Patent Number: 044938128
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to a nuclear reactor valve and more particularly to a breeder reactor blanket fuel assembly coolant system valve which increases coolant flow to the blanket fuel assembly to minimize long-term temperature increases caused by fission of fissile fuel created from fertile fuel through operation of the breeder reactor. Valves may be used for many applications in nuclear reactors. Currently an important use of valves is in the nuclear reactor coolant system. However, no self-actuating valves are presently used to control coolant flow to each of the many fuel assemblies which form the core of the reactor. The present state-of-the-art uses a fixed-size orifice in each fuel assembly to provide the entrance for coolant flow to the fuel rods contained therein, and use of a check valve to prevent reverse flow has been considered. In certain circumstances, varying the size of each fuel assembly coolant entrance orifice may be desirable. For example, in breeder reactors the blanket fuel assemblies experience a long-term increase in temperature due to fuel rod power increase caused by an increase in fissile fuel content. This is brought about by the breeder reactor's operation in converting the blanket fuel assemblies' fertile fuel into fissile fuel. The long-term temperature increase may be different for each fuel assembly. Blanket fuel assemblies are designed to operate within a certain temperature range. Higher temperatures will degrade the fuel assembly by shortening material life. Lower temperatures will degrade the reactor's performance by lowering its power for a given coolant flow. Thus, the inherent problem of breeder reactor blanket assemblies is that, with a fixed-size coolant entrance orifice, beginning-of-life temperatures are too low with acceptable end-of-life temperatures, or end-of-life temperatures are too high with acceptable beginning-of-life temperatures. A size-varying orifice valve could increase coolant flow to the blanket fuel assembly to keep long-term temperature increases to a minimum. Another example where a size-varying fuel assembly coolant entrance orifice may be desirable is in a fissile fuel assembly designed for long life, where the fuel rod power, and hence temperature, will decrease as more of the fissile fuel is depleted over the long-term operation of the nuclear reactor. Here a size-varying orifice valve could decrease coolant flow to minimize temperature decreases. Some ways of changing the size of the fuel assembly's coolant entrance orifice include shutting down the reactor to change the orifice unit with one of different size, or equipping the fuel assembly coolant entrance with an externally controlled valve. A mechanically or electrically actuated valve for each fuel assembly making up the nuclear reactor core would pose serious design and operation problems because of the hostile environment. Also, a self-contained temperature-actuated valve would follow the short-term temperature fluctuations of plant startup, power transients and plant shutdown, and pose time-lag and fail-safe problems. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear radiation actuated valve. It is another object of the invention to provide a self-contained nuclear reactor coolant system valve to control coolant flow. It is an added object of the invention to provide a breeder reactor blanket fuel assembly coolant flow entrance valve to minimize long-term temperature increases due to increases in fissile fuel formed from fertile fuel through breeder reactor operation. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. In one aspect thereof, the valve of the present invention is utilized in a nuclear reactor and is characterized by first and second members arranged in a manner to form an orifice for the passage of a fluid therethrough. The members are formed of materials, respectively, having different nuclear radiation swelling properties whereby exposure to nuclear radiation effects differential swelling and relative movement of said members to vary the size of said orifice. Several benefits and advantages are derived from the invention. The valve's nuclear radiation actuation feature provides a simple, reliable, and self-contained valve suitable for use in a nuclear reactor core. The invention may be used as the coolant inlet orifice for a fuel assembly where long-term temperature changes are to be minimized without responding to short-term temperature fluctuations. The nuclear radiation actuated valve allows a long-life core fissile fuel assembly not to significantly decrease temperature as fissile fuel is depleted. It also allows a breeder reactor blanket fertile fuel assembly not to drastically increase temperature as fertile fuel is converted into fissile fuel and hence not to degrade the useful material life of the fuel assembly or adjacent reactor vessel internals.