At the spent nuclear fuel pools in the Fukushima Daiichi Nuclear Power Plant, hydrazine has been added to reduce dissolved oxygen in the pool water containing salts. The reduction behavior of dissolved oxygen in seawater with hydrazine in the presence of radiation is unknown. The effect of gamma ray irradiation on deoxygenation by hydrazine in artificial seawater was investigated at room temperature. We placed the artificial seawater with a small amount of hydrazine under gamma ray irradiation at dose rates of 0.3-7.5 kGy/h. The concentration of dissolved oxygen in the solutions was measured before and after the irradiation. The concentration of dissolved oxygen hardly decreased in the absence of gamma radiation in a few hours, whereas it markedly decreased in the presence of gamma radiation. The concentration of dissolved oxygen decreased with irradiation time. At this moment, hydrazine concentration decreased more than twice the dissolved oxygen concentration. This shows that some gamma radiolysis products of hydrazine act as deoxidizers. The concentration of dissolved oxygen in artificial seawater could be decreased by the addition of a small amount of hydrazine in the presence of gamma radiation at room temperature.
In the radiation dosimetry of radiocesium in Iitate, Fukushima, the level of radiocesium around the environment did not exceed the criteria in liquid phases such as puddle water, but was distributed in solid phases such as some soil types and organic matter. On the other hand, retting of the cut bamboo grass and hemlock fir in water allowed the release of radiocesium, about 230 Bq/kg exceeding the criteria for a bathing area. The flow-thru test using zeolite showed the removal of radiocesium from the liquid phase. The wet classification test was performed for 3 types of radiocesium-contaminated soil. According to the results of wet classification, radiocesium was detected and its level exceeded the cropping restriction level in almost all classified particle fractions. The decontamination effect of wet classification on radiocesium contamination was smaller than that on heavy metal contamination. Specifically, the wet classification could not induce volume reduction. Accordingly, preprocessing and intermediate treatments such as dispersion or attrition by vibration or mixing in the wet classification process were devised and examined as improved processing techniques. As a result, the effectual volume reduction of the radiocesium-contaminated soil was confirmed by adding an intermediate process such as the surface attrition in the vibrator.
Decontamination work following radiocesium exposure requires a vast reduction in the amount of contaminated soil generated. The current study subjected 4 types of contaminated soil with different properties to multistep soil washing under the same conditions. This study also determined the effectiveness of radiocesium decontamination and the extent to which the amount of contaminated soil was reduced. In addition, the effectiveness of plant matter separation, adsorbent addition, and grinding as part of multistep soil washing was determined using the same contaminated soil. Results of testing indicated that the rate of radiocesium decontamination ranged from 73.6 to 89.2% and the recovery rate ranged from 51.5 to 84.2% for twice-treated soil, regardless of the soil properties or cesium level. Plant matter in soil had a high radiocesium level. However, there was little plant matter in our soil sample. Therefore, plant matter separation had little effect on the improvement in the percentage of radiocesium decontamination of twice-treated soil. Soil surface grinding improved the rate of radiocesium decontamination of twice-treated soil. However, radiocesium in soil tightly bound with minerals in the soil; thus, the addition of an adsorbent also failed to improve the rate of radiocesium decontamination.
In this paper, the authors propose long-term projections of global nuclear power generation, uranium production, and uranium enrichment capacities by region, and estimate the trade flows of natural uranium and uranium enrichment activities in 2020 and 2035. In spite of the rapid nuclear power generation capacity growth expected especially in Asia, the natural uranium and uranium enrichment trade will not be tightened by 2020 due to the projected increase in both natural uranium production and uranium enrichment capacities, which may cause a drop in natural uranium and uranium enrichment prices. Thus, there is a great possibility that the current projects for capacity expansion will be delayed considerably. However, in the “high-demand scenario”, where nuclear expansion will be accelerated due to growing concerns about global warming and energy security issues, additional investments in uranium production and enrichment facilities will be needed by 2035. In Asia, the self-sufficiency ratio for both natural uranium supply and uranium enrichment activities will remain relatively low until 2035. However, the Herfindahl-Hirschman (HH) index of natural uranium and uranium enrichment activity trade to Asia will be lowered considerably up to 2035, indicating that nuclear capacity expansion can contribute to enhancing energy security in Asia.
If an emergency event occurs in a nuclear power plant, appropriate action is selected and taken in accordance with the plant status, which changes from time to time, in order to prevent escalation and mitigate the event consequences. It is thus important to predict the event sequence and identify the plant behavior resulting from the action taken. In predicting the event sequence during a loss-of-coolant accident (LOCA), it is necessary to estimate break diameter. The conventional method for this estimation is time-consuming, since it involves multiple sensitivity analyses to determine the break diameter that is consistent with the plant behavior. To speed up the process of predicting the nuclear emergency event sequence, a new break diameter estimation technique that is applicable to pressurized water reactors was developed in this study. This technique enables the estimation of break diameter using the plant data sent from the safety parameter display system (SPDS), with focus on the depressurization rate in the reactor cooling system (RCS) during LOCA. The results of LOCA analysis, performed by varying the break diameter using the MAAP4 and RELAP5/MOD3.2 codes, confirmed that the RCS depressurization rate could be expressed by the log linear function of break diameter, except in the case of a small leak, in which RCS depressurization is affected by the coolant charging system and the high-pressure injection system. A correlation equation for break diameter estimation was developed from this function and tested for accuracy. Testing verified that the correlation equation could estimate break diameter accurately within an error of approximately 16%, even if the leak increases gradually, changing the plant status.
It is of importance for stable operations of sodium-cooled fast reactors (SFRs) to prevent gas entrainment (GE) phenomena due to free surface vortices. The entrained gas flow rate should be below an allowance level. However, theoretical determination of universal onset conditions of GE is difficult due to nonlinear characteristics of GE phenomena. Therefore, the authors have been developing an evaluation method for GE based on computational fluid dynamics (CFD) methods. In this study, we determine a suitable CFD method for GE phenomena from several candidates through some numerical benchmarks. As a result, we obtain the following guideline for the vortex-induced gas entrainment. The free vortex flow around the vortex core can be correctly evaluated by using appropriate numerical models, such as sufficient mesh resolution, suitable advection solver, suitable turbulence and free surface modeling. From the geometrical viewpoint, the jagged description of the curved boundary using a rectangular mesh is not suitable since it damps rotating flow. As for turbulence modeling, which is especially investigated in this paper, direct numerical simulation (DNS) without any turbulent model is strongly recommended, but RNG k-ε and LES are acceptable. Lastly, we apply the recommended methods to the numerical analysis of a large-scale (>1/2) test experiment. The numerical results show good agreement with the onset condition of the GE observed in the experiments. This fact indicates that our recommended CFD methods are applicable to the GE phenomena in SFRs. In the next paper, a GE evaluation method is developed, which can calculate GE occurrences based on the results of numerical simulations performed in accordance with the simulation guideline proposed in this paper.
For the stable operation of fast breeder reactors (FBRs), the occurrences of gas entrainment (GE) phenomena should be suppressed below an allowance level. Therefore, a reliable evaluation method for the GE phenomena is necessary to determine the operating conditions of FBRs. However, such a method has not yet been established, especially for the vortex-type GE. In this paper, the authors propose a GE evaluation method in which free surface vortices are identified from velocity fields by using the second invariant of the velocity gradient tensor, and GE evaluation parameters, e.g., gas core length, are calculated by using the Burgers vortex model. In addition, the standard method for the prevention of three kinds of vortex-type GE is shown by considering experimental data, evaluation results obtained by the proposed method and comparison results. Finally, it is confirmed that the onset conditions of the vortex-type GE can be evaluated by the proposed method.