Fuel Debris Retrieval Research Articles

Towards safe, efficient long-reach manipulation in nuclear decommissioning: a case study on fuel debris retrieval at Fukushima Daiichi

ABSTRACT The potential of nuclear power in tackling the global energy crisis hinges on the development of effective decommissioning strategies. While the use of human workforce is still common practice, robotics has the potential to revolutionise decommissioning tasks. However, the restricted access, crammed environments, and harsh environmental conditions of nuclear power plants are not suited for conventional robots. In this scenario, the slender design and overall length of long-reach manipulators represent a promising solution. Yet, the same features that make these manipulators a suitable candidate, also introduce significant engineering challenges in design and control. The key challenges and hazards of nuclear decommissioning are summarized using the decommissioning of the Fukushima Daiichi nuclear power plant as a representative use case, in which a manipulator over 20 m long with 18 degrees of freedom is planned to be used for fuel debris retrieval. Critical research gaps on the state-of-the-art of long-reach manipulators for nuclear decommissioning are presented to summarise the focused key research areas ongoing and foster further research engagement within the community.

Criticality Evaluation During Fuel Debris Particle Sedimentation Using Coupled DEM-MPS Code

From the viewpoint of criticality management in the fuel debris retrieval operation at the Fukushima Daiichi Nuclear Power Station, it is important in criticality safety analyses to consider the behavior of fuel debris particles as they fall into the water, given that the neutron moderation condition of the fuel debris can dramatically change. In this study, we evaluated a reactivity insertion while fuel debris particles dropped into the water. Specifically, we considered the effects of the fuel debris particle-size distribution in either an erroneous operation or a postulated accident in the fuel debris retrieval operation. Three types of fuel debris particle-size distribution were assumed: monodisperse, uniform, and Rosin-Rammler. The behaviors of the fuel debris particles during sedimentation were evaluated using the coupled Distinct Element Method–Moving Particle Simulation (DEM-MPS) code. The multiplication factors corresponding to the behaviors of the falling fuel debris were calculated by a continuous-energy Monte Carlo code MVP3.0 with JENDL-4.0. Consequently, the multiplication factors changed with the particle motions during the sedimentation, and the trends of the multiplication factors differed between the particle-size distributions. Especially, the 2-cm monodisperse particle-size distribution showed the highest multiplication factor during sedimentation, the trend of which differed from the others in the fuel debris particles dispersing and piled-up phases in the water.

Fuel debris retrieval and robot technology

Fuel debris retrieval and robot technology

Development of Robot Simulating Fuel Debris Retrieval

A creative robot contest for decommissioning was held as part of human resources development for decommissioning of the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Co., Inc. In the 5th creative robot contest for decommissioning, robots were developed for fuel-debris retrieval from the lower part of the pedestal. Fukushima KOSEN developed a robot consisting of a primary robot that moves to an access port on the platform while a secondary robot is stored inside the main body. The robot collects fuel debris by lowering the secondary robot 3.2 m downward. Fukushima KOSEN also developed a graphical user interface (GUI) to visually express the motion of the robot.

Assessment of Spray and Pool Scrubbing Efficiencies for Means of Mitigation Against Aerosol Dispersion in the Context of Fuel Debris Retrieval at Fukushima Daiichi: Part I

Abstract The general context of this paper is an evaluation of strategies that can be used to mitigate aerosol dispersion during fuel debris or corium retrieval in damaged Fukushima Daiichi reactors. Knowledge of the aerosol source terms released during fuel debris retrieval operations is one of the key factors for assessing aerosol dispersion leading to the potential dissemination of radionuclides into the environment. Our approach is to couple experimental results from integral tests obtained during laser cutting experiments, analytical tests performed in a dedicated facility to reproduce two-phase flow such as flows representative of pool scrubbing and spray scrubbing conditions, and numerical simulations. Integral tests provide relevant information on the airborne particle release fraction during laser cutting for underwater conditions at different water depths, such as the particle concentration and particle size distribution. However, the detailed characterization of two-phase flows, such as the size and velocity of gas bubble and water droplets, is not possible during laser cutting integral tests. Therefore, a more analytical approach is necessary to obtain detailed information on two-phase flow, composed of bubbles in water, inducing pool scrubbing phenomenon, and droplets in gas generated by spray scrubbing systems, which are essential to the physical mechanisms of both processes and enable their respective efficiencies to be evaluated. The main objectives of this work were to develop models and ensure their validation based on experimental approach for predicting the pool scrubbing and spray scrubbing efficiencies in the context of fuel debris removal at Fukushima Daiichi.

Assessment of Spray and Pool Scrubbing Efficiencies for Means of Mitigation Against Aerosol Dispersion in the Context of Fuel Debris Retrieval at Fukushima Daiichi: Part II

Abstract The general context of this paper is an evaluation of strategies that can be used to mitigate aerosol dispersion during fuel debris or corium retrieval in damaged Fukushima Daiichi reactors. Knowledge of the aerosol source terms released during fuel debris retrieval operations is one of the key factors for assessing aerosol dispersion leading to the potential dissemination of radionuclides into the environment. Our approach is to couple experimental results from integral tests obtained during laser cutting experiments, well-controlled analytical tests with separated effects performed in a dedicated facility to reproduce two-phase flow such as flows representative of pool scrubbing and spray scrubbing conditions, and numerical simulations. Integral tests provide relevant information on the airborne particle release fraction during laser cutting for underwater conditions at different water depths, such as the particle concentration and particle size distribution. However, the detailed characterization of two-phase flows, such as the size and velocity of gas bubble and water droplets generated by spray systems, is not possible during laser cutting integral tests. Therefore, a more analytical approach is necessary to obtain detailed information on two-phase flow, composed of bubbles in water, inducing pool scrubbing phenomenon, and droplets in gas generated by spray scrubbing systems used for mitigating dust dispersion, which are essential to the physical mechanisms of both processes and enable their respective efficiencies to be evaluated. The main objectives of this work were to develop models and ensure their validation based on experimental approach for predicting the pool scrubbing and spray scrubbing efficiencies in the context of fuel debris removal at Fukushima Daiichi.

Mechanical properties and thermal conductivity of (U,Zr)SiO4

During the core meltdown at the Fukushima Daiichi Nuclear Power Plant (1F), molten core materials penetrated the reactor pressure vessel (RPV), and they may have reacted with the concrete at the bottom of the primary containment vessel (PCV). Through this molten core concrete interaction (MCCI), uranium could potentially dissolve into ZrSiO4 and form a (U,Zr)SiO4 solid solution. For fuel debris retrieval to proceed smoothly, detailed information on the physical properties and critically characteristic of all possible fuel debris, including MCCI products, is necessary. However, no studies have reported on the physical properties of uranium in ZrSiO4 because it is difficult to synthesize. In this study, we fabricated dense (U,Zr)SiO4 samples through the spark plasma sintering (SPS) of SiO2 and (U,Zr)O2 and analyzed the solubility of uranium in ZrSiO4. The solubility limit of uranium in ZrSiO4 was found to be within 1.6 – 2.6 at%. The mechanical properties of the (U,Zr)SiO4 solid solution were calculated using the samples’ sound velocities. To understand the effect zirconium substitution by uranium has on heat transport, the thermal conductivity was calculated using the thermal diffusivity data measured with laser flash. This is the first study on the thermophysical properties of (U,Zr)SiO4, and it provides us with a better understanding of the properties of MCCI products.

Characteristics of reactivity change as fuel debris falls in water

We investigated the characteristics of reactivity change due to fuel debris falling into water. The impact of each parameter on criticality was evaluated using a basic parametric survey. The results clearly indicated that when a large number of fuel debris pieces suddenly sediment in water, the reactivity increases rapidly. It suggested that this situation must be avoided to prevent criticality accidents during fuel debris retrieval operations. On the other hand, when fuel debris pieces gradually sediment, falling from a low height onto a floor with high friction, the sedimentation shape can easily condense to a significant height. In such cases, the criticality risk might become large once the sedimentation process is complete. Our findings indicate that this situation should also be avoided.

Study on strategy construction for dismantling and radioactive waste management at Fukushima Daiichi Nuclear Power Station

Upon the commencement of the phase-three decommissioning of Fukushima Daiichi Nuclear Power Station (FDNPS), strategy construction is a necessary step for effective radioactive waste management as well as safe project implementation. In this study, we evaluate safety and environmental detriment (SED) score of radioactive materials in the expected major stages of the project for the strategy construction. Following assumptions are made for the evaluation. The phase-three decommissioning, which is defined as active decommissioning, proceeds in the order of fuel debris retrieval, core component removal, piping & equipment dismantlement, and building structures demolition. The radioactive waste management proceeds in the order of pre-treatment, treatment, and conditioning. SED scores are calculated for each of the four groups of objects (object-base) and their total (plant-base) at a specific point of time during the active decommissioning, considering the progress of the radioactive waste management. The calculation results indicate following suggestions for strategy construction. First, treatment and conditioning of fuel debris and core components make a large contribution for reducing plant-base SED score during the active decommissioning. Second, nearly 90% of achievable amount of reduction in plantbase SED score could be realized without piping & equipment dismantlement and building structures demolition. Third, since plant-base SED score could be hugely influenced by physical form of fuel debris, it may be necessary to consider work plan from the viewpoint of cutting and containing methods. Those perspectives would be useful input to construct active decommissioning strategies together with project management parameters such as staffing, technical capability, and financial readiness.

Development and validation of an agglomeration model for CFD simulations of aerosol dispersion in the frame of Fukushima fuel debris retrieval

ABSTRACT This article is devoted to demonstrate the feasibility of laser cutting technique for the fuel debris retrieval into the Fukushima Daiichi damaged reactors. IRSN is involved in a project led by ONET Technologies, in collaboration with CEA, to evaluate the risk occurred by the dispersion of aerosols emitted by the dismantling operations [4].During the laser cutting operations of fuel debris in air condition, particles will be produced, inducing a potential risk of dispersion into the environment. Hence, evaluating the fate of these particles after their emission is one of the safety key issues for these dismantling operations. In this objective, IRSN performed CFD simulations of dispersion of particles representative of inactive fuel debris simulants (1)(2)(3)(4).The first numerical results (5) showed a quite good agreement but some improvements were needed to take into account complex particle size distributions (PSD). Consequently, this article proposes an alternative method to better initialize the agglomeration calculation, in order to take account for different kinds of particle size distribution in the CFD simulations and to better evaluate the fate of aerosols produced by the debris cutting in the damaged reactors of Fukushima-Daiichi pedestal.

Microparticles with diverse sizes and morphologies from mechanical and laser cutting of fuel debris simulants and geopolymer as a covering material

ABSTRACT During the decommissioning of Fukushima Daiichi nuclear power plant, the dismantling of massive fuel debris is important for the defueling process. The production and dispersion of radioactive microparticles highly depends on the cutting technique implemented. Previous studies have been conducted on the development of cutting techniques and the treatment of radioactive particles generation during the cutting process. Besides, adequate understanding of the microparticle products during the cuttings of fuel debris is of vital necessity. Nowadays, geopolymer application is proposed as a covering material during the fuel debris retrieval in order to keep the structural integrity of damaged components. In this study, the microparticle products during mechanical and laser cutting of fuel debris simulants (Hafnium oxide and Tungsten(IV) oxide pellets) and geopolymer material were investigated. The cuttings have been carried out for samples of the simulants, geopolymer, and simulants with geopolymer covering. The generated particles were collected and investigated by laser diffraction particle size analyzer and Scanning Electron Microscope with X-ray spectrometry. Particles with diverse sizes and morphologies were observed from the products of each sample. It also appears that particles with unique sizes and morphologies can generate from the laser cuttings due to the thermal effect of laser cutting.

Fukushima Daiichi fuel debris retrieval: results of aerosol characterization during laser cutting of non-radioactive corium simulants

ABSTRACT The production and dispersion of contaminated aerosols during the laser cutting of corium can potentially provide useful insights into the dispersion of contamination during the evacuation of damaged reactors during decommissioning. Quantitative assessments of contamination dispersion are fundamental to the development of a safety case for the decommissioning of the Fukushima Daiichi plant. This collaborative work between IRSN, ONET Technologies and CEA, managed by the Mitsubishi Research Institute on behalf of the Japanese Ministry of Economy, Trade and Industry, presents the characterization of aerosols generated during laser cutting of corium simulants both in air and under water. The objective is to obtain quantitative data for risk assessment related to the contamination released and disseminated when implementing this technique, over the next few years, in the process of decommissioning the damaged reactors. This paper presents a part of the results stemming from this project, focused on the characterization of aerosols produced during laser cutting of two representative corium simulants in air and underwater conditions. The experimental configuration also enabled investigation of the production of other material residues such as particle dross and water purity on the particulate composition of the aerosols. Ultimately, the radioisotope concentration distribution in the aerosols are transposed to radioactivity in order to assess the risk to radiation workers during decommissioning.

Numerical Simulation and Validation of Aerosol Particle Removal by Water Spray Droplets With OpenFOAM During the Fukushima Daiichi Fuel Debris Retrieval

In the decommissioning of damaged Fukushima Daiichi reactors, the melted and re-solidified fuel debris in the bottom of the reactor pressure vessel and primary containment vessel need to be cut into small pieces before removing them from reactor buildings. During the cutting operations, submicron radioactive aerosol particles are expected to be generated and dispersed into the atmosphere of the primary containment vessel. Those suspended particles must be removed from the air atmosphere inside the containment before escaping to the environment. The water spray system in the upper part of the primary containment vessel is an effective and applicable method to remove airborne radioactive aerosol particles. Computational Fluid Dynamics simulation of aerosol scavenging by spray droplets is complicated but necessary to investigate the aerosol removal process inside the vessel. In this paper, a numerical model was developed and implemented into an open-source computational fluid dynamic code OpenFOAM to simulate the aerosol removal by water spray droplets with considering the collection mechanisms of inertial impaction, interception, and Brownian diffusion. In this model, the dispersed spray droplets were described using the Lagrangian particle tracking method, the continuous particle-laden gas was described using the Eulerian method, and a two-way interaction between dispersed and continuous phases was considered. The polydisperse aerosol particles at different diameters from 0.2 – 1 μm were treated as different gas species of the continuous phase. Continuity equations of each gas specie were solved using a passive scalar transport equation. The numerical model was validated by comparing the simulation results with the experimental data obtained from UTARTS facility. Simulation results agreed well with the experimental results. The simulation results provided more insights to better understand the aerosol removal process, including the time evolution of aerosol mass fraction and flow field of the gas phase.

Characterization of chemical composition and particle size distribution of aerosols released during laser cutting of fuel debris simulants

We present here the results regarding the characterization of chemical composition and size distribution of aerosols released during laser cutting of two types of fuel debris simulants (Ex-Vessel and In-Vessel scenarios) in air and underwater conditions in the context of Fukushima Daiichi dismantling. The aerosols have systematically an aerodynamic mass median diameter below 1 μm, with particle sizes generally comprised between 60 nm and 160 nm for air cutting conditions, and larger diameters (300−400 nm) for underwater experiments. Regarding the chemical composition, iron, chromium and nickel are mainly found by more than 50 % in the samples whereas radioactive surrogates of Uranium (Hafnium) are undetectable. When compositions are transposed to radioactivity, taking into account radioisotope inventories 10 years after the accident, it is well evidenced that the radioactivity is carried out by small particles in air condition tests (median size around 100 nm) than underwater (median size around 400 nm): 50 % of the radioactivity is present in particles below 90 nm, and 99 % below 950 nm. Caesium carries the largest part of the radioactivity at all sizes below 1 μm in the case of an Ex-Vessel fuel debris simulant. For the In-Vessel, the aerosol median size for the radioactivity is situated around 100 nm, with 59 % of the radioactivity is carried by strontium, 17 % by barium and 16 % by minor actinides (modelled by cerium) and 7% by the caesium. For sizes above 1.6 μm, cerium representing alpha particles (surrogate of plutonium) is almost the only radioactivity-bearing element (96–97 % of the radioactivity). The data produced here could already be used for modelling or designing development of strategies to implement in-situ the laser cutting for fuel debris retrieval and safety associated strategies.

Phase analysis of uranium oxides after reaction with stainless steel components and ZrO2 at high temperature by XRD, XAFS, and SEM/EDX

In the Fukushima Daiichi nuclear power station accident in March 2011, fuel debris was formed when fuel materials reacted with various structural materials in reactor core. Fuel debris retrieval is expected to start in 2021 for decommissioning of the damaged plants. To perform this activity safely, it is necessary to know the properties of the fuel debris. In this study, we investigated the reaction of UO2, stainless steel (SS) components, and ZrO2 at high temperature under oxidizing and reducing conditions, and determined the valence state of uranium in the products, such as CrUO4 and (Fex, Cr1−x)UO4, by X-ray absorption near edge structure (XANES) spectroscopy. Mixed powders of UO2 and SS, Fe, and/or Cr were heated in Ar + 2% O2 (oxidizing condition) or Ar + 10% H2 (reducing condition) at a flow rate of 20 mL/min for 2 h at 1473 or 1673 K. After heat treatment, the phase relation of the products was analyzed by powder X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Under reducing conditions, UO2 did not react with SS, Fe, Cr, or ZrO2. In contrast, under oxidizing conditions, UO2 reacted with SS and Cr to form CrUO4 or (Fex, Cr1−x)UO4 at 1473 and 1673 K. When the UO2 and Fe mixture was heated under oxidizing conditions, the Fe2O3 phase coexisted with U3O8 at 1473 K, whereas FeUO4 formed at 1673 K. When the UO2, Fe, and Cr mixtures were heated at 1473 K under the oxidizing condition, the molar ratio of Fe/Cr in the (Fex, Cr1−x)UO4 phase corresponded to the initial molar ratio in the sample. As the iron content of these samples increased, all three lattice parameters of (Fex, Cr1−x)UO4 approached those of FeUO4. XANES spectra revealed that the oxidation state of uranium in CrUO4 and (Fex, Cr1−x)UO4 is pentavalent.

Proposal of a Statistical Evaluation Method for the Criticality of the Fukushima Daiichi Nuclear Power Plants

The Fukushima Daiichi Nuclear Power Plants (1FNPPs) are thought to be subcritical, but the condition will be changed during the fuel debris retrieval. Subcriticality control is one of the most important processes to eliminate the possibility of criticality through the decommissioning. For the subcriticality control, it is important to properly evaluate the status of criticality. We propose a statistical evaluation method for the criticality of the 1FNPPs with various uncertainties. Although physical parameters related to the criticality are still uncertain, conservative assumptions may lead to excessive requirements for the criticality control system. The goal of the proposed method is to construct a methodology to evaluate the realistic status of the plants based on useful information about the fuel debris observed by current and future in-core investigations and obtained by accident analysis codes. The method is composed of sampling methods for physical parameters, a criticality evaluation method based on a continuous-energy Monte Carlo code, and processing methods to evaluate the results. Physical parameters related to criticality such as debris size, porosity fraction, structure material contamination, corrosion depth, and so on are sampled from predetermined probability distributions based on knowledge for the in-core status. Calculated results are processed statistically to give probability distributions of neutron multiplication factors. From these results, physical parameters that have strong correlations with the neutron multiplication factor can be identified. In the case that the neutron multiplication factor is estimated from some other observation results, posterior distribution of physical parameters can be determined by the Bayesian estimation method. To demonstrate the method, statistical criticality evaluations are made for 1FNPP Unit 1. The fuel debris of the 1FNPP is assumed to be located at the lower plenum, the pedestal, and the drywell. The distribution of the fuel debris is located by the results of the severe accident code MAAP. Physical parameters are determined according to the characteristics list given by the fuel debris characterization project. The Bayesian estimate of stainless steel fraction based on the neutron multiplication factor evaluated by the ratio of 88Kr to 135Xe was reported. The results suggest that the criticality risk is extremely small for 1FNPP Unit 1.

Simulating covering material flow for stable fuel debris retrieval at Fukushima Daiichi Nuclear Power Station

Simulating covering material flow for stable fuel debris retrieval at Fukushima Daiichi Nuclear Power Station

A Development of Fuel-Debris Retrieval Robot for the Decommissioning Robot Contest

A Development of Fuel-Debris Retrieval Robot for the Decommissioning Robot Contest

Supercritical transient analysis in hypothetical fuel-debris systems by multi-region approach based on integral kinetic model

Criticality safety during fuel debris retrieval from the damaged cores of the Fukushima Daiichi nuclear power station (1FNPS) Units 1–3 is an important topic for the overall activity of the plant’s decommissioning. Analyses of possible criticality accidents in the fuel debris and its consequences are crucial for safe criticality control. In order to analyze a possible criticality accident, the condition of fuel debris and also potential changes in the fuel debris in terms of, for example, its geometry and the amount of water during fuel debris retrieval must first be known in detail; however, the condition of fuel debris at the 1FNPS is currently unknown. Second, an appropriate method for use in such analyses is crucial. In this study, hypothetical fuel-debris systems, i.e., systems that are to some extent possible at the 1FNPS, were considered. It is reasonable that fuel debris at the 1FNPS is coupled, whereby physically or compositionally distinct debris regions interact with one another via neutron transport. In such coupled systems, a multi-region approach is appropriate as it models a system in a time- and space/region-dependent manner. Accordingly, in the current work, we used the Multi-region Integral Kinetic (MIK) code, a code with a multi-region approach that we had previously developed. The purpose of this study was to confirm the applicability of the multi-region approach to supercritical transient in hypothetical fuel-debris systems with more complicated geometry and composition than those of our previous studies. This study also aimed to investigate the relative impact of several fuel-debris parameters on the supercritical transient. A total of fourteen hypothetical systems were made by varying the three considered parameters. As a result of this study, a region-wise power profile, temperature profile, and energy release were obtained for each system. For the systems studied herein the parameter with the largest impact on the supercritical transient was found to be the enrichment of 235U, followed in most cases by the angle of repose of the conical fragmented debris region, while the height of the consolidated debris region generally had the least impact. Our study showed that the multi-region approach can be a useful tool for analysis of supercritical transient in fuel-debris systems with rather complicated geometry and composition.

Developments of Robots for Fuel Debris Retrieval by Side Entry Method

Developments of Robots for Fuel Debris Retrieval by Side Entry Method