Phase 5—VEEGA. The VEEGA phase starts when the upper stage with spacecraft escapes Earth gravity and ends when the second Earth flyby is completed three years after launch. (VEEGA is an acronym for Venus-Earth-Earth Gravity Assist.) Environments Several types of accidents could occur during the six phases of the mission. The complete set of accident environments include fire, explosion, overpressure, impelled fragments, reentry over-heating and ground impacts. The accident analyses evaluated the response of the generators to these accident environments. A statistical basis for estimating the likelihood of a release of fuel and the quantities of radioactive material released for each type of accident was developed. Results were calculated based on the location of the release (on or near the ground, at high altitude or upon reentry), the particle size distribution (as based on experimental tests) and knowledge of population distributions and meteorological conditions. The results included: (1) the most probable impact of a release, should one occur; (2) the maximum credible impact of a release; and (3) the expected value for all releases for each mission phase. Environments created by the accidents depend, in general, on the source of the accident and the time that it occurs. Time is important because it may affect the character of the source or the resulting secondary environments. For example, the shuttle solid rocket booster (SRB) fragments will achieve higher velocity if a case failure occurs near the end of the burn when less propellant is available to be accelerated along with the case wall. Liquid propellant explosions are more severe near the ground where the ground promotes mixing. Early failures can result in ground impacts, while failures above the upper atmosphere can result in reentry heating and subsequent ground or water impact. The explosion environments can have multiple elements as seen by the RTGs and RHUs. The sudden release of energy in air will drive a shock-wave that can distort or break up the RTG case, depending on the shock severity. The same explosive energy can push fragments of structure into the RTG. Finally, the resulting fire associated with accidents on or near the ground can provide thermal stresses on the RTG elements. Safety Tests The DOE extensively tested the response of RTGs, RHUs and their components to conditions characteristic of potential accident environments. The bulk of this testing occurred at Los Alamos National Laboratory in two phases. First, the general purpose heat source (GPHS) was tested under conditions representative of Space Shuttle/Cen- taur accident environments. Then, after the Challenger accident, testing was expanded to accommodate new accident environments identified based on the Challenger and Titan 34 D-9 accident analyses. This expansion entailed further study of SRB fragment effects and basic heat-source response data for model calibrations. Together, the two phases of testing established an internally consistent database that allows response predictions for the accident environments now postulated. Table II summarizes the nature and results of the testing. The table is organized according to test category, test description, purpose, conditions and results. Test category refers to the type of safety hazard investigated. Six types of safety hazards were examined: explosion overpressure, compact and flyer plate-type fragments, large fragment impacts, module reentry impact, fire and bare clad impacts. The specific tests
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