JP2007256230A - Coolant separation type fused nuclear fuel reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To safely annihilate spent nuclear fuel containing Pu240 and/or Pu242 at a low cost. <P>SOLUTION: The nuclear fuel is separated from a coolant, and the nuclear fuel is brought into a fused state. A power is controlled by opening and closing a neutron absorption contact. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a nuclear reactor that burns spent nuclear fuel safely and inexpensively.

A nuclear reactor is a device in which nuclear fuel, coolant, and other components such as uranium (U), plutonium (Pu), and minor actinide (MA) are arranged so that a controlled fission chain reaction can be sustained for a long time.
Although there are various types of nuclear reactors, light water reactors (pressurized water reactor (PWR) and boiling water reactors) that use micro-enriched uranium or plutonium mixed with micro-enriched uranium as a heat source and light water as a coolant (BWR)) is the mainstream.
FIG. 1 is a schematic view of a conventional boiling water nuclear power generator. The nuclear reactor pressure vessel (1) is loaded with a solid nuclear fuel rod (2) as a heat source and a control rod (3) for controlling the reaction. The heat generated in the solid nuclear fuel rod (2) is transferred to the liquid water that has entered the reactor pressure vessel (1) by the feed water pump (4), and the hot water becomes saturated steam as a turbine ( 5) Rotate. The rotational movement is transmitted to the generator (6) to generate electricity. The output control is performed by changing the neutron absorption amount by moving the control rod (3) up and down by the drive motor M. The low-temperature and low-pressure steam that exits the turbine (5) passes through the condenser (7) to become liquid water and goes to the feed water pump (4).
: Sho 61-37591, “Nuclear Fuel Assembly”. : Corona, author Toko “Atomic power” 117, 120 pages.

Spent nuclear fuel (SF) from which solid waste such as gaseous waste and cladding tube has been removed from spent nuclear fuel assemblies from light water reactors using uranium (U) oxide (UOX) as nuclear fuel Among them, U is recessive uranium (DU) including uranium 234 (U234) and uranium 236 (U236) in addition to uranium 235 (U235). U234 and U236 interfere with fission in order to be reused in a light water reactor mainly composed of thermal neutrons. It is difficult to use plutonium (Pu) in SF repeatedly in a light water reactor mainly composed of thermal neutrons. So-called depleted plutonium (DPu), which has increased content of plutonium 240 (Pu240) and plutonium 242 (Pu242), which are Pu isotopes, hinders fission when used in light water reactors mainly composed of thermal neutrons.
DU and DPu are fissioned by fast neutrons, but there is a safety problem because the void reactivity coefficient tends to be positive. Various measures have been taken to make the void reactivity coefficient negative.
MA also fission with fast neutrons like DPu, but there is a safety problem because the void reactivity coefficient tends to be positive.
DU, DPu, and MA are being accumulated. Eventually, SF must be burned off.

SF does not fission for thermal neutrons but fission for fast neutrons. A coolant such as water has the effect of slowing down fast neutrons, so if there are few coolants near the SF, it can be a fast neutron-based reactor. For heat removal from SF, if SF is in the form of molten metal, SF itself also acts as a coolant.
If the void reactivity coefficient is set to a negative or very small value from the viewpoint of safety, the output will not increase rapidly, but the coolant and nuclear fuel will not be able to cause an event that increases the effective multiplication factor keff more reliably. Isolate.

In the past, nuclear fuel and coolant were in contact and interacted. As a result, the upper limit of the cooling water temperature had to be set low in order to protect the nuclear fuel. In the present invention, since the nuclear fuel and the coolant are separated, the nuclear fuel can be heated to a temperature higher than the melting point, so that high temperature steam can be obtained, energy efficiency is increased, and power generation cost is reduced.
In the present invention, a cladding tube is not required and the cost is reduced.
Since a coolant with a neutron moderating action is not in contact with nuclear fuel, it can continue fission sufficiently with fast neutrons even with Pu242 or Pu240 having a high fission energy threshold.

Du, DPu, and MA without compromising safety and without significantly increasing power generation costs
Reactor that burns and extinguishes can be provided.

FIG. 2 is a schematic view of the coolant separated molten nuclear fuel reactor of the present invention during operation. Gas atmosphere containing high-temperature molten nuclear fuel (44) and fission product gas (FP) on a liquid with a low melting point by mixing U, DPu, or MA with iron or zirconium in a nuclear fuel container (43) ) Exists. The nuclear fuel container (43) is housed in the pressure-resistant coolant container (71). There is a heat insulating material partition wall (55) between the nuclear fuel container (43) and the pressure-resistant coolant container (71), which separates the cooling water (53) and the two-phase flowing water (56). Water entering the cooling water (53) region from the water supply pipe (60) flows from the bottom of the heat insulating partition wall (55) as indicated by the solid line arrow and enters the two-phase flow water (56) region, and enters the nuclear fuel container (43). Heat is absorbed from the molten nuclear fuel (44) through the wall, flows upward, flows as shown by broken arrows, and exits to the vapor (52) region. The steam (52) further absorbs heat from the neutron absorber heat transfer fin (41) and goes out from the steam pipe (51) to the turbine. The neutron absorber heat transfer fin (41) penetrates the nuclear fuel container (43) to the molten nuclear fuel liquid level (45) and transfers heat from the molten nuclear fuel (44) to the vapor (52). The reactor power is controlled by opening and closing a neutron absorbing tentacle (33) made of hafnium or sintered boron carbide. During normal operation, the neutron absorption tentacle (33) is closed in the control rod guide tube (32) made of neutron reflector and has a small surface area, so the neutron absorption effect is small. Since the control rod guide tube (32) is made of iron or zirconium neutron reflector, neutrons do not reach the neutron absorbing tentacle (33) but are reflected into the molten nuclear fuel (44). Numerous neutron absorbing tentacles (33) are bundled and passed through the control rod guide tube (32) and opened and closed by being operated up and down by the motor (M) of the control rod drive mechanism and FP exhaust mechanism (31). The The FP in the gas atmosphere (42) is exhausted to the outside by the control rod drive mechanism / FP exhaust mechanism (31). The cooling water (53) also serves as a neutron reflector.
When an event occurs in which water from the water supply pipe (60) stops, the result is as shown in FIG. Since the coolant level (54) falls and less neutrons are reflected, more neutrons leak from the nuclear fuel container (43), so the reaction of the nuclear fuel is reduced and the output is reduced. On the other hand, the decrease in the water level leads to a decrease in the two-phase flow water (56) and the heat removal from the nuclear fuel container (43) decreases, so the temperature of the molten nuclear fuel (44) rises and the volume of the molten nuclear fuel (44) becomes The density of the molten nuclear fuel (44) increases, the reaction of the nuclear fuel decreases and the output decreases. The increase in the volume of the molten nuclear fuel (44) also leads to an increase in the liquid level of the molten nuclear fuel liquid level (45), neutron leakage increases, and the reaction of the nuclear fuel decreases and the output decreases. Furthermore, the tip of the neutron absorber heat transfer fin (41) made of a neutron absorber is immersed in the molten nuclear fuel (44) and absorbs neutrons, so that the reaction of the nuclear fuel is reduced and the output is reduced. As an alternative to the neutron absorber heat transfer fin (41), heat removal with a heat pipe is also promising. When the neutron absorbing tentacle (33) is pushed out of the control rod guide tube (32) by the motor (M) of the control rod drive mechanism and FP exhaust mechanism (31), the surface area increases and the neutron absorption action increases and the reaction of nuclear fuel decreases. Then lower the output.
Although the soundness of the nuclear fuel container (43) wall is maintained by water injection from the emergency water supply pipe (131), even if the nuclear fuel container (43) wall is torn, the molten nuclear fuel (44) will remain in the pressure-resistant coolant container (71 The reaction of nuclear fuel further decreases and the output further decreases, and the wide wall of the pressure-resistant coolant container (71) absorbs heat.
When the turbine is stopped, the steam pressure in the pressure-resistant coolant container (71) does not rise excessively if steam is automatically flowed to the condenser (7) by a three-way valve or the like.

  According to the present invention, DU, DPu, and MA, which are difficult to handle in disposal of spent nuclear fuel, can be effectively utilized and extinguished. The problem of the final disposal site is also reduced. If this is the case, power generation costs will be reduced.

Fig. 2 is an overview of a conventional boiling water nuclear power plant. FIG. 2 is an overview diagram of the coolant separated molten nuclear fuel reactor of the present invention during operation. Fig. 2 is an overview of the situation at the time of the accident of the coolant-separated molten nuclear fuel reactor.

Explanation of symbols

1 is a reactor pressure vessel.
2 is a solid nuclear fuel rod.
3 is a control rod.
4 is a water supply pump.
5 is a turbine.
6 is a generator.
7 is a condenser.
31 is a control rod drive mechanism and FP exhaust mechanism.
32 is a control rod guide tube.
33 is a neutron absorbing tentacle.
41 is a neutron absorber heat transfer fin.
42 is a gas atmosphere.
43 is a nuclear fuel container.
44 is a molten nuclear fuel.
45 is a molten nuclear fuel liquid level.
51 is a steam pipe.
52 is steam.
53 is cooling water.
54 is a coolant level.
55 is a heat insulating partition.
56 is two-phase running water.
60 is a water supply pipe.
71 is a pressure-resistant coolant container.
131 is an emergency absorption tube.

Claims (1)

Between the pressure-resistant coolant container (71) and the nuclear fuel container (43) containing the nuclear fuel container (43) in which a gas atmosphere (42) containing high-temperature molten nuclear fuel (44) and fission product gas (FP) exists. Has a partition wall (55) and separates cooling water (53) and two-phase flow water (56), and on the nuclear fuel container (43), the heat of molten nuclear fuel (44) is transferred to the steam (52). The neutron absorber heat transfer fin (41) is made of, and passes through the control rod guide tube (32) made of neutron reflector to move up and down by the motor (M) of the control rod drive mechanism and FP exhaust mechanism (31) A coolant-separated molten nuclear fuel reactor, whose output is controlled by a neutron absorption tentacle (33) opened and closed by

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