JP2020134425A - Fuel debris separation device and method for separation - Google Patents

Abstract

To prevent the device from being damaged when fuel debris generated in a nuclear reactor is subjected to a fluorination treatment.SOLUTION: The fuel debris separation device includes: a fluorination reaction unit for reacting fuel debris and fluorine with each other; a measurement unit for measuring the temperature of the fluorination reaction unit; and a controller. The controller controls the rate of supplying fluorine to be supplied to the fluorination reaction unit so that the temperature of the fluorination reaction unit will be a predetermined value or lower.SELECTED DRAWING: Figure 1

Description

The present invention relates to a fuel debris sorting device and a sorting method.

In past nuclear power plant accidents, the loss of the core cooling function caused the fuel assembly and internal structures to melt, and the core melt containing nuclear fuel, fuel cladding, control materials, internal structures, etc. (hereinafter referred to as "fuel debris"). ".) Has occurred. If a similar accident occurs in another light water reactor or an innovative reactor such as a fast reactor, it is expected that the above fuel debris will occur.

In Japan, accurate measurement control of nuclear fuel material is required, so it is necessary to take out the generated fuel debris from the reactor, store it stably after that, and manage it appropriately.

In addition to the above substances, fuel debris contains fission products (FP), minor actinides (MA), concrete components, etc., and various substances interact with each other to create complex and inhomogeneous substances. It is assumed that the composition is as good as possible. If the nuclide content and chemical form in fuel debris are stored unclear, there are safety risks such as criticality, shielding, and measurement control. Therefore, it is desirable to stabilize the fuel debris taken out from the reactor into a manageable form.

In Non-Patent Document 1, as an example of past treatment of fuel debris, an attempt was made to experimentally treat fuel debris generated at the Three Mile Island Nuclear Power Station with a nitric acid-based acid solution, but the solubility was low. Therefore, it is stated that it is difficult to dispose of fuel debris. To date, no effective means for stabilizing fuel debris has been known.

Patent Document 1 discloses a method for treating damaged or melted nuclear fuel, which is a method for treating fuel debris into powder by halogenating the fuel debris. In the treatment method described in this document, fluorine (F) is taken as an example of halogen, and uranium (U) in fuel debris is uranium hexafluoride (UF) which is a volatile gas by a reaction represented by the following formula or the like. It changes to ₆ ) and is volatilized and separated from fuel debris.

UO ₂ + 3F ₂ = UF ₆ + O ₂
It is also described that if the powdered fuel debris residue is then oxidized, the residue can be made into a stable and nitric acid-soluble chemical form.

Non-Patent Document 2 describes a specific device configuration assumed for the above processing. That is, it is a debris treatment to which the fluoride volatilization method is applied. Fluorine has a very high reaction activity. In this document, fuel debris having various compositions is loaded into a reactor having a heating function, the temperature is raised to a predetermined temperature, and then fluorine gas is introduced into the reactor, and the difference in the vapor pressure of fluoride is determined. Fluoride such as U and plutonium (Pu), which are nuclear fuel components, is volatilized as a gas and condensed and recovered by a cold trap, etc., and zirconium (Zr), which is a fuel cladding component, and components of the structure inside the furnace. It is described that impurities such as iron (Fe) are left as solid fluoride in the reaction furnace, and that the obtained fluoride is oxidized and converted into a stable and nitrate-soluble form.

Non-Patent Document _3, UO in fluoride test using U-containing simulated debris containing 2 -ZrO ₂ massive simulated debris, solid solution, compound, even in bulk, fluoride is 99% U a reaction time of about 30 minutes It is stated that the compound volatilized and the prospect of separating most of the impurities was obtained.

Non-Patent Document 4 describes a fluorination experiment in which the highest set temperature as an experimental condition is 550 ° C. in clean up (fluorination) of Pu. The fluidized bed fluorination column used for this treatment is made of nickel (Ni).

Japanese Unexamined Patent Publication No. 2014-29319

D. W. Akers, Core Debris Chemistry and Fission Product Behavior. ACS Series 293, pp.146-167 (2008) Hitachi-GE Nuclear Energy, Inc .: 2013 Ministry of Education, Culture, Sports, Science and Technology National Issue-based R & D Promotion Project Nuclear System R & D Project, R & D Results Report on Stabilization of Fuel Debris Using Fluoride Technology (2014) March) Kuniyoshi Hoshino et al .: Development of fuel debris stabilization treatment technology using the fluorination method (20) Simulated debris fluorination test (6), Atomic Energy Society of Japan Fall 2018 IG08 Japan Atomic Energy Research Institute Reprocessing Laboratory: Development of F2 two-stage fluorination method in dry reprocessing, JAERI-M6393, February 1976

In equipment design, when deciding the material of a certain reactor, it is common to select a material that has a margin with respect to the normal operating temperature of the plant.

Regarding the example of the reaction apparatus using fluorine, in the case of Non-Patent Document 4 assuming dry reprocessing of spent fuel, the set temperature at the time of operation of the fluorination tower (reactor) of the fluidized bed is 550 ° C. .. Therefore, the fluorination tower is made of Ni, which has a margin at 550 ° C. This is no exception in the fluorination treatment of fuel debris, and it is necessary to apply a material having a margin for a certain normal operating temperature to the reactor for fluorination treatment of fuel debris.

However, even if a material having a margin is selected, it is not desirable to continue the operating state for a long time, which greatly exceeds the operating temperature assumed in advance. In the example of the fluorination tower of the fluidized bed, since the properties and composition of the spent fuel to be treated can be grasped in advance, the amount of heat of reaction generated by the fluorination reaction can also be grasped in advance. Therefore, it is possible to estimate how the temperature inside the fluorination tower changes, and the risk of equipment damage is small.

However, when the object to be treated is fuel debris, it is assumed that various substances interact with each other to form a complex and inhomogeneous composition. In addition, the properties and composition of fuel debris are greatly affected by the reactor type, fuel form, progress of accidents, and the like.

Therefore, when processing fuel debris, the composition will differ depending on the reactor type, accident conditions, etc., and the composition distribution will also occur, so it is possible that local heat generation will occur. This may deviate from the expected reactor temperature and lead to equipment damage.

Therefore, in order to prevent equipment damage in the fluorination treatment of fuel debris generated by a core meltdown accident in any nuclear reactor, it is necessary to control the temperature of the reactor so that it does not reach the temperature that leads to equipment damage. ..

An object of the present invention is to prevent damage to the apparatus during fluorination of fuel debris generated in a nuclear reactor.

The fuel debris sorting apparatus of the present invention includes a fluorination reaction unit that reacts fuel debris with fluorine, a measurement unit that measures the temperature of the fluorination reaction unit, and a control unit, and the control unit includes fluorine. The supply rate of fluorine supplied to the fluorine reaction unit is controlled so that the temperature of the reaction unit becomes equal to or lower than a predetermined temperature.

According to the present invention, it is possible to prevent the device from being damaged when the fuel debris generated in the nuclear reactor is fluorinated.

It is a schematic block diagram which shows the fuel debris sorting apparatus of this invention. It is a flow chart which shows the method of separating fuel debris of this invention. It is a schematic block diagram which shows more concretely the debris processing part of the sorting apparatus of FIG.

The present invention relates to an apparatus and method for sorting fuel debris by fluorination treatment.

Fluorine is an element with extremely high reactive activity and reacts with all substances.

Table 1 shows the calculation results of the Gibbs energy change (ΔG) in the fluorination reaction of the substances contained in the fuel debris. This calculated value is a value calculated by the thermodynamic database MALT, and if ΔG shows a negative value, the fluorination reaction proceeds.

From this table, ΔG at T = 900K and 600K shows a negative value for all the substances. Therefore, it can be seen that the fluorination reaction proceeds in all the components contained in the fuel debris.

The fluorination reaction is an exothermic reaction, and heat of reaction is generated as the fluorination reaction progresses.

Table 2 shows the two types of oxides of U, which are the main components of fuel debris that can be generated in light water reactors and fast reactors, and the heat of reaction (metal atoms at 900 K) in the fluorination reaction of metals and oxides of Fe and Zr. The change in enthalpy per mole) is shown. This calculated value is also a value calculated by the thermodynamic database MALT.

From this table, the amount of change differs depending on the compound, and the heat of reaction tends to be relatively larger for metals than for oxides. If the heat of reaction of the object to be treated increases due to the fluorination reaction, the thermal effect on the reactor increases, which may lead to damage to equipment such as the reactor.

In this treatment method, after supplying the object to be treated into the reactor, the fluorine reaction is started by introducing fluorine gas set at a certain temperature and fluorine supply rate into the reactor. When the fluorination reaction starts, heat of reaction is generated as the reaction progresses, and the object to be treated generates heat. The calorific value of the object to be treated depends not only on the metal / oxide amount ratio contained in the object to be treated, but also on the magnitude of the reaction rate.

The fuel debris sorting device according to the embodiment of the present invention will be described. The method for separating fuel debris according to the embodiment of the present invention follows the steps performed by using the separation device.

(1) The fuel debris sorting device includes a fluorination reaction unit that reacts the fuel debris with fluorine, a measurement unit that measures the temperature of the fluorination reaction unit, and a control unit. The control unit controls the supply rate of fluorine supplied to the fluorine reaction unit so that the temperature of the fluorine reaction unit becomes equal to or lower than a predetermined temperature.

(2) The fuel debris sorting device has a volatile substance recovery unit including a cold trap that condenses and recovers volatile substances generated in the fluorination reaction unit, and an oxidation that converts the recovered volatile substances into oxides. It also has a processing unit.

(3) The fluorine supply rate is controlled by adjusting the fluorine supply flow rate or the fluorine concentration.

(4) The temperature of the fluoride reaction unit is measured by a thermocouple.

(5) The rate of increase in temperature of the fluorine reaction section is detected, and the rate of supply of fluorine is adjusted based on the rate of increase.

(6) The fuel debris sorting device measures the amount of fluorine consumed and adjusts the fluorine supply rate based on the amount of consumption.

Hereinafter, examples of the present invention will be described.

FIG. 1 shows an outline of a fuel debris sorting device.

In this figure, the sorting device 100 includes a fluorination reaction unit 1 including a reactor, a volatile substance recovery unit 2 including a cold trap, a non-volatile substance recovery unit 3, an oxidation treatment unit 4, an alkaline scrubber, and the like. The off-gas processing unit 5 including the above, the measurement unit 11, and the control unit 12 are provided. Fuel debris is charged into the fluorine reaction unit 1 and heated to supply fluorine gas and an inert gas. Fuel debris is lumpy or powdery.

The valve 21 is arranged in the fluorine gas supply pipe. The valve 22 is arranged in the supply pipe of the inert gas. As the inert gas, nitrogen, argon (Ar) or the like can be used.

The fluorination reaction unit 1 is provided with a temperature sensor for measuring the temperature of the fuel debris, its support portion, or the inner wall surface or outer wall surface of the reactor. The signal from the temperature sensor is sent to the measuring unit 11. The measuring unit 11 monitors the measured value so as not to exceed a predetermined temperature.

As the temperature sensor, a thermoelectric pair or the like is used. The inside of the reactor has a highly corrosive fluorine atmosphere. Therefore, for example, a sheath thermocouple using highly corrosive Ni is suitable. Such a sheath thermocouple can be used by welding to the inner wall surface of the reactor or a tray of fuel debris provided in the reactor. The temperature of the reactor measured in this way or the temperature near the fuel debris may be used as the representative temperature (index temperature). Further, the temperature of the reactor may be measured with a thermocouple other than the sheath thermocouple embedded inside the reactor wall. Further, a radiation thermometer may be used instead of the thermocouple.

The control unit 12 receives a necessary signal from the measurement unit 11, performs a predetermined calculation, and adjusts the opening degrees of the valves 21 and 22 based on the result to increase or decrease the supply amount of the fluorine gas and the inert gas. .. As a result, the supply rate of fluorine to the fluorine reaction unit 1 is adjusted. In other words, the fluorine concentration in the fluorine reaction unit 1 is adjusted.

In this figure, the solid arrow indicates the flow of the substance, and the broken line arrow indicates the flow of the signal. That is, the sorting device 100 is roughly classified into a debris processing unit and a measurement / control unit. The debris processing unit is a part where substances actually move, and is connected by piping or the like. The debris treatment unit includes a fluorination reaction unit 1, a volatile substance recovery unit 2, a non-volatile substance recovery unit 3, an oxidation treatment unit 4, and an off-gas treatment unit 5. The measurement / control unit includes the temperature sensor of the fluoride reaction unit 1, the measurement unit 11 and the control unit 12, and is connected by a communication line for transmitting / receiving signals.

The volatile substance generated in the fluorination reaction unit 1 is sent to the volatile substance recovery unit 2, and is condensed and recovered by a cold trap. The temperature of the cold trap can be changed according to the boiling point of the object to be recovered, and fluoride and fluorine gas having higher volatility than UF ₆ can be transferred to the subsequent stage without being condensed and recovered. The highly volatile fluoride or fluorine gas transferred to the subsequent stage may be stabilized by a predetermined off-gas treatment method such as an adsorption method using an adsorbent or the like, or may be transferred to the subsequent stage in some cases. Fluorine gas or the like may be refluxed to the fluoride reaction unit 1. The condensed / recovered volatile substance is sent to the oxidation treatment unit 4.

On the other hand, the non-volatile substance remaining in the fluorination reaction unit 1 is sent to the oxidation treatment unit 4 via the non-volatile substance recovery unit 3.

The oxidation treatment unit 4 converts the recovered volatile substances and non-volatile substances into oxides in a more stable form. Since solid oxides and gas oxides are generated during the oxidation treatment, the gas oxides are sent to the off-gas treatment unit 5 and stabilized by a method such as an alkaline scrubber. Oxides of U and Pu and other solid oxides are reused or stored after undergoing predetermined treatment.

The gas oxide is treated with an alkaline scrubber or the like of the off-gas treatment unit 5.

FIG. 2 is a flow chart showing an example of a method for separating fuel debris.

As shown in this figure, in the method for separating fuel debris, the fuel debris is supplied to the reactor and heated (S110). At this time, fluorine gas and an inert gas are supplied to the reactor. Then, the temperature of the wall surface of the reactor or the fuel debris is measured (S120). The supply rates of the fluorine gas and the inert gas are adjusted so that the index temperature such as the temperature of the wall surface of the reaction furnace or the temperature of the fuel debris falls within a predetermined temperature range (S130).

It is known that the magnitude of the fluorine supply rate determines the reaction rate of the fluorine reaction. Therefore, if the fluorine supply rate is adjusted, the reaction rate can be controlled at the same time. In general, collisions between reactants are indispensable for the progress of a chemical reaction, and the larger the number of particles colliding per unit time, the higher the reaction rate. That is, in a reaction in a gas in which particles easily move, if the supply rate of the reaction gas is increased, the probability that the reactants collide with each other increases, and the reaction rate increases. The fluorine supply rate can be adjusted by adjusting the fluorine supply flow rate or the fluorine concentration, and the fluorine supply flow rate may be adjusted by adjusting the opening degree of the flow valve or the like. May be adjusted by, for example, introducing an inert gas such as Ar into the reaction furnace to dilute fluorine.

FIG. 3 shows more specifically the debris processing unit of the sorting device of FIG.

In this figure, the sorting device 300 includes a fluorine reaction furnace 31, a cold trap 32 that condenses and recovers the volatile substances generated in the fluorine reaction furnace 31, and a volatile substance recovered by the cold trap 32. An oxidation treatment unit 33 that oxidizes with steam or the like (wet gas), an oxidation treatment unit 34 that oxidizes the non-volatile residue generated in the fluoride reaction furnace 31 with steam or the like (wet gas), and a cold trap 32 as a gas. It includes an off-gas treatment unit 35 for absorbing the remaining substance into an absorbing liquid or the like. Here, the volatile substance is U, Pu, a trace amount of FP, fluoride such as boron (B), carbon (C), chlorine (Cl) and the like. The non-volatile residue is a fluoride such as Zr, FP, MA, sodium (Na) and the like.

The volatile substances recovered in the cold trap 32 are fluorides of U and Pu. On the other hand, what is not condensed and recovered by the cold trap 32 and is sent to the off-gas treatment unit 35 as a gas is fluoride such as B, C and Cl, F _{2 and the} like.

The oxides obtained in the oxidation treatment units 33 and 34 can be flexibly dealt with in all disposal cases such as long-term storage, reprocessing, and final disposal by performing a predetermined treatment. For example, U and Pu can be used again as fuel for light water reactors and the like. Other components are vitrified and finally disposed of as waste. In addition, the oxide may be stored for a long period of time in an appropriate mixed state.

In this embodiment, the measuring unit 11 of FIG. 1 detects the temperature rise rate (temperature rise rate) of the fluoride reaction unit 1 (reactor).

In the first embodiment, the temperature of the reactor is measured so that the temperature of the apparatus is not damaged. On the other hand, in this embodiment, both the temperature and the elapsed time of the reactor are measured by the measuring unit 11, and the temperature rise rate of the reactor is monitored from the elapsed time and the temperature difference.

For example, in the present invention, lumpy fuel debris is treated, but when the lumpy sample is crushed and pulverized in the process of treatment, a rapid fluorination reaction occurs due to an increase in the surface area of the object to be treated, and the reactor It is assumed that the temperature will jump over the predetermined temperature through the rapid temperature rise of the above, and the fluorination reaction condition control will not catch up and the reactor will be damaged.

Therefore, it is possible to prevent damage to the apparatus by monitoring the temperature rise rate of the reactor, predicting the future temperature rise tendency, and quickly controlling the fluorination reaction conditions by the control unit 12. On the other hand, when the measured temperature rise rate becomes smaller than expected, it is possible to carry out control that promotes the fluorination reaction by increasing the fluorine supply rate.

In this embodiment, the measurement unit 11 in FIG. 1 measures the amount of fluorine consumed and the reaction rate of the object to be treated. In the present embodiment, as in the second embodiment, the temperature jumps over a predetermined temperature due to a rapid temperature rise of the reactor, the fluorination reaction condition control cannot catch up, and the reactor is prevented from being damaged.

Specifically, the reaction rate of the object to be treated is measured by simultaneously measuring the amount of fluorine consumed and the elapsed time. For example, fuel debris, which is the object to be treated, is a substance in which metals and oxides are inhomogeneously mixed, but due to the different reaction rates of metals and oxides, rapid heat generation is generated from a certain point in time. It is also assumed that the sudden temperature rise of the reactor causes the temperature to jump over a predetermined temperature, and the fluorination reaction condition control cannot catch up and the reactor is damaged. Therefore, by monitoring the elapsed time of the fluorination reaction rate, it is possible to predict the future fluorination tendency and to control the fluorination reaction conditions as soon as possible to prevent damage to the device.

The fluorine consumption of the reaction reactor is measured, for example, by the difference in fluorine concentration at the interface (boundary portion) of the front stage (fluorine gas, inert gas supply unit) and the rear stage (volatile substance recovery unit) of the reactor. By measuring the reaction time at the same time, it is possible to obtain the fluorine consumption from the elapsed time and the concentration difference. The fluorine concentration may be measured by Fourier transform infrared spectroscopy or the like.

Further, when the reaction rate becomes smaller than the expected amount of the measured fluorine consumption, it is possible to control the promotion of the fluorine reaction by increasing the fluorine supply rate.

1: Fluorination reaction unit 2: Volatile substance recovery unit 3: Non-volatile substance recovery unit 4: Oxidation treatment unit 5: Off-gas treatment unit, 11: Measurement unit, 12: Control unit, 21, 22: Valve , 31: Fluorination reactor, 32: Cold trap, 33, 34: Oxidation treatment unit, 35: Off-gas treatment unit, 100, 300: Separation device.

Claims (11)

A fluorine reaction unit that reacts fuel debris with fluorine,
A measuring unit that measures the temperature of the fluoride reaction unit and
With a control unit
The control unit is a fuel debris sorting device that controls the supply rate of fluorine supplied to the fluorine reaction unit so that the temperature of the fluorine reaction unit becomes equal to or lower than a predetermined temperature. A volatile substance recovery unit including a cold trap that condenses and recovers volatile substances generated in the fluorination reaction unit, and
The fuel debris sorting apparatus according to claim 1, further comprising an oxidation treatment unit that converts the recovered volatile substance into an oxide. The fuel debris sorting apparatus according to claim 1, wherein the supply rate of the fluorine is controlled by adjusting the supply flow rate of the fluorine or the concentration of the fluorine. The fuel debris sorting apparatus according to claim 1, wherein the temperature of the fluoride reaction unit is measured by a thermocouple. The fuel debris sorting apparatus according to claim 1, wherein the rate of increase in temperature of the fluorine reaction unit is detected, and the rate of supply of fluorine is adjusted based on the rate of increase. The fuel debris sorting apparatus according to claim 1, wherein the consumption of fluorine is measured, and the supply rate of the fluorine is adjusted based on the consumption. The fuel debris and fluorine are reacted in the fluorine reaction section,
The temperature of the fluoride reaction part was measured and
A method for separating fuel debris, which controls the supply rate of fluorine supplied to the fluorine reaction unit so that the temperature of the fluorine reaction unit becomes equal to or lower than a predetermined temperature. The volatile substances generated in the fluoride reaction section are condensed and recovered, and then
The method for separating fuel debris according to claim 7, wherein the recovered volatile substance is converted into an oxide. The method for separating fuel debris according to claim 7, wherein the supply rate of the fluorine is controlled by adjusting the supply flow rate of the fluorine or the concentration of the fluorine. The method for separating fuel debris according to claim 7, wherein the rate of increase in temperature of the fluorine reaction unit is detected, and the rate of supply of fluorine is adjusted based on the rate of increase. The method for separating fuel debris according to claim 7, wherein the consumption of fluorine is measured, and the supply rate of the fluorine is adjusted based on the consumption.

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