Method of treating radioactive nuclear fuel cladding

Abstract

A method of treating a radioactive nuclear fuel cladding is provided. The method comprises the following steps: oxidizing the nuclear fuel cladding with oxygen-containing gas at 500-1200 DEG C to obtain an oxidized nuclear fuel cladding; and carrying out acid immersion cleaning on the oxidized nuclear fuel cladding. According to the method, the radioactive level of the nuclear fuel cladding can be reduced.

Description

Technical Field

[0001] The invention relates to the technical field of nuclear fuel cladding treatment, and in particular to a method for treating radioactive nuclear fuel cladding. Background Art

[0002] After reactor irradiation, nuclear fuel assemblies contain a large amount of fission products and transuranic products. The nuclear fuel cladding is composed of residual materials attached to the inner wall of the fuel cladding and the cladding. The residual materials include undissolved nuclear fuel uranium, plutonium and fission products. The outer wall of the cladding is a 20-30μm thick oxide layer, and the interior is dense metal zirconium. Some of the fission products (such as 3 H. 137 Cs, 94 Nb, 125 Sb, 60 Co and 241 Am) and some uranium and plutonium will migrate into the cladding. In the traditional nuclear fuel reprocessing process, the nuclear fuel assembly is first sheared and dissolved to separate the cladding from the fuel. The cladding is then rinsed, but this does not reduce the radioactivity inside the cladding. According to current processes and regulatory requirements, the rinsed cladding is compacted and directly landfilled as intermediate-level waste, which not only has high disposal costs but also poses potential harm to the environment.

[0003] In order to reduce the radioactivity level of the cladding, there are prior art reports that use HF solution to dissolve and decontaminate the cladding surface, dissolving 30-40μm of the cladding surface to reduce the radioactivity of the cladding. However, this method has the following problems: first, the dissolution process is uneven, resulting in inconsistent decontamination effects; second, this method produces a large amount of hydrogen, increasing safety risks; in addition, the use of HF solution requires high corrosion resistance of the container. Most importantly, the fission products 137 Cs, 60 Co and 94 Nb is almost evenly distributed in the entire cladding, and tritium and zirconium alloy will also form tritiated zirconium that is difficult to remove and evenly distributed in the zirconium cladding. This surface dissolution method cannot completely remove the radioactive contamination inside the cladding, so the HF surface dissolution decontamination technology has not been finally applied.

[0004] In order to improve the economic efficiency of the operation of spent fuel reprocessing plants and reduce potential harm to the environment, it is urgent to develop a method for processing radioactive nuclear fuel cladding. Summary of the invention

[0005] In view of this, the main object of the present invention is to provide a method for treating radioactive nuclear fuel cladding to reduce the radioactivity level.

[0006] To this end, the present invention provides a method for treating radioactive nuclear fuel cladding, the method comprising: oxidizing the nuclear fuel cladding with oxygen-containing gas at 500° C.-1200° C. to obtain oxidized nuclear fuel cladding; and acid leaching the oxidized nuclear fuel cladding.

[0007] In some embodiments, the oxygen-containing gas is a mixed gas of nitrogen and oxygen.

[0008] In some embodiments, the oxidation temperature is 800-1200°C, more preferably 1000-1200°C.

[0009] In some embodiments, the volume fraction of oxygen in the oxygen-containing gas is 10%-100%, preferably, the volume fraction of oxygen is 10%-90%.

[0010] In some embodiments, the oxidation time is 1 h-8 h; preferably 3 h-8 h.

[0011] In some embodiments, the acid is nitric acid.

[0012] In some embodiments, the concentration of the acid is 1 mol / L-6 mol / L.

[0013] In some embodiments, the radioactive nuclear fuel cladding is a spent UO2 fuel zirconium cladding.

[0014] In some embodiments, the spent UO2 fuel zirconium cladding includes one or more of Zr, H, Tc, Ru, U, Pu, Cs, Sr, Nb, Co, Sb and Am.

[0015] The method of the present invention firstly performs high temperature oxidation treatment on the radioactive nuclear fuel cladding, so that the high-activity volatile nuclides inside the cladding, such as 3 H is released in the gas phase, along with semivolatile fissionable elements, such as 137 Cs and 94 Part of Nb and the like is released in the form of gas phase; in addition, the radioactive nuclear fuel cladding and the fissile elements therein form oxides, and the radioactive nuclear fuel cladding is powdered due to oxidation. Further, the oxidized radioactive nuclear fuel cladding is acid leached to remove the oxides of nuclides such as U, Pu, Cs, Co, etc. in the cladding, thereby obtaining a low-radioactive oxidized nuclear fuel cladding. The method of the present invention utilizes a combined process of high-temperature oxidation and acid leaching to fully release and dissolve the fission products trapped in the zirconium cladding, and has the advantages of high purification factor, convenient centralized management of volatile radioactive fission elements, convenient classification and treatment, and few types of nuclear waste dissolution products. The treatment method of the present invention not only reduces the radioactivity level of the nuclear fuel cladding, but also reduces the potential harm to the environment. DETAILED DESCRIPTION

[0016] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0017] Throughout the specification, unless otherwise specifically stated, the terms used herein should be understood as meanings commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. In the event of a conflict, the present specification takes precedence.

[0018] It should be noted that, in the embodiments of the present invention, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a method or device including a series of elements includes not only the elements explicitly stated, but also other elements not explicitly listed, or also includes elements inherent to the implementation of the method or device. In the absence of further restrictions, an element defined by the sentence "includes a ..." does not exclude the existence of other related elements in the method or device including the element.

[0019] It should be noted that the terms "first\second\third" involved in the embodiments of the present invention are only used to distinguish similar objects, and do not represent a specific order for the objects. It is understandable that "first\second\third" can be interchanged with a specific order or sequence where permitted. It should be understood that the objects distinguished by "first\second\third" can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those described herein.

[0020] Nuclear fuel assemblies after reactor irradiation contain a large amount of fission products and transuranic products. These nuclear fuel assemblies have high radioactivity levels, high processing costs and harm to the environment. Therefore, it is necessary to reduce their radioactivity levels before processing.

[0021] In view of this, the present invention provides a method for treating radioactive nuclear fuel cladding, the method comprising: oxidizing the nuclear fuel cladding with oxygen-containing gas at 500°C-1200°C to obtain oxidized nuclear fuel cladding; and acid leaching the oxidized nuclear fuel cladding.

[0022] In the present invention, the nuclear fuel cladding is first oxidized by treating it with oxygen-containing gas at 500°C-1200°C, so that the high-activity volatile nuclides inside the cladding, such as 3H is released in the gas phase, along with semivolatile fissionable elements, such as 137 Cs is partially released in the form of gas phase; in addition, the nuclear fuel cladding and the fissile elements therein form oxides, and the nuclear fuel cladding is pulverized due to oxidation. Further, the oxidized nuclear fuel cladding is acid leached to remove the oxides of nuclides such as U, Pu, Cs, Co, etc. in the cladding, thereby obtaining a low-radioactive oxidized nuclear fuel cladding. The method of the present invention utilizes a combined process of high-temperature oxidation and acid leaching to fully release and dissolve the fission products trapped in the zirconium cladding, and has the advantages of high purification factor, convenient centralized management of volatile radioactive fission elements, convenient classification and treatment, and fewer types of nuclear waste dissolution products. The treatment method of the present invention not only reduces the radioactivity level of the nuclear fuel cladding, but also reduces the potential harm to the environment.

[0023] In the present application, radioactive nuclear fuel cladding refers to a nuclear fuel assembly that has been irradiated by a reactor. In some embodiments, the radioactive nuclear fuel cladding is a spent UO2 fuel zirconium cladding. The spent UO2 fuel zirconium cladding is composed of residual material attached to the inner wall of the nuclear fuel cladding and the zirconium cladding, which contains a large amount of fission products and transuranic products. The outer wall of the zirconium cladding is a 20-30μm thick oxide layer, and the interior is dense metallic zirconium. The residual material includes undissolved nuclear fuel uranium, plutonium and fission products, and some fission products (such as 3 H. 137 Cs, 94 Nb, 125 Sb, 60 Co and 241 In some embodiments, the spent UO2 fuel zirconium cladding includes one or more of Zr, H, Tc, Ru, U, Pu, Cs, Sb, Sr, Nb, Co and Am.

[0024] In the present invention, the term "purification factor" is an indicator to measure the degree of removal of certain radioactive impurities in the process of removing radioactive substances deposited on the inner and outer surfaces of nuclear facility structures, materials, and equipment by chemical or physical methods. In the present invention, purification factor = radioactive element concentration of the cladding before treatment / radioactive element concentration of the cladding after treatment. The larger the value of the purification factor, the higher the degree of removal of radioactive impurities. Exemplarily, the purification factor can be measured by the following method: cut the cladding under the same working condition into multiple claddings of about 1g, dissolve all the claddings before and after treatment in HF solution, determine the concentration of radioactive elements by ICP, and calculate the purification factor using the above formula.

[0025] In the present invention, the oxidation temperature is 500°C-1200°C. By making the oxidation temperature within the above range, on the one hand, it is beneficial for the radioactive nuclear fuel cladding and the radioactive fission elements therein to fully form oxides, which is beneficial for the subsequent acid leaching treatment to remove these radioactive fission elements. On the other hand, it is beneficial for the high-activity volatile fission elements inside the cladding, such as 3 H is released in the gas phase, and semi-volatile fission elements such as Cs are partially released in the gas phase. In the present invention, in the oxidation step at the above temperature, more than 99.6% of 3 H is released in the gas phase, a small amount, for example about 30% 137 Cs is released in the form of gas, and Tc, Nb, etc. are oxidized to Tc2O7 and Nb2O5 through an intermediate state to achieve final volatilization. Exemplarily, the oxidation temperature can be 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, 1200°C, or a value between the ranges formed by any two of them. In some preferred embodiments, the oxidation temperature is 800°C-1200°C, more preferably 1000°C-1200°C.

[0026] In some embodiments, the oxygen-containing gas is a mixed gas of nitrogen and oxygen. In the oxygen-containing gas, oxygen acts as an oxidant to react with metal elements in the nuclear fuel cladding to generate oxides, and nitrogen is an inert gas that does not participate in the oxidation reaction and can be used to adjust the oxygen content in the oxygen-containing gas, which is beneficial for controlling the high-temperature oxidation reaction rate as needed.

[0027] In some embodiments, in the oxygen-containing gas, the volume fraction of oxygen is 10%-100%. By making the volume fraction of oxygen in the oxygen-containing gas within the above range, it is beneficial for the high-temperature oxidation reaction to be fully carried out, and on the other hand, the rate of the high-temperature oxidation reaction can be controlled as needed. Exemplarily, in the oxygen-containing gas, the volume fraction of oxygen can be 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% or a value between the ranges composed of any two of them. In some preferred embodiments, in the oxygen-containing gas, the volume fraction of oxygen is 10%-90%.

[0028] In some embodiments, the oxidation time is 1h-8h. By making the oxidation time within the above range, it is beneficial to fully oxidize the zirconium cladding and the radioactive fission elements therein, and the zirconium cladding is fully pulverized for subsequent acid leaching steps. Exemplarily, the oxidation time can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h or a value between the ranges consisting of any two of them. In some preferred embodiments, the oxidation time is 4h-8h, more preferably 4h-6h.

[0029] In some embodiments, the acid is nitric acid. Nitric acid (HNO3) has a high coordination coefficient with nuclides such as uranium and plutonium, and can be fully dissolved under the action of nitric acid, while the completely oxidized zirconium oxide is insoluble in the nitric acid solution, thereby achieving effective separation of the oxides of radioactive elements and the oxidized zirconium cladding. In the present invention, the residual nuclides such as U, Pu, Cs, and Co in the oxidized radioactive nuclear fuel cladding exist in the form of stable oxidation states of U3O8, PuO2, Cs2O, and CoO, respectively, and these oxides have sufficient solubility in nitric acid, thereby these radioactive nuclides can be removed by nitric acid leaching, thereby achieving a reduction in the radioactivity level of the zirconium cladding.

[0030] In some embodiments, the concentration of the acid is 1mol / L-6mol / L. By making the acid concentration within the above range, it is beneficial to fully dissolve the oxides of the radioactive nuclides in the oxidized nuclear fuel cladding and provide a controllable reaction time. Those skilled in the art can adjust the concentration of the acid as needed. For example, the concentration of the acid can be 1mol / L, 2mol / L, 3mol / L, 4mol / L, 5mol / L, 6mol / L or a value between the ranges consisting of any two of them. In some preferred embodiments, the concentration of the acid is 2-6mol / L.

[0031] In the method for treating radioactive nuclear fuel cladding of the present invention, after the nuclear fuel cladding is treated by oxidation and further treated by acid leaching, the total purification factor of the nuclear fuel cladding reaches above 0.9E+7. The method of the present application has a high purification factor, which can effectively reduce the radioactivity level of the radioactive nuclear fuel cladding and reduce the harm to the environment.

[0032] The method of the present application is described in more detail below through specific examples.

[0033] Example

[0034] Example 1

[0035] Step 1) oxidizing the nuclear fuel cladding at 1200° C. for 4 hours in a pure oxygen atmosphere to obtain an oxidized nuclear fuel cladding.

[0036] Step 2) The oxidized nuclear fuel cladding is rinsed with a 6 mol / L nitric acid solution.

[0037] The ICP method was used to measure the concentration of each element in the nuclear fuel cladding before treatment, after step 1) treatment and after step 2) treatment, and the purification factor of the treatment method was calculated according to purification factor = radioactive element concentration of the cladding before treatment / radioactive element concentration of the cladding after treatment. The results are recorded in Table 1.

[0038] Embodiment 2-4

[0039] The nuclear fuel cladding was treated in a similar manner to that of Example 1, except that an oxygen-containing gas was used for oxidation treatment in the oxidation step, and the oxygen content in the oxygen-containing gas was the value described in Table 1. The purification factor was measured and the results are recorded in Table 1.

[0040] Table 1:

[0041]

[0042] Example 5

[0043] Step 1) oxidizing the nuclear fuel cladding at 1200° C. for 6 hours in a nitrogen-oxygen mixed gas atmosphere with an oxygen volume fraction of 90% to obtain an oxidized nuclear fuel cladding.

[0044] Step 2) The oxidized nuclear fuel cladding is rinsed with a 6 mol / L nitric acid solution.

[0045] The ICP method was used to measure the concentration of each element in the nuclear fuel cladding before treatment, after step 1) treatment and after step 2) treatment, and the purification factor of the treatment method was calculated according to purification factor = radioactive element concentration of the cladding before treatment / radioactive element concentration of the cladding after treatment. The results are recorded in Table 2.

[0046] Embodiment 6-8

[0047] The nuclear fuel cladding is treated in a manner similar to that of Example 5, except that, in the oxidation step, the oxidation temperature is adjusted according to the values ​​shown in Table 2.

[0048] Table 2

[0049]

[0050] Examples 9-12

[0051] The nuclear fuel cladding is treated in a manner similar to that of Example 5, except that, in the oxidation step, the oxidation time is adjusted according to the values ​​described in Table 3.

[0052] Table 3

[0053]

[0054] Examples 13-15

[0055] The nuclear fuel cladding was treated in a manner similar to that of Example 5, except that, in the oxidation step, the nitric acid concentration was adjusted according to the values ​​described in Table 4.

[0056] Table 4

[0057]

[0058] It can be seen from the above results that the method for treating radioactive nuclear fuel cladding of the present application can effectively reduce the content of radioactive nuclides, and can treat multiple radioactive nuclides, effectively improving the overall purification factor.

[0059] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for treating radioactive nuclear fuel cladding, characterized in that: The method comprises: oxidizing the nuclear fuel cladding with oxygen-containing gas at 500° C.-1200° C. to obtain an oxidized nuclear fuel cladding; and acid leaching the oxidized nuclear fuel cladding.

2. The method according to claim 1, wherein: The oxygen-containing gas is a mixed gas of nitrogen and oxygen.

3. The method according to claim 1 or 2, wherein: The oxidation temperature is 800-1200°C, more preferably 1000-1200°C.

4. The method according to claim 1 or 2, wherein: In the oxygen-containing gas, the volume fraction of oxygen is 10%-100%, preferably, the volume fraction of oxygen is 10%-90%.

5. The method according to claim 1 or 2, wherein: The oxidation time is 1 h-8 h, preferably 3-8 h.

6. The method according to claim 1, wherein: The acid is nitric acid.

7. The method according to claim 1, wherein: The concentration of the acid is 1 mol / L-6 mol / L.

8. The method according to claim 1, wherein: The radioactive nuclear fuel cladding is a spent UO2 fuel zirconium cladding.

9. The method according to claim 8, wherein: The spent UO2 fuel zirconium cladding includes one or more of Zr, H, Tc, Ru, U, Pu, Cs, Sr, Nb, Co, Sb and Am.

Images