JPS58140673A - Fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel assembly for a Benteng Water Reactor (BWR).

[Technical background of the invention]

Figure 881 shows a horizontal cross section wAt of a conventional BWR fuel assembly.
show. The fuel rods are arranged in 8 rows and 8 columns and include 1 or 2 9jita rods each consisting of a hollow tube made of Zircaloy through which water can flow. Fuel rod l is a spacer (1 not shown)
They are bundled together and surrounded by a rectangular cylindrical channel box 3). Also channel box 3
A cross-shaped control rod 4 is inserted into the outside of the control rod 4 as required. A normal fuel rod 1 is placed inside a Zircaloy cladding tube.
O, pellets are enclosed. Furthermore, in order to suppress the excess reaction W at the initial stage of phosphorus combustion, several tons of gadolinia (GdlOs) as a burnable poison were mixed into UO, and UO,-
There are several gadolinia-filled fuel rods 5 (8 in Fig.
There is a book and it is indicated by W. ) is used. Gd1O
, Gd (gadolinium) is a natural substance, and its composition is as shown in the table below.

Among these Gd isotopes, the neutron absorption cross section (#,)
Gd-155 and Gd-157 have the highest values, each with 61
There are 000 burns and 240000 burns. G
The reason why d is used as a burnable poison is that the two isotopes Gd-155 and Gd-157 have very large σ1, which suppresses the excess reactivity of the fuel at the initial stage of combustion and prevents neutron absorption. Gd-155→Gd-156゜G
Converting from d-157 to Gd-158, the generated Gd
-156 and Gd-158 are small, so fewer neutrons are wasted in the final stage of combustion. (G
σ1 of d-156 and Gd-158 is about several burns. ) [Problems with the background technology] However, in recent years, due to the economical nature of fuel, U-23 fuel
58,411 degrees Celsius, and combined with the long-term operation of the nuclear reactor, the surplus reactivity is further increasing.In order to suppress this, the number of Gd fuel rods and the Gd concentration tend to increase. For this reason, Gd-156, Gd-
The wasteful consumption of neutrons of magnitude 158 is becoming impossible to ignore.

Figure 2 shows a Vk fuel assembly containing no Gdt, and 5-
Gd of 0. UO, -Gd mixed with 0. fuel rod t8
This is a comparison of the KoO (infinite multiplication factor) of eight fuel assemblies. It can be seen that there is a difference in 19bΔ at the end of combustion, for example at 20GWD/T. In terms of fuel costs, this difference amounts to 100%, which is a huge loss.

By the way, KooO! in Figure 2! (1%ΔK) Breakdown t- Looking at the above-mentioned Gd-156, Gd-15
In addition to the reactivity loss due to neutron absorption of 8, in fact, Gd-155-) Ga-156 → Gd-157,
A small amount of Gd-157 generated in a chain remains, and 4IL
Although it is a large amount, σ is huge, so this Gd-157
was found to have the largest contribution to reactivity loss.

[Purpose of the invention]

The present invention has been made in view of the following circumstances.
By reducing the amount of Gd-157 produced from Gd-155 and Gd-156 compared to the conventional method, the purpose is to prevent wasted neutrons due to Gd at the final stage of combustion, that is, reactivity loss.

[Invention 1! [Summary] The present invention resides in a fuel assembly containing an industrial fuel poison enriched with Gd-157.

[Embodiments of the invention]

The external shape is the same as that of the conventional one (FIG. 1); the fuel rod 5 containing i-trinia is filled with UO and pellets containing the number S of gadolinia (Gd*Os).

The feature of the present invention is that the isotopic composition ratio of Gd in this gadolinia is different from that of the natural one (Table 1), and that Gd-157 is the main component. In natural products, Gd
The composition ratio of -157 is set at 16%, but the higher the ratio, the greater the effect of the present invention, and it is preferable to set it to about 50 to 90, for example. The isotopic composition ratio of Gd changes, and G
Even if it is used as the main component of d-157e, the physical properties of gadolinia, such as thermal conductivity and compatibility with UO, are the same as when natural ones are used. However, Ga-157
Since σ1 of is as large as 240,000 Pans, Gd2O3
The amount of mixed into UO2 depends on the existence ratio of Gd-157,
It needs to be reduced somewhat.

[Action of the invention]

In the gadolinia-containing fuel in the fuel assembly of the present invention, Gd-157t-Gd is used as the main component.

Neutron absorption cross section of Gd-157 (6a') ti
Since it is sufficiently large at 240,000 burns, the neutron absorption capacity is sufficient, and the reactivity control capacity at the initial stage of combustion is sufficient. Gd-157 absorbs neutrons and converts them into Gd-158, but since the absorption cross section of Gd-158 is very small, only a few burns, at the final stage of combustion, almost no neutrons are absorbed by Gd.

#I3 diagram: Gd-157t is almost 100% and L is Gd
tOst.

)(oo (infinite multiplication factor)) of a fuel assembly having eight fuel rods containing 1% UO.

Compared to the conventional combination of natural Gd, at the end of combustion, for example, Z
It can be seen that the reactivity is close to 1-Δ after OGWT)/T, and the reactivity is high.

In addition, in the example mentioned above, Uo! Gd mixed in, 0. Since the amount of Yr is reduced to 115, the thermal conductivity of the fuel is UO.

It's almost the same as when it was only, Gd, O.

The deterioration in thermal conductivity caused by the contamination of the meat can be alleviated, making it a healthy meat to eat as a fuel.

〔Effect of the invention〕

In the fuel assembly of the present invention, the reactivity is sufficiently low at the early stage of combustion and can be suppressed to the level of conventional Gd-containing fuel.
At the final stage of combustion, neutron absorption of Gd isotopes, particularly Gd-157, which has conventionally been produced in trace amounts, is prevented, and wasteful consumption of neutrons, that is, loss of reactivity, can be prevented.

J: In the above-described embodiment, the reactivity is 1 in 1 inch Δ, that is, the fuel cost is 1091 G, and the effect is large. Also, Gd!
The amount of O, O mixed can be reduced, and Gd, 0. Deterioration of thermal conductivity of fuel due to contamination can be alleviated.

[Brief explanation of drawings]

Figure 1 is a schematic horizontal cross-sectional view of a conventional fuel assembly, Figure 2 is a Koo (infinite multiplication factor) combustion characteristic diagram of the conventional fuel assembly, and Figure 3 is a Koo (infinite multiplication factor) combustion characteristic diagram of the fuel assembly of the present invention. (multiplication factor)
FIG.

Claims (2)

[Claims] (1) A fuel assembly characterized by having a Gd ratio of 1111 compared to Gd-157. (2) The fuel assembly according to claim 1, wherein the enriched Gd is housed in a fuel rod.