The low flux of high energy neutron sources requires optimum utilization of the available neutron... more The low flux of high energy neutron sources requires optimum utilization of the available neutron field. A furnace system has been developed in support of the US DOE fusion materials program which meets this challenge. Specimens positioned in two temperature zones just 1 mm away from the outside surface of a neutron window in the furnace enclosure can be irradiated simultaneously at two independent, isothermal (+- 1/sup 0/C) temperatures. The temperature difference between these closely spaced isothermal zones is controllable from 0 to 320/sup 0/C and the maximum temperature is 400/sup 0/C. The design of the system also provides a controlled specimen environment, rapid heating and cooling and easy access to heaters and thermocouples. This furnace system is in use at the Rotating Target Neutron Source-II of Lawrence Livermore National Laboratory.
The objective of the several RTNS-II irradation programs is to maximize information gained from t... more The objective of the several RTNS-II irradation programs is to maximize information gained from the small test volume available in this unique irradiation facility for application in the fusion materials program. While this facility provides the highest 14 MeV neutron flux available, the flux is generally too low and the irradiation volume too small for testing of engineering materials. Emphasis, therefore, is on identifying damage mechanisms of high energy neutrons and correlating them quantitatively with effects produced by fission neutrons. The information gained will be used to evaluate and calibrate damage and correlation models under development. The scope of the program includes in-situ experiments, postirradiation experiments, irradiation temperatures ranging from 4/sup 0/K to 1,000/sup 0/K, and fluences ranging from 3 x 10/sup 16/ to about 3 x 10/sup 19/ n/cm/sup 2/.
Candidate first wall fusion reactor materials were subjected to a deuterium bombardment comparabl... more Candidate first wall fusion reactor materials were subjected to a deuterium bombardment comparable to that of anticipated reactors. Significant microstructural and mechanical damage modes were observed. The mechanisms involved and the relevance of deuterium interactions to fusion is discussed. (MOW)
Abstract We have determined the damage kinetics of unipolar arcing in a shock tube environment wi... more Abstract We have determined the damage kinetics of unipolar arcing in a shock tube environment with an ion temperature of about 600 eV and a particle flux of about 10 23 ions/cm β s. Included in our study are the stainless steels AISI 304, 316 and Nimonic PE-16; the refractory metals Nb, Nb-751, T-111, Mo-TZM and W and isotropic reactor grade graphite. We find, for example, that unipolar arcing is a significant damage mechanism that is observed to occur on all materials tested; that arcing produces cathode spots preferentially at microstructural heterogeneities, that, with certain exceptions, differences in the chemistry of widely different metals and alloys did not result in resolvable differences in area coverage owing to arcing and that powder metallurgical W, martensitic 4130 and isotropic, bulk graphite are much more susceptible to arcing than the other materials. The results are discussed with respect to anticipated impurity fluxes into plasmas and with respect to arc theory.
A miniature tensile specimen technology has been developed in support of high energy neutron irra... more A miniature tensile specimen technology has been developed in support of high energy neutron irradiated material testing. The work includes miniature specimen design and fabrication technique development, as well as both baseline and irradiated tensile testing. Baseline data obtained using miniature tensile specimens are in good agreement with published values obtained using larger specimen geometries. Tensile data from miniature specimens irradiated up to a high energy, E approx. 14 MeV, dose level of 1 10/sup 18/ n/cm/sup 2/ or 0.003 dpa, have been obtained for annealed and cold worked AISI 316 stainless steel.
Theoretical and experimental studies have been made of phenomena which occur when a hot (T/sub 1/... more Theoretical and experimental studies have been made of phenomena which occur when a hot (T/sub 1/ approximately equal to 6 x 10/sup 6/ /sup 0/K), dense (n approximately equal to 10/sup 16/ cm/sup -3/), deuterium plasma containing a transverse magnetic field is brought into sudden contact with a cold metal wall. These studies are motivated by the need to understand plasma and metallurgical conditions at the first-wall of a fusion reactor. Experiments were carried out in the Columbia high energy electromagnetic shock tube. Computational simulation was used to investigate the detailed physics of the fusion plasma boundary layer which develops at the wall. The rate of energy transfer from the plasma to the wall was calculated and conditions under which surface melting occurs are estimated. Experimental measurements of plasma-wall heat transfer rates up to 3 x 10/sup 5/ watts/cm/sup 2/ were obtained and agreement with computed values are good. Fusion reactor first-wall materials have been exposed to 6.0 x 10/sup 21/ eV cm/sup -2/ (1,000 shots) of deuterium plasma bombardment. Scanning electron micrograph photographs show preferential erosion at grain boundaries, formation of deuterium surface blisters, and evidence of local surface melting. Some cracking is observed along grain boundaries, and amore » decrease in tensile ductiity is measured.« less
A method of and apparatus for heating test specimens to desired elevated temperatures for irradia... more A method of and apparatus for heating test specimens to desired elevated temperatures for irradiation by a high energy neutron source. A furnace assembly is provided for heating two separate groups of specimens to substantially different, elevated, isothermal temperatures in a high vacuum environment while positioning the two specimen groups symmetrically at equivalent neutron irradiating positions.
The feasibility of using the intense synchrotron X-radiation to microradiographically image both ... more The feasibility of using the intense synchrotron X-radiation to microradiographically image both surface and internal cracks in reasonably thick (200 micron) specimens of aluminum base alloys is investigated. It is concluded that synchrotron microradiography can be successfully used to image such defects as fatigue microcracks, creep induced wedge cracks and notches with a reasonable contrast and within exposure times of
The low flux of high energy neutron sources requires optimum utilization of the available neutron... more The low flux of high energy neutron sources requires optimum utilization of the available neutron field. A furnace system has been developed in support of the US DOE fusion materials program which meets this challenge. Specimens positioned in two temperature zones just 1 mm away from the outside surface of a neutron window in the furnace enclosure can be irradiated simultaneously at two independent, isothermal (+- 1/sup 0/C) temperatures. The temperature difference between these closely spaced isothermal zones is controllable from 0 to 320/sup 0/C and the maximum temperature is 400/sup 0/C. The design of the system also provides a controlled specimen environment, rapid heating and cooling and easy access to heaters and thermocouples. This furnace system is in use at the Rotating Target Neutron Source-II of Lawrence Livermore National Laboratory.
The objective of the several RTNS-II irradation programs is to maximize information gained from t... more The objective of the several RTNS-II irradation programs is to maximize information gained from the small test volume available in this unique irradiation facility for application in the fusion materials program. While this facility provides the highest 14 MeV neutron flux available, the flux is generally too low and the irradiation volume too small for testing of engineering materials. Emphasis, therefore, is on identifying damage mechanisms of high energy neutrons and correlating them quantitatively with effects produced by fission neutrons. The information gained will be used to evaluate and calibrate damage and correlation models under development. The scope of the program includes in-situ experiments, postirradiation experiments, irradiation temperatures ranging from 4/sup 0/K to 1,000/sup 0/K, and fluences ranging from 3 x 10/sup 16/ to about 3 x 10/sup 19/ n/cm/sup 2/.
Candidate first wall fusion reactor materials were subjected to a deuterium bombardment comparabl... more Candidate first wall fusion reactor materials were subjected to a deuterium bombardment comparable to that of anticipated reactors. Significant microstructural and mechanical damage modes were observed. The mechanisms involved and the relevance of deuterium interactions to fusion is discussed. (MOW)
Abstract We have determined the damage kinetics of unipolar arcing in a shock tube environment wi... more Abstract We have determined the damage kinetics of unipolar arcing in a shock tube environment with an ion temperature of about 600 eV and a particle flux of about 10 23 ions/cm β s. Included in our study are the stainless steels AISI 304, 316 and Nimonic PE-16; the refractory metals Nb, Nb-751, T-111, Mo-TZM and W and isotropic reactor grade graphite. We find, for example, that unipolar arcing is a significant damage mechanism that is observed to occur on all materials tested; that arcing produces cathode spots preferentially at microstructural heterogeneities, that, with certain exceptions, differences in the chemistry of widely different metals and alloys did not result in resolvable differences in area coverage owing to arcing and that powder metallurgical W, martensitic 4130 and isotropic, bulk graphite are much more susceptible to arcing than the other materials. The results are discussed with respect to anticipated impurity fluxes into plasmas and with respect to arc theory.
A miniature tensile specimen technology has been developed in support of high energy neutron irra... more A miniature tensile specimen technology has been developed in support of high energy neutron irradiated material testing. The work includes miniature specimen design and fabrication technique development, as well as both baseline and irradiated tensile testing. Baseline data obtained using miniature tensile specimens are in good agreement with published values obtained using larger specimen geometries. Tensile data from miniature specimens irradiated up to a high energy, E approx. 14 MeV, dose level of 1 10/sup 18/ n/cm/sup 2/ or 0.003 dpa, have been obtained for annealed and cold worked AISI 316 stainless steel.
Theoretical and experimental studies have been made of phenomena which occur when a hot (T/sub 1/... more Theoretical and experimental studies have been made of phenomena which occur when a hot (T/sub 1/ approximately equal to 6 x 10/sup 6/ /sup 0/K), dense (n approximately equal to 10/sup 16/ cm/sup -3/), deuterium plasma containing a transverse magnetic field is brought into sudden contact with a cold metal wall. These studies are motivated by the need to understand plasma and metallurgical conditions at the first-wall of a fusion reactor. Experiments were carried out in the Columbia high energy electromagnetic shock tube. Computational simulation was used to investigate the detailed physics of the fusion plasma boundary layer which develops at the wall. The rate of energy transfer from the plasma to the wall was calculated and conditions under which surface melting occurs are estimated. Experimental measurements of plasma-wall heat transfer rates up to 3 x 10/sup 5/ watts/cm/sup 2/ were obtained and agreement with computed values are good. Fusion reactor first-wall materials have been exposed to 6.0 x 10/sup 21/ eV cm/sup -2/ (1,000 shots) of deuterium plasma bombardment. Scanning electron micrograph photographs show preferential erosion at grain boundaries, formation of deuterium surface blisters, and evidence of local surface melting. Some cracking is observed along grain boundaries, and amore » decrease in tensile ductiity is measured.« less
A method of and apparatus for heating test specimens to desired elevated temperatures for irradia... more A method of and apparatus for heating test specimens to desired elevated temperatures for irradiation by a high energy neutron source. A furnace assembly is provided for heating two separate groups of specimens to substantially different, elevated, isothermal temperatures in a high vacuum environment while positioning the two specimen groups symmetrically at equivalent neutron irradiating positions.
The feasibility of using the intense synchrotron X-radiation to microradiographically image both ... more The feasibility of using the intense synchrotron X-radiation to microradiographically image both surface and internal cracks in reasonably thick (200 micron) specimens of aluminum base alloys is investigated. It is concluded that synchrotron microradiography can be successfully used to image such defects as fatigue microcracks, creep induced wedge cracks and notches with a reasonable contrast and within exposure times of
Uploads
Papers by N. Panayotou