JPH0820537B2 - Control rod for pressurized water reactor - Google Patents

Description

The present invention relates to a control rod of a pressurized water reactor (PWR),
More specifically, a PWR that improves the structure of the neutron absorber and prevents cracking of the cladding due to swelling of the neutron absorber.
Control rod for a vehicle.

[Prior Art] FIG. 4 shows a schematic configuration of a conventionally known control rod cluster (RCC) for PWR.

In the figure, the RCC designated as a whole by 30 is 16-24.
The control rod 20 of the book is attached to the spider joint 31 to be integrally configured. Where spider fitting 31 and control rod
Install with the vane 32 of the spider joint 31 as shown in the figure.
This is done by attaching the upper end plug 21 of the control rod 20 to the finger 33 provided on the.

As shown in FIG. 5, each control rod 20 has a coating tube 22 made of stainless steel and a silver-indium-cadmium alloy (usually silver (Ag): 80%, indium (In): 15%, cadmium ( Cd): 5%) and is loaded with a solid rod-shaped neutron absorber 23. As a design example of this solid absorber 23, one having an absorber radius of 4.27 mm and an overall length in the axial direction of the absorber of about 3.7 m is known.

In FIG. 5, 24 is a lower end plug and 25 is a plenum spring.

The RCC 30 configured as described above is pulled out to the upper side of the core during normal operation, and only some of the control rods 20 of the RCC 30 are inserted into the core.

[Problems to be Solved by the Invention] In the RCC 30 configured as described above, the Ag—In—Cd alloy forming the neutron absorber 23 causes volume expansion, so-called swelling, due to its main purpose of neutron absorption.

On the other hand, the outer diameter of the cladding tube 22 made of stainless steel gradually decreases due to creep down due to the pressure difference between the pressure of the primary coolant of PWR (about 157 Kg / cm ² G) and the pressure inside the cladding tube and neutron irradiation.

As a result, when the RCC 30 is used for a long period of time, the inner wall of the coating pipe 22 of the control rod 20 and the outer periphery of the absorber 23 come into contact with each other, and a circumferential tensile strain is generated in the coating pipe 22. Moreover, since the ductility of the stainless steel forming the cladding tube 22 is lowered by neutron irradiation, the cladding tube 22 often cracks. Particularly, in the conventional control rod 20, the fact that the absorber 23 is solid over the entire length in the axial direction is a cause of the above-mentioned cracking. That is,
When the absorber 23 is solid, there is no portion that absorbs the absorber swelling amount after the contact between the inner wall of the cladding tube 22 and the outer periphery of the absorber 23. Therefore contact pressure ~300Kg / cm ²
It will be a high value.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to reduce the tensile strain in the circumferential direction of the cladding due to the swelling of the absorber to prevent cracking of the cladding. It is to provide a control rod for PWR capable of.

[Means for Solving the Problems] In order to achieve the above object, a pressurized water nuclear reactor control rod according to the present invention includes a rod-shaped neutron absorber made of a silver-indium-cadmium alloy in a stainless steel cladding tube. In the loaded control rod, the rod-shaped neutron absorber is solid on the axially upper side of the control rod and hollow on the axially lower side.

[Action] Swelling of Ag-In-Cd alloy is mainly caused by the composition change of Ag and In due to neutron absorption.

Generally, when a metal is irradiated with fast neutrons, lattice defects are generated, but in the Ag-In-Cd alloy, which is a problem here, (1) PWR operating temperature (core inlet temperature about 290 ° C, outlet temperature (About 320 ° C) is higher than the recrystallization temperature (about 275 ° C) of Ag-In-Cd alloy.

(2) The fast neutron absorption cross section of Ag, In, Cd is small.

It is considered that the swelling mainly depends on the thermal neutron irradiation dose. Therefore, in the following description, fast neutron irradiation will not be considered.

The graph of FIG. 2 shows an example of the calculation result of the thermal neutron flux distribution in the absorber radial direction in PWR. The horizontal axis of this graph is the distance from the surface of the absorber to the center of the absorber, where 0 is the surface and 1 is the center (for fuel assemblies, 17 ×
17 type, absorber radius 4.27mm). The vertical axis represents the relative value of the thermal neutron flux at each radial position when the thermal neutron flux in the coolant is 1.

As is clear from this graph, the thermal neutron flux is considerably small inside the absorber due to the shielding of the absorber itself. Focusing on this point, by replacing the solid shape of the absorber with a hollow cylindrical shape, it is possible to absorb the volume increase due to the swelling of the absorber without significantly affecting the controllability, that is, the covering tube of the absorber. It can be seen that the contact pressure on the can be reduced.

The present invention is configured such that only the tip portion, which is inserted into the core for a long time, that is, the cumulative dose of neutrons is relatively high and swelling is likely to occur, is hollow. In this case, since the hollow portion is a very small portion, it is possible to prevent an adverse effect due to insufficient capacity of the Ag-In-Cd alloy due to hollowing.

To further clarify the features and advantages of the present invention, the preferred embodiments will be described below with reference to the accompanying drawings.

[Embodiment] FIG. 1 shows a configuration example of a control rod 20 according to an embodiment of the present invention. The same reference numerals as those of the above-mentioned conventional technique indicate the same components, and the description thereof will be omitted.

In the control rod 20 indicated as a whole by reference numeral 10, Ag-In-
The rod-shaped neutron absorber made of Cd alloy is composed of a lower absorber (hollow absorber) 11a occupying a region of about 25 cm in the lower axial direction and an upper absorber 1 occupying other regions of the total length of about 3.7 m in the axial direction.
It is composed of 1b (solid absorber).

Here, the solid absorber 11b has the same radius as the conventional absorber 23.
Although it is solid at 4.27 mm, the hollow absorbent body 11a has a void s inside as shown in the drawing, and has an outer radius of 4.27 mm and an inner radius of 2.56 mm.
Is formed into a hollow cylindrical body.

Next, the contact pressure of the hollow absorbent body 11a with the swelling to the coating tube 22 will be described. Note that documents cited in the following description are collectively shown at the end of this section.

According to the post-irradiation test results in the United States shown in Document 1, the volume swelling rate (that is, the volume increase rate ΔV / V (%)) of the Ag—In—Cd alloy is 2.6 × 10 ²¹ pieces / cm ² It is reported to be 0.7-1.2% of the neutron irradiation dose.

As a result, the volume swelling rate of the Ag-In-Cd alloy is
ΔV / V = 3.0 × 10 ^-24 φ _t (1.1) (however, the neutron energy E <1.85 eV) for the cumulative irradiation φ _t , and the PWR
ΔV / V = 5.87 × 10 ^-7 hr ^-1 (1.2) for the standard thermal neutron flux of. Converting this into a diameter change rate, Δr / r = 1.96 × 10 ⁻⁷ hr ⁻¹ (1.3) is obtained.

According to Reference 2, the relationship between the creep rate and the creep stress of Ag-In-Cd alloy is as shown in Fig. 3. The creep rate is about 1 Ksi (0.7 Kg / mm ² ) at creep stress. Is at least 2 × 10 ^-7 hr ^-1 or more. In FIG. 3, the vertical axis represents creep stress and the horizontal axis represents creep rate.

On the other hand, the creep of a cylinder that receives external pressure is Where P: contact pressure r ₀ , r _i : outer diameter, inner diameter of cylinder σ _r , σ _θ , σ _z : radial direction, circumferential direction, axial direction _θ : stress _g : circumferential strain rate ε _g : equivalent strain rate σ _g : given by equivalent stress.

Here, by substituting the values of the hollow absorber 11a for r ₀ and r _i , r _i = 2.56 × 2 = 5.12 = (4.27 × 2) × 0.6 = 0.6r ₀ (2.6) is obtained. Substituting this into equations (2.2) and (2.3), (2.
When the circumferential strain rate _θ and the equivalent stress σ _g are obtained from the equations 1) and (2.5), _θ = 0.99 _g (2.7) σ _g = 1.62P (2.8) is obtained.

Therefore, when the circumferential strain rate _θ is _θ = △ r / r = 1.96 × 10 ^-7 hr ^-1 from the equation (1.3), the equivalent strain rate _g is _g 2 × 10 ^-1 hr from the equation (2.7). ^-1 . Equivalent stress σ that causes this equivalent strain rate _g
g becomes σ _g 1ksi according to the data of the above-mentioned reference 2 (Fig. 3). The contact pressure P at this time is P616psi (~ 0.2Kg / mm ² ) (2.9) from the formula (2.8), which is lower than the pressure of the primary coolant of PWR. Therefore, the hollow hollow absorber 11a cannot withstand the external pressure creep caused by the coating tube 22, and as a result, no circumferential tensile stress is generated in the coating tube 22.

Although the region forming the hollow absorbent body 11a in the above-described embodiment is a region of about 25 cm in the tip portion where the volume is easily increased by swelling, the present invention is not limited to this.

References Reference 1 ... "IRRADIATION BEHAVIOR OF PRESSURIZ
ED REACTOR CONTOROL MATERIALS ", Nuclear Technology,
VOL.62 July 1983) Document 2 ····· EPRI (E lectric P ower R esearch
I nstitute; US Electric Power Research Institute) NP-1972: "Contorol Rod Materials and Burnable Poisons", An E
valuation of the State of the Art and Need for Tec
hnology Development, July 1980) [Advantages of the Invention] As described above, according to the PWR control rod of the present invention, the swelling of the absorber is achieved by adopting the neutron absorber having a hollow lower axial side. It is possible to reduce the tensile stress in the cladding tube circumferential direction due to or reduce the compressive stress. Therefore, there is an effect that cracking of the cladding tube can be prevented.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing the structure of a PWR control rod of the present invention,
FIG. 2 is a diagram showing an example of the calculation result of thermal neutron flux distribution in the absorber radial direction in PWR, FIG. 3 is a diagram showing the relationship between creep rate and creep stress of Ag-In-Cd alloy,
FIG. 4 is a schematic configuration diagram of the RCC, and FIG. 5 is a longitudinal sectional view showing the structure of a conventional PWR control rod. [Explanation of symbols of main parts] 10 ... Control rod 11a ... Hollow neutron absorber 11b ... Solid neutron absorber 22 ... Cladding tube In the drawings, the same reference numerals indicate the same or corresponding portions.

Claims (1)

[Claims] 1. A control rod in which a rod-shaped neutron absorber made of a silver-indium-cadmium alloy is mounted in a stainless steel cladding tube, wherein the neutron absorber has a tubular shape with a part of the axial direction being hollow. A control rod for a pressurized water reactor, which is formed and in which the hollow cylindrical portion is arranged on a lower side in the axial direction of the neutron absorber.