SEDL / STP / STP1046V2-EB / STP49486S

The Influence of Creep Damage and Neutron Irradiation on the Tensile Properties of Type 304 Stainless Steel

Schaaf, Bob van der
Head, mechanical testing and microscopy laboratory, Netherlands Energy Research Foundation, ECN, LE Petten,

Pages: 15    Published: Jan 1990

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Abstract

In addition to the data base used to assess normal operating renditions, licensing authorities need postirradiation mechanical properties to evaluate the effects of hypothetical core disruptive accidents in liquid metal fast breeders such as the SNR-300. In particular, the synergistic effects of creep and neutron radiation damage on tensile properties constitute a major concern. The structural steel DIN 1.4948, similar to Type 304, is used for the permanent primary structures such as reactor vessel and core shield of the SNR-300.

Tensile/creep specimens have been irradiated in the HFR, Petten, at 823 K to a total neutron fluence of 1024n×cm−2, the end-of-life condition of permanent primary structures. Thermal control specimens were heat-treated parallel to the irradiation. Both material conditions were subjected to a stress of 155 MPa at 823 K and to creep strains in the range from 0.5 to 2.0%. Afterwards, tensile tests were conducted with strain rates in the range of from 10⁻⁶ s⁻¹ to 1 s⁻¹ at a temperature of 823 K.

We observed that radiation reduces the elongation and subsequently ultimate tensile strength for strain rates below 10⁻⁴ s⁻¹. The creep damage reduces the ductility and strength by a smaller, but fairly constant amount, which has a relatively large effect at low strain rates. At a tensile rate of 10⁻⁶ s⁻¹, the postirradiation ductility is reduced from 10 to 3% by the creep damage effect. The helium transmutation product from the interaction of thermal neutrons with the ¹⁰B (boron) isotope promotes the initiation and propagation of intergranular cracks under creep loads and low-rate tensile testing. The ability of the steel to absorb energy by plastic deformation is strongly reduced by the combined effect of creep and radiation damage.