Development of Controlled Temperature-Cycle Irradiation Technique in JMTR

The effects of cyclic temperature changes during neutron irradiation upon radiation induced microstructure evolution and resulting property changes of materials is very important from both fundamental and engineering viewpoints. Therefore, a technique that allows us to do the controlled temperature-cycle irradiation was developed in the Japan Materials Testing Reactor (JMTR). The technique meets the following requirements : (1) the temperature-cycle irradiation is to be performed under three different conditions by changing lower and upper temperatures; 200 --- 400℃, 300 --- 400℃ and 300 --- 450℃. (2) the number and period of the temperature-cycles are to be six for 24-day full irradiation and approximately 44 h/44 h at the lower/upper temperatures. (3) the temperatures of each specimen assembly are to be maintained at the lower temperatures before start-up of the reactor and at the upper temperatures during shut-down until the complete absence of reactor power. In this paper, the details of the irradiation rig, successful results and several problems to be overcome for future improvement are presented

Study of Radiation Induced Electrical Degradation of Alumina in a Dynamic Pumping Condition in a Fission Reactor

The electrical conductivity of two ceramic insulators, alumina, and silicon nitride was measured in a fission reactor, JMTR under a dynamic vacuum condition. The instrumented irradiation rig was dynamically evacuated during fission reactor operation. The uppermost vacuum level attained was better than 10^ Torr. In this experiment, we attempted to reveal effects of the gaseous environment on the measurement of electrical conductivity under ionizing irradiation. The results showed that the gaseous environment has a most hazardous effect in a certain gas pressure range. In the second experiment, an electrical resistivity of polycrystal α-alumina was measured at about 680K in a dynamic pumping condition. In a good vacuum, we observed smaller RIC(radiation induced conductivity) than our previous results. We observed RIED(radiation induced electrical degradation)-like behavior. The results suggested that RIED would take off faster at a higher ionizing dose rate. In the meantime, the take-off occurred at about the same displacement damage of about 0.03-0.05dpa in the range of fast neutron flux of 3.4-15.1x10^n/cm^2s

Irradiation Behavior of developed radiaiton resistance optical-fibers and observed optical radiation from their SiO_2 cores under reactor irradiation

Two kinds of optical fibers were irradiated in a fission reactor, JMTR(Japan Materials Testing Reactor), up to a 1.55x10^n/cm^2 fast neutron fluence and a 3.3x10^9Gy ionizing dose at 370K. Optical transmission spectra were measured in the wavelength range of 450-1750nm, in-situ. Growth of strong optical absorption bands were observed in the range of wavelength shorter than 750nm. In the meantime, the fibers showed good radiation-resistance in the range of wavelength longer than 750nm. Optical radiations were observed from SiO_2 optical fibers under irradiation. A major part of the observed optical radiations is thought to be composed of broad optical radiation in the whole wavelength range studied in the present experiment. This broad optical radiation will be generated by the process of so-called Cerenkov radiation. Also, a sharp optical radiation peak was found at 1270nm on a F-doped fiber. This peak is thought to relate with doped Fluorine ions and ionizing gamma-ray irradiation

Development of Irradiation Techniques for Material Study in JMTR

The Oarai Branch has been carrying out the irradiation of materials using the JMTR, for these twenty years. We have made efforts to improve the irradiation conditions and to satisfy. the various demands evoked by our users. Here, we describe our efforts to improve the irradiation rigs and the irradiation techniques

Electrical current and voltatge induced in MI-cable under irradiation in JMTR

Radiation induced electrical current and voltage in MI-cables were measured under JMTR irradiation. The negative voltage of about 10V was generated in the center lead. The voltage increased with increasing irradiation dose with about -14V with a total dose of 1.1x10^Gy ionizing irradiation and 1.1x10^n/m^2 fast neutron irradiation. The observed electromotive force was a so-called current driven source and the measured current was linearly dependent on the reactor power and was about 50nA for the 0.7m long MI-cables at 573K. Under a reactor full power, irradiation conditions were 6 W/g of a gamma flux(6x10^3Gy/s ionizing dose rate) for iron and 5x10^n/m^2s and 1.1x10^n/m^2s fast and thermal neutron fluxes, respectively

Neutron Irradiation Effects on Optical Fibers

Irradiation effects on optical fibers were observed with 14 MeV fusion neutron and fission neutron. As a result, the following characteristics have become clear. Permanent absorption loss after fission and fusion neutron irradiation, were greater than that of gamma ray irradiation. In case of fission neutron, large absorption loss appeared at wavelength of shorter than 700 nm and loss peak of 600 nm, and absorption loss increased exponentialy to the fluence of 10^ n/cm^2. During irradiation, the light emission was observed in the wavelength of 400 to 1700 nm and peak of light at about 1200 nm was appeared. Two kinds of the SiO_2 fibers were survived irradiated up to 3 x 10^ n/cm^2

Dose dependence of irradiation hardening of neutron irradiated vanadium alloys by using temperature control rig in JMTR

TEM observation and tensile test were examined for vanadium alloys irradiated in a temperature control rig in JMTR at 290°C with damage level ranged from 0.003 to 0.06dpa. With the increase of the neutron dose, irradiation hardening could be observed in all the vanadium alloys except for the V–5Nb alloy. In the case of pure vanadium, the relationship between irradiation hardening and neutron dose was described as Δσ ∝ (ϕt)0.35-0.53. For V–5Cr alloy and V–4Cr–4Ti–0.1Si alloy, the dose dependence on irradiation hardening increase was shown as Δσ ∝ (ϕt)0.8 and Δσ ∝ (ϕt)0.8-1.0, respectively. From the TEM observation, the hardening source of radiation-induced defects was mainly determined to be dislocation loops for pure vanadium, loops with voids for V–5Cr and, loops and {100} precipitates for V–4Cr–4Ti–0.1Si and V–3Fe–4Ti–0.1Si alloys. From the strain rate dependence of 8% stress for V–4Cr–4Ti–0.1Si alloys tested at RT, the strain rate sensitivity, m=1/σ*(dσ/dln(dε/dt)) shows positive. Therefore, the dynamic interaction between interstitial impurities and dislocation is not strong in V–4Cr–4Ti alloys in the temperature range from RT to 290°C. A discrepancy of deformation mode of irradiated V–4Cr–4Ti–0.1Si alloys with 0.068dpa could be seen when the charpy impact test indicated the brittle behavior and the tensile test indicated the ductile behavior at room temperature. It can be explained by the difference of strain rate for the value of yield stress between tensile test and charpy test and the critical fracture stress

Effect of Specimen Geometry on Charpy Impact Test Results for Ferritic Steel Irradiated in JMTR

In order to develop the small-scale specimen technology in Charpy impact testing for ferritic steels, the effects of specimen size and notch geometry on the upper shelf energy (USE) and ductile-to-brittle transition temperature (DBTT) were investigated for Japanese Ferrite/Martensite Dual Phase Steel (JFMS). Miniaturized specimens with different sizes and notch geometry, together with full size specimens, were irradiated to 3x10^n/m^2 in the Japan Materials Testing Reactor (JMTR) and were Charpy impact tested. The USE for miniaturized specimens, normalized by Bb^2 or (Bb)^ (B is the specimen thickness, b the ligament size), was essentially independent of specimen size and notch geometry and decreased by the irradiation, but the decrease was larger in full size specimens than in miniaturized specimens; the normalized USE for miniaturized specimens was distinctly higher than that for full size specimens . The DBTT of miniaturized specimens was strongly dependent on notch geometry, but its dependence decreased as compared with that for unirradiated JFMS . It is shown that these results may be useful in determining the USE and DBTT for full size specimens from those for miniaturized specimens