1. Field of the Invention
The present invention relates to the coolant with dispersed neutron poison micro-particles, used in an SCWR emergency core cooling system.
2. Description of the Related Art
Generally, in nuclear power generation, thermal energy is generated by the chain fission of a fissionable material such as thorium, uranium, plutonium or the like, and power necessary for electric power generation is derived from the thermal energy. At this time, the fissionable material is prepared in the form of a sintered body and contained in a nuclear fuel rod. Nuclear fuel rods are arranged in a bundle to form a nuclear fuel assembly. In a nuclear reactor, a control rod and a moderator are generally used to prevent a chain reaction (reactivity: >1) of fissionable materials. The number and speed of extra neutrons can be appropriately controlled by a control rod, a moderator/coolant and the like, and accordingly, energy generated from nuclear fission reactions is used for electric generation. The moderator generally includes heavy water (D2O), light water (H2O), graphite, beryllium and the like. Depending on the moderator/coolant, nuclear reactors may be classified into a light-water type nuclear reactor (LWR: PWR, BWR), a heavy-water type nuclear reactor (HWR), a high-temperature gas-cooled reactor (HTGR), and the like.
For example, in a light-water reactor, ordinary water (light water) is used as the moderator and coolant of a nuclear reactor. The basic operating principle of the light-water reactor is that steam is generated by heating water using nuclear reactions in the nuclear reactor, and electric power is generated using the steam. Therefore, the light-water reactor requires a nuclear reactor, a steam generator, a recirculation pump, a turbine, a condenser and the like, used in a nuclear power generation system.
However, up to the present, nuclear power has had not only its convenience but also its problems such as the risk of radiation accident and the treatment of radioactive waste. Therefore, requirements for fourth-generation nuclear reactor (GEN IV) have been increased to solve such problems. In relation to the GEN IV, theoretical interests on a supercritical water-cooled reactor (SCWR) have been increased.
The SCWR is a system for generating high-temperature water (350 to 500° C.) by pressurizing core cooling water to above the critical pressure of water and heating the core cooling water in a core. Since the SCWR forms a pressure greater than a critical pressure of 22.1 MPa in a nuclear reactor and generates supercritical water that is a fluid at a high temperature of about 375° C. or more, the steam generator and recirculation pump used in the conventional light-water reactor are unnecessary. Accordingly, a nuclear power generation system can be simply manufactured. Furthermore, since the supercritical water has a high temperature, generation efficiency can be increased up to about 44%. Accordingly, the SCWR has more improved characteristics than an improved thermal power plant.
The operating temperature of a coolant in the SCWR is generally in the range of about 280 to 550° C. In this case, the pressure is higher than 22.1 MPa that is the critical pressure of water. Under conditions of such a nuclear reactor core, the density of water is greatly changed in the range of about 0.1 to 0.7 g/cc or less. Therefore, the density of water has a much lower value than that in a conventional pressurized or boiling water reactor. The reason why the density of water is lowered in a supercritical state is that hydrogen bonding formed between water molecules is broken. At this time, the basic chemical property of water is changed from polar to non-polar such as that of organic oil.
Such a supercritical water state generally has two problems. A first problem is moderation of neutrons due to the decrease in density of water for changing fast neutrons to thermal neutrons. A second problem is a low solubility of boric acid used as a neutron poison micro-particles. The problem of neutron moderation can be solved by adding a water rod in a non-critical state into the core or by using a solid moderator such as zirconium hydride. However, an appropriate method of substituting for soluble neutron poison materials has not been developed yet.
Since safety is more important than economical efficiency in nuclear power plants, strict regulations related to safety are applied in all processes of design, construction, operation and destruction of the nuclear power plants. A nuclear reactor has intrinsic safety governed by the natural laws, and a linkage system is provided to automatically control the nuclear reactor not to be operated out of a predetermined safety range. If the nuclear reactor is operated out of the safety margin, it is designed to stop immediately.
To stop a nuclear reactor, control rods formed of the solid neutron poison materials is generally inserted into a core. However, an emergency core cooling system is provided to the nuclear reactor for the case where the control rods are not operated. The emergency core cooling system functions to stop operations of the nuclear reactor and to prevent an increase in temperature of the nuclear reactor.
Specifically, the emergency core cooling system shown in FIG. 1 comprises an accumulation tank 3 for automatically injecting the coolant containing neutron poison micro-particles into a nuclear reactor 2 when the pressure in the nuclear reactor 2 is lower than a predetermined pressure; a refueling water storage tank 1 in which the neutron poison micro-particles are contained; and a high-pressure injection pump 7 for re-injecting refueling water and the water gathered in a drainage tank at the bottom of a nuclear reactor building. The neutron poison micro-particles contained in the water are injected into the nuclear reactor through the emergency core cooling system, so that nuclear fission reaction is stopped.
At this time, boric acid solution has been used as cooling water containing neutron poison micro-particles in the conventional light-water or boiling nuclear reactor.
Generally, boric acid as soluble neutron poison material used in a nuclear reactor is well dissolved in a polar solvent. Therefore, the boric acid is well dissolved in water with a non-critical state of 374° C. or lower but is not easily dissolved in water with a supercritical state higher than the non-critical state. If boric acid solution is used in the SCWR, the dissolved boric acid can be extracted by a solubility difference when the boric acid solution is changed into the supercritical state. Such a phenomenon is well known in a salt solution such as salt water. If the dissolved boric acid is extracted when the boric acid solution is changed into the supercritical state, functions for the control and stop of the nuclear reactor using cooling water may be limited. Therefore, there is required a new coolant used in a SCWR emergency core cooling system.
Accordingly, while studying a method of uniformly dispersing neutron poison materials in cooling water even though the polarity of water is changed, the present inventors have developed a coolant for a SCWR, with uniformly dispersed neutron poison micro-particles. Based on these findings, the present invention was completed.