Studies on IASCC initiation (interim results from IFA-618)

The main objective of the investigation being performed in IFA 618 is to study the effects of material, stress level and fluence on the initiation of cracking in pressurised tube specimens in a representative BWR environment. The test was operated on a combined Joint programme and bilateral basis from the start of IFA 618 (January 1996) to December 1997. Since January 1998, the test has been conducted on a joint programme only basis. The tube specimens of IFA-618 are arranged in a total of six strings, each containing six tube specimens (i.e. a total of 36 tube specimens). The tube specimens, which are prepared from sensitised 304 SS, cold worked 347 SS and solution annealed 304 SS, 316L SS and 347 SS have been pressurised to stress levels ranging from 0.8 to 2.75 times the yield stress of the unirradiated material. Three of the strings are arranged in the upper test section and three of strings are arranged in the lower test section, which is surrounded by booster rods, which create a high fast neutron flux. Each of the six strings is equipped with a cladding elongation detector for on-line monitoring of the string elongation and for the detection of tube specimen rupture. In addition, the tube ruptures are determined by increased activity levels which are measured in the coolant as a result of the release of the tube filler gas (activated Ar-41) into the water. The tube specimens have been subjected to five interim inspections since the loading of IFA 618. The interim inspections were performed to determine post-irradiation behaviour and to identify ruptured tube specimens in cases where in-pile data (increases in gamma activity levels and/or changes in string elongation detector signals) indicated rupture in specific strings. By the end of 1998 the fluence accumulated for the low fast flux and high fast flux specimens was 3.9 x 1015 - 6.0 x 1018 n/cm2 and 1.2 x 1021 - 1.6 x 1021 n/cm2, respectively. Irradiation of IFA 618 will be continued to higher fluence levels in 1999 (author) (ml)