[en] Oak Ridge National Laboratory (ORNL) is completing a major task for the Department of Energy (DOE) in the demonstration that the primary piping of the New Production Reactor-Heavy Water Reactor (NPR-HWR), with its relatively moderate temperature and pressure, should not suffer an instantaneous Double-Ended-Guillotine-Break (DEGB) under design basis loadings and conditions. The growth of possible small pre-existing defects in the piping wall was estimated over a plant life of 60 years. This worst case flaw was then evaluated using fracture mechanics methods. It was established that this worst case flaw would increase in size by at least 14 times before pipe instability during an earthquake would even begin to be possible. The approach to showing the improbability of an instantaneous DEGB for HWR primary piping required a major facility (Pipe Impact Test Facility, PITF) to apply all possible design loads, including an equivalent major earthquake (called the SSE earthquake). The facility was designed and built at ORNL in six months. The test article, a 20-ft (6.1 m) long 16-in. (406 mm) diameter SCH-40 pipe of stainless steel 316LN material was fabricated to exacting standards and inspections following the nuclear industry standard practices. A flaw was machined and fatigued into the pipe at a TIG butt weld (ER316L weld wire) as an initial conditions. The flaw/crack was sized to be beyond the worst-case flaw that HWR piping could see in 60 years of service-if all leak detection systems and if all crack inspection systems failed to notice the flaw's existence. The pipe was impacted 104 times at levels equal and well beyond the SSE loadings. In addition, over 560,000 fatigue cycles and numerous purposeful static overloads were applied in order to extend the flaw to establish the data necessary to confirm fracture mechanics theories, and, more importantly, to simply demonstrate that instantaneous DEGB is highly improbable for the relatively moderate energy system