JPS6346387B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for automatically shutting down a nuclear fast breeder reactor in an emergency when the temperature of a coolant for cooling the reactor core rises above a set value due to an unexpected accident.

During operation of a nuclear reactor, if the flow rate of the coolant temporarily decreases for some reason, or if the output of the fuel assemblies that make up the reactor core increases due to the addition of reactivity, the temperature of the coolant will rise rapidly. This could lead to boiling, which could lead to melting of the fuel assembly and even lead to an accident in the entire reactor core. Usually, a nuclear reactor is equipped with a reactor shutdown device that can immediately detect such an emergency situation and safely shut down the reactor by emergency insertion of a control rod or the like.

However, this type of reactor shutdown device is configured to operate by signals sent from a neutron detection device inserted into the reactor, so it is important to ensure the safety of the reactor. Reliability becomes an issue.

The object of the invention is to, independently of the operation of the detection device,
To provide a device that automatically stops a nuclear reactor in an emergency when the temperature of a coolant flowing through a reactor core rises above a preset value.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a longitudinal sectional view of a fuel element 1 of a fast breeder reactor. Normally, the fuel element 1 consists of an elongated sealed metal cladding tube 2, which is arranged from bottom to top: a lower blanket part 3 made of natural uranium or depleted uranium, a core part 4 made of a mixed oxide of uranium and plutonium, and a reactor core part 4 made of natural uranium or depleted uranium. An upper blanket section 5 consisting of a fuel body and a gas plenum section 6 for storing fission product gas released from the fuel body are arranged.

FIG. 2 is a longitudinal cross-sectional view of the emergency nuclear reactor automatic shutdown device 7 according to the present invention. The internal arrangement of the device 7 according to the invention is closely related to that of the fuel element 1 described above. That is, the device 7 of the present invention has almost the same shape as the fuel element 1, and a neutron shield 9 is provided inside the sealed metal cladding tube 8 at a portion corresponding to the lower blanket section 3 in FIG. A hollow chamber 10 is provided in a corresponding portion, a neutron shielding body 11 is provided in a portion corresponding to a part of the upper blanket portion 5, and a strong neutron absorbing material storage chamber 12 is provided in a portion corresponding to the remainder of the upper blanket portion 5 and the gas plenum portion 6. It is located. The shield 11 is provided with a hole in which an actuating body 15 (described later) is installed, and the inner diameter of the hole is larger than the outer diameter of the actuating rod 15, so that the vertical movement of the actuating rod 15 is not restricted. Strong neutron absorption material storage chamber 12
(hereinafter referred to as the poison storage room) contains liquid lithium,
Neutron poisons 13 such as B ₄ C particles and BF ₃ (gas) are stored. The neutron poison 13 is kept outside the reactor core during normal operation of the nuclear reactor so as not to affect the core characteristics. That is, the hollow chamber 10 and the poison storage chamber 12 are mechanically separated by the partition wall 14. The aforementioned neutron shield 11 located directly below the bulkhead 14 suppresses reactor neutrons from reacting with the neutron poison 13 during normal operation of the reactor, and the lower neutron shield 9 prevents reactor neutrons from streaming downward. serve to prevent The presence of these shields 9 and 11 is essential when the cross-sectional area of the stop device 7 is large, but it is not necessarily necessary to provide them when the cross-sectional area is small.

During normal operation, the hollow chamber 10 is maintained in a vacuum state or at a microatmospheric pressure by an inert gas, and an operating rod 15, which is made of a material with a different expansion coefficient from that of the metal clad tube 8 and has a sharp tip, is attached to the metal clad tube 8. 8 and a stand 16 made of a material with good thermal conductivity (preferably the same material as the cladding tube 8) so as to be in thermal equilibrium, and its tip is opposed to the partition wall 14 with a small distance D left. be. Further, the partition wall 14 is constituted by a rupture disk.

The operation of this device with the above structure will be explained next. During transit operation of the reactor, the bulkhead 14 retains the neutron poison 13 in the poison containment chamber 12, but if an abnormality occurs in the plant and the coolant flow rate decreases or reactivity increases, the temperature of the coolant may change. When the temperature rises above the design allowable temperature, the temperature of the cladding tube 8 of this device becomes the same temperature. Therefore, the platform 16 and the actuating rod 15, which are in thermal equilibrium with the cladding tube 8, expand thermally and extend;
The cladding tube 8 and the working rod 15 have different expansion coefficients, and the working rod 15 is larger. Therefore, when the reactor reaches the maximum allowable temperature, the gap D at the tip of the working rod 15 becomes zero, and the rupture disk 14 Upon destruction, the neutron poison 13 flows into the hollow chamber 10 through the gap in the neutron shield 11 .

Here, an example of calculation for determining the gap D is as follows.

The average temperature _T0 of the reactor during normal operation is 450℃, and the cladding tube 8 is made of invar type stainless steel.
Also, if SUS316 is used as the actuating rod 15, the linear expansion coefficient a ₁ = 1 × 10 -6 /℃ of the former is 1 × 10 ^-6 /℃, and that of the latter is
a ₂ =20×10 ^-6 /°C. If the length of the operating rod is L, the reactor temperature T ₀ is, for example, the maximum temperature T _nax
Assuming that the temperature rises to =800°C, the gap D is calculated using the following formula.

D=L(a ₂ −a ₁ )×(T _nax −T ₀ ) If calculated by substituting the above-mentioned numerical value into the above equation with L=80 cm, D≈5.3 mm.

As explained in detail above, according to the present invention,
Since it is possible to obtain a device that automatically shuts down a nuclear reactor in an emergency without the aid of an external detector, it is a shutdown device that has the stability inherent in a nuclear reactor and is extremely useful in terms of ensuring the safety of a nuclear reactor. be. Furthermore, since this device does not require any piping system or cables connected to the outside of the reactor vessel, it can be easily inspected and replaced at the time of fuel replacement.

The device of the present invention has the following applications. In other words, it can be used not only as a reactor shutdown device, but also as a control device for local power peaks of a nuclear reactor. Normally, the power distribution inside a nuclear reactor fluctuates greatly due to the insertion and withdrawal of control rods during reactor operation, but there is a possibility that higher-than-expected power peaks may occur in the fuel assembly due to design/calculation errors or manufacturing errors. be. Since the position where such an output peak is likely to occur can be known in advance by calculation, if the device of the present invention is loaded at the predicted position described above, if an output peak higher than expected occurs, the surrounding cooling When the material temperature rises and reaches the maximum permissible temperature, the device of the present invention is activated and neutron poisons flow into the hollow chamber in the core. If the cross-sectional area of the device of the present invention and the amount of neutron poison are appropriately selected, the reactor operation can be continued safely by reducing peripheral output peaks without significantly reducing the reactivity of the reactor. As the combustion progresses, the neutron poison that has entered the hollow chamber is reduced as burnable poison.

The embodiments described above are merely illustrative and not limiting. Therefore, many changes and modifications can be made without departing from the scope and spirit of the invention.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a fuel element of a fast breeder reactor, FIG. 2 is a longitudinal sectional view of an automatic emergency reactor shutdown system according to the present invention, and FIG. 3 is a longitudinal sectional view of its essential parts. The symbols in the drawings indicate the following members, respectively. 1: fuel element, 2: cladding tube, 3: lower blanket part,
4: Reactor core, 5: Upper blanket, 6: Gas plenum, 7: This device, 8: Cladding tube, 9: Lower neutron shield, 10: Hollow chamber, 11: Neutron shield, 12: Neutron absorbing material storage Chamber, 13: Neutron poison, 14: Bulkhead (rapture disk), 1
5: Operating rod, 16: Stand.

Claims (1)

[Claims] 1 A sealed metal cladding tube is provided with a hollow portion corresponding to the reactor core, a storage chamber adjacent to the hollow portion for storing a strong neutron absorbing substance, and a partition wall separating the hollow chamber and the storage chamber. In the above, the partition wall is constituted by a rupture disk, and the tip of an actuating rod fixed to a stand attached to the sealed metal cladding tube is installed so as to be directly below the rupture disk; When the temperature of the cladding tube reaches the maximum allowable temperature, the tip of the actuating rod destroys the rupture disk by utilizing the difference in expansion coefficient between the actuating rod and the cladding tube, and the strong neutron absorbing material in the containment chamber is An emergency nuclear reactor automatic shutdown device characterized by allowing the flow to flow into a hollow chamber.