41 1968 Semiscale‘Blowdown’Tests To validate computer models predicting the behavior of coolant during a loss-of-coolant accident, engineers built Semiscale, an experimental reactor simulator that showed how coolant water would behave as it depressurized following a pipe break. “Blowdown” tests and computer analysis of the simulated accidents led to the formulation of computer codes that could predict the performance of backup cooling systems during a blowdown.The codes originated at the NRTS Supercomputing Center, which went into operation in 1968.Tests in 1970 and 1971 revealed that, in certain cases, steam pressure in coolant pipes would prevent any emergency water from getting to the core. Margins of safety that had been assumed were revised downward.Tests were performed at Semiscale until 1986.