JP2017156283A - Neutron absorber, and criticality accident prevention method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To input neutron absorbing substance securely into the vicinities of fuel debris thereby to prevent criticality accident by reducing reactivity during operation to take out the fuel debris.SOLUTION: A neutron absorber uses solid-state depleted uranium as neutron absorbing substance for absorbing neutrons generated in fuel debris. The neutron absorbing substance may also contain substance smaller in neutron absorbing cross section than elements or radionuclides used in controlling the reactivity of the reactor but equal to or larger in this respect than other elements except depleted uranium.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a neutron absorber for preventing a criticality accident during a fuel debris retrieval operation and a method for preventing the criticality accident.

  At the Fukushima Daiichi Nuclear Power Station, fuel debris (Debris), which is a solid substance containing fissile material melted and produced in the accident, will be taken out. When removing fuel debris, it is important not to cause an accident that reaches or exceeds the criticality (hereinafter referred to as a criticality accident). Alternatively, it is important to quickly return to the subcritical state even if the critical state is reached.

  Prompt criticality is a state in which the fission chain reaction is dominated by neutrons (prompt neutrons) that occur almost simultaneously during fission. In this state, the change of the event progresses very quickly, human control does not reach, and it becomes an explosive reaction, and enormous energy and radiation are released by the fission reaction in a short time.

  In order to prevent such a prompt critical state from occurring, a technique for estimating the burnup degree of fuel debris and a technique for estimating the subcriticality degree of fuel debris are known. In addition, it has been considered to introduce a neutron absorber into the area of the fuel debris to be handled during the fuel debris retrieval operation.

A typical neutron absorber is a substance containing boron (B). Natural boron (B) is composed of boron isotope ¹⁰ B at a ratio of 19.9 atom% and ¹¹ B at a ratio of 80.1 atom%. Of these, the cross-sectional area of ¹⁰ B neutron absorption is large. That is, the absorption cross section of ¹⁰ B thermal neutrons is about 3800 barn (= 3800 × 10 ⁻²⁴ cm ² ), and about 760 barn with natural boron containing about 20% of ¹⁰ B.

Since the fission cross section of uranium 235 ( ²³⁵ U) for thermal neutrons is about 580 barn, natural boron is more capable of reacting with neutrons than ²³⁵ U. That is, as used in control materials as neutron absorbers during normal operation, boron-containing substances inhibit the ²³⁵ U fission reaction. Therefore, the substance containing boron serves to prevent the fission reaction from reaching a critical state, and to reduce and stop the fission reaction in the event of a critical accident.

Japanese Patent Laying-Open No. 2015-227817 JP, 2006-024154, A

Boron is the most commonly used neutron absorber in nuclear reactor facilities. Typical substances containing boron include boron carbide (B ₄ C) and boric acid (H ₃ B0 ₃ ). Of these, B ₄ C (boron carbide) is often used.

There are plans to use this boron compound as a neutron absorber, including boric acid, at the time of fuel debris retrieval at the Fukushima Daiichi Nuclear Power Station.
(1) Boron carbide has a relatively low specific gravity. Although boron carbide is heavier than water, the specific gravity is about 2.5, and it is not always clear whether debris fuel can be reached quickly after injection.
(2) There is no denying the possibility of hoisting during drilling when removing fuel.
(3) It is unclear whether the neutron absorber moves in the same direction when the debris collapses.
(4) It is unclear whether the neutron absorber will enter the cracks of the debris quickly.
(5) Boron carbide is very hard, and there is a risk of damaging equipment such as a drill when touched by a drill or the like.
(6) If it changes to boric acid etc., the chemical influence on an apparatus and material is not necessarily preferable.

In addition, when boric acid is used, there are the following problems.
(1) The solubility of boric acid in water is limited, and the concentration of boric acid that can be used is limited in terms of chemical reaction with peripheral devices.
(2) Although boric acid is a very weak acid, it may still react undesirably with a chemical reaction to the material of the device.
(3) When it is necessary to inject boric acid into the work area (underwater) during the debris retrieval operation, in order to ensure time responsiveness as an emergency response, a high-pressure injection device for increasing the injection speed, etc. Special equipment is considered necessary.

As for neutron absorbers that are urgently injected to prevent criticality accidents in the fuel debris retrieval operation at the Fukushima Daiichi Nuclear Power Station, conventional neutron absorber materials (hereinafter referred to as normal neutron absorbers) are as described above. There are the following problems.
(1) Since the density is not necessarily sufficiently high, it is unclear whether or not the molten fuel portion can be reached quickly in time when it is introduced from the top away from the work position.
(2) In the case of a liquid such as boric acid, additional equipment such as a high-pressure injection device is required.
(3) There is a risk of deteriorating environmental chemical properties such as water quality where fuel debris is present.
(4) There is a risk of damaging the device when the molten fuel is subsequently taken out.

  In view of the above situation, the embodiment of the present invention is to ensure that a neutron absorber is introduced in the vicinity of the fuel debris to reduce the reactivity in order to prevent a criticality accident during the fuel debris retrieval operation. Objective.

  In order to achieve the above object, the neutron absorber according to the present embodiment is characterized by using solid deteriorated uranium as a neutron absorbing material that absorbs neutrons generated in fuel debris.

  In addition, the criticality accident prevention method according to the present embodiment includes a fuel debris retrieval step for extracting a part of fuel debris, and a neutron absorber that inputs a neutron absorber to the fuel debris retrieval location after the fuel debris retrieval step After the injection step and the neutron absorber injection step, it is determined whether or not the neutron flux level in the vicinity of the fuel debris has sufficiently decreased. If it is determined that the neutron absorber level has not sufficiently decreased, the neutron absorber input And a neutron absorber effect determination step that repeats the steps below.

  According to the embodiment of the present invention, during the fuel debris retrieval operation, it is possible to reliably introduce the neutron absorber in the vicinity of the fuel debris to reduce the reactivity in order to prevent a criticality accident.

It is an external view which shows the case where the neutron absorber which concerns on embodiment is a granular material. It is sectional drawing which shows the structure of the granule of the neutron absorber which concerns on embodiment. It is sectional drawing which shows the structure of the modification of the granule of the neutron absorber which concerns on embodiment. It is the table | surface which compared the characteristic of each nuclide. It is a graph which shows notionally the neutron energy dependence characteristic of the neutron absorption cross section of uranium 238. It is a graph which shows the example of the neutron energy spectrum of BWR. It is a flowchart which shows the procedure of the criticality accident prevention method which concerns on embodiment.

  Hereinafter, a criticality accident preventing neutron absorber and a criticality accident preventing method according to an embodiment of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

  FIG. 1 is an external view showing a case where the neutron absorber according to the embodiment is a granular body. FIG. 2 is a cross-sectional view illustrating a configuration of a neutron absorber granule according to the embodiment.

  The neutron absorber 1 is granular, and a plurality of granular bodies 10 are stored in a storage bag 12. In the present embodiment, as a main component of the neutron absorber 1, a neutron absorbing material that is a solid having a large specific gravity and has a certain amount of neutron absorption cross section will be described as an example. Hereinafter, such a neutron absorbing material is referred to as a fast sinking neutron absorbing material. The specific gravity of the neutron absorbing material for high-speed settlement is assumed to exceed 10.

  Here, having a certain neutron absorption cross section means that, for example, boron (B) or cadmium (used for control rods or liquid control materials for reactor control or emergency reactor shutdown, etc. Elements or nuclides used for reactor reactivity control, such as elements or nuclides such as Cd), or elements or nuclides such as gadolinium (Gd) used for reactivity control poisons during fuel combustion This means that the neutron absorption cross section is small compared to, but the neutron absorption cross section is large compared to other elements.

Specifically, the fast sinking neutron absorbing material is depleted uranium, that is, uranium having a small proportion of uranium 235 and a large proportion of uranium 238 ( ²³⁸ U) compared to natural uranium. For example, uranium oxide can be used as the uranium of the neutron absorbing material for fast settlement.

  Here, the granular material 10 is substantially spherical, for example. There are a plurality of types of the diameters of the granular material 10, and therefore, when the granular material 10 is stored in the storage bag 12, the gaps between the large granular material 10 are filled with the small granular material 10. . For this reason, compared with the case of only the thing of the same particle size, the filling rate is large. The particle size of the granular material 10 is distributed from 1 to several tens of mm.

  The storage bag 12 may be, for example, a metal or synthetic fiber net. The mesh size is set smaller than the minimum particle size.

  FIG. 3 is a cross-sectional view illustrating a configuration of a modified example of the granular material of the neutron absorber according to the embodiment. In this variation, the uranium is either metal uranium, uranium carbide, or uranium nitride. Compared with the case of uranium oxide, the surface of the neutron absorber 1 is covered with the covering portion 11 because there is a possibility of reaction with the coolant around the fuel debris. For example, the covering portion 11 may be a synthetic resin.

  It is considered that the fuel debris is taken out by a method such as drilling a hole in the fuel debris, rolling the surface, or widening the hole. For example, by drilling a cylindrical shape with a drill, holes are formed in the remaining fuel debris, and the surrounding coolant in which the fuel debris is immersed penetrates to increase the neutron moderation effect and increase the reactivity. To increase. In this case, it is effective to put a neutron absorber into the formed hole.

  Although the case where the granular material 10 is a spherical shape has been shown, for example, a rod-like shape, a rectangular parallelepiped shape, or the like may be used to insert into the formed hole. Also, the particle size is not limited to the range of 1 to several tens of millimeters, but by setting the optimum particle size based on the volume, caliber, depth, etc., excavated at one time when collecting fuel debris. Good.

  Moreover, when taking out the surface of the fuel debris by rolling, it is also effective to insert the neutron absorber 1 so that the upper surface of the fuel debris is covered with the neutron absorber 1. In this case, the neutron absorber 1 may be plate-shaped.

  FIG. 4 is a table comparing the characteristics of each nuclide. The fast sinking neutron absorbing material used for the neutron absorber 1 has a specific gravity exceeding 10 in order to reliably reach the fuel debris, and has a certain neutron absorption cross section to reduce the reactivity in the fuel debris. There is a need. Boron and cadmium have been used as neutron absorbers (hereinafter referred to as normal neutron absorbers) that have been used to control the reactivity of nuclear reactors from the standpoint of neutron absorption cross-sections and do not contain fissile materials In addition, gadolinium and tungsten (W) and lead (Pb) are shown as substances having a large specific gravity in order to surely sink. For comparison, uranium (U) is also shown.

Boron has a neutron absorption cross section of about 760 barn when ¹⁰ B is 20%. In the case of B ₄ C, the melting point is about 2450 ° C. and the specific gravity is about 2.5.

  Cadmium has a melting point of about 321 ° C., a specific gravity of about 8.6, and a neutron absorption cross section of about 2450 barn.

Gadolinium in the form of gadolinia, melting point of about 1312 ° C., iron and comparable specific gravity of about 7.4, the neutron absorption cross ^section, about 60,900barn in the case of ^{155 Gd,} about 254 in the case of ¹⁵⁷ Gd, 000 barn. Slightly water-soluble and easily soluble in hydrochloric acid. Natural gadolinium is composed of an isotope ¹⁵⁵ Gd having a large neutron absorption cross section of 14.80 atom% and ¹⁵⁷ Gd of 15.65 atom%. The absorption cross section of thermal neutrons is about 60,900 barn for ¹⁵⁵ Gd and about 254,000 barn for ¹⁵⁷ Gd. Natural gadolinium has a thermal neutron absorption cross section of about 46,000 barn. This value is natural boron containing about 20% of ¹⁰ B, and is extremely large compared to about 760 barn. However, boron absorbs neutrons with high energy other than thermal neutrons quite well, so gadolinium oxide generally absorbs neutrons. The overall ability to absorb neutrons is slightly inferior to boron carbide.

  Tungsten is a metal and has a melting point of about 3422 ° C. and a specific gravity of about 17-19.

  Lead is a metal having a melting point of about 328 ° C. and a specific gravity of about 11.3.

  Uranium is a metal, has a melting point of about 1132 ° C., a specific gravity of about 17 to 19, and a neutron absorption cross section of about 280 barn (however, resonance integration).

  As described above, from the viewpoint of specific gravity and neutron absorption cross section, boron, cadmium, and gadolinium have large neutron absorption cross sections, but the specific gravity is less than 10. Tungsten and lead have a specific gravity exceeding 10, but do not become neutron absorbers. In other words, the specific gravity of the normal neutron absorber is less than 10.

  On the other hand, uranium constituting the neutron absorber 1 has a specific gravity of about 17 to 19 and a neutron absorption cross section of about 280 barn, which satisfies both conditions of specific gravity and neutron absorption cross section.

FIG. 5 is a graph conceptually showing the neutron energy dependence characteristic of the neutron absorption cross section of uranium 238 ( ²³⁸ U). The horizontal axis represents neutron energy E (eV), and the vertical axis represents neutron absorption cross section (barn).

In the region where the neutron energy is about 10 eV or less, the characteristic is approximately proportional to E ^−1/2 , that is, approximately proportional to 1 / v, where neutron velocity is v.

In the region where the neutron energy is about 10 eV to about 10 ⁴ eV, there are many resonance absorption peaks, and the peak value reaches 10 ⁴ barn.

  FIG. 6 is a graph showing an example of a neutron energy spectrum of BWR. In the case of the neutron energy spectrum around the fuel debris, it is considered that the ratio of the cooling water is larger than this. Therefore, it can be considered that the ratio of the lower neutron energy is larger than the energy spectrum shown in FIG. 6, that is, the neutron spectrum is shifted to the lower energy side.

Fission generates neutrons on the order of 10 ⁶ eV to 10 ⁷ eV, and the generated neutrons are decelerated to thermal equilibrium energy. The energy in the thermal equilibrium state is about 0.025 eV at room temperature. In this deceleration process, the energy in the region of about 10 eV to about 10 ⁴ eV, that is, the resonance absorption region passes, so that neutrons are mainly absorbed by ²³⁸ U in this energy region (neutron absorption cross section is about 280 barn).

  FIG. 7 is a flowchart showing the procedure of the criticality accident prevention method according to the embodiment.

  In order to take out the fuel debris, a neutron absorber is usually put on the fuel debris in advance. Then, a part of the fuel debris is removed and taken out with a drill, a turning jig, or the like (step S01).

  As a result, when the cooling water flows into the part where the fuel debris is partially removed and the neutron moderating effect increases, the reactivity increases. Increasing the reactivity increases the output of the neutron detector that monitors the status of the reactivity in the fuel debris, that is, the neutron flux level. For this reason, a normal neutron absorber is put into a part where fuel debris is partially taken out (step S02). Here, the normal neutron absorber is introduced by a method of dropping the normal neutron absorber from above the cooling water in which the fuel debris is immersed or from the upper part in the cooling water. Or you may carry out by transferring with the guide pipe etc. to the vicinity of fuel debris.

  As a result of adding the normal neutron absorber, the neutron flux level decreases, but it is determined whether or not the neutron flux level has been sufficiently reduced (step S03). When it is determined that the neutron flux level is not sufficiently lowered (step S03 NO), the neutron absorber 1 is introduced (step S04). Here, the neutron absorber 1 is introduced by a method of dropping from above the cooling water in which the fuel debris is immersed or from the upper part in the cooling water. Or you may carry out by transferring with the guide pipe etc. to the vicinity of fuel debris.

  By introducing the neutron absorber 1, a neutron absorption effect is added and a diluting effect of the fissile material in the fuel debris is added, and the reactivity is lowered. As a result, it is determined whether the neutron flux level has sufficiently decreased (step S05). When it is determined that the neutron flux level has not been sufficiently lowered (NO in step S05), step S04 and subsequent steps are repeated.

  When it is determined that the neutron flux level is sufficiently lowered (step S05 YES), it is determined whether or not the fuel debris removal is completed (step S06). Also, if it is determined in step S03 that the neutron level has sufficiently decreased (YES in step S03), it is determined whether or not the fuel debris removal has been completed (step S06).

  If it is determined that the fuel debris removal has not been completed (NO in step S06), steps S01 and after are repeated. If it is determined that the fuel debris removal has been completed (step S06 YES), the step is terminated.

  In the above procedure, the case where the neutron absorber 1 is used when the normal neutron absorber is first introduced and the effect is not sufficient is shown, but the present invention is not limited to this. For example, the neutron absorber 1 may be used from the beginning without using a normal neutron absorber.

As described above, the neutron absorber 1 according to the present embodiment has the following effects.
(1) In order to prevent a criticality accident, a neutron absorber can be reliably urgently charged.
(2) Depleted uranium is softer than boron compounds and does not hurt work tools and tools.
(3) There is no fear of rolling up due to water flow during the debris retrieval operation.
(4) Even if the debris collapses or moves, it is easy to follow the movement of the debris because it is heavy.
(5) Large shielding effect of radiation such as gamma rays.
(6) Since it is an inorganic substance, no chemical change other than oxidation occurs. What is necessary is just to surface-treat against oxidation.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. For example, in the embodiment, the case where the neutron absorbing material of the neutron absorber is a deteriorated uranium alone is shown, but the present invention is not limited to this. It can also be combined with depleted uranium and materials used for reactor reactivity control.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Neutron absorber, 10 ... Granule, 11 ... Covering part, 12 ... Storage bag

Claims (10)

  A neutron absorber characterized by using solid depleted uranium as a neutron absorbing material that absorbs neutrons generated in fuel debris.   The neutron-absorbing material is small compared to the elements or nuclides used for reactor reactivity control, but has a large neutron absorption cross-section or a neutron absorption cross-section larger than that of other elements excluding the depleted uranium. The neutron absorber according to claim 1, comprising a substance having   The neutron absorber according to claim 1 or 2, wherein the neutron absorber has a specific gravity exceeding 10.   The neutron absorber according to any one of claims 1 to 3, wherein the neutron absorbing material is at least one of a metal, an oxide, a carbide, or a nitride.   The neutron absorbing material is a metal, carbide, or nitride, and further includes a covering portion that covers the surface of the neutron absorbing material in order to eliminate the possibility of a reaction between the neutron absorbing material and a surrounding coolant. The neutron absorber according to any one of claims 1 to 3, wherein the neutron absorber is characterized by the following.   The neutron absorber according to any one of claims 1 to 5, wherein the neutron absorber is formed into a plurality of grains.   The neutron absorber according to claim 6, wherein the neutron absorber is stored in a net-like storage bag.   The neutron absorber according to claim 6 or 7, comprising a plurality of types having different particle sizes. A fuel debris removal step for removing a portion of the fuel debris;
A neutron absorber injection step of introducing a neutron absorber into the fuel debris extraction site after the fuel debris extraction step;
After the neutron absorber injection step, it is determined whether or not the neutron flux level in the vicinity of the fuel debris has sufficiently decreased. If it is determined that the neutron absorber level has not sufficiently decreased, the steps after the neutron absorber injection step are repeated. Neutron absorber effect determination step;
A method for preventing a criticality accident characterized by comprising: After the fuel debris retrieval step,
A normal neutron absorbing material charging step of charging a normal neutron absorbing material into the fuel debris removal point;
After the normal neutron absorber injection step, determine whether or not the neutron flux level in the vicinity of the fuel debris has sufficiently decreased. If it is determined that the neutron flux level has not sufficiently decreased, proceed to the neutron absorber injection step. A normal neutron absorber effect determination step,
The method for preventing a criticality accident according to claim 9, further comprising:

Images