A pulse-controlled modified-burst test instrument for accident-tolerant fuel cladding

Pellet-cladding mechanical interaction due to thermal expansion of nuclear fuel pellets during a reactivity-initiated accident (RIA) is a potential mechanism for failure of nuclear fuel cladding. To investigate the mechanical behavior of cladding during an RIA, we developed a mechanical pulse-controlled modified-burst test instrument that simulates transient events with a pulse width from 10 to 300 ms. This paper includes validation tests of unirradiated and pre-hydrided ZIRLO™ cladding tubes. A ZIRLO™ cladding sample with 168 wt. ppm of hydrogen showed ductile behavior and failed at the maximum limits of the mechanical test setup; hoop strain to failure was greater than 9.2%. ZIRLO™ samples showed high resistance to failure even at very high hydrogen contents (1466 wt. ppm). When the hydrogen content was increased to 1554 wt. ppm, “brittle-like” behavior was observed at a hoop strain of 2.5%. Preliminary scoping tests at room temperature of tubes fabricated from FeCrAl, a candidate material for accident-tolerant cladding were conducted to reproduce the pulse behavior of transient test reactors during integral tests. The preliminary FeCrAl tests are informative from the perspective of characterizing the test rig and supporting the design of RIA integral tests for current cladding materials and for candidate accident-tolerant cladding materials.