JP2021148515A - Nuclear transformation aggregate - Google Patents

Abstract

To provide a nuclear transformation aggregate formed by easily obtainable materials, which can prevent an output peak from occurring in adjacent fuel rods because a neutron uninvolved in nuclear transformation in an LLFP aggregate is leaked from the LLFP aggregate.SOLUTION: According to the present invention, a nuclear transformation aggregate 102A includes a nuclear transformation part 105 for performing nuclear transformation of nuclear fission products, and a second neutron absorption part 106 arranged around the nuclear transformation part 105 to absorb second neutrons N2 uninvolved in nuclear transformation among first neutrons N1. The second neutron absorption part 106 is composed of materials having quality in which a neutron capture cross-sectional area to the first neutrons N1 is 10b or below when the first neutrons N1 having the energy of average 2MeV are irradiated by the nuclear transformation aggregate 102A, and a neutron capture cross-sectional area to the second neutrons N2 is 103b or more when the second neutrons N2 having the energy equal to or less than thermal neutron energy by being decelerated by a nuclear transformation part 105 are irradiated.SELECTED DRAWING: Figure 2

Description

The present invention relates to transmutation assemblies for fast reactors for the extinction of long-lived fission products.

Among the fission products generated in the fission reactor, there are nuclides (Long-live-Fission Products: LLFP) having a long half-life. Typical examples are selenium ( ⁷⁹ Se), technetium ( ⁹⁹ Tc), palladium ( ¹⁰⁷ Pd), iodine ( ¹²⁹ I), zirconium ( ⁹³ Zr), and cesium ( ¹³⁵ Cs) in the distant future in geological disposal. It is said that the risk may become apparent.

Therefore, transmutation technology has been studied as one of the measures to reduce the volume of radioactive waste and reduce the environmental load. These transmutation techniques convert LLFP nuclides into short-lived nuclides or stable nuclides, and methods such as accelerators, nuclear reactors, and accelerator-driven subcritical reactors have been studied. , LLFP nuclides need to be separated into odd nuclides (even-odd separation), and a large current (1-10A) is required.

On the other hand, in the method using a nuclear reactor, the moderator and the LLFP nuclide are mixed or made into separate rods to form an LLFP aggregate, which is loaded into the nuclear reactor for transmutation. Fast neutrons generated in the core are decelerated by moderators in the LLFP aggregate and absorbed by LLFP nuclides for transmutation, but some of the decelerated neutrons leak to the core and output from adjacent fuel rods. Raises and produces a large output peak. Therefore, conventionally, by arranging a Tc rod using Tc, which is one nuclide of LLFP having a large resonance absorption cross section from the resonance energy region to the vicinity of the thermal energy region, in the outermost layer of the LLFP aggregate, the adjacent fuel rods are used. Studies have been conducted to suppress the output peak of. However, since Tc is an LLFP nuclide and does not exist in nature, it is necessary to reprocess spent fuel or extract and purify it from medical radioactive material waste, and a reprocessing process for this or a facility for that purpose is required separately. You will need it.

Japanese Unexamined Patent Publication No. 8-194802

The present invention has been made in view of the above circumstances, and low-energy neutrons composed of easily available substances while achieving a high transmutation rate and decelerated inside the LLFP aggregate leak to the outside of the aggregate. It is an object of the present invention to provide a transmutation aggregate capable of suppressing the above.
Since the reaction cross section of neutrons increases from the resonance energy region to the thermal energy region rather than the fast energy region, it is necessary to slow down the fast neutrons in order to carry out transmutation efficiently. For this purpose, hydrogen having a proton as a nucleus, which has the same mass as a neutron, can be used to slow down the neutron most efficiently and generate many low-energy neutrons.
On the other hand, some of the decelerated neutrons leak from the LLFP aggregate to increase the fission of adjacent fuel rods and increase the fuel rod output (output peak). Increased fuel rod output should be avoided as it raises the fuel core temperature and can lead to fuel melting. In order to obtain such a high transmutation rate, it is necessary to efficiently generate a large amount of decelerated neutrons, but this also increases the decelerated neutrons leaking to the fuel rods. Fuel rod output peaks occur. Therefore, it is an object of the present invention to provide an LLFP aggregate capable of suppressing leakage of decelerated neutrons from the LLFP aggregate by using an easily available substance while achieving a high transmutation rate. ..

In order to solve the above problems, the present invention employs the following means.

(1) The nuclear conversion aggregate according to one aspect of the present invention is arranged around the nuclear conversion unit and the nuclear conversion unit that irradiates the first neutron to perform nuclear conversion of the nuclear fission product, and the first neutron. Of these, a second neutron absorbing unit that absorbs a second neutron that is not involved in nuclear conversion is provided, and when the second neutron absorbing unit is irradiated with the first neutron having an average energy of 2 MeV. The neutron capture cross-sectional area for the first neutron is 10b (1b = ^10-24 cm ² ) or less, and the second neutron has an energy of thermal neutron energy (about 0.0253 eV) or less after being decelerated by the nuclear conversion unit. There upon irradiation, have the property of neutron capture cross section for the second neutron it becomes 10 3 ^b or more.

(2) In the transmutation aggregate according to (1) above, it is preferable that the second neutron absorber contains at least one of gadolinium, samarium, europium, and cadmium as a main component.

(3) In the transmutation aggregate according to any one of (1) or (2) above, it is preferable that the transmutation unit includes a moderator containing hydrogen together with the fission product.

(4) In the transmutation aggregate according to any one of (1) to (3) above, the second neutron absorbing portion is formed thicker as the irradiation amount of the second neutron is larger. Is preferable.

(5) In the transmutation aggregate according to any one of (1) to (4) above, the transmutation portion is a prism extending in one direction, and the second neutron absorption portion is the said. It may consist of one or more prisms extending in the same direction as the transmutation section.

(6) In the transmutation aggregate according to any one of (1) to (4) above, the transmutation unit is a prism extending in one direction, and the second neutron absorption unit is the above. It may be a plate extending along the circumferential direction of the transmutation part.

In the transmutation assembly of the present invention, a second neutron absorption unit having a property that the neutron capture cross-sectional area is small in the high energy region and the neutron capture cross-sectional area is large in the low energy region is arranged around the transmutation unit. .. Therefore, when installed around the core, high-speed first neutrons leaking from the core can be guided into the transmutation section without being absorbed by the second neutron absorption section, contributing to the nuclear reaction. Can be done. On the other hand, the low-energy second neutron that is decelerated inside the LLFP aggregate and leaks to the outside of the aggregate is absorbed frequently by the second neutron absorber. The substance of the second neutron absorber having such properties is, for example, gadolinium produced from gadolinium which is relatively abundant in the crust, and is used as a neutron poison in medical treatment and fuel for nuclear reactors. Therefore, the transmutation aggregate of the present invention is composed of easily available substances, and can suppress the leakage of decelerated neutrons without being involved in the transmutation.

It is sectional drawing of the nuclear reactor which concerns on one Embodiment of this invention. It is an enlarged view of one LLFP aggregate which constitutes the nuclear reactor of FIG. It is a graph of the neutron capture area of the LLFP nuclide. It is a graph of the nuclear reaction cross section of ^{155 Gd.} It is a graph of the nuclear reaction cross section of ^{157 Gd.} It is a graph of the nuclear reaction cross section of ^{149 Sm.} It is a graph of the nuclear reaction cross section of ^{151 Eu.} It is a graph of the nuclear reaction cross section of ^{113 Cd.} It is an enlarged view of the structure in the vicinity of the LLFP aggregate. It is a graph of the peak output of the fuel rod adjacent to the LLFP aggregate which concerns on Example 7 and Comparative Example 7. It is a graph of the output peak of the fuel rod adjacent to the LLFP assembly which concerns on Examples 8-12.

Hereinafter, the transmutation aggregate according to the embodiment to which the present invention is applied will be described in detail with reference to the drawings. In addition, in the drawings used in the following description, in order to make the features easy to understand, the featured parts may be enlarged for convenience, and the dimensional ratio of each component may not be the same as the actual one. No. Further, the materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

FIG. 1 is a cross-sectional view of a nuclear reactor (LLFP loading furnace) 100 according to an embodiment of the present invention. The nuclear reactor 100 is mainly composed of a core 101, an LLFP aggregate 102, a diameter blanket 103, and a shield 104. The core 101 is columnar, and the LLFP aggregate 102, the diameter blanket 103, and the shield 104 are configured to cover the core 101 in this order. The core 101 is composed of several regions (in the case of two regions, called an inner core and an outer core) having different degrees of enrichment of fissile material, and heat is generated by a fission chain reaction by neutrons. Some of the neutrons that do not contribute to fission leak from the core. The diameter blanket 103 is composed of a parent material of fuel, and newly generates fuel by capturing neutrons leaked from the core. The shield 104 is made of stainless steel and shields neutron rays and gamma rays from the core to the reactor vessel and the main equipment inside the reactor vessel.

FIG. 2 is an enlarged view of one LLFP aggregate 102A among the plurality of LLFP aggregates (transmutation aggregates) 102 constituting the reactor 100 of FIG. 1. The LLFP aggregate 102A mainly includes a transmutation unit 105 and a second neutron absorption unit 106.

The transmutation unit 105 is one of fission products (LLFP nuclides) having a long half-life, such as selenium (Se), technetium (Tc), palladium (Pd), iodine (I), zirconium (Zr), and cesium (Cs). , Is an aggregate of a plurality of pillars including at least one. _{By irradiating the transmutation unit 105 with a high-speed neutron (first neutron) N 1} generated in the core 101 and having an energy of 2 MeV on average, the LLFP nuclide is converted into another nuclide having a short half-life or a stable nuclide. ..

The second neutron absorption unit 106 is arranged around the transmutation unit 105. The transmutation unit 105 is a prism extending in one direction, and the second neutron absorbing unit 106 is, for example, from one or more prisms (rod-shaped members) extending in the same direction as the transmutation unit 105. Become. The layer formed by the rod-shaped member around the transmutation portion 105 may be a single layer or a plurality of layers. Further, the rod-shaped member may be a part of the inside of the layer. The shape of the second neutron absorbing unit 106 may be a plate (cylinder) extending along the circumferential direction of the transmutation unit 105 and continuously covering the outer periphery of the prismatic transmutation unit 105. .. The thickness of the second neutron absorbing section 106 is preferably equal to or greater than the mean free path of neutrons inside the second neutron absorbing section 106. Further, it is preferable that the second neutron absorption unit 106 is formed thicker as the irradiation amount of _{the second neutron N 2 is larger.}

In the second neutron absorption unit 106, when the first neutron N _{1 is irradiated, the second neutron N 2} having a neutron capture cross-sectional area of 10 b or less with respect to the first neutron N _{1 and an energy equal to or less than the thermal neutron energy is generated.} upon irradiation, have the property of neutron capture cross section of the second neutron becomes 10 3 ^b or more. That is, the second neutron absorption unit 106 has a small neutron capture cross section in the high energy region and a large neutron capture cross section in the low energy region.

By setting the neutron capture cross-sectional area for the first neutron N _{1 to 10b or less, the first neutron N 1} generated from the core 101 is not absorbed by the second neutron absorbing unit 106 and reaches the transmutation unit 105. It is possible to perform transmutation of the LLFP nuclide in the transmutation unit 105.

By the neutron capture cross section for the second neutron N ₂ and 10 3 ^b above, it is possible to absorb a second neutron (thermal neutrons) N _2, which is decelerated in the transmutation unit 105, the second neutron N ₂ Leakage to the outside of the LLFP assembly (on the core 101 side) can be suppressed. As a result, the output peak due to thermal neutrons of the fuel rods adjacent to the LLFP aggregate can be suppressed.

The second neutron absorber 106 having the above properties can be realized, for example, by a compound or alloy containing at least one of natural gadolinium, samarium, europium, and cadmium. For example, _{chemical forms such as Gd 2} O ₃ , Sm ₂ O ₃ , Eu ₂ O ₃ , and Cd, respectively. From the graph of FIG. 4-8, each element may, neutron capture cross section in the low energy region is not less 10 3 ^b or more, is considered to be suppressed thermal neutron leakage.

It is preferable that the transmutation unit 105 contains a moderator containing hydrogen together with the fission product. By including the moderator, neutrons are efficiently decelerated, so that the transmutation rate of the LLFP nuclide can be increased. As the moderator, for example, YH ₂ or the like can be used.

As described above, the transmutation aggregate 102A according to the present embodiment has a property that the neutron capture cross section is small in the high energy region and the neutron capture cross section is large in the low energy region around the transmutation unit 105. The two neutron absorption unit 106 is arranged. Therefore, when placed around the core 101 of the reactor 100, the first neutron N ₁ Fast emitted directly from the core 101 without being absorbed by the second neutron absorbing part 106, the transmutation 105 Can be led to and contribute to the nuclear reaction. _{On the other hand, the second neutron N 2} that tries to jump out to the core 101 side without contributing to the nuclear reaction can be absorbed by the second neutron absorbing unit 106 with high frequency. The material of the second neutron absorber having such properties is a stable nuclide such as gadolinium, which is relatively abundant in the crust and is used as a neutron poison in medical treatment and nuclear reactor fuel. Therefore, the transmutation aggregate 102A of the present invention is made of an easily available material, and can suppress the leakage of decelerated neutrons without being involved in the transmutation.

In the present invention, gadolinium is used as the neutron absorber. _{In commercial light water reactors, Gd 2} O ₃ with a weight ratio of 10 wt% or less is mixed with UO ₂ fuel in consideration of the decrease in thermal conductivity, and it is put into practical use as a means for controlling the excess reactivity of the core as a fuel containing gadlinear. ing. In the present invention, Gd ₂ O ₃ using natural Gd is independently loaded in the cladding tube as a rod and used. _{In the reactor, small diameter Gd 2} O ₃ rods and (Gd, Zr) Oy rods were manufactured for irradiation tests for the purpose of loading the hollow part of MOX fuel. _{Gd 2} O ₃ rods with a diameter of 1.23 mm to 1.26 mm and a density of 95.4 to 96.7% were produced by the extrusion molding method, and the chemical composition, dimensions, density, and straightness were all sufficiently satisfied with the design specifications. Has been reported to have been obtained. (PNC TN1340 95-004, Power Reactor Technique No. 96 1995.12). _{From this, it can be said that the Gd 2} O ₃ rod having a high theoretical density that can be loaded into the nuclear reactor can be sufficiently designed and manufactured, and the present invention can be realistically embodied.

In the LLFP aggregate according to the present invention, hydrogen can be loaded as a moderator inside except for the outermost circumference, so that a high transmutation rate can be achieved. As a result, by loading this LLFP aggregate into the first layer of the blanket, which has a large amount of neutron leakage from the core, the amount of LLFP waste can be significantly reduced, and the potential radioactivity in waste volume reduction and geological disposal can be reduced. Risk can be reduced.

In the transmutation of LLFP nuclides using leaked neutrons from the core, a small fast breeder reactor (about 700 MWt) with many leaked neutrons gives a high transmutation rate. Using this small fast breeder reactor, for transmutation of LLFP nuclides such as I, Pd, and Se, it is innovative that the LLFP nuclides generated by oneself can be extinguished while propagating the fuel. Allows the design of nuclear reactors.

In addition, if five small fast breeder reactors of this class are used for transmutation, three LLFP nuclides emitted by a large fast breeder reactor (about 3500 MWt) to be introduced in the future can be eliminated. In other words, a nuclear system that processes ultra-long-lived fission products while generating electricity will be constructed, and it will be possible to achieve both energy supply, reduction of waste treatment, and reduction of potential risk, which is a negative legacy for the future. Since it is possible to avoid the above, sustainable development of humankind can be expected.

Hereinafter, the effects of the present invention will be further clarified by examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

FIG. 9 is an enlarged view of the vicinity of the LLFP assembly loading unit 102. A plurality of LLFP rods 102B including LLFP nuclides and moderators are arranged in each LLFP aggregate 102A. The inside of the LLFP assembly 102A (center side of the core) is composed of a plurality of fuel assemblies 101A, and a plurality of fuel rods 101B containing plutonium are arranged in each fuel assembly 101A. There is. The outside of the LLFP aggregate 102A is composed of a plurality of blanket aggregates 103A, and a plurality of blanket rods 103B containing depleted uranium or natural uranium pellets are arranged in each blanket aggregate 103A. There is.

(Example 1)
The fuel rod 101B adjacent to the LLFP aggregate 102A has a high output because a part of the decelerated neutrons leaks from the LLFP aggregate to increase fission. In particular, a high output peak appears on the fuel rods 101C (fuel rods 101C located at the corners of the fuel assembly 101A) that are in contact with the LLFP assembly 102A on two surfaces. When the average output of the fuel rods 101B of the adjacent fuel assembly is standardized to 1.0, the fuel rod output peak shows a high value of 2.44. In order to suppress this output peak, it is necessary to block the leakage of thermal neutrons from the LLFP aggregate 102A. Therefore, a substance that absorbs thermal neutrons is loaded on the outermost pin of the LLFP aggregate 102A on the entire circumference or a part thereof. In this case, the fast neutrons from the core are not prevented from entering the transmutation portion 102B of the LLFP assembly 102A, and the thermal neutrons generated by the LLFP assembly 102A are blocked from leaking to the fuel assembly side. Must be a substance. Such substances are substances containing elements such as gadolinium, samarium, europium and cadmium.

(Example 2)
In the configuration in which the second neutron absorption unit 106 of the LLFP aggregate of the above embodiment is changed to the transmutation unit 105, the neutron behavior simulation in the reactor is performed by the cylindrical system shown in FIG. Evaluation was performed. The transmutation section had a three-layer structure in which the LLFP assembly loading section 102 and the diameter blanket 103 of FIG. 1 were two layers, and _{the volume ratio of YH 2} functioning as a moderator was set to 90% in each layer. Further, it is assumed that the LLFP aggregate is arranged from a position about 90 (cm) away.

Even in the configuration in which the LLFP assembly loaded in the LLFP assembly loading section 102 in FIG. 1 was changed to the diameter blanket assembly 103A, the output of the adjacent fuel rods was simulated.

FIG. 10 is a graph showing the simulation results in each case. The horizontal axis of the graph shows the distance from the center of the core, and the vertical axis of the graph shows the output of the fuel rods standardized in the fuel assembly (the average output of the fuel rods is standardized to 1.0).
In the former simulation result, a large output peak of 2.74 is generated in the vicinity region of the LLFP aggregate. From this result, it can be seen that the thermal neutrons decelerated at the transmutation part leak to the adjacent fuel rods and locally increase their output.
On the other hand, in the latter simulation result in which the diameter blanket is loaded, the output is smoothly reduced, and the output peak as generated in the former simulation result is not observed.

(Example 3)
_{The volume ratio of Gd 2} O ₃ contained in the second neutron absorber 106 and the irradiation time dependence of the output peak generated in the fuel rods adjacent to the LLFP aggregate were investigated. FIG. 11 is a graph showing the result. The horizontal axis of the graph shows the irradiation time of neutrons, and the vertical axis of the graph shows the output peaks of adjacent fuel rods. In either case, since the strong absorbers ¹⁵⁵ Gd and ¹⁵⁷ Gd are reduced by neutron irradiation, the output peak tends to increase in proportion to the irradiation time. However, _{it can be seen that the larger the volume ratio of Gd 2} O _3, the slower the rise, and the lower the output peak can be suppressed for a long period of time. On the other hand, _{it can be seen that even if the volume ratio of Gd 2} O ₃ is small, the output peak can be suppressed low if the irradiation time is short. From these results, it can be seen _{that an appropriate volume ratio of Gd 2} O ₃ can be selected according to the target irradiation time and allowable line output. For example, when the permissible output peak is 1.6, _{the volume ratio of Gd 2} O ₃ must be 100% in order to irradiate for 1200 days.

100 ... Reactor 101 ... Core 101A ... Fuel assembly 101B ... Fuel rod 102 ... LLFP assembly (loading section)
102A ... LLFP aggregate 102B ... LLFP rod 103 ... Diameter blanket 103A ... Blanket aggregate 103B ... Blanket rod 104 ... Shield 105 ... Transmutation unit 106 ... Two neutron absorber N ₁ ... First neutron N ₂ ... Second neutron

Claims (6)

A transmutation unit that irradiates the first neutron and performs transmutation of fission products,
It is provided with a second neutron absorption unit, which is arranged around the transmutation unit and absorbs a second neutron that is not involved in the transmutation among the first neutrons.
When the second neutron absorption unit is irradiated with the first neutron having an average energy of 2 MeV, the neutron capture cross-sectional area for the first neutron is 10b or less, and the neutron capture cross section is decelerated by the nuclear conversion unit to be less than the thermal neutron energy. of when the second neutron is irradiated with energy, transmutation aggregates neutron capture cross section for the second neutrons and having the property of the 10 3 ^b or more. The transmutation aggregate according to claim 1, wherein the second neutron absorber contains at least one of gadolinium, samarium, europium, and cadmium as a main component. The transmutation aggregate according to claim 1 or 2, wherein the transmutation unit includes a moderator containing hydrogen together with the fission product. The transmutation aggregate according to any one of claims 1 to 3, wherein the second neutron absorbing portion is formed thicker as the irradiation amount of the second neutron is larger. It is a prism in which the transmutation part extends in one direction.
The transmutation aggregate according to any one of claims 1 to 4, wherein the second neutron absorption unit is composed of one or more prisms extending in the same direction as the transmutation unit. .. It is a prism in which the transmutation part extends in one direction.
The transmutation aggregate according to any one of claims 1 to 5, wherein the second neutron absorbing unit is a plate body extending along the circumferential direction of the transmutation unit.

Images