Plasma and Fusion Research: Regular Articles

Volume 11, 2405077 (2016)

Conceptual Design of Temporally Storage Area in Hot Cell for
Fusion DEMO Reactor∗)
Masatoshi KONDO1) , Youji SOMEYA2, a) , Mitsuyo TSUJI3) , Satoshi YANAGIHARA4) ,Hiroyasu UTOH2, a) , Takashi KATO, Kenji TOBITA2, a) and Shinzaburo MATSUDA1)
1)

2)

Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan
Japan Atomic Energy Agency, 2-166 Omotedate, Obuchi, Rokkasyo, Aomori 039-3212, Japan
3)
Tokai University, 4-1-1 Kitakaname, Hiratuka, Kanagawa, 502-5292, Japan
4)
University of Fukui, Fukui 914-0055, Japan(Received 22 November 2015 / Accepted 26 March 2016)

In the maintenance procedure of fusion DEMO reactor, used in-vessel components are removed from thereactor vessel to be replaced. The temporally storage area for the used blanket segments is necessary. In thepresent work, the temporally storage area is conceptually designed as the used segments are stored dispersively
in the plural compartments in the hot cell for the following dismantling procedure. The space dose rate of thecompartment, where more than one segment is installed, was evaluated by means of the gamma ray transport
calculation with PHITS Monte Calro analysis code. The space dose rate in the compartment decreases with time.
For example, by the temporally storage of five used segments in one compartment for 20 years, the space dose ratein the compartment becomes lower than 250 Gy/hr, which is the limited value proposed for the remote handling in
the ITER. The decay heat of the segments during the temporally storage is removed by flowing He. The segments
are cooled at the temperature lower than 550 ◦ C. Then, the used back plates can be reused in the other operationof the reactor, since the mechanical properties of the back plates are not aﬀected by the heat treatment during the
temporally storage.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: hot cell, segment, waste management, low-level waste, remote handling
DOI: 10.1585/pfr.11.2405077

1. Introduction
The maintenance scenario and the waste managementof fusion DEMO reactor have been proposed [1–3]. In themaintenance procedure of the reactor, used in-vessel components are removed from the reactor vessel to be replaced.
Then, the low-level radioactive waste (LLW) of approximately 6000 t is generated. The quantity of the radioactive
waste can be reduced by 60 % in total, if some parts of
the used blanket components are reused and recycled forthe other operation of the reactor [4]. However, these areradioactivated and having a large decay heat. Their dismantlement is available by the remote handling (RH) [5]
only after their temporally storage for a certain period in
the hot cell until the radiation level becomes low to be allowed for the RH [6]. Therefore, the temporally storage ofthe blanket components in the hot cell is the essential procedure for the waste management of the fusion reactors.
However, the study on the temporally storage of the usedblanket components in the hot cell is quite limited.
author’s e-mail: kondo.masatoshi@lane.iir.titech.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).
a) Present aﬃliation: National Institutes for Quantum and Radiological
Science and Technology, 2-166 Omotedate, Obuchi, Rokkasho, Aomori
039-3212, Japan

It is proposed that the used blankets are removed fromthe vacuum vessel (VV) though the vertical port [7, 8] asmulti module segments, which are the back plate integrated
blanket module. These segments must be stored temporally in the hot cell after the removal of residual tritiumby thermal detritiation process [9]. During the temporally
storage of the segments, their decay heat must be removed.
The segments must be cooled at the adequate temperature
to suppress the eﬀect of the heat treatment on their mechanical properties for their reuse and recycle.
In the present study, it is proposed that the used blanket segments are stored dispersively in the plural compartments of the temporally storage area in the hot cell. The
motivation for this system is to achieve the reduction of
the risk and the mitigation of the cooling conditions of
the decay heat without the increase of total floor space required for their storage area. The changes of the space dose
rate and the temperature condition in the compartment withtime were investigated by means of gamma ray transport
calculation and thermofluid calculation. The purpose of
the present study is to propose the conceptual design of thetemporally storage area in the hot cell.

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

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2. Conceptual Design of Temporally
Storage Area in Hot Cell
Main parameters of the fusion DEMO reactor are presented in Table 1 [5]. This reactor is assumed to be periodically maintained every two years. After the shutdown of
the reactor, the in-vessel components are cooled in the VV
for one month. 80 pieces of the multi module segments(i.e., back plate integrated blanket segments) are removed
from the reactor every two years. The weight of the segment is approximately 67 t.
The temporally storage area for the used segments in
the hot cell is divided into small compartments. Figure 1
Table 1 Main parameter of fusion DEMO reactor [5].

(a) shows the simplified model of the segment, which has
a rectangular parallelepiped shape, though the real shapeof the segments has an arch-shape. This model is used toevaluate the space dose rate and the temperature conditionsin the temporally storage compartment.
Figure 1 (b) shows the structure of the compartment.
The wall of the compartment is made of a heavy concrete. The inner surface of the wall is covered by the lining made of stainless steel to suppress the transport of tritium released from the segment during the storage. The
coolant for the decay heat removal of the used segments
is a flowing He because tritium should be recovered fromthe coolant during the storage. The heavy concrete wall iscooled by flowing water using embedding piping system.
The plural segments are placed in the compartment
and arranged as their blanket sides facing each other tohave the beneficial eﬀect of the radiation shielding by theirback plates. In this case, the dose rate in the backward
space of the back plates is low. This space can be available
for the installation of the devices required for the monitoring system, such as the temperature measurement of thesegments. The segments are arranged as a circle, and theFig. 1 Structure of (a) simplified model of the segment having a rectangular parallelepiped shape and (b) temporally storage compartment.

Fig. 2 Plan view of segment installation with interval of 0.25 m in compartment (a) 3 segments installation, (b) 4 segments installation,(c) 5 segments installation, (d) 8 segments installation.

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example for the installation of plural segments in one compartment is shown in Fig. 1 (b) and Fig. 2. Then, the total
floor space, S total , required for the temporary storage of all
the segments is expressed as2n
S total = (W + 1) × + (F + D + G) × 2
π×

Nsegments.
n(1)Here, the width, W, and the thickness, D, of the segment
are 1.5 m and 1 m, respectively. The interval between theadjoining segments is given as I. The number of the segments installed in one compartment is given as n. The
number of the segments in total, Nsegments , is 80. The thickness of the heavy concrete wall, F, is 1 m. The gap between the segment and the wall, G, is 1 m. The height ofthe compartment assumed as 22 m considering the space
for the installation device of the shipping cask [8].
The total floor space required for the storage of 80
segments is evaluated by Eq. (1), and the results are shownin Fig. 3. The total floor area for the temporally storage
becomes larger by increasing the number of segments installed in one compartment as shown in Fig. 3 (a). It is also
found that the curve of the graph has a downward convex when the number of segments and/or the interval aresmall. Then, there are some cases which can achieve thespace-saving by the installation of plural segments in thecompartment.
Figure 3 (b) shows the influence of the interval (I)on the total floor area of the temporally storage. The arrangement of the segments with smaller interval results tosmaller total area. The increasing ratio of the total floorarea by increasing the interval becomes larger when thenumber of segments installed in one compartment is larger.
On the contrary, the influence of the interval on the total
storage area is negligibly small when the interval is lessthan 0.5 m. The decay heat removal by flowing He shouldbe achieved even when the segments are installed with thesmall interval. The conditions of the interval: I = 0.25 m
and the gap: G = 1 m are selected as one example in thecurrent work based on above mentioned consideration.

3. Conceptual Design of Temporally
Storage Compartment in Hot Cell
3.1 Evaluation of space dose rate in temporally storage compartment in hot cell
Figure 4 shows the concentration of radioactive nuclides in the blanket and the back plate after the shutdown, which were obtained by the neutronic calculation
by THIDA-2 code with the nuclear data library FENDL2.0 [5]. The radio nuclides, which mainly influence on thedose rate and the decay heat, are 54 Mn, 55 Fe and 56 Mn.
56
Mn has short half-life of 2.5785 h. 54 Mn and 55 Fe emit
gamma ray of 0.8 MeV and characteristics X ray of 6 keV,
respectively. The contribution of latter one on the dose rate
and the decay heat is small. Therefore, the space dose ratein the temporary compartment was evaluated as the segments emit 0.8 MeV gamma ray in the present work.
The wall of the compartment is made of a heavy concrete for the purpose of the radiation shielding. Table 2
presents the chemical composition of the heavy concrete.
The linear attenuation coeﬃcient of the heavy concrete is(a)Fig. 4 Induced radioactivity of blanket and back plate.

(b)
Table 2 Chemical compositions of heavy concrete (wt%).
Fig. 3 Total floor space required for temporally storage (a) effect of installation number, (b) eﬀect of interval between
segments.

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Fig. 5 Space dose rate on horizontal plane at level of 5 m from floor surface in compartment after one month of shut down, (I) 4 segments
installation, (II) 5 segments installation, (III) 8 segments installation, (IV) 10 segments installation, (V) 16 segments installation,(VI) 20 segments installation and (VII) 5 segments installation with larger interval and gap.

estimated as 1.53 × 10−1 cm−1 . The thickness of the heavy
concrete is 1 m, and then the energy of gamma ray can be
reduced to less than 1 eV by this heavy concrete wall.
The space dose rate in the compartment time is simulated by the gamma ray transport calculation with PHITS
Monte Calro analysis code. The geometries are shownin Figs. 1 and 2. The segments are cooled in the VV for
one month. Therefore, the gamma ray transport calculation was performed with the conditions, where the induced
radioactivity of the used segments corresponds to that after
one-month cooling in VV (Fig. 4).
Figure 5 shows some examples of the gamma raytransport calculation of the temporally storage compartment. Fig. 5 indicated the distribution of the space dose
rate on a horizontal plane at the level of 5 m from the floor
surface. The total floor space of the temporary storage
area in these cases is indicated in Fig. 3 (a). The dose ratearound the center region is higher than the other part in thecompartment. The space dose rate in a backward space ofthe back plate is lower than the other part. The space dose
rate in the backward space is lower than 250 Gy/h even at
the start of the temporally storage. Figure 5 (VII) shows the
distribution of the space dose rate when the segments are
installed with larger interval and gap. The backward spaceof the segments with small dose rate is larger than that by
the arrangement with narrow interval and gap (Fig. 5 (II)).
However, total floor space of the temporally storage area

becomes larger as shown in Fig. 3 (b).
The pentagonal arrangement of the segments has theadvantage as all the diagonal lines between the segments
are the same length. These diagonal lines do not intersect
in one point. These features make the gentle distributionof the dose rate in the compartment, and result to suppress
the formation of the hot spot in the compartment. Thisfeature may allow the margin for the consideration of thedismantle procedures in the hot cell and/or the transport ofthe dismantled segments. The calculation results for thedose rate and the decay heat removal in the case of pentagonal arrangement of the segments in the compartment are
mainly reported in this paper.
Figures 6 (a-1) and (a-2) show the distribution of thespace dose rate in the compartment one month after the
shutdown of the reactor. The dose rate at the center region is higher than the other part in the compartment. The
dose rate in the center region has the peak around thelevel of 5 m from the floor surface, where is indicated as
Point D in Fig. 1. The dose rate in the VV of the ITER
after the plasma shutdown is estimated as 250 Gy/h, andthe in-vessel components are going to be maintained by
RH [4]. Therefore, it is assumed that the segments canbe monitored and dismantled by RH under the dose rateof 250 Gy/h based on the technological basis for the ITER
RH. Figures 6 (b-1) and (b-2) shows the dose rate in thecompartment 10 years after the shutdown. The dose rate

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Fig. 6 Space dose rate of temporally storage compartment (a-1) X-Z cross section after one month from shutdown (a-2) X-Y cross section
after one month from shutdown (b-1) X-Z cross section after 10 years from shutdown (b-2) X-Y cross section after 10 years fromshutdown.

Fig. 7 Dose rate at center of compartment and at Point D in
Fig. 1 (b).

becomes lower than 250 Gy/h except for the center region,and the distribution of the space dose rate becomes small.
Figure 7 shows the change of the dose rate at the Point
D with time. The space dose rate in all the part of thecompartment becomes lower than 100 Gy/h after the temporally storage for 20 years.

3.2

Removal of decay heat in temporally
storage compartment of hot cell

Figure 8 shows the decay heat for one segment. After the cooling of the segments in the VV of the reactor,
the used segments are transported from the reactor vesselto the compartment using the shipping cask [8]. The total floor space of the temporary storage area is estimatedas less than 1500 m2 (Fig. 3). For example, the maximum
moving distance of the segment in the temporally√ storage area is roughly estimated as 77.5 m (= 2 × 1500)
based on the assumption as the temporary storage area has

Fig. 8 Decay heat of blanket and back plate in one segment.

a square shaped floor. It is assumed that the transport speedof the segment by an overhead crane may not be less than
1 m/min considering the transport speed of the spent fuelin the spent fuel reprocessing plant [10]. Therefore, thetime required for the segment transport must be less than
1.5 hour. Then, the time for the transport of the segmentby the cask including the mounting and the dismounting of
the segment into/from the cask can be roughly estimated asless than 5 hours. The temperature increase of the segmentby the decay heat in this procedure is less than 30 K.
Architectural Institute of Japan [11, 12] proposed thatthe limiting temperature of the heavy concrete wall is
338 K. Therefore, the temperature of the heavy concrete
wall (T2 in Fig. 1 (b)) in the temporally storage compartment should be lower than 65 ◦ C. The temperature of thesegments installed in the compartment is evaluated by the
CFD code PHOENICS. Here, the decay heat after the cooling for one month (Fig. 8) in the VV is applied in the cal-

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Table 3 Thermal properties of materials used in layer structure of wall.

culation to check the coolability of the compartment. The
decay heat of the segment is removed by flowing He in
the compartment. The inlet port having a rectangular crosssectional channel is located on the top of the compartment
as shown in Fig. 1 (b). Two outlet ports having a rectangular cross sectional channel are located at the bottom partof the side walls. The size of these ports is 1 m2 . The
flow rate of the flowing He is 60 m3 /min. The wall of thecompartment has the layer structures as shown in Fig. 1 (b).
The thermal properties of the materials used in the wallis summarized in Table 3. The coolability of the heavy
concrete wall is roughly evaluated with one-dimensionalmodel. Here, the Reynolds number of flowing water is2567, and the bulk temperature (Tb in Fig. 1 (b)) is 303 K.
Then, the surface temperature of the concrete wall (T2 in
Fig. 1 (b)) can be kept as less than 313 K by the flowing water, even when the temperature of the T1 is 473 K. Therefore, it is found that the temperature of the concrete wall is
kept at the temperature less than 313 K by the water cooling.
Figure 9 shows the temperature distribution in thecompartment. The temperature of the back plates should
be lower than 550 ◦ C to suppress the eﬀect of this heat
treatment on their mechanical properties for their reuse. It
was found that the temperature of the segments was aroundthe 150 ◦ C at the steady state condition.

4. Conclusion
The temporally storage area for the used blanket segments is conceptually designed to investigate in detail themaintenance procedure of fusion DEMO reactor. Major
conclusions are follows;(1) The used segments are stored dispersively in the plural compartments in the hot cell for the following dismantling procedure. The total floor space required forthe temporally storage of 80 segments are estimatedas less than 1500 m2 .
(2) The wall of the temporally storage compartment,
which is made of a heavy concrete, has high shielding
performance for the γ ray emitted from the used segments. The linear attenuation coeﬃcient of the heavy
concrete is estimated as 1.53 × 10−1 cm−1 .
(3) The distribution of the space dose rate in the compartment, in which five segments were installed, wasevaluated by means of the gamma ray transport cal-Fig. 9 Temperature distribution in compartment by 5 segments
installation.

culation with PHITS Monte Calro analysis code. The
backward space of the segments has lower space dose
rate than that in the other part. This distribution isdue to the beneficial eﬀect of the back plate as theradiation shielding. The space is available for the installation of some devices required for the monitoringof segment’s temperature.
(4) The space dose rate in the temporally storage compartment decreased according to the decay of the radioactive nuclides in the used segments. The spacedose rate in the compartment becomes lower than thevalue proposed for the remote handling in the ITER
when five segments are cooled in the compartment for
10 years.
(5) The results of the thermofluid simulation using the
PHOENICS code showed that the decay heat during the temporally storage of the segments can be removed by flowing He. It was found that the segments
can be kept at the temperature lower than 550 ◦ C tosuppress the eﬀect of the heat treatment on the me-

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chanical properties of the back plate. Then, the reuseof the used back plates in the other operation of thereactor is available, and significantly reduces the generation of the low-level radioactive waste.

Acknowledgments
This work was partially supported by the Japan
Atomic Energy Agency under the Joint Work contract
27K422, as a part of Broader Approach activities. The
author would thank Mr. Y. Nakajima in Tokai university
for the technical support on gamma ray transport calculation and Mr. Y. Noguchi in QST for the fruitful discussion
about the remote maintenance technologies of fusion reactors.
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