thermal-hydraulic integration of weapon cooling with the power system is a major factor in the thermal design of the power system. This is especially true for the normal rapid startups and shutdowns, and thermal implications of load rejection resulting from faults which will impact the state of the hydrogen entering the power system. 3. SDI has very restrictive vibration requirements that dictate the fluid conditions in some portions of the weapon system. Supercritical hydrogen is required for accelerator cooling to eliminate the flow induced vibration that could result from boiling in the coolant passages and thus partially establishes the reactor system fluid inlet conditions. 4. The need for an SDI system to be tested regularly and to tolerate very rapid load rejection has prompted the use of dummy electrical loads which may be hydrogen cooled in open cycle systems. These dummy loads are heaters that are integrated into the hydrogen flow path and thus provide an additional thermal consideration. 5. The concerns of hydrogen acquisition and pumping noted above are increased because of SDI spacecraft maneuvering requirements. Under these conditions the potential for flow reduction, instability, and interruption to both the cooling circuits and the power system could occur as a result of ineffective fluid acquisition within the tanks and/or hydrogen pump cavitation. Any pumping system within the power system, as with a closed cycle, must address this problem. Thus the microgravity considerations are often exacerbated. Safety Related Considerations 1. Launch safety considerations require that liquid metal systems be frozen during launch. This requirement leads to complex thaw systems and analytical methods to evaluate the process. 2. The multi-megawatt space reactor power system will be designed to ensure that the reactor remains structurally sound for all normal operating modes and credible accidents, including loss of primary flow or coolant. 3. The multi-megawatt space reactor will be designed to remain subcritical even if a launch accident or inadvertent reentry were to occur. It is also the objective of the space reactor program to avoid, or if that is not possible, minimize, the use of toxic materials, such as beryllium, which could be dispersed during a launch accident or an inadvertent reentry. 4. Multi-megawatt space reactor power systems will not be operated until planned orbits are achieved. The lifetime of an orbit is usually defined as the time it takes a spacecraft in a specific orbit to reenter the earth’s atmosphere and is a direct function of orbit altitude. The planned orbit lifetime (and altitude) of a space multi-megawatt reactor system will be carefully selected, either initially or by subsequent boost, to ensure sufficient time for radioactive materials to naturally decay in space to acceptable levels. The selected orbit altitude (lifetime) will depend on the operating history and fission product inventory of the reactor at the end of life. 5. The functional and operating requirements for multi-megawatt space reactors specifies that the reactor, either by placement in an initial long life orbit or boost to high orbit after operation, not reenter the earth’s atmosphere. However, as an added safety measure, design specifications for the reactor also require that the reactor reenter intact in the event of an inadvertent reentry. This will result in reactor burial in soil, pavement, or water upon impact and will ensure that reactor material would be contained within a small area.
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