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Metals that fracture with a relatively small or negligible amount of plastic strain exhibit _________
Brittle fracture
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Metals that can sustain substantial plastic strain or deformation before fracturing exhibit _________
Ductile fracture
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An indication of the amount of stress required to propagate a pre-existing flaw
Fracture toughness
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The temperature above which a material is ductile and below which it is brittle
Nil-Ductility Transition Temperature (NDTT)
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The temperature below which a material fails by brittle fracture. It is determined using both the Drop Weight and the Charpy V-Notch Tests.
Reference Temperature (RTNDT)
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Describe the two changes made to reactor pressure vessels to decrease (NDTT)
Use of smaller grain size & small additions of selected alloying elements: nickel, manganese to low carbon steels
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State the effect of fast neutron irradiation on reactor vessel metals
Heavier particles, such as alpha particles, neutrons, protons, and fission fragments, produce significant changes in the properties of metals. As a result of elastic collisions, appreciable energy can be transferred from the particle of radiation to the atoms of the metal. If the energy is sufficiently high, these atoms can be displaced from their equilibrium position within the lattice, and permanent physical changes are noted. This effect is known as radiation damage
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State how grain size and irradiation effects NDTT
- Small grain size increases fracture toughness resulting in lower NDTT
- The reactor vessel is continuously exposed to fast neutrons that escape the core. As a result the NDTT increases steadily
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List the conditions needed for brittle failure
- Flaw such as a crack or notch resulting from design, fabrication, system modifications
- Stress of sufficient intensity to develop a small deformation at the crack tip
- Temperature low enough to promote brittle fracture
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State the conditions that tend to mitigate crack formation
Large grain size, lower temperatures, and higher stress tend to favor crack propagation
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List the factors that determine the fracture toughness of a material
Metal Composition, Metal Temperature, Extent of deformations to the crystal structure, Metal grain size, Metal Crystalline form
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Stress Temperature Diagram
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State the two basics used for developing a minimum pressurization-temperature curve
They are based on reactor vessel and head stress limitation and the need to preclude reactor vessel and head brittle fracture
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- Note that the safe operating region is to the right of the reactor vessel MPT curve. The reactor vessel MPT
- curve ensures adequate operating margin away from the Crack Arrest Curve discussed above. The curves used by operations also incorporate instrument error to ensure adequate safety margin. Because of the embrittling effects of neutron irradiation, the MPT curve will shift to the right over core life to account for the increased brittleness or decreased ductility
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List the normal actions taken in sequence, if the minimum pressurization-temperature curve is exceeded during critical operations
Scram reactor, Cool down, depressurize the RCS, conduct an engineering evaluation prior to the further operation
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State the precautions of hydrostatic testing
Make sure the pressure is consistent with plant temperatures
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