This report was prepared as an account of work sponsored by an agency of the United States Government.

The isothermal test series is part of the Semiscale Blowdown and Emergency Core Cooling (ECC) Project conducted by Aerojet Nuclear Company for the U. S. Atomic Energy Commission. The test series consisted of ten blowdown tests and five hot-wall tests with emphasis on emergency core coolant delivery. The blowdown tests were conducted to investigate the effects of lower plenum geometry, heat transfer configuration, ECC injection location, downcomer gap size, and break size. The hot-wall tests were conducted to investigate the effects of residual vessel wall heat on ECC delivery after blowdown was completed. The test system included a pressure vessel with internals; an operating loop with steam generator, pump, and pressurizer; a.blowdown loop with rupture assembly, simulated pump, and simulated steam generator; a pressure suppression system with suppression tank and header, and a simulated ECC system with accumulators and injection pumps. The isothermal test program, 1-112-loop semiscale system, experimental instrumentation, operating procedures, and test conditions are described in this report to provide a reference document for support of ,the data reports specific to one or more of the isothermal tests. Presented are descriptions of the test program, test apparatus, instrumentation, operating procedures,.and test conditions.

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Gas-liquid countercurrent flow tests were performed as part of the Semiscale Blowdown and Emergency Core Cooling (ECC) Project. These tests included: (a) preliminary tests to establish the lower plenurn liquid levcl for subsequent tests and to investigate bypass flow phenomena; (b) steady state tests to investigate the relation between steam flow and ECC entrainment; and (c) transient tests to investigate the effect of steam generation and high wall heat flux on countercurrent flow behavior. The tests were conducted using the semiscale vessel, a phase separator, and a storage tank. Resl~lts are presented as dimensionless volumetric flux graphs, computer printout of digital data at discrete time intervals, and computer-produced graphs ot anal6g data as a function of time. Descriptions of the test facility, instrumentation, operating procedures, and test conditions are presented. Rlaximurr~ errors are identified .and an error analysis is presented for the volumetric flux presentations. The purpose of this document is to present the data with minimum interpretation for use by the nuclear community and for support of subscquent interpretative analyses.