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.
Nuclear Engineering and Design 374 (2021) 111066 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Study on strategy construction for dismantling and radioactive waste management at Fukushima Daiichi Nuclear Power Station Akira Asahara *, Daisuke Kawasaki , Satoshi Yanagihara University of Fukui, 1-3-33 Kanawa-cho, Tsuruga-shi, Fukui 914-0055, Japan A R T I C L E I N F O A B S T R A C T Keywords: Radioactive waste management Decommissioning SED score 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, pro ceeds 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 con struction 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 man agement parameters such as staffing, technical capability, and financial readiness Introduction The decommissioning project of Fukushima Daiichi Nuclear Power Station (FDNPS) has been steadily progressing under the Mid-and-LongTerm Roadmap towards the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station Units 1–4 (The Inter-Ministerial Council for Contaminated Water and Decommissioning Issues, 2019) The roadmap defines three phases in the decommissioning project; phase one is for preparation of spent fuel removal from spent fuel pool; phase two is for spent fuel removal and preparation of fuel debris retrieval; phase three is major part of the decommissioning project, in which the fuel debris is retrieved, structures, systems, and components of facilities are dismantled and removed, and finally the buildings are demolished to complete the decommissioning project Those activities are defined as active decommissioning in this paper Up to the present, progress has been made toward starting the phase three especially for characteriza tion of the core part; robotic systems have been deployed to survey the conditions inside primary containment vessel (PCV) of damaged units (IRID and IAE, 2018; Yamashita et al., 2020) Also, scenarios of fuel debris retrieval have been studied, which include top access with sub mersion, top access with partial submersion and side access with partial submersion to retrieve fuel debris by remote handling technologies (NDF, 2019) However, the detailed plan of the phase three after fuel debris retrieval have not been described in the roadmap On the other hand, current efforts on radioactive waste management have been devoted mainly for managing the radioactive wastes generated until now, estimating radionuclide composition in the wastes, and planning of treatment of radioactive wastes to be generated within around 10 years ahead (Sugiyama et al., 2019; TEPCO, 2019) Strategy construction is a necessary step for planning of radioactive Abbreviations: TEPCO, Tokyo Electric Power Company; FDNPS, Fukushima Daiichi Nuclear Power Station; SED, Safety and environmental detriment; RHP, Radiological hazard potential; FD, Facility descriptor; WUD, Waste uncertainty descriptor; FF, Form factor; CF, Control factor * Corresponding author E-mail address: aasahara@u-fukui.ac.jp (A Asahara) https://doi.org/10.1016/j.nucengdes.2021.111066 Received 30 September 2020; Received in revised form 30 December 2020; Accepted January 2021 Available online 29 January 2021 0029-5493/© 2021 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 waste management in the active decommissioning Prioritization of the active decommissioning is one of the significant challenges in strategy construction for the decommissioning project UK Nuclear Decom missioning Authority (NDA) has developed a prioritization measure called Safety and Environmental Detriment (SED) score for effective decommissioning and site remediation, including decontamination, dismantling, waste management, etc (NDA, 2011) Since calculation of SED scores considers both the intrinsic properties of a material and the current conditions of the material and its confining facility, it could be applied to decommissioning, site remediation, radioactive waste man agement where hazardous materials will be treated We can see some applications in literature (Jarjies et al., 2013; Utkin and Linge, 2019) Nuclear Damage Compensation and Decommissioning Facilitation Cor poration of Japan (NDF), which has a responsibility to develop technical strategic plan, has applied SED scores for prioritizing on-site activities to reduce various radiation risk relating to such as spent fuel, fuel debris, secondary waste arising from treatment of contaminated water, and concrete rubbles (NDF, 2019) However, its application is limited only to D&D activities for the time being In this study, we have studied SED scores in order to obtain useful input to strategy construction of active decommissioning and radioac tive waste management of FDNPS Since the main purpose of this study is to see the applicability of SED scores as a prioritization measure, only reactor building of unit is considered as a case study The active decommissioning is assumed to consist of fuel debris retrieval, core components removal, piping & equipment dismantlement, and building structures demolition The radioactive waste management is categorized into pre-treatment, treatment, and conditioning Details of the active decommissioning and radioactive waste management are explained in the following section By modifying evaluation criteria for the applica tion to FDNPS, 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 radioactive waste management The analysis of SED scores would be useful to construct active decommissioning strategies together with project management parameters such as staffing, technical capability, and financial readiness 2.2 Radioactive waste management Methods SEDscore = RHP × (WUD × FD)n , This section describes how SED scores are calculated for unit of FDNPS First, assumptions are made on how the active decommissioning and radioactive waste management proceed Then, the process of SED score calculations are described After that, assumptions about radio active materials and confining facilities for them are made as an input for calculating SED scores where RHP denotes the degree of hazard originating from radionuclides in materials, WUD the potential for dispersion of the radioactive mate rial due to degradation with time, FD the degree of confinability by the facility containing radioactive materials RHP is calculated based on the total radioactivity in materials and estimated to range from 100 to 1012 in the case of FDNPS WUD and FD are scored ranging from 100 to 102, respectively The n is a balancing factor so that effect of WUD and FD becomes dominant on prioritization SED score is a multi-attribute scoring method that takes account of potential impact of stored mate rials being released into the environment, not only simple radiotoxicity or surface radiation dose In this study, SED scores are calculated for each object, namely, fuel debris, core components, piping & equipment, and building structure RHP is derived from the following equation: ∑ RHP = Ai × Pi × FF × CF − , (2) During each of the above-mentioned stages of the active decom missioning, radioactive materials are generated by removing from their original places and they need to be managed as radioactive waste Ac cording to the IAEA, radioactive waste management could be catego rized into six steps: pre-treatment, treatment, conditioning, interim storage, transport and disposal Interim storage must be provided for untreated/unconditioned waste as well as for conditioned waste (IAEA, 2017) In this study, an SED score is calculated for each radioactive material before pre-treatment (defined as ’as-it-is’) and in storages after each of the three steps: pre-treatment, treatment, and conditioning, respectively Pre-treatment consists of removal of the materials from its original place by cutting/dismantling/demolishing/segregating and placement of the removed materials into temporary containers Treat ment consists of desiccation process or volume reduction process and packaging into storage containers Desiccation is applied to fuel debris and core components, while volume reduction applied to piping & equipment and building structure Conditioning is solidification or sta bilization process for onsite long-term storage Vitrification is assumed as a conditioning method for fuel debris because it is applied to high level radioactive waste produced by reprocessing of spent fuel in Japan Similarly, cementation is assumed for core components and piping & equipment as it is applied to low-level radioactive waste The condi tioning is not applied to building structure, which is to be packaged into flexible containers after being demolished In this paper, we focus on the activities that are likely to be done within the site of FDNPS, and we exclude the transport and disposal 2.3 SED score calculations SED score consists of three attributes: the intrinsic properties of a material, the current conditions of the material, and the current condi tions of a facility that contains it It can be therefore generally used as a prioritization measure in the active decommissioning and radioactive waste management The scores are calculated using the following equation: 2.1 Four stages in active decommissioning Generally speaking, decommissioning process proceeds from ’hot to cold’, namely from higher radioactivity to lower radioactivity The four stages in this assumption are made based on this approach The active decommissioning proceeds in the following order of four stages: fuel debris retrieval, core components removal, piping & equipment dismantlement, and building structures demolition The objects of those four stages are fuel debris, core components, piping & equipment and building structures, respectively The fuel debris is defined as fuel as sembly, control rod and structures inside reactor that have melted and solidified together (NDF, 2019) The core components are equivalent to equipment and structures inside the PCV such as reactor pressure vessel, steam dryer, shroud, etc The piping & equipment are components located outside PCV but inside reactor building The building structures are mainly made of reinforced concrete that makes up biological shielding and reactor building (1) all nuclides i where, Ai denotes the radioactivity (TBq) of radionuclide i, Pi the amount of water required to dilute radionuclide i for safe drinking (m3/ TBq), FF (Form Factor) the fraction of radionuclides being released if its confinement is completely lost, and CF (Control Factor) the length of time a material itself can maintain the current stabile state if safety measures against its chemical or radiological properties such as flam mability, reactivity with air or water, heat generation, etc are completely lost The worst and the best FF are 100 and 10− 6, respec tively The worst FF is given to radionuclides in the form of gas or liquid, A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 100% of which could be dispersed, while the best FF in the form of large monolithic solid or activated component, almost 0% of which could be dispersed The worst and the best CF are 100 and 105, respectively The worst CF is given to radionuclides that become unstable in ‘hours’ if its safety measure is lost, while the best CF to radionuclides that maintain its stability for ‘decades (more than 87,600 h ≈ 105 h)’ Pi of each radionuclide has been calculated using an equation in the NDA report (NDA, 2010) Tables for FF and CF are provided in Appendices Evaluation criteria of WUD and FD have been modified to meet the conditions of FNDPS 10 years after the accident, rather than legacy nuclear sites that NDA are targeting For scoring WUD in this study, considered evaluation criteria are a) generation of hazardous gases, b) dispersion due to physical degradation, c) lack of containers, and d) lack of monitoring For scoring FD, considered evaluation criteria are a) presence of significant defects, b) unsatisfaction of safety standards, c) insufficient layers of boundary, d) excess of remaining building life, and e) loss of its boundary by failures of neighbouring facilities Tables and show evaluation criteria and the corresponding numbers of WUD and FD, respectively The value of n is determined so that RHP and WUD × FD have the same contributions to SED scores in average as a whole process of radioactive waste management, using the following equation: n= ∑N=4 ∑M=4 i=object log(RHPi ) /(N × M) i ) + log(FDi ) dispersed if their containment is lost in as-it-is After pre-treatment where piping & equipment and concrete structure are cut into pieces, their FF becomes 10− as their internal surface contamination becomes dispersible CF is set based on the reactivity with other materials, considering hydrogen and hazardous gas production due to radiation decomposition of water and metallic corrosion Fuel debris in both as-it-is and pretreatment have possibility to produce hydrogen because of existing cooling water, and droplet adhered to the materials after segmentation Desiccation in treatment reduces probability of hydrogen production by reducing the amount of water contents The possibility of hydrogen production remains low after the treatment CF represent the frequency of human intervention such as monitoring or ventilation, namely, 100 for fuel debris in as-it-is, 101 in pre-treatment, 102 in treatment, and 105 in conditioning, respectively CF of core components in as-it-is is set as 103 considering the possibility of hazardous gas production due to metallic corrosion Desiccation in treatment reduces probability of hazardous gas production by reducing the amount of water contents Therefore, CF of core components in pre-treatment, treatment, and conditioning is set as 104, 105, and 105, respectively CF of the other objects is set as 105 because they not have probability of hydrogen or hazardous gas production 2.5 Radioactive inventory (3) j=step log(WUD Radioactivity of all objects in as-it-is is estimated at 10 years after the accident, when the active decommissioning is planned to begin For estimating radioactive inventory of fuels, burnup calculation results for unit of FDNPS are referred (Nishihara et al., 2012) For the other ob jects, the radioactive inventory is considered to be from two sources: activation and contamination Radioactive inventory due to activation is referred from the calculation of a reference BWR reactor (Oak et al., 1980) and that due to contamination is referred from the study on release fraction of radionuclides from the reactor core (Shibata, et al 2016) According to the reference, core shroud, piping in the primary system, and biological shielding have the dominant radioactivity in each of the three objects, respectively The radioactive inventory of the three objects is extracted from that of those dominant components As a source of contamination, Cs-137 is taken into account as the dominant radionuclide released from the fuels For estimating its radioactivity, the assumption is made that the contamination distribu tion is 0.3, 0.2, and 0.1 among core components, piping & equipment, and building structure, respectively, considering the closer the object to the fuel, the higher the degree of contamination with Cs-137 Further more, the release fraction of Cs-137 from the core is estimated to be 60% (Sugiyama et al., 2019) Table shows radioactivity and specific toxic potential (Pi) of the radionuclides that have dominant contribution to SED scores of each object where N is the number of objects i and M the number of radioactive waste management steps, respectively 2.4 FF and CF FF is set depending on physical form of object The physical form of fuel debris in as-it-is is assumed to be either sludge, powder, or discrete solid That of core components is discrete solid and large monolithic components That of piping & equipment and building concrete is large monolithic components Physical form can be changed in some steps in radioactive waste management The physical form of fuel debris does not change by pretreatment, while those of the other objects change from large mono lithic components to discrete solid by pre-treatment The physical form of all objects does not change by treatment and change into large monolithic solid by conditioning In the case of building structure, however, its physical form does not change by conditioning where crushed concrete rubbles are packaged into flexible containers FF represent the size of the physical form (NDA, 2010), namely 10− for sludge and powders, 10− for relatively large (< ton) solid, and 10− for very large (> ton) components FF of fuel debris in as-it-is is conservatively assumed to be 10− FF of core components in as-it-is are assumed to be 10− since there is external surface contamination that is more dispersible than discrete solid but less than powder FF of piping & equipment and concrete structure is assumed to be 10− although they are contaminated on their internal surface that is unlikely to be 2.6 Confining facilities for radioactive materials The potential of loss of confinement is evaluated as FD in SED cal culations The confining facilities are specified by postulating that new facilities for confining the objects will be constructed in the process of the active decommissioning An auxiliary building adjacent to the reactor building will to be constructed prior to fuel debris retrieval for receiving pieces of fuel debris, core components, and pipes & equip ment A covering structure will be installed as a containment for building structures prior to demolition Waste storage facilities will be constructed to receive wastes that have been treated or conditioned Table lists those facilities for each object Table WUD evaluation criteria and corresponding numbers Criteria a) Generation of hazardous gases b) Potential impact of degradation c) Lack of packaging d) Lack of monitoring WUD Categories 10 Y Y Y Y N N Y Y N N Y Y N N Y Y Y N Y N Y Y Y N Y Y Y N Y Y Y N N N N N N N N N 100 90 74 50 30 17 Y = yes, N = no A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 Table FD evaluation criteria and corresponding numbers Criteria Categories a) Presence of significant defects b) Unsatisfaction of safety standards c) Insufficient layers of boundary d) Excess of remaining building life e) Loss of its boundary by failures of neighbouring facilities FD 10 L L L L L 100 L L P L L 91 U L L L L 74 U L P L L 52 L L U L L 29 U L U L L 15 U U U L L U U U P L U U U U L U U U U U L = likely, P = possible, U = unlikely Table Radioactivity and specific toxic potential of dominant radionuclides of FDNPS unit-1 Objects Radionuclides Radioactivity (Ai) (TBq) Specific toxic potential (Pi) (m3/TBq) Fuel debris Pu-238 Pu-241 Am-241 Sr-90 Cs-137 Fe-55 Co-60 Ni-63 Cs-137 Co-60 Cs-134 Cs-137 Fe-55 Co-60 Eu-152 Cs-137 4.72E+03 1.38E+05 3.37E+03 1.18E+05 1.61E+05 8.90E+03 1.23E+04 1.13E+04 4.83E+04 1.97E+01 1.06E-01 3.22E+04 3.49E-01 2.77E-02 9.46E-03 1.61E+04 1.38E+08 2.88E+06 1.20E+08 1.68E+07 7.80E+06 1.98E+05 2.04E+06 9.00E+04 7.80E+06 2.04E+06 1.14E+07 7.80E+06 1.98E+05 2.04E+06 8.40E+05 7.80E+06 Core components Piping & equipment Building structure Table FF, CF and RHP of objects in different steps of radioactive waste management Objects Steps of radioactive waste management FF CF RHP Fuel debris As-it-is 1.00E01 1.00E01 1.00E01 1.00E06 1.00E03 1.00E03 1.00E03 1.00E06 1.00E06 1.00E03 1.00E03 1.00E06 1.00E06 1.00E03 1.00E03 1.00E03 1.00E+00 4.69E+11 1.00E+01 4.69E+10 1.00+02 4.69E+09 1.00E+05 4.69E+01 1.00E+03 4.05E+05 1.00E+04 4.05E+04 1.00E+05 4.05E+03 1.00E+05 4.05E+00 1.00E+05 2.51E+00 1.00E+05 2.51E+03 1.00E+05 2.51E+03 1.00E+05 2.51E+00 1.00E+05 1.26E+00 1.00E+05 1.26E+03 1.00E+05 1.26E+03 1.00E+05 1.26E+03 Pre-treatment Treatment Conditioning Core components Steps of radioactive waste management Confining facilities Fuel debris As-it-is Pre-treatment Treatment PCV Auxiliary building Waste storage facility Waste storage facility PCV Auxiliary building Waste storage facility Waste storage facility Reactor building Auxiliary building Waste storage facility Waste storage facility Covering structure Covering structure Waste storage facility Waste storage facility Conditioning Core components As-it-is Pre-treatment Treatment Conditioning Piping & equipment As-it-is Pre-treatment Treatment Conditioning Building structure As-it-is Pre-treatment Treatment Conditioning Pre-treatment Treatment Conditioning Piping & equipment Table Facilities confining objects in different steps of radioactive waste management Objects As-it-is As-it-is Pre-treatment Treatment Conditioning Building structure As-it-is Pre-treatment Treatment Conditioning debris and core components, RHP steadily decrease by ten and five or ders of magnitude from as-it-is to conditioning, respectively On the other hand, RHP of both piping & equipment and building structure does not decrease by completing waste management up to conditioning As for fuel debris and core components, the decrease in RHP from as-it-is to treatment is a change in CF because of the reduction in the amount of water surrounding those objects by their retrieval from the core and the desiccation process, while the decrease from treatment to conditioning is a change in FF as a result of solidification process As for the other two objects, the increase in RHP by completing pre-treatment is a change in FF because of their surface contamination becoming dispersible by segmentation RHP of building structure does not change by condi tioning because the process does not change its physical form In addi tion, by completing conditioning, RHP of core components, piping & equipment, and building structures are almost the same order of magnitude as that of fuel debris This is because Cs-137 originating from the accident has the dominant contribution in radioactive inventory of the former three objects In other words, RHP of piping & equipment and building structures in as-it-is after planned shutdown are three and six orders of magnitude less than the case of FDNPS, respectively Results and discussion 3.1 RHp RHP is calculated for all objects with different in radioactive waste management The calculation results are shown in Table As for fuel A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 3.2 WUD and FD connection with reactor building for transporting those objects Covering structure of reactor building will meet safety standards as a storage facility for demolished concrete but will be temporarily pre pared to demolish reactor building and likely to have limited design life as a storage facility The other confining facilities are supposed to have sufficient protections of wastes and not potential to be affected by fail ures of other on-site facilities Consequently, for the PCV, a) presence of significant defects is figured as likely, and c) insufficient layers of boundary as possible, and FD is set as 91 For reactor building and the auxiliary building, 15 is set because criteria a) presence of significant defects and c) insufficient layers of boundary are figured as unlikely with b) unsatisfaction of safety standards being likely For covering structure, is set because a) presence of significant defects and b) unsatisfaction of safety standards are figured as unlikely and d) excess of remaining building life is likely For the other confining facilities, is set because a) presence of significant defects, b) unsatisfaction of safety standards, d) excess of remaining building life, and e) loss of its boundary by failures of neighbouring facilities are figured as unlikely The setting results of FD are shown in Table WUD is set assuming characteristics of objects The setting is made by whether each criterion is judged as yes or no by referring Table The characteristics of the objects in as-it-is are postulated as follows Fuel debris and core components have a possibility of physical degra dation due to contact with water Piping & equipment are not expected to progress significant physical degradation, which is expected in building structures due to physical damage at the time of the accident Consequently, WUD of each object in as-it-is is given as follows Fuel debris and core components are given 90 because of the criteria a) generation of hazardous gases, b) dispersion due to physical degrada tion, and c) lack of waste containers being figured as yes with d) lack of monitoring being no Piping & equipment is given because of a) generation of hazardous gases, b) dispersion due to physical degrada tion, and c) lack of waste containers being figured as no Building structures is given 17 because of b) dispersion due to physical degra dation and c) lack of waste containers being figured as yes and a) gen eration of hazardous gases and d) lack of monitoring being no By completing pre-treatment, WUD of both fuel debris and core components decrease to because of providing temporary containers WUD of building structures decrease to because of the same reason as in the case of fuel debris and core components By completing condi tioning, WUD of fuel debris and core components decrease to because of solidification or stabilization process The setting results of WUD are shown in Table FD setting is done by assuming the confinability by the facility containing radioactive materials The setting is made by whether each criterion is judged as likely, possible, or unlikely by referring to Table The confinability of facilities is postulated as follows PCV has been damaged to have a possible leak of radioactive nuclides but the release of radionuclides from PCV could be to some extent contained in the reactor building Reactor building and the auxiliary building not meet safety standards as a storage facility for the objects to be removed from reactor building because part of reactor building is open due to spent fuel retrieval operation and the auxiliary building has a 3.3 SED scores Based on the calculation of RHP and the setting of FD and WUD described in Sections 3.1 and 3.2, n value is calculated to be 3.30 Fig shows SED scores of each object SED scores of fuel debris and core components have relatively high in as-it-is but hugely decrease in each step of radioactive management On the other hand, those of the other two objects have relatively low in as-it-is, even lower than that of fuel debris in conditioning, and increase in pre-treatment As for the effect of radioactive waste management, SED scores of fuel debris decreases by 21 orders of magnitude (from 24 to 3) with the process from as-it-is to conditioning, while those of building structure decrease by orders of magnitude (from to 5) only This is due to higher FF, lower CF, higher FD, and higher WUD of fuel debris and core components Plant-base SED score is calculated by summing up SED scores of each object, considering two cases where different steps of radioactive waste management are reached Fig shows the case where each object after completing pre-treatment in the process of radioactive waste manage ment Plant-base SED score decreases by the order of (from 24 to 17) and its value remained whether the active decommissioning proceeds or not This is because that fuel debris and core components own large SED scores in as-it-is, and plant-base SED score could not be decreased by progress of active decommissioning, where piping& equipment and building structure contribute little to decreasing plant-base SED score Fig shows the case where each object after completing conditioning in the process of radioactive waste management Plant-base SED score further decreases by the order of 19 (from 24 to 5) with progress of radioactive waste treatment and conditioning for fuel debris and core components Comparison between the two cases indicates that core components removal could reduce a relatively large amount of plant-base SED score under the conditions that treatment and conditioning of fuel debris are done When stabilization of radioactive waste is realized by condition ing, nearly 90% of achievable amount of reduction in plant-base SED score will take place during stage and It indicates that fuel debris and core components have inherent higher RHP than that of the other objects, which could be minimized if robust confinement of the con tainers is provided after retrieval Therefore, the priority may be given fuel debris retrieval and core components removal as early as possible in the active decommissioning from the viewpoints of keeping plant-base SED score minimum In addition, physical size of radioactive materials is expected to change in the process of pre-treatment Taking fuel debris as an example, SED scores of fuel debris is calculated hypothesizing that all the fuel debris becomes discrete solids and powders (Fig 4) It is shown that plant-base SED score in the case of powder form is four orders of Table FD and WUD in different steps of radioactive waste management Objects Steps of radioactive waste management Confining facilities WUD FD Fuel debris As-it-is Pre-treatment PCV Auxiliary building Waste storage facility Waste storage facility PCV Auxiliary building Waste storage facility Waste storage facility Reactor building Auxiliary building Waste storage facility Waste storage facility Covering structure Covering structure Waste storage facility Waste storage facility 90 91 15 2 90 91 15 2 2 15 15 2 2 17 8 3 Treatment Conditioning Core components As-it-is Pre-treatment Treatment Conditioning Piping & equipment As-it-is Pre-treatment Treatment Conditioning Building structure As-it-is Pre-treatment Treatment Conditioning A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 Fig SED scores of each object in different steps of radioactive waste management Fig SED scores of each object after completing pre-treatment in the process of radioactive waste management Fig SED scores of each object after completing conditioning in the process of radioactive waste management A Asahara et al Nuclear Engineering and Design 374 (2021) 111066 active decommissioning compared with the core components removal This is due to inherent large RHP of the fuel debris, which could be minimized by robust confinement by the containers after retrieval 2) Since nearly 90% of achievable amount of reduction in plant-base SED score could be realized by fuel debris retrieval and core com ponents removal without addressing piping & equipment and building structure, the priority will be given the efforts to deal with the fuel debris and core components as early as possible in the whole decommissioning period 3) Plant-base SED score will be hugely influenced by physical form of fuel debris It may be necessary to consider for developing work plan from the viewpoints of cutting and containing methods 4) These perspectives would be useful to construct active decom missioning strategies together with project management parameters such as staffing, technical capability, and financial readiness Fig SED scores of fuel debris with different physical form after completing pre-treatment in the process of radioactive waste management magnitude higher than that of discrete solid This means physical form of fuel debris will hugely impact plant-base SED score It may be therefore necessary to consider work plan for fuel debris retrieval from the viewpoints of cutting and containing methods CRediT authorship contribution statement Akira Asahara: Conceptualization, Methodology, Writing - original draft, Software, Investigation Daisuke Kawasaki: Writing - review & editing Satoshi Yanagihara: Writing - review & editing, Supervision Conclusions The calculation of SED scores indicates the following suggestions for strategy construction Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper 1) Treatment and conditioning of fuel debris retrieved make a large contribution for reducing plant-base SED score under proceeding the Appendix I FF of each physical form FF (Release fraction) Physical form 1 10− 10− 10− 10− Gas Liquid Powder Sludge Discrete solid Large monolithic, activated component 1 Appendix II CF of each length of time that represents the frequency of human intervention CF 10− 10− 10− 10− 10− Length of time Hours (0 – 24 h) Days (24 – 168 h) Weeks (168 – 720 h) Months (720 – 8760 h) Years (8,760 – 87,600 h) Decades (87,600 h –) References NDA, 2011 NDA Prioritisation – Calculation Of Safety And Environmental Detriment Scores, EGPR02, Revision NDA, 2010 Instruction for the calculation of the Radiological Hazard Potential, EGPR02WI01, Revision NDF, 2019 Technical Strategic Plan 2019 for Decommisioning of the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company Holdings, Inc Nishihara, K., Iwamoto, H., Suyama, K., 2012 Estimation of fuel compositions in Fukushima-Daiichi Nuclear Power Plant JAEA-Data/Code 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Tsukamoto, M., 2019 Development of calculation methodology for estimation of radionuclide composition in wastes generated at Fukushima Daiichi nuclear power station J Nucl Sci Technol 56, 881–890 https://doi.org/10.1080/00223131.2019.1595765... dismantlement, and building structures demolition The radioactive waste management is categorized into pre-treatment, treatment, and conditioning Details of the active decommissioning and radioactive waste. .. and disposal Interim storage must be provided for untreated/unconditioned waste as well as for conditioned waste (IAEA, 2017) In this study, an SED score is calculated for each radioactive material