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

Nuclear Engineering and Design 374 (2021) 111066

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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.

1. 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: (A. Asahara).
/>Received 30 September 2020; Received in revised form 30 December 2020; Accepted 6 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 ( />

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 1 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

2. Methods

SEDscore = RHP × (WUD × FD)n ,

This section describes how SED scores are calculated for unit 1 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 − 1 ,
(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,
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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 1 and
2 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− 3 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 do 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 1 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 3 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− 1
for sludge and powders, 10− 5 for relatively large (< 1 ton) solid, and
10− 6 for very large (> 1 ton) components. FF of fuel debris in as-it-is is
conservatively assumed to be 10− 1. FF of core components in as-it-is are
assumed to be 10− 3 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− 6 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 4 lists those facilities for each object.

Table 1
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
1

2

3

4

5

6

7

8

9

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

9

5

3

2

Y = yes, N = no.
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Nuclear Engineering and Design 374 (2021) 111066

Table 2
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

1

2

3

4

5

6

7

8

9

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
8

U
U
U
P
L
5

U
U
U
U
L
3

U
U
U
U
U
2

L = likely, P = possible, U = unlikely.
Table 3
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 5
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 4
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.

3. 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 5. As for fuel
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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,
8 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, 2 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 6.

WUD is set assuming characteristics of objects. The setting is made by
whether each criterion is judged as yes or no by referring Table 1.
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 2 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 9 because of providing temporary containers.
WUD of building structures decrease to 3 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 2 because
of solidification or stabilization process. The setting results of WUD are
shown in Table 6.
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 2.
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 do 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. 1
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 2 orders of
magnitude (from 7 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. 2 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 7 (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. 3 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 1 and 2. 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 6
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
9

91
15

3

2

2

2

90
9

91
15

3

2

2

2

2
2

15
15

2

2

2

2

17

8

3

8

3

2

3

2

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

5

A. Asahara et al.

Nuclear Engineering and Design 374 (2021) 111066

Fig. 1. SED scores of each object in different steps of radioactive waste management.

Fig. 2. SED scores of each object after completing pre-treatment in the process of radioactive waste management.

Fig. 3. SED scores of each object after completing conditioning in the process of radioactive waste management.

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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. 4. 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.

4. 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
1
10−
10−
10−
10−

Gas
Liquid
Powder
Sludge
Discrete solid
Large monolithic, activated component

1
1
5
6

Appendix II
CF of each length of time that represents the frequency of human intervention

CF

1
10−
10−
10−
10−
10−

Length of time
1
2
3
4
5

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 –)

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