Patent Number: 052951707
Section: summary

BACKGROUND OF THE INVENTION This invention relates to techniques for adjusting the pH of post accident containment water in nuclear power plants, and in particular passive techniques. Nuclear power plants are generally provided with reactor containment vessels that surround the nuclear reactor core and steam generators attached to the core. Water, either pressurized or boiling, circulates through the core, extracting heat from the core for the generation of power. Following an accident that involves the loss of reactor coolant, an emergency core cooling system (ECCS) fills the reactor core with water to a level such that the core is covered. It is possible for the core to suffer some damage in such an accident, and, in the case where the safety injection system fails, to suffer significant damage. After an accident, radioactive iodine present in the containment vessel can change from a particulate form, such as CeI salts, to a more volatile organic form. The organic form of iodine behaves like a gas and is more likely to leak out of the containment vessel, thereby resulting in greater releases of radioactivity from the plant. Adjusting the pH of the water in the containment vessel prevents the radioactive iodine from changing into the organic form. Therefore, during and after such accidents, it is necessary to adjust the pH of the water that is in the containment sump in order to limit the amount of radiation released from the plant. Initially after an accident, the pH of the water in the containment vessel may be as low as 3. In order to prevent the formation of significant amounts of organic iodine the pH must be raised to a value of about 7. In current plants, the pH of the post accident water is adjusted by the addition of chemicals such as sodium hydroxide. This is typically accomplished by the use of containment spray pumps to inject sodium hydroxide along with the spray water from the ECCS. This provides an effective means of adjusting the pH. However, this means of injection is subject to some of the same failure mechanisms that can cause the failure of the safety injection system. Some of the common cause failure mechanisms include the complete loss of all AC power, the complete loss of all cooling water, or the complete loss of all ventilation systems. In some recent advanced nuclear reactors, pumps are not used in the safety systems, and in some passive plants there is no spray system at all. Therefore, a different means of adjusting the pH is required. One approach is to place water permeable sacks of NaOH crystals in the ECCS sump. When the sump is flooded, the NaOH dissolves in the water. However, this passive system suffers serious drawbacks. First, the NaOH is very caustic and must be isolated from equipment. Second, accidental water spills in the containment vessel could activate this type of passive system, necessitating expensive clean-up procedures to remove the NaOH and to repair any damaged equipment. Third, the sacks must be checked for leaks and replaced frequently. Therefore, it would be advantageous to provide a passive pH adjustment system that overcomes the above stated problems. SUMMARY OF THE INVENTION An object of this invention to provide a method of adjusting the pH of post accident water in a nuclear reactor containment vessel. An additional object of this invention is to provide a nuclear reactor having a means of adjusting the pH of post accident water in a nuclear reactor containment vessel that does not depend upon the safety injection system. It is a further object of this invention to provide a passive means of adjusting the pH of post accident water in the nuclear reactor containment vessel that will be activated only in an emergency situation. According the invention, a nuclear reactor is placed within a containment vessel. Means are provided for elevating the pH of post accident water in the containment vessel. A tank containing a basic liquid, such as liquid sodium hydroxide, is located within the containment vessel at an elevation above the maximum post accident water level. A sensor provides a signal indicative of an elevated radiation condition within the containment vessel that is greater than a predetermined, normal operating radiation level. If the sensor detects a radiation condition much greater than the normal operating range of the nuclear reactor, it triggers an explosive squib valve attached to the bottom of the tank. This allows the basic liquid contained within the tank to drain by gravity into the containment vessel. The basic liquid is directed by routing means from the tank to two spaced apart pH sumps in the containment vessel. Each of the pH sumps is located proximate to a safety injection sump. The pH sumps are capable of holding the entire contents of the tank. A vacuum breaker is positioned near the top of the tank. Opening the vacuum breaker allows the basic liquid to more easily drain from the tank after the explosive valve has opened.