The present invention relates generally to thermionic energy conversion systems, and more specifically to a novel pressure vessel for nuclear energy powered thermionic fuel elements that, as part of a nuclear reactor core made of an array of thermionic fuel elements, provides a nuclear reactor core with true redundancy and that is highly resistant to single point failures.
Thermionic converters directly convert thermal energy, or heat, into electrical energy, or electricity, by heating a metallic emitter to sufficiently high temperatures so that electrons escape the emitter and flow to a cooler collector. The heat source for conversion to electricity may be any of several types, including exothermic chemical reactions and the heat of nuclear fission. In order to promote efficient operation of a thermionic converter to generate useful amounts of electrical power, the converter must not only generate the large amounts of heat needed to energize the hot side thermionic emitter, but must also provide means for rejecting waste heat from the cold side collector.
Nuclear fission powered thermionic converters often have a core made of an array of individual thermionic fuel elements. Each thermionic fuel element, or TFE, typically is a cylinder having a center of fissionable fuel surrounded, in outward order, by cylindrical layers of an emitter, a gap, a collector, an insulator and an outer protective shell. A cylindrical vent, or snorkle, generally extends through the centerline of the fuel for venting gaseous fission products. Conventional cooling systems for cooling the converters enclose the core array of thermionic fuel elements inside a single large pressure vessel. Liquid metal coolant, typically a sodium-potassium alloy or lithium, is circulated through the core around and between the individual thermionic fuel elements, and then between the core and a remote heat exchanger. A single electromagnetic coolant pump and expansion accumulator is generally used to circulate the coolant. This conventional system is, however, subject to single point failure in that a leak anywhere in the cooling system, or a pump or accumulator failure, destroys the entire system. This is particularly undesirable because the primary expected use for nuclear fission powered thermionic converters is in outer space, where extreme reliability is required.
Prior art attempted solutions include adding heat pipes to the core to remove waste heat, thereby eliminating the single liquid metal loop. Unfortunately, these attempts have merely created new problems. For example, to accommodate the heat pipes, particularly heat pipes placed inside individual thermionic fuel elements, the elements have had to be substantially redesigned away from successful state-of-the-art designs. Also, in the event of failure of a single heat pipe, waste heat will travel through an alternate path that typically will result in a several hundred degree increase in the temperature of nuclear fuel in that path. Further, in the event of a possible space launch accident, in which the heat pipes may be sheared off at the reactor, the resulting large voids may fill with water causing the reactor to possibly go critical. The presence of unplanned or unwanted water in a reactor is dangerous because water generally acts as a neutron moderator, slowing down the fission-produced high velocity neutrons, which normally escape the reactor without reaction, to slower thermal neutrons which can trigger further fission reactions and cause unwanted or runaway chain reactions. This danger is particularly acute with so-called out-of-core systems where the heat absorbing evaporator end of a heat pipe is positioned inside a reactor core and the heat emitting condenser end of the heat pipe is the inside heat source of a thermionic converter.
Thus it is seen that there is a need for nuclear fission powered thermionic fuel elements and cores that provide redundancy and do not have single point failure modes.
It is, therefore, a principal object of the present invention to provide a nuclear fission powered thermionic fuel element that, combined with other thermionic fuel elements in a reactor core, has redundancy and eliminates single point failure modes.
It is another object of the present invention to provide a nuclear fission powered thermionic fuel element that combines its other features and advantages with high resistance to runaway chain reactions in the event of non-operational sudden and accidental destruction.
It is a feature of the present invention that it provides an extremely robust reactor, in that the reactor will be able to endure much operational damage and still continue to work adequately and, if the damage is too severe, fail gradually without catastrophic failure.
It is another feature of the present invention that it provides a modular reactor that can easily be scaled up or down to meet different power requirements.
It is an advantage of the present invention that it minimizes the needed volume of coolant making an efficient thermionic reactor of reasonable volume and weight.
It is a another advantage of the present invention that its construction and operation will be straightforward and uncomplicated.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.