Design of nuclear-powered rocket motors

Abstract

At the beginning of the development of nuclear power for space, motors and electric-power plants were produced separately: first in the form of nuclear rocket motors (NRMs) with a thrust of 40-100 tonnes (project NERVA in the USA) or about 4 tonnes (project IRGIT in the USSR [I]) and second - reactors for producing 5-100 kW of electric power by means of thermoelectric, thermionic, or turbine machine with a Brayton energy-conversion cycle. Examples are SNAP-100 in the USA and the Topaz reactor in the USSR. It was soon found, however, that this approach is too costly, since, having solved the problem posed, the NRM reactor remained throughout the entire life of the space vehicle a useless load, while in order to provide for the electric requirements, it was necessary to produce in addition a power reactor, similar in size to the NMR reactor and it takes mass away from the useful load. For this reason, there arose the idea of using a single reactor both for the motor and for generating electric power, i.e., to develop a nuclear motor-power plant (NMPP). At first glance it seems quite simple to design a NMPP on the basis of the heterogeneous reactor NRM IRGIT to obtain an electric power of 5 kW, which is produced by the Topaz thermionic converter-reactor. For this, the reactor must develop a power of 100 kW in the power mode with an efficiency of 10%. Electricity is generated by means of a Brayton gas-turbine cycle. It can be organized in two ways. In the first method (Fig. 1) gas is pumped along a closed loop, and passes through a throttle, and the thermionic converter contains either a vacuum or a standing gas atmosphere. In the second scheme (Fig. 2) gas also passes through the fuel elements, for which the skirt-nozzle is closed with a cover. This simplest NMPP already illustrates the main problems: the contradictory requirements which are imposed on the construction by the motor and power modes. For example, in the NMPP considered here a hydride-zirconium moderator with low porosity was chosen for the motor mode; this made it possible to produce a small intermediate reactor with a small uranium load. The moderator temperature should not exceed 600~ For the needs of the power mode, however, the temperature must be raised in order to achieve a high efficiency, since increasing the efficiency by, for example, 10-20% decreases by approximately a factor of two the area of the radiator-cooler as a result of a corresponding decrease of the required thermal power of the reactor with constant electric power. The area of the radiator-cooler is decreased even more by increasing the temperature of its surface; this requires a further increase of the gas temperature in the power mode, for which zirconium hydride is no longer suitable: it is necessary to use yttrium or beryllium hydride. To close the gas-turbine cycle, the pressure losses over a cycle should be several percent, which requires a higher moderator porosity. Both requirements, which follow from the power mode - replacing the moderator by a worse moderator and increasing the porosity - make it necessary to increase the size of the reactor without changing the load. However, this is undesirable, since the mass of the radiation shield increases markedly and the gain in mass achieved by replacing separate motors and power plants with the NMPP is lost. For this reason, preference is given to increasing the uranium concentration in the fuel. As a result, a switch is made to a fast reactor. In the schemes considered the NMPP does not generate electricity in the motor mode, i.e. the reactor operates serially in each regime. It is now required that both regimes operate together, i.e. electricity is generated for the needs of the space vehicle during the motor regime also. An example of a reactor for such a power plant is shown in Fig. 3 [2]. Thermionic converters are installed in a homogeneous core with a dual radial flow. In the first half the slit channel near it is the collecting channel of the lower part of the reactor and in the second half it operates as a distributing channel. On account of this, the temperature along the channel is constant and equal to 2000 K, which is necessary for operation of the thermionic converter. In the upper part of the reactor the gas is heated up to 2800 K. Combining of the regimes is achieved in this manner.