JPH0339695A - Fast breeder reactor - Google Patents

Abstract

PURPOSE:To obtain the fast breeder reactor which can suppress the decrease of a negative reactivity feedback effect by loading fuel successively from the central part of the core toward the peripheral part of the core in order of the higher ratios of <239>Pu with respect to the Pu isotope element ratio of the reactor using an oxide mixture composed of plutonium Pu and uranium U as a fuel. CONSTITUTION:The core fuel having the relatively low ratio of <239>Pu like Pu obtd. from a light water reactor having an high degree of burnup is disposed on the relatively peripheral part of the core (core fuel 12 of the peripheral region) and the fuel having the relatively high ratio of <239>Pu is disposed as the core fuel 11 of the central region. The part which has a high neutron flux and makes an important contribution to a reactivity feedback is the central part of the core and, therefore, the negative reactivity feedback increases and the safety intrinsic to the core is enhanced as compared to the case of loading only the Pu from the light water reactor. In addition, the effective utilization of Pu, etc., which are recycled while the safety is sufficiently assured is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to fast breeder reactors, and more particularly to fast breeder reactors that use mixed oxides of plu)nium and uranium as core fuel.

"Prior art" Fast breeder reactors use fast neutrons to produce uranium-235 ("
'U) 'P Furtoni ft mu 239 (2316 p u
) to cause a nuclear fission reaction. Some of the fast neutrons generated at this time were again 23%
U and 23! It collides with Pu and nuclear fission occurs. In addition, some of the remaining fast neutrons are absorbed by uranium-238 (23'U), which becomes 2"sPu and causes a fission reaction. In this way, in fast breeder reactors, 23B, which was hardly used in light water reactors, is absorbed by uranium-238 (23'U). (It has the excellent property of multiplying fuel by changing J to 23 mpu.

The core of such fast breeder reactors is loaded with mixed oxides of plutonium and uranium as fuel. For the time being, plutonium can be obtained by reprocessing fuel extracted from combustion in light water reactors. The ratio of isotopes in the plu)nium composition, i.e. 23'a p u
1 Plutonium-240 (''Pu), Plutonium-241, (241pu) and Plutonium-24
The ratio of 2 (242 pu) depends on the burnup of the fuel.

In addition, when recycling in a light water reactor, changes depending on the number of times of recycling are shown.

Therefore, as the burn-up of light water reactors increases and plutonium recycling progresses, plutonium with various isotope ratios will be obtained. Plu)nium with various isotope ratios obtained in light water reactors will be used in fast breeder reactors in the future. Therefore, when plutonium is used in a fast breeder reactor, plutonium with these various isotope ratios is (i) mixed at the reprocessing stage, produced as a fuel with a single isotope ratio, and then fed into the reactor core. or (ii) separately reprocessed to produce fuels each having a different isotope ratio, and (a) collecting and loading fuels with the same isotope ratio for each core; Or (b) it is conceivable to uniformly load these fuels into the reactor core.

``Problem to be solved by the invention'' By the way, compared to plutonium extracted from current light water reactors, plutonium obtained from high-burnup light water reactors and plutonium recycled from light water reactors has a
With the proportion of 3IPu decreasing and the proportion of 241 p u increasing, the sum of the plutonium proportions of these fissile materials is decreasing. Conversely, the proportion of 240 pu and 242 Pu, which are neutron absorbing substances, is increasing.

Plutonium obtained from these high-burnup light water reactors has a lower reactivity value than plutonium extracted from current light water reactors. For this reason, if they are loaded into part or all of the core of a fast breeder reactor, the overall plutonium enrichment must be made higher than if only plutonium extracted from current light water reactors is loaded. If the enrichment of plutonium is increased, relatively 23
Since the proportion of 8 U is reduced, the Doppler effect with negative reactivity feedback is reduced. Moreover, the sodium density coefficient with positive reactivity feedback increases mainly because these plutoniums contain a large amount of 240 pu. As a result, negative reactivity feedback effects are reduced in this sense as well, compared to the case of using plunium extracted from current light water reactors. A significant reduction in the negative reactivity feedback effect is undesirable from the standpoint of core safety.

Therefore, the object of the present invention is to develop a fast breeder reactor that can suppress the decrease in the negative reactivity feedback effect as much as possible even when plutonium obtained from a light water reactor with a high burnup or plutonium recycled from a light water reactor is used. Our goal is to provide the following.

"Means for Solving the Problems" The fast breeder reactor of the present invention is made of a mixed oxide of plutonium and uranium, and has a plutonium isotope ratio of 23
The core fuels with higher iPu ratios are loaded from the center of the core to the periphery of the core. To this end, plutonium with various isotope ratios extracted from multiple light water reactors is separated and reprocessed to produce separate plutonium fuels.

That is, in the present invention, core fuel with a relatively low IS*pu ratio, such as plutonium obtained from a light water reactor with a high burnup or plutonium recycled from a light water reactor, is arranged relatively on the same side of the reactor core. Core fuel having a relatively high wasp u ratio is placed in the center of the core. Since the central part of the reactor core has a high neutron flux and greatly contributes to reactivity feedback, it may be necessary to load only plutonium from high-burnup light water reactors or plutonium recycled from light water reactors, or to mix it with plutonium from current light water reactors. Compared to the case of uniform loading in the core, the present invention increases negative reactivity feedback. This makes it possible to further enhance the inherent safety of the reactor core compared to these cases.

"Example" Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 1 shows a longitudinal section of a fast breeder reactor in one embodiment of the present invention. The thermal output of this fast breeder reactor is approximately 2
It has a capacity of 600MW and generates 1 million KW of electrical output. In this fast breeder reactor, an upper blanket 13 as an axial blanket is arranged above the central region core fuel 11 and the peripheral region core fuel 12, as in the conventional example, and a lower blanket 14 is arranged below. Further, a radial blanket 15 is arranged around both core fuels 11 and 12, as in the conventional example. Here, the length of the central region core fuel 11 and the peripheral region core fuel 12 in the core axial direction is set to 1000 mm, and the length of the radial direction blanket 15 is set to 1700 mm. Also,
The length in the radial direction of the core consisting of the central region core fuel 11 and the peripheral region core fuel 12 is set to 3300 mm, and the length in the radial direction of the core including the radial blanket 15 is 390 mm.
It is set to 0mm.

In a fast breeder reactor with such a configuration, the central region core fuel 11
The isotope ratio (weight ratio) of plutonium in the peripheral region core fuel 12 is as shown in Table 1 below.

Here, the central region core fuel 1 is extracted from a current light water reactor and has a burnup of 320 wd/l (giga watt day/ton), and the peripheral region core fuel 12 is a fuel extracted from a light water reactor with a high burnup (burnup is 45 (II, wd/l
) is the fuel extracted from In this embodiment, these core fuels 11 and 12 are loaded in the core of a fast breeder reactor with an electrical output of 1000 MWe (megawatts) at a volume ratio of 1:1.

As described above, in the fast breeder reactor of this embodiment, plutonium extracted from current light water reactors is loaded only in the center of the core, and other plutonium is loaded only in the periphery of the core. Table 2 below compares the Doppler coefficients and IJum density coefficients of the fast breeder reactors of this example and the conventional example.

(Margin below) Table 2 However, in this table, "first Pl" refers to a fuel with the same -Pu isotope ratio as the central region core fuel 11, and "second Pl"
"refers to fuel with the same -Pu isotope ratio as the peripheral region core fuel 12."

As can be seen from Table 2, the fast breeder reactor of this example (bottom column of this table) is different from the conventional fast breeder reactor of only Pu'' (second column from the top of this table). In comparison, the absolute value of the Doppler coefficient increases significantly and the sodium density decreases significantly.Compared to the conventional fast breeder reactor with homogeneous plutonium mixing (third column from the top of this table). It can be seen that the Doppler coefficient and sodium density have been improved in the direction of increasing the inherent safety of the reactor core.In other words, the fast breeder reactor of this example is a fast breeder reactor loaded only with plutonium extracted from current light water reactors. (The top column of this table) is close to the case of 1, and the negative feedback is increasing.

In the example described above, it was decided to load the core fuel into the reactor core by dividing it into two parts, one with a relatively high ratio of 239 pu and the other with a relatively low ratio, but the core fuel was divided into three parts according to the ratio of 239 pu. Of course, it is also within the scope of the present invention to divide the fuel into the above parts and load the fuel from the center of the core to the periphery in descending order of the ratio of 239 p u.

Furthermore, it is natural that the ratio between those having a relatively high ratio of 239Pu and those having a relatively high ratio of 239Pu is not limited to this example.

"Effects of the Invention" As described above, according to the present invention, f of plutonium and uranium is
Containing plutonium oxide, the core fuel with the highest plutonium isotope ratio of 239 pu was loaded into the core from the center to the periphery of the core, ensuring the safety of the fast breeder reactor. At the same time, plutonium obtained from high-burnup light water reactors and plutonium recycled from light water reactors can be effectively used.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a fast breeder reactor in one embodiment of the present invention. 11...Central region core fuel (fuel with a higher proportion of 23"Pu in the loaded plutonium), 12.
... Peripheral area core fuel (fuel with a lower proportion of 231 p u in the loaded plutonium).

Claims (1)

[Claims] A fast breeder reactor comprising a mixed oxide of plutonium and uranium, and loaded from the center of the core to the periphery of the core in order of increasing plutonium isotope ratio of ^2^3^9Pu.