Gray control rod having a neutron absorber comprising terbium and dysprosium

Abstract

A gray control rod having a neutron absorber comprising terbium and dysprosium is provided. The neutron absorber comprises at least one first component and at least one second component, the reactivity worth of the first component increases as the service time of the neutron absorber increases, the reactivity worth of the second component decreases as the service time of the neutron absorber increases; the reactivity worth of the neutron absorber varying no more than 15% within the service time of the neutron absorber. By using the first component and the second component to form the neutron absorber, and adjusting the proportion of the respective components in the neutron absorber, the neutron absorber having a substantially planar reactivity worth loss characteristic can be obtained. The gray control rod and the assembly having required reactivity controlling ability can be obtained by increasing or decreasing the material dosage of the neutron absorber.

Claims

1. A neutron absorber of a gray control rod, comprising a first absorber material and a second absorber material, wherein reactivity worth of the first absorber material increases as service time of the neutron absorber increases, and reactivity worth of the second absorber material decreases as the service time of the neutron absorber increases; and reactivity worth of the neutron absorber varies no more than 15% within the service time of the neutron absorber; wherein the first absorber material is metal terbium, or a compound of terbium, or an alloy comprising terbium; and the second absorber material is metal dysprosium, or a compound of dysprosium, or an alloy comprising dysprosium; wherein the first absorber material is metal terbium, terbium oxide, terbium titanate, or terbium alloy; wherein the neutron absorber is terbium-dysprosium alloy, sinter of mixture of terbium oxide and dysprosium oxide, or sinter of mixture of dysprosium titanate and terbium titanate; wherein the neutron absorber is a cylinder with diameter of D, where 1.0 mmD8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where 0.0688D+0.6388x0.0026D+0.8626; the reactivity worth of the neutron absorber varies no more than 10% within the service time of the neutron absorber.

2. The neutron absorber of the gray control rod as claimed in claim 1, wherein the neutron absorber is a cylinder with diameter of D and the mass fraction of element terbium in the neutron absorber is x, where 0.0571D+0.7371x0.0039D+0.7261; the reactivity worth of the neutron absorber varies no more than 5% within the service time of the neutron absorber.

3. The neutron absorber of the gray control rod as claimed in claim 1, wherein the neutron absorber is a cylinder with diameter of D, where 1.3 mmD3.3 mm.

4. The neutron absorber of the gray control rod as claimed in claim 3, wherein the neutron absorber is a cylinder with diameter of D, where 1.8 mmD3.0 mm.

5. The neutron absorber of the gray control rod as claimed in claim 4, wherein the neutron absorber is a cylinder with diameter of D, where D=2 mm; wherein the mass fraction of element terbium in the neutron absorber is x, where x=70%; the reactivity worth of the neutron absorber varies no more than 2.8% within the service time of the neutron absorber.

6. A gray control rod, comprising a cylindrical cladding tube, an upper end plug and a lower end plug for sealing two ends of the cladding tube, a neutron absorber being encapsulated in the cladding tube, wherein the neutron absorber comprises a first absorber material and a second absorber material, reactivity worth of the first absorber material increases as service time of the neutron absorber increases, reactivity worth of the second absorber material decreases as the service time of the neutron absorber increases; and reactivity worth of the neutron absorber varies no more than 15% within the service time of the neutron absorber; wherein the first absorber material is metal terbium, or a compound of terbium, or an alloy comprising terbium; and the second absorber material is metal dysprosium, or a compound of dysprosium, or an alloy comprising dysprosium.

7. The gray control rod as claimed in claim 6, wherein the neutron absorber is terbium-dysprosium alloy, sinter of mixture of terbium oxide and dysprosium oxide, or sinter of mixture of dysprosium titanate and terbium titanate.

8. The gray control rod as claimed in claim 7, wherein the neutron absorber is a cylinder with diameter of D, where 1.0 mmD8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where 0.0688D+0.6388x0.0026D+0.8626; the reactivity worth of the neutron absorber varies no more than 10% within the service time of the neutron absorber.

9. The gray control rod as claimed in claim 8, wherein the neutron absorber is a cylinder with diameter of D and the mass fraction of element terbium in the neutron absorber is x, where 0.0571D+0.7371x0.0039D+0.7261; the reactivity worth of the neutron absorber varies no more than 5% within the service time of the neutron absorber.

10. The gray control rod as claimed in claim 8, wherein the neutron absorber is a cylinder with diameter of D, where 1.3 mmD3.3 mm.

11. The gray control rod as claimed in claim 10, wherein the neutron absorber is a cylinder with diameter of D, where 1.8 mmD3.0 mm.

12. The gray control rod as claimed in claim 11, wherein the neutron absorber is a cylinder with diameter of D, where D=2 mm; wherein the mass fraction of element terbium in the neutron absorber is x, where x=70%; the reactivity worth of the neutron absorber varies no more than 2.8% within the service time of the neutron absorber.

13. A gray control rod assembly, comprising a plurality of gray control rods, each gray control rod comprising a cylindrical cladding tube, an upper end plug and a lower end plug for sealing two ends of the cladding tube, a neutron absorber being encapsulated in the cladding tube, wherein the neutron absorber comprises a first absorber material and a second absorber material, reactivity worth of the first absorber material increases as service time of the neutron absorber increases, reactivity worth of the second absorber material decreases as the service time of the neutron absorber increases; and reactivity worth of the neutron absorber varies no more than 15% within the service time of the neutron absorber; wherein the first absorber material is metal terbium, or a compound of terbium, or an alloy comprising terbium; and the second absorber material is metal dysprosium, or a compound of dysprosium, or an alloy comprising dysprosium.

14. The gray control rod assembly as claimed in claim 13, wherein the neutron absorber is terbium-dysprosium alloy, sinter of mixture of terbium oxide and dysprosium oxide, or sinter of mixture of dysprosium titanate and terbium titanate; the neutron absorber is a cylinder with diameter of D, where 1.0 mmD8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where 0.0688D+0.6388x0.0026D+0.8626; the reactivity worth of the neutron absorber varies no more than 10% within the service time of the neutron absorber.

15. The gray control rod assembly as claimed in claim 14, wherein the neutron absorber is a cylinder with diameter of D and the mass fraction of element terbium in the neutron absorber is x, where 0.0571D+0.7371x0.0039D+0.7261; the reactivity worth of the neutron absorber varies no more than 5% within the service time of the neutron absorber.

16. The gray control rod assembly as claimed in claim 14, wherein the neutron absorber is a cylinder with diameter of D, where 1.3 mmD3.3 mm.

17. The gray control rod assembly as claimed in claim 16, wherein the neutron absorber is a cylinder with diameter of D, where 1.8 mmD3.0 mm.

18. The gray control rod assembly as claimed in claim 17, wherein the neutron absorber is a cylinder with diameter of D, where D=2 mm; wherein the mass fraction of element terbium in the neutron absorber is x, where x=70%, the reactivity worth of the neutron absorber varies no more than 2.8% within the service time of the neutron absorber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the relationship curves of reactivity worth with respect to service time for the gray control rod assemblies using pure Hf rod, pure Dy rod and AgInCd alloy rod with standard diameter as their neutron absorbers. The reactivity worth showed in the figure are relative values compared with the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber.

(2) FIG. 2 shows the relationship curves of reactivity worth with respect to service time for the gray control rod assemblies using tungsten rod, Dy-50Tb alloy rod and (AgInCd)Tb alloy rod as their neutron absorbers. The reactivity worth showed in the figure are relative values compared with the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber.

(3) FIG. 3 shows the relationship curves of reactivity worth with respect to service time for the gray control rod assemblies using DyTb alloy rod with different diameter and mass fraction of Tb as their neutron absorbers. The reactivity worth showed in the figure are relative values compared with the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber.

(4) FIG. 4 shows the relationship curves of amplitude of variation with respect to mass fraction of Tb for the gray control rod assemblies using DyTb alloy rod with diameter of 1.0 mm and 8.7 mm as their neutron absorbers, the amplitude of variation is the amplitude of variation of the reactivity worth relative to the service time.

(5) FIG. 5 shows the value range of the diameter of DyTb alloy rod and mass fraction of Tb of the DyTb alloy rod which serves as the neutron absorber, when its amplitude of variation of reactivity worth with respect to service time is no more than 5% and 10%.

(6) FIG. 6 shows the relationship curve of reactivity worth with respect to diameter of the DyTb alloy rod for the gray control rod assembly using DyTb alloy rod as the neutron absorber. The reactivity worth showed in the figure are relative values compared with the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber.

(7) FIG. 7 is a schematic view of a neutron absorber of a gray control rod in accordance with the invention.

(8) FIG. 8 is a partially sectioned elevational view of a gray control rod in accordance with the invention.

(9) FIG. 9 is a partially sectioned elevational view of a gray control rod assembly in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) In the present invention, using DyTb alloy rod as neutron absorber 1 has been analyzed. In particular, for the situation of terbium with different mass percentage (30%, 50%, 68%, 70% and 90%) and DyTb alloy rod having different diameters (1 mm, 2 mm, 3 mm and 8.7 mm), the reactivity worth of the gray control rod assembly using DyTb alloy rod as neutron absorber 1 has been calculated at different service time. The calculated result has been shown in FIG. 3. In the figure, Dy-50Th (1.0 mm) represents dysprosium terbium alloy rod with Tb mass fraction of 50% and the diameter of 1.0 mm. Representation of other alloy rods is the same and will not be described repeatedly here.

(11) The figure shows the relationship curves of reactivity worth with respect to service time for the gray control rod assemblies using DyTb alloy rods with different diameter and mass fraction of Tb as their neutron absorbers. The reactivity worth showed in the figure is a relative value compared with the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber. That is to say, the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber is 1.0. The following reactivity worth (relative value), which will not be specifically described, and the relative reactivity worth are all relative to the initial reactivity worth of the gray control rod assembly using AgInCd alloy rod with standard diameter as the neutron absorber. The service time is not longer than 20 years, since the neutron absorber is encapsulated in a cladding tube of the gray control rod, and the gray control rod is a part of the gray control rod assembly, the service time can be the service time of the neutron absorber, and can also be the service time of the gray control rod or the gray control rod assembly.

(12) The ratio of the difference between the maximum and the minimum value to the minimum value (i.e., the ratio of the maximum value to the minimum value minus 1) of the reactivity worth of the gray control rod assembly during the whole lifetime (20 years) is called the amplitude of variation of the reactivity worth relative to the service time (amplitude of variation or amplitude of variation of the reactivity worth for short). In general, when the amplitude of variation is less than 10%, the variation of the reactivity worth will not affect the operating mode of mechanical compensation strategy, and will not significantly increase the risk of PCI. Calculating the amplitude of variation of the reactivity worth of the gray control rod assembly using DyTb alloy rod with different diameter and mass fraction of Tb as the neutron absorbers shown in FIG. 3, the relationship curves of the amplitude of variation of reactivity worth with respect to the mass fraction of Tb can be fitted and obtained for the gray control rod assemblies using DyTb alloy rods with diameter of 1.0 mm and 8.7 mm as the neutron absorbers as shown in FIG. 4.

(13) The following conclusions can be derived from the fitting curves of FIG. 4: for DyTb alloy rod of 1 mm, the amplitude of variation is no more than 10% when mass fraction of Tb is between 57 to 86%, for DyTb alloy rod of 8.7 mm, the amplitude of variation is no more than 10% when mass fraction of Tb is between 4 to 84%. That is, an amplitude of variation of no more than 10% can be obtained by combining the composition (the mass fraction of Tb is x) which is in the region enclosed by the solid squares and lines in FIG. 5 and the diameter D (unit: mm) of the DyTb alloy rod. The region can also be expressed by the formula as: 0.0688D+0.6388x0.0026D+0.8626, 1.0 mmD8.7 mm.

(14) The following preferred conclusions can be derived from the fitting curves from FIG. 4: for DyTb alloy rod of 1 mm, the amplitude of variation is no more than 5% when mass fraction of Tb is between 68 to 73%, for DyTb alloy rod of 8.7 mm, the amplitude of variation is no more than 5% when mass fraction of Tb is between 24 to 76%. That is, an amplitude of variation of no more than 5% can be obtained by combining the composition (the mass fraction of Tb is x) which is in the region enclosed by the solid circles and lines in FIG. 5 and the diameter D (unit: mm) of the DyTb alloy rod. The region can also be expressed by the formula as: 0.0571D+0.7371x0.0039D+0.7261, 1.0 mmD8.7 mm.

(15) The relationship curves of reactivity worth with respect to diameter for the gray control rod assemblies using DyTb alloy rods as the neutron absorbers are shown in FIG. 6. It can be seen from FIG. 6, when the reactivity worth (relative value, or so-called relative reactivity worth) is 0.20, the corresponding diameter of Dy-70Tb alloy rod is 1.3 mm; when the relative reactivity worth is 0.45, the corresponding diameter of Dy-70Tb alloy rod is 3.3 mm. When the relative reactivity worth is 0.25, the corresponding diameter of Dy-70Tb alloy rod is 1.8 mm, when the relative reactivity worth is 0.40, the corresponding diameter of Dy-70Tb alloy rod is 3.0 mm.

(16) It can be known from FIG. 3, the main factor affecting the reactivity worth of a gray control rod assembly is the diameter of DyTb alloy rod which is used as the neutron absorber. As long as the amplitude of variation of the reactivity worth of the gray control rod assembly is no more than 10%, even the Tb content changes greatly, the diameter of the alloy rod can still be calculated through the curve shown in FIG. 6 by using the initial reactivity worth of the Dy-70Tb alloy rod.

(17) Thus, the results in FIG. 5 are further defined to the relative reactivity worth of 0.20 to 0.45 which the gray control rod assembly requires, and to the preferred relative reactivity worth of 0.25 to 0.40. That is to say, the diameter of the TbDy alloy rod corresponding to the relative reactivity worth of 0.20 to 0.45 is: 1.3 mmD3.3 mm; the diameter of the TbDy alloy rod corresponding to the preferred relative reactivity worth of 0.25 to 0.40 is: 1.8 mmD3.0 mm.

(18) The parameter range of TbDy alloy rod meeting the needs of the relative reactivity worth of 0.20 to 0.45 and amplitude of variation no more than 10% of the gray control rod assembly is: 0.0688D+0.6388 x0.0026D+0.8626, 1.3 mmD3.3 mm. The parameter range of TbDy alloy rod meeting the needs of the relative reactivity worth of 0.25 to 0.40 and amplitude of variation no more than 10% of the gray control rod assembly is: 0.0688D+0.6388x0.0026D+0.8626, 1.8 mmD3.0 mm.

(19) The parameter range of TbDy alloy rod meeting the needs of the relative reactivity worth of 0.20 to 0.45 and amplitude of variation no more than 5% of the gray control rod assembly is: 0.0571D+0.7371x0.0039D+0.7261, 1.3 mmD3.3 mm. The parameter range of TbDy alloy rod meeting the needs of the relative reactivity worth of 0.25 to 0.40 and amplitude of variation no more than 5% of the gray control rod assembly is: 0.0571D+0.7371x0.0039D+0.7261, 1.8 mmD3.0 mm.

(20) In addition to the above TbDy alloy, sinter of the mixture of terbium oxide and dysprosium oxide can also be used as the neutron absorber. The sinter is a ceramic material having a better corrosion resistance than DyTb alloy. Since the neutron absorption capacity of oxygen is almost zero, it is easy for those skilled in the art to calculate and obtain the parameter range of the diameter of the terbium oxide required by the gray control rod assembly through using the above parameter range of the Tb content and diameter of the DyTb alloy.

(21) The neutron absorber comprising element Dy and Tb and other elements which have little effect on reactivity worth such as Zr, Fe, Ni, Nb and Mo also can be used. Since the neutron absorption capacity of these elements are so weak, it is easy for those skilled in the art to calculate and obtain the parameter range of the diameter of the Tb alloy required by the gray control rod assembly through using the above parameter range of the Tb content and diameter of the DyTb alloy.

(22) A more preferable solution can be used, that sinter of the mixture of dysprosium titanate and terbium titanate is used as the neutron absorber, wherein the unique structure of the material has a strong resistance to irradiation swelling. Since the neutron absorption capacity of oxygen is almost zero, and the neutron absorption capacity of titanium is so weak, it is easy for those skilled in the art to calculate and obtain the parameter range of the diameter of the terbium titanate required by the gray control rod assembly through using the above parameter range of the Tb content and diameter of the DyTb alloy.

(23) Besides the second component of metal dysprosium or compound of dysprosium or alloy comprising dysprosium, other second component can be used, such as the second component of AgInCd alloy. The neutron absorber that is of AgInCd alloy being added with metal Tb to a mass fraction of 50% has been shown in FIG. 2. It has an improved amplitude of variation of reactivity worth relative to service time.

(24) Besides the first component of metal terbium, terbium oxide, terbium titanate and terbium alloy, other first component can also be used, such as metal praseodymium, metal nickel, or compound of praseodymium, nickel, or alloy comprising praseodymium, nickel. They all have the characteristic that the reactivity worth increases as the service time increases.

(25) Accordingly, the present invention provides a gray control rod 2, which consists of an elongated cladding tube 3 and an upper end plug 4 and a lower end plug 5 for sealing the two ends of the cladding tube 3. A cylindrical neutron absorber is encapsulated in the cladding tube which is made of DyTb alloy. The outside of the gray control rod is covered with a cladding tube made of stainless steel or nickel-base alloy.

(26) Preferably, the mass fraction of Tb in the DyTb alloy rod is 70%, the diameter of the DyTb alloy rod is 2 mm. Both the reactivity worth of the gray control rod assembly using this alloy rod as the neutron absorber and the reactivity worth of the gray control rod assembly using tungsten rod are 0.27, but the amplitude of variation of the former is 2.8%, better than that of the latter of 3.9%, as shown in FIG. 3.

(27) The TbDy alloy may also comprises 2% of impurities such as Ho, Fe, Ca, Si, Cl, O and so on.

(28) The advantages of using TbDy alloy as the neutron absorber of the gray control rod is that the property of the neutron absorption material changes little during the designed lifetime of the gray rod. After TbDy alloy has been irradiated in the reactor core for a long time, element Tb decreases, element Dy increases, element holmium has been created. The density of Dy and Ho are all larger than that of Tb, their crystal structure are the same and they are solid dissolved to each other, no other phases will be generated, thus the volume expansion caused by the change of material density will not happen.

(29) The present invention also provides a gray control rod assembly 6 for reactor, comprising 24 gray control rods. Each of them consists of an elongated cladding tube and an upper end plug and a lower end plug for sealing the two ends of the cladding tube. A cylindrical neutron absorber is encapsulated in the cladding tube which is made of DyTb alloy. The outside of the gray control rod is covered with a cladding tube made of stainless steel or nickel-base alloy.

(30) Preferably, the mass fraction of Tb in the DyTb alloy rod is 68%, the diameter of the DyTb alloy rod is 3 mm. The reactivity worth of the gray control rod assembly using this alloy rod is 0.40, which is larger than that of tungsten rod of 0.27, and the amplitude of variation is only 1.8%, as shown in FIG. 3.

(31) The advantage of the gray control rod assembly is that it can meet the requirement of the reactivity worth in the large reactor with great power, and significantly reduce stepping number of the gray control rod assembly, reduce the wear of the cladding tube of the gray control rod and extend the life of the gray control rod assembly.

(32) The present invention also provides a gray control rod assembly for reactor, comprising 24 gray control rods. Each of them consists of an elongated cladding tube and an upper end plug and a lower end plug for sealing the two ends of the cladding tube. A cylindrical neutron absorber is encapsulated in the cladding tube which is made of AgInCdTb alloy. The outside of the gray control rod is covered with a cladding tube made of stainless steel or nickel-base alloy.

(33) Preferably, the mass fraction of Tb in the AgInCdTb alloy rod is 50%, the diameter of the AgInCdTb alloy rod is 3 mm. The reactivity worth of the gray control rod assembly using this alloy rod is 0.32, and the amplitude of variation is 8%.

(34) The foregoing described the preferred embodiments of the present invention. It should be understood that an ordinary one skilled in the art can make many modifications and variations according to the concept of the present invention without creative work. Therefore, any person skilled in the art can get any technical solution through logical analyses, deductions and limited experiments, which should fall in the protection scope defined by the claims.