Masato Koshiishi

Re-weldability of neutron-irradiated stainless steels studied by multi-pass TIG welding

Weldability of neutron-irradiated stainless steel (SS) has been studied by multi-pass bead-on-plate and build-up tungsten inert gas (TIG) welding, simulating the repair-welding of reactor components. Specimens were submerged arc welding (SAW) joint of Type 304 SS containing 0.5 appm helium (1.8 appm in the SAW weld metal). Sound welding could be obtained by one- to three-pass welding on the plates at weld heat inputs less than 1 MJ/m in the irradiated 304 SS base metal. In the case of the build-up welding of a groove, no visible defects appeared in the specimen at a heat input as low as 0.4 MJ/m. However, build-up welding at a high heat input of 1 MJ/m was prone to weld cracking, owing to the formation of helium bubbles on grain boundaries of the base metal or dendrite boundaries of pre-existing SAW weld metal, in the area within 0.6 mm from the fusion line.

Modification of the FRI Crack Growth Model Formulation from a Mathematical Viewpoint

The FRI model of crack growth, which incorporates mechanical properties into the slip oxidation mechanism of crack advance, is an extension of the well-known Ford-Andresen model. When the exponent of the oxidation current decay curve is set close to 1, however, the FRI model gives an infinite crack growth rate. Here, the oxidation decay curve integral is revised to eliminate this divergence, and modified crack growth rate equations are derived. Also presented here is a procedure for determining the oxidation current parameters from the curve-fitting to measurements of crack growth rate. Parameter value determination and crack growth calculations are illustrated for cold-worked Type 316L stainless steel.

Study on laser surface modification and repair welding techniques for neutron irradiated austenitic stainless steels

The deterioration of weldability caused by helium is an important issue to be addressed in planning of proactive maintenance of light water reactor core internal components. Helium concentration, generated by neutron irradiation, was calculated to be about 20 appm in a core shroud after the 40-years nuclear power plant operation. The helium bubbles in grain boundaries (GBs) of the weld heat affected zone is one of the causes of weld cracking. In fact, there are some reports that application of conventional Gas Tungsten Arc Welding (GTAW) for neutron irradiated austenitic stainless steels caused cracks1,2. Yttrium-Aluminum-Garnet (YAG) Laser technique is one of the candidate method for surface modification and repair welding of the irradiated stainless steels, because the cooling rate of YAG Laser technique is fairly fast, the helium bubbles could not grow enough in grain boundary during process. Therefore, in the s study, the test of Laser surface modification (LSM) treatment, which is same as Laser de-sensitized treatment (LDT) and Laser repair welding (LRW) for neutron irradiated Type 304 and Type 316L austenitic stainless steels, were conducted. The applicability of LSM and LRW with optimized condition for neutron irradiated to 5E24 n/m2 (>1MeV) was demonstrated. The present study was carried out by the Japan Power Engineering and Inspection Corporation (JAPEIC) as one of the research works entrusted by the Ministry of Economy, Trade and Industry (METI).The deterioration of weldability caused by helium is an important issue to be addressed in planning of proactive maintenance of light water reactor core internal components. Helium concentration, generated by neutron irradiation, was calculated to be about 20 appm in a core shroud after the 40-years nuclear power plant operation. The helium bubbles in grain boundaries (GBs) of the weld heat affected zone is one of the causes of weld cracking. In fact, there are some reports that application of conventional Gas Tungsten Arc Welding (GTAW) for neutron irradiated austenitic stainless steels caused cracks1,2. Yttrium-Aluminum-Garnet (YAG) Laser technique is one of the candidate method for surface modification and repair welding of the irradiated stainless steels, because the cooling rate of YAG Laser technique is fairly fast, the helium bubbles could not grow enough in grain boundary during process. Therefore...