Experimental investigation on effective aerosol scavenging using different spray configurations with pre-injection of water mist for Fukushima Daiichi decommissioning

During the decommissioning of the Fukushima Daiichi nuclear power plant,it is important to consider the retrieval of resolidified debris both in air and underwater configurations.For the subsequent retrieval of debris from the reactor building,the resolidified debris must be cut into smaller pieces using various cutting methods.During the cutting process,aerosol particles are expected to be generated at the submicron scale.It has been noted that such aerosols sizing within the Greenfield gap(0.1-1μm)are difficult to remove effectively using traditional spraying methods.Therefore,to improve the aerosol removal efficiency of the spray system,a new aerosol agglomeration method was recently proposed,which involves injecting water mist to enlarge the sizes of the aerosol particles before removing them using water sprays.In this study,a series of experiments were performed to clarify the proper spray configurations for effective aerosol scavenging and to improve the performance of the water mist.The experimental results showed that the spray flow rate and droplet characteristics are important factors for the aerosol-scavenging efficiency and performance of the water mist.The results obtained from this study will be helpful for the optimization of the spray system design for effective aerosol scavenging during the decommissioning of the Fukushima Daiichi plant.

Investigation on Cutting Stability of Jacket in Decommissioning Process

Jacket cutting operation is one of the most complicated and highest risk operations in the process of decommissioning offshore piled platform, the security and stability of which must be assured. In this paper, the current research on offshore structure removal and jacket cutting is introduced, on the basis of which the types of load along with the load calculation method are determined. The main influences on the stability of a jacket in cutting are analyzed. The experiment test plan is drawn by using orthogonal testing method, and the formula of critical load during the cutting procedure is deduced by differential evolution algorithm. To verify the method and results of this paper, an offshore piled platform to be decommissioned in the South China Sea is taken for an example, and the detailed schedule for jacket cutting is made with the three-dimensional finite element model of the jacket established. The natural frequency, stress, strain and stability of the jacket during cutting process are calculated, which indicates that the results of finite element analysis agree well with that of the deduced formula. The result provides the scientific reference for guaranteeing the safety of jacket in cutting operation.

Estimation of exposure dose for decontamination workers from contaminated soil at a nuclear decommissioning site in Korea

Assessment of the exposure dose for workers is crucial to protecting workers from the radiological risk.This preliminary study estimates the potential radiological exposure for a soil remediation worker at a nuclear decommissioning site contaminated with Cs-137 in Korea,and then calculates the maximum workable soil concentration to comply with the occupational dose constraint of 20 mSv per year.The Korean characteristic data,detailed exposure scenarios for workers by the type of work,and relevant exposure pathways were used in the dose estimation.As a result,the most severe exposure-induced work type was identified as the excavator operation with an annual individual dose of 5.92×10^-5 mSv for a unit concentration of soil,from which the derived maximum workable soil concentration was 3.38×105 Bq/kg.Furthermore,dose contribution by each exposure pathway was found to be decreased in the following order:external radiation exposure,soil ingestion,dust inhalation,and skin contamination.The results of this study are expected to be used effectively to optimize radiation protection for workers and establish appropriate work procedures for future site remediation.

Prototype of an Echo-PIV Method for Use in Underwater Nuclear Decommissioning Inspections

The ongoing decommissioning of the Fukushima Daiichi (1F) nuclear power plant requires the inspection of the inside of containment vessels that have been submerged in water. These inspections must locate leaks and map the distribution of fuel debris in water with very low visibility. This paper reports the design and testing of an echo-PIV system that uses a single divergent signal wave and delay-and-sum processing to efficiently map the interior and fluid flow within a submerged vessel. The diverging wave and delay-and-sum processing improve upon the performance of conventional ultrasound PIV methods specifically to meet the demands of containment vessel inspections. The imaging method uses an ultrasonic linear sensor array that emits a diverging wave that covers a wide angle with a single transmission. The delay-and-sum algorithm combines echo signals recorded by each element of the sensor array. We optimized the design of an echo-PIV system in laboratory-scale tests, and then tested the prototype with a mockup of a containment vessel in a water tank. The small-scale prototype successfully located a mock leak and mapped the surface of a piece of mock debris. This prototype can be scaled up readily for inspections at the Fukushima Daiichi plant.

A review of multi-attributes decision-making models for offshore oil and gas facilities decommissioning

With the development of the offshore fossil energy industry,the designing life of many offshore oil and gas facilities will end.The decommissioning of these facilities has become an urgent task due to unpre-dictable costs,high risks,and environmental protection issues of public concern.Decision-making,as the core in the pre-decommissioning stage,plays a decisive role in the cost,risk,and impact of the entire de-commissioning.Therefore,the multi-attribute decision-making model has attracted much attention from industry and academia.An efficient,accurate,and simply using multi-attribute decision-making model can enable governments,energy companies,other marine users,and environmental protection organi-zations to reasonably fulfill their concerns.It is of great significance to all parties.This review mainly studies the multi-attribute decision-making models that have been used in the decommissioning of off-shore oil and gas facilities,and conducts a more detailed interpretation of them,including the relevant regulations,frameworks,methodology,preferences and advantages and disadvantages of different mod-els.In addition,a more comprehensive review of the cost assessment model,an important part of the decision-making model,is carried out,including the general framework and methodology of the cost as-sessment model,and the accuracy of the models is explored.And then the current evaluation method of accuracy of the cost assessment model raises the author’s personal doubts.At the end of the article,this paper names two core problems of the current decision-making model,that is,the lack of basic data and the incomplete MCDA(Multi-criteria Decision Analysis)method.This review can provide a comprehen-sive reference and feasible research directions for future scholars who aims to study the decommissioning of offshore oil and gas facilities especially in the North Sea in the UK and point out the direction for the industry to improve its current multi-attribute decision-making models.

Resource and waste quantification scenarios for wind turbine decommissioning in the United Kingdom

Wind power produces more electricity than any other form of renewable energy in the United Kingdom(UK)and plays a key role in decarbonisation of the grid.Although wind energy is seen as a sustainable alternative to fossil fuels,there are still several environmental impacts associated with all stages of the lifecycle of a wind farm.This study determined the material composition for wind turbines for various sizes and designs and the prevalence of such turbines over time,to accurately quantify waste generation following wind turbine decommissioning in the UK.The end of life stage is becoming increasingly important as a rapid rise in installation rates suggests an equally rapid rise in decommissioning rates can be expected as wind turbines reach the end of their 20-25-year operational lifetime.Waste data analytics were applied in this study for the UK in 5-year intervals,stemming from 2000 to 2039.Current practices for end of life waste management procedures have been analysed to create baseline scenarios.These scenarios have been used to explore potential waste management mitigation options for various materials and components such as reuse,remanufacture,recycling,and heat recovery from incineration.Six scenarios were then developed based on these waste management options,which have demonstrated the significant environmental benefits of such practices through quantification of waste reduction and greenhouse gas(GHG)emissions savings.For the 2015-2019 time period,over 35 kilotonnes of waste are expected to be generated annually.Overall waste is expected to increase over time to more than 1200 kilotonnes annually by 2039.Concrete is expected to account for the majority of waste associated with wind turbine decommissioning initially due to foundations for onshore turbines accounting for approximately 80%of their total weight.By 2035-2039,steel waste is expected to account for almost 50%of overall waste due to the emergence of offshore turbines,the foundations of which are predominantly made of steel.

Development of Measurement System for Flow and Shape Using Array Ultrasonic Sensors

In Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the fuel debris formed in the Reactor Pressure Vessel (RPV) and Primary Containment Vessel (PCV) at Unit 1 - 3. To accelerate and decide further decommissioning steps of the FDNPP, it is crucial to obtain realistic information of the debris and localize contaminated water leakage from PCV. Due to high radiation and dark environment inside the PCV, investigating instruments and techniques should necessarily to meet specification of radiation resistance, waterproofness, dust resistance and so on. This study focuses on development of ultrasonic measurement system using a couple of sectorial array sensors to localize contaminated water leakage and visualize shape of object that repre- senting fuel debris, simultaneously. In this study, Total Focusing Method (TFM) and Ultrasonic Velocity Profiler (UVP) methods are considered to visualize object shape and flow pattern around it, respectively. To demonstrate applicability and reliability of developed measurement system with sectorial array sensors, a mock-up experiment result of simulated water leakage and fuel debris shape were discussed in this paper.

Nondestructive high-sensitivity measurement method for activation estimation in accelerator room concrete

This study established a method for easily and quickly estimating the specific activity produced in the concrete walls and floors of accelerator rooms during long-term operation of accelerator,for advanced zoning of activated/nonactivated areas in planning the decommissioning of an accelerator.We propose a new,highly sensitive method for nondestructively estimating the specific activity in concrete that can be applied to activation zoning.In this method,instead of direct determination of the specific activities of important long-half-life radionuclides for decommissioning,such as 152 Eu and 60 Co,we determine the specific activities of short-half-life radionuclides,24 Na and 56 Mn,in situ to obtain neutron flux.The obtained neutron flux and accelerator operation history yield the specific activities of 152 Eu and 60 Co for the advance zoning of activated/non-activated concrete.This method is a powerful long-term prediction tool for concrete activation.

Evaluation of activated area in the electrostatic accelerator facilities

In order to clear the activated area in electrostatic accelerator facilities,four accelerator facilities were selected and typical neutron emission experiments were performed.Neutron flux during operation and induced activity caused by charged particles on the accelerator and its surrounding area after irradiation were measured.Also the monitored neutron flux and calculated value by Monte Carlo calculation using PHITS code were compared.It was confirmed that the results between calculated data and measured data showed the good agreement with each other.Finally,it was concluded that we have to take care the activation of beam line and target.But,it is not necessary to treat accelerator tank,surrounding materials,and building concrete as radioactive materials in case of decommissioning.

Analysis of molten metal spreading and solidification behaviors utilizing moving particle full-implicit method

To retrieve the fuel debris in Fukushima Daiichi Nuclear Power Plants(1F),it is essential to infer the fuel debris distribution.In particular,the molten metal spreading behavior is one of the vital phenomena in nuclear severe accidents because it determines the initial condition for further accident scenarios such as molten core concrete interaction(MCCI).In this study,the fundamental molten metal spreading experiments were performed with different outlet diameters and sample amounts to investigate the effect of the outlet for spreading-solidification behavior.In the numerical analysis,the moving particle full-implicit method(MPFI),which is one of the particle methods,was applied to simulate the spreading experiments.In the MPFI framework,the melting-solidification model including heat transfer,radiation heat loss,phase change,and solid fraction-dependent viscosity was developed and implemented.In addition,the difference in the spreading and solidification behavior due to the outlet diameters was reproduced in the calculation.The simulation results reveal the detailed solidification procedure during the molten metal spreading.It is found that the viscosity change and the solid fraction change during the spreading are key factors for the free surface condition and solidified materials.Overall,it is suggested that the MPFI method has the potential to simulate the actual nuclear melt-down phenomena in the future.