An essential component of a horizontal, underground nuclear test setup at the Nevada Test Site is the auxiliary closure system. The massive gates that slam shut immediately after a device has been detonated allow the prompt radiation to pass, but block debris and hot gases from continuing down the tunnel. Thus, the gates protect experiments located in the horizontal line-of-sight steel pipe. Sandia " National Laboratories has been the major designer and developer of these closure systems. This report records the history of SNL's participation on in and contributions to the technology of auxiliary closure systems used in horizontal tunnel tests in the underground test program.

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