Patent Number: 
Section: 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 mm≤D≤8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where −0.0688×D+0.6388≤x≤−0.0026×D+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 −0.0571×D+0.7371≤x≤0.0039×D+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 mm≤D≤3.3 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 mm≤D≤3.0 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 mm≤D≤8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where −0.0688×D+0.6388≤x≤−0.0026×D+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 −0.0571×D+0.7371≤x≤0.0039×D+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 mm≤D≤3.3 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 mm≤D≤3.0 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 mm≤D≤8.7 mm, and unit of D is millimeter; mass fraction of element terbium in the neutron absorber is x, where −0.0688×D+0.6388≤x≤−0.0026×D+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 −0.0571×D+0.7371≤x≤0.0039×D+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 mm≤D≤3.3 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 mm≤D≤3.0 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.