Pneumatic servo-controlled material testing device capable of operating at high temperature water and irradiation conditions: Dissertation

Special requirements set for the constructional materials used in energy production have strongly influenced the challenges in the component design work. Many material parameters such as corrosion, fracture mechanics, fatigue and oxide films are needed as input data for such work. The generation of reliable data calls also for more sophisticated testing systems. The newly developed pneumatic loading technology (patented in Finland and international patents pending) provides important potential benefits and has already been successfully applied to testing many kinds of materials in different test environments. As the moving parts that penetrate the pressure boundary are not needed, the friction force at the sealing element location is avoided. Therefore the load control with pneumatic loading unit is more accurate than with the conventional servo-hydraulic devices. This enables testing of small size samples which is an advantage e.g. when testing irradiated materials or testing materials inside the reactor core of a nuclear power plant and especially in determining environmentally assisted crack growth rates of structural materials. Furthermore, the new design enables simultaneous testing of several samples, which helps produce more reliable statistical data in a more economical way. This thesis summarizes my work done at the Technical Research Centre of Finland over the past 9 years to increase the knowledge of factors affecting material testing devices and material properties in gas and aqueous coolants at high temperatures. The developed pneumatic servo-controlled material testing device has been used to perform crack growth rate tests as a function of stress intensity factor K for AISI 316 stainless steel and Inconel 182 weld metal in Boiling Water Reactor (BWR) coolant by using small size (10x10x55 mm) three point bending specimens. The load and displacement were controlled during these tests by the pneumatic servo-controlled fracture measuring device (PSCFM device) and based on test results crack growth rates for Inconel weld metal and AISI 316 have been calculated. During the tests, accuracy with very slow constant displacement rates of 1.2 10-5 mm/min and 1.2 10-6 mm/min and long term stability of the pneumatic material testing system under high temperature water environment, has been verified. Furthermore, the crack growth rate for Inconel 182 weld metal as a function of sulphate content (10, 20, 30 ppb) of BWR coolant has been determined. These results shows that the crack growth rate of Inconel 182 weld material is strongly affected by the sulphate content of BWR coolant. Furthermore, prototypes of pneumatically powered fatigue (PSCFAT-device), controlled distance electrode (PSCCDE-device) and tensile (PSCINCORE-device) devices have been designed and tested. The units, which were based on proven technology, operated well and gave reliable test results for the material design parameters. The PSCFAT device was tested at room temperature and the degree of specimen bending and general system performance were measured. A strain controlled axial fatigue test in a 12 MPa autoclave at 100*C was also performed. The test material was AISI 316 stainless steel and the specimen failed in its midsection due to fatigue after 15600 cycles. The preliminary tests with pneumatically powered controlled distance electrode arrangement equipment (PSCCDE) is presented. The equipment was tested in air at room temperature and in typical BWR environments. The test material was pure nickel and its oxide film properties were tested in BWR environment as a function of potential. The design work for the pneumatic tensile testing device which is capable of working in a real reactor core is introduced. Prototype design, load calibrations, load frame for thin specimen tensile testing, and reactor installation with 30 m gas lines is presented. Furthermore, the PSCINCORE device was used to determine stress and strain curves for pure Cu specimen in the BR-2 reactor at Mol in Belgium. Reactor pool water temperature was 90 C, neutron flux ~0.3*1014 n cm-2s-1s (E>1 MeV) and damage rate ~2*10-4 h-1. The test type was constant displacement rate test with strain rate ~10-7 1/s. Some of the results gained in this work are unique, whenever possible however, the test results have been verified by comparison to earlier results with alternative techniques. Invariably, the comparisons prove applicability of the pneumatically powered technology developed in this work.