Tritiated water vapor (HTO) is the dominant source among the radioactive airborne effluent from commercial nuclear power plants. Gas or liquid metal coolant is recognized to be widely used in the advanced nuclear energy systems and fusion reactor. Elemental tritium (HT) would be the dominant form regarding the environmental effluents of advanced nuclear energy systems under the oxygen-free environment. Additionally, tritium release amount would also be higher than current pressurized water reactor. A dynamic modeling scheme was proposed to simulate tritium migration and conversion behaviors in the atmosphere-soil compartments. Atmospheric tritium concentration level was predicted by hourly-resolution wind field data. With regard to HTO discharge source, HTO concentration level in soil moisture was evaluated with consideration of daily-resolution atmospheric HTO concentration data, daily-resolution precipitation and evapotranspiration data. With regard to HT discharge source, HT concentration in air-filled soil porosity, conversion product known as HTO concentration in air-filled and water-filled soil porosity was predicted from the viewpoint of time series. Influence of key parameters such as HT conversion rate and gaseous fraction of conversion product in the soil on HTO distribution was discussed at last. It was indicated that environmental factors such as wind field, conversion rate, precipitation and evapotranspiration rate, gaseous fraction have a significant influence on tritium distribution in atmosphere-soil compartment. Development of time-varying simulation method adopting the above dynamic environmental factors as input data is beneficial for achieving a refined environmental impact assessment.

Tritium discharge from the fusion system and its environmental impact receive wide attention. Considerable deviation exists in various atmospheric dispersion models for evaluating near-surface tritium concentration distribution. To quantitatively find out the diversity of these models, a performances comparison was performed based on the case of tritium discharge from the ITER site. Wind tunnel experiments were preliminarily performed to test the performances of these models. Annual average tritium concentration and individual radiation dose were assessed for some selected residential areas adjacent to the ITER site. It is indicated that atmospheric dispersion simulation by the CFD method is significantly influenced by the modeling of the turbulence. Atmospheric dispersion predicted by the CFD-RANS model with default parameters is weaker, resulting in higher downwind pollutant concentration, compared with the CFD-LES model, Gaussian plume model and Lagrangian puff model. Wind tunnel experimental results relatively support the CFD-LES model with strong turbulent dispersion. CFD-LES model is superior in reflecting the effects of complex topography in high resolution and shows strong turbulent dispersion. Individual radiation dose under normal operation at selected residential areas near ITER was estimated to be much lower than the natural radiation level and also the ITER dose limit even considering the uncertainty margin.

Tungsten is considered to be the most promising material for plasma-facing components of fusion reactors in addition to other industrial and military uses. Activated tungsten dust produced by plasma-wall interaction and released under accidental conditions would be the emerging environmental pollutant with both chemical and radiation toxicity. For the first time, atmospheric dispersion and deposition characteristics were investigated for tungsten dust with various particle size spectrum similar as generated in fusion reactors. Euler-Lagrange approach was adopted to describe parcels motion and deposition behaviors. Proper numbers of parcels were determined for the atmospheric dispersion and various parcel diameters scale. Near-surface concentration distribution and dry deposition velocity were predicted for tungsten dust with both single size dust and realistic particle size spectrum dust. Influences of chemical forms and release factors on atmospheric dispersion and deposition of tungsten dust were preliminary identified at last. It is indicated that 150 million parcels are needed for 10 mu m dust under 5 km scale. Dry deposition velocity of tungsten dust is up to 11.3 cm s(-1) and increase along with particle diameter because of its high density and gravitational deposition effect. The critical diameter judging whether gravitational deposition could appear for tungsten dust is 1 mu m. Deposited particle size spectrum would be completely different along the downwind distance which would result in differences in subsequent behaviors in other environmental compartments.

Various types of radionuclides have different atmospheric dispersion characteristics, such as buoyancy and gravitational deposition phenomenon of light gas and heavy particles, respectively. Gaussian plume model was widely used to describe atmospheric dispersion behaviors of radioactive effluents, particularly for the purpose of engineering environmental impact assessment or nuclear emergency support. Nonetheless, buoyancy and gravitational deposition were rarely reported in previous work for tritium in particular, which might cause a deviation in evaluating near-surface concentration distribution and radiation dose to the public. Based on the multi-form tritium case, we made a quantitative description for the buoyancy and gravitational deposition phenomenon and discussed the feasibility of developing an improved Gaussian plume model to predict near-surface concentration distribution. Firstly, tritium concentration distribution near to the surface was predicted by using computational fluid dynamics method (CFD) and standard Gaussian plume model to reach consistency without consideration of buoyancy and gravitational deposition effects. Secondly, effects of buoyancy and gravitational deposition were identified by species transport model for gaseous tritium and discrete phase model for droplet tritium with integrating the buoyancy force caused by density variation of gaseous tritium and gravitational force of droplet tritium with enough size. Thirdly, buoyancy and gravitational deposition correction factors were obtained to modify the standard Gaussian plume model. Lastly, predictive results by improved Gaussian plume model were compared with CFD method. It was proved the improved correction method enables higher accuracy in predicting the atmospheric concentration distribution of gaseous pollutants with density variation or particles with gravitational deposition properties.

Under natural circulation, reverse flow may occur on the primary side of inverted U-tube steam genera-tors (UTSGs). This phenomenon is significantly affected by the operating parameters of primary and sec-ondary loops of the reactor and the design parameters of the steam generator. In this study, according to the design parameters of the UTSG in the Pressurized Water Reactor Parallel Channel Test Loop (PWR PACTAL), the reverse flow phenomenon in the primary side of U-tubes under the condition of flow decline is simulated by using the computational fluid dynamics (CFD) software FLUENT. Further, the effects of the inlet temperature and pressure at the primary loop, wall thickness, inner roughness, and thermal conduc-tivity of U-tubes, feed water temperature, and flow rate of the secondary loop on the reverse flow phe-nomenon are investigated. The results show that the critical mass flow rate of the steam generator can be increased by increasing the temperature of the primary side, the operating pressure of the primary side, and the thermal conductivity of U-tubes, which boosts the flow reversal. The critical mass flow rate decreases and the reverse flow phenomenon is inhibited by increasing the mass flow rate and tempera -ture of secondary side and the inner wall roughness of U-tubes. Further, changing the wall thickness of U-tubes has a negligible effect on the reverse flow phenomenon. Compared with the temperature of the sec-ondary circuit, the temperature of the primary circuit has a more substantial effect on the reverse flow phenomenon. The conclusions can be used as reference for UTSG design and its operation under natural circulation.(c) 2022 Elsevier Ltd. All rights reserved.