Unirradiated Particles Research Articles

Comparison of unirradiated and irradiated AGR-2 TRISO fuel particle oxidation response

The silicon carbide (SiC) coating in a tristructural isotropic (TRISO) particle acts as a barrier to fission product release during reactor operation and accident scenarios. Oxidation and subsequent failure of the SiC layer during a rare air ingress event is a proposed mechanism for fission product release in a high-temperature gas-cooled reactor (HTGR). Although previous oxidation studies have analyzed unirradiated TRISO particle response, this study compared the oxidation behavior of irradiated and unirradiated TRISO particles from the second Advanced Gas Reactor Fuel Development and Qualification Program irradiation experiment (AGR-2). Particles with exposed SiC were subjected to six varying oxidizing tests in the Furnace for Irradiated TRISO Testing (FITT), examined for failure fraction with the Irradiated Microsphere Gamma Analyzer (IMGA) and characterized with focused ion beam and scanning/transmission electron microscopy techniques to analyze the oxide layer. Uncorrelated unirradiated particle failures throughout the series of exposures suggests that external factors inherent to the experiment increased particle failure sensitivity. However, irradiated particle observations indicated an increased failure response at 400 h 1400 °C in both 2% and 21% O2 atmospheres above failure associated with external factors. Oxide thickness measurements after 400 h at 1400 °C revealed a greater oxidation rate than predicted by parabolic growth, which was attributed to the increased complexity of the oxide structure at longer exposure times. Altering the atmosphere from 21% to 2% O2 reduced the average oxide thickness by approximately 12%–14% in both irradiated and unirradiated particles at 400 h 1400 °C. Overall, the minor variations observed between irradiated and unirradiated particles in this study led to the conclusion that unirradiated TRISO particles can be used to approximate irradiated TRISO oxidation kinetics.

X-ray tomography study on the crushing strength and irradiation behaviour of dedicated tristructural isotropic nuclear fuel particles at 1000 °C

Two types of dedicated Tristructural isotropic (TRISO) nuclear fuel particles, PyC-1 (Kernel/Buffer/PyC) and PyC-2 (Kernel/Buffer/SiC/PyC) from PYCASSO (Pyrocarbon irradiation for creep and swelling/shrinkage of objects) neutron irradiation experiments, were studied. For unirradiated particles, crushing experiments using a unique hot cell, combined with in situ X-ray computed micro-tomography (XCT) imaging, were conducted at room temperature (RT) and at 1000 °C. Although the SiC layer on the particles is presumed to provide ‘mechanical stability’ to the TRISO particles, results showed a remarkable reduction (~45%) in the crushing strength of the PyC-2 particles at 1000 °C compared to RT. The fracture patterns of the two types of particles, both at the contact zone and on subsequent propagation, differ significantly at RT and 1000 °C. Further, irradiated particles (irradiation temperature: 1000 ± 20 °C; irradiation doses: 1.08–1.23 dpa and 1.49–1.51 dpa) were imaged by XCT; 250 PyC-1 particles and 223 PyC-2 particles were studied in total and the change in radius/layer thickness in each type was examined. It was found that the buffer densification was lower in PyC-1 particles compared to PyC-2 particles, and the PyC layer shrank in the PyC-I particles, whereas it expanded in PyC-2. Results are discussed in terms of how the residual stresses can impact the high-temperature and post-irradiation behavior of these particles.

Comparative study of microscopic, spectroscopic and magneto-optic response of ferrofluids subjected to γ-radiation

Present work reports a comparative study of spectroscopic and magneto-optic properties of Fe3O4 based ferrofluids prepared with two different carrier fluid namely milli-Q-water and kerosene. The ferrofluids are labelled as FFW and FFK respectively. Both ferrofluids are subjected to γ-irradiation of dose: 2,635 Gy. Transmission electron microscopy and dynamic light scattering studies reveal that γ-photon interaction could lead to substantial growth of the dispersed nanoparticles of ferrofluids. In case of FFW, particle sizes are ~9 nm (pristine) and ~48 nm (irradiated) whereas, in FFK unirradiated particles of size ~9 nm show growth to ~18 nm after the irradiation. The role of different physical parameters (viz. viscosity, density) of carrier fluid on the growth process is considered in this regard. Viscosity of the carrier fluid is found noteworthy in the measurement of Faraday rotation response of the ferrofluids. In addition to low viscosity of water, irradiation induced particle growth as well as improved size distribution with larger particles leads to a comparatively larger enhancement of Faraday rotation of FFW than that of FFK. Consequently, as a result of γ-irradiation, Verdet constant is modified by ~70 % in the former case and by ~60 % in the latter case.

Analysis of oxygen partition equilibrium within HTGR low-enriched UO2 coated particles.

To evaluate the temperature- and burnup-dependent intraparticle CO/CO2 pressures for the design of HTGR low-enriched UO2 coated particles, a thermodynamic model has been developed, based on the equilibrium oxygen partition between UO2, carbon oxide gases and fission product oxides. The model takes into account a change in kernel stoichiometry due to temperature change and fuel burnup, and involves oxygen release correlations established by referring to measured CO content data in the literature.The analyses of measured CO contents below 1, 400°C for unirradiated particles show that the 0/U ratio of as-coated particle kernel is 2.00012.001. The 0/U ratio in irra-diated particles has been predicted to range from 2.0001 to 1.999 below 2, 000°C, indicating that the oxygen liberated by fission is consumed nearly all in forming fission product oxides and CO/CO2 gases under normal reactor operating conditions, but at extreme tem-peratures above 2, 000°C the fuel kernel will be reduced to highly hypostoichiometric composition.

Abortive transcription products of vaccinia virus are guanylylated, methylated, and polyadenylylated.

Abortive transcription products were synthesized in vitro by UV-irradiated vaccinia virus particles that were incubated with all four ribonucleoside triphosphates or by unirradiated particles that were incubated in reaction mixtures deficient in CTP or UTP. The RNA sedimented at 4 to 6S in sucrose gradients, suggesting that premature termination had occurred, presumably in one case because the DNA contained UV-induced pyrimidine dimers and in the other case because of ribonucleoside triphosphate was present at limiting concentration for transcription. Nevertheless, the short transcripts were capped, methylated, and polyadenylylated, indicating that neither completion of an RNA chain nor processing from a polycistronic precursor was required for modification of either end of the RNA. In addition, the finding of m7G(5')pppAm and m7G(5')pppGm at the 5' ends of the short RNA molecules implied that transcription was initiated with both ATP and GTP. The presence of the polyadenylic acid tract suggested that a slow-down or cessation of transcription, rather than a specific 3'-terminal sequence, served as a signal for polyadenylylation.

The Indirect Action of γ-rays on the Coat Protein of Bacteriophage Particles

SummarySuspensions of bacteriophage T4 in 25 mM phosphate buffer, pH 7·1, were irradiated with 60Co γ-rays. Measurements of the viscosity of irradiated samples before and after treatment with DNA-ase and trypsin showed that DNA is released from damaged phage particles. The phage debris acts as an effective radical scavenger and at higher doses protects the residual phage, thereby decreasing the inactivation rate. In irradiated suspensions of T4 and ΦX174 phage particles biologically-active subviral particles able to infect bacterial spheroplasts were found. Sedimentation in a sucrose gradient showed that the subviral particles of T4 have nearly the same sedimentation coefficient as unirradiated particles. Examination of irradiated samples with the electron microscope revealed that they contained many particles with the sheath contracted. The subviral particles produced by irradiation resemble those obtained with urea but do not show the tail clusters found in samples treated with urea.