa because there are two partially burnable poison-bearing fuel rods having gadolinia only in the central region IIa. high. Therefore, in the first half of the cycle, the reactivity in the central region IIa is suppressed by gadolinia, the axial power distribution of the core reaches the lower peak, and the core average void fraction increases, so that the accumulation of plutonium is promoted. On the other hand, since gadolinia is almost burned out at the end of the cycle, the axial power distribution of the core shifts to the central region IIa where the uranium enrichment is high, the core average void fraction decreases, and the reactivity loss due to voids decreases, At the same time, the plutonium accumulated in the first half of the cycle also burns. And from these spectrum shift effects, the core reactivity at the end of the cycle increases,
Fuel economy will be improved.

In the upper region Ia, the uranium enrichment is 3.33%, which is lower than that in the central region IIa, and the gadolinia concentration is lower than that in the central region IIa by the amount of the two fuel rods containing partially burnable poisons.
By reducing the uranium enrichment in the upper region Ia having a high neutron importance at the cold temperature, the reactor shutdown margin is improved. By setting the length of the upper region Ia to about 1/8 of the active fuel length, the central region II
It is possible to increase the reactor shutdown margin without impairing the spectrum shift effect produced by making the difference between the uranium enrichment and the number of gadolinia-containing fuel rods in a and the lower region IIIa.

In short, in this embodiment, it is possible to improve the fuel economy as in the embodiment of FIG. 1 and also improve the reactor shutdown margin.

Further, the effect obtained when the gadolinia of 2.0% concentration is placed only in the central region IIa as in this embodiment can be explained as follows.

For comparison with the fuel assembly shown in FIG. 5, a fuel assembly having a gadolinia of 2.0% concentration in the upper region Ia is assumed as shown in FIG. That is, in the fuel assembly shown in FIG. 6, the upper region I is attached to the two fuel rods containing the partially burnable poison.
2.0% over the two areas a and the central area IIa
It is considered that the concentration of gadolinia is added.

Therefore, the effectiveness of the fuel assembly shown in FIG. 5 (designated as bundle V) and the fuel assembly shown in FIG. 6 (designated as bundle VI) in operating the reactor. Comparing the multiplication factors (Keff) is as shown in FIG.

As is clear from the comparison of FIG. 7, in the bundle VI, the effective multiplication factor (Keff) does not reach the target value at the beginning of the cycle. When the effective multiplication factor (Keff) is small as described above, it is difficult to control the core output by the control rod even at the beginning of the cycle, which is not preferable. Looking at the effective multiplication factor (Keff) at the end of the cycle, it can be seen that the bundle V is superior to the bundle VI in terms of fuel economy.

The second invention is not limited to the above embodiment. For example, as shown in FIG. 8, any one of natural uranium, slightly enriched uranium and depleted uranium may be put in the lower end portion,
As shown in FIG. 9, they may be placed at both upper and lower ends.

〔The invention's effect〕

As described above, according to the first invention, the fuel economy can be improved in the fuel assembly loaded in the core of the boiling water reactor.

According to the second aspect of the present invention, it is possible to increase the spectrum shift effect of the boiling water reactor while ensuring a sufficient reactor shutdown margin for improving the safety of the reactor, and to improve the fuel economy. Further, according to the first invention and the second invention, the number of the fuel rods containing the partial-length combustible poison is made smaller than the number of the fuel rods containing the full-length combustible poison, and the flammability of the fuel rod containing the partial-length combustible poison is also increased. By making the poison concentration lower than the burnable poison concentration of the fuel rod containing the full-length burnable poison, the reactivity change amount of the burnable poison in combustion is delicately controlled, the spectrum shift effect is enhanced, and the fuel economy is further improved. Can be improved.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a fuel assembly of an embodiment according to the first invention, and FIG. 2 is a diagram showing a relationship between infinite multiplication factor and burnup for explaining the effect of the embodiment, FIGS. 3 and 4 are diagrams schematically showing the fuel assemblies of other embodiments according to the first invention, and FIG. 5 is a schematic view of the fuel assemblies of the embodiment according to the second invention. FIG. 6 is a diagram schematically showing a fuel assembly to be compared with the fuel assembly of the same embodiment, and FIG. 7 is an effective multiplication factor and a cycle burnup for explaining the effect of the same embodiment. FIG. 8, FIG. 8 and FIG. 9 are views schematically showing the fuel assemblies of other embodiments according to the second invention. I ... upper area, II ... lower area, Ia ... upper area,
IIa: central area, IIIa: lower area.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shunsuke Ogiya 1-1-1, Shibaura, Minato-ku, Tokyo Inside Toshiba Headquarters office (72) Inventor Isamu Toyoyoshi 1-1-1, Shibaura, Minato-ku, Tokyo Shares Company Toshiba Headquarters Office (56) Reference JP 54-64287 (JP, A) JP 58-196483 (JP, A) JP 59-84184 (JP, A) JP 60-117182 ( JP, A)

Claims (2)

[Claims] 1. A fuel assembly comprising a large number of fuel rods arranged in a vertical direction and loaded in a core part of a boiling water reactor, at least one of upper and lower ends of which is enriched with natural uranium and slightly enriched. Put either one of uranium or depleted uranium and divide the other area into upper area and lower area,
The average uranium enrichment of the fuel assembly in the upper region is made higher than the average uranium enrichment of the fuel assembly in the lower region, and some fuel rods contain burnable poison over their entire length. The number of fuel rods that are smaller than the fuel rods containing burnable poisons in the full length is the fuel rods containing burnable poisons in a partial length in which the concentration of burnable poisons is lower than that in the fuel rods containing burnable poisons in the upper region only. A fuel assembly characterized by the following. 2. A fuel assembly comprising a large number of fuel rods arranged in the vertical direction and loaded in the core of a boiling water reactor, at least one of upper and lower ends of which is enriched with natural uranium and slightly enriched. Put either one of uranium or depleted uranium and divide the other area into upper area, central area and lower area, and calculate the average uranium enrichment of fuel assembly in the central area as fuel in upper area and lower area. The fuel concentration should be higher than the average uranium enrichment of the aggregate, and several fuel rods should be fuel rods containing full-length combustible poisons containing burnable poisons over their entire length. A fuel assembly characterized in that the rod is a fuel rod containing a partial-length burnable poison containing a lower concentration of burnable poison than the fuel rod containing the full-length burnable poison only in the central region.