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Thesis. 1976. M.S. cn--Massachusetts Institute of Technology. Dept. of Nuclear Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / M.S.cn

In order to increase the usefulness of the nuclear research reactor, it is necessary to enhance the neutron fluxes available in the experimental facilities. One method of attaining this objective is to increase the power level of the reactor. The VPI&SU 100 KW research reactor has been studied to investigate the feasibility of operation of 500 KW. From a thermal-hydraulic point of view, the following limiting conditions should not be exceeded for a 500 KW operation: 1) bulk boiling of the coolant should not occur, 2) surface boiling should not occur on any fuel plate, 3) fuel meltdown must not occur after a loss-of-flow or loss-of-coolant accident, 4) fuel plate vibration problems should not be significant. The results of the investigation indicate that the above limiting conditions will not be exceeded provided that certain system modifications are made. Thus the operation of the VPI&SU reactor at 500 KW is feasible as far as thermal-hydraulics is concerned. / Master of Science

In the event of a small break loss of coolant accident (SBLOCA) in a nuclear reactor, cold fluid is injected through the reactor system high pressure injector to compensate for the coolant loss. When this flow rate is less than a critical value, however, the hot fluid in the cold leg penetrates into the vertical injection line in a process called buoyant backflow. Because the resulting penetrations induce thermal stresses in the pipe, the presence of backflow in the injection lines is potentially significant. Since these penetrations could potentially damage the pipe, it was the purpose of this study to evaluate the backflow behavior. To this end, both the critical injection conditions and the subcritical penetration depth were experimentally determined through flow simulation in a 1/5 scale model. In addition, the experimental trends wi-re modeled theoretically. By matching the theoretical results to the experimental data, it was determined that backflow began below a critical Froude number of .65 and increased in depth with the negative logarithm of the injection velocity. The agreement between theory and experiment was excellent. For a certain class of reactor systems, the full scale Froude numbers were then compared to the critical value obtained in the analysis. For the systems involved in this comparison, the full scale Froude numbers were shown to be less than .65 for all practical flow rates. As a consequence, buoyant backflow is expected within the injection lines of these reactors, under safety injection conditions. / Graduation date: 1991

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