04pSIII-4
核融合炉の安全性と安心感
Safety and Reassurance Aspects of a Fusion Reactor
小川 雄一
Yuichi OGAWA
東大新領域
Graduate School of Frontier Sciences, the University of Tokyo
1. Introduction
The accident of nuclear power plant at
Fukushima Daiichi has brought terrible damages,
and a lot of public people have been evacuated.
Since a fusion reactor is a plant to harness fusionenergy, we should carefully pay attention to safetyissues related to nuclear energy, as well. It isworthwhile to reconsider the safety issues related toa fusion reactor. In addition, since the accident of
nuclear power plant has drawn attention to energy
policy in Japan, we should explain the role offusion energy to the public.
From these viewpoints the JSPF has organized
the task force committee, in which these issues (i.e.,safety problem in the fusion reactor and the role ofthe fusion energy) have been discussed so as tosummarize an assessment to the development offusion energy. A content of the report[1] is asfollows;
1. Role of fusion energy in 21st Century
1.1 Energy problem and energy policy
1.2 Characteristics of fusion energy andintroduction scenario
2. Evaluation on safety issues for fusion plant
2.1 Safety issue on ITER
2.2 Safety issue on fusion plant
3. Radioactivity on a fusion reactor
3.1 Decay heat problem of a fusion reactor
3.2 Radioactive waste
4. Safety analysis for a fusion reactor
4.1 Safety analysis codes and V&V experiments
4.2 Safety issues for solid breeder blankets
4.3 Safety issues for liquid breeder blankets
5. Safety aspect on tritium
5.1 Environmental behavior of tritium
5.2 Biological effect of tritium
5.3 Measurement of environmental tritium
5.4 Safety analysis of tritium
6. Summary
In fission reactors, three functions (i.e., (i) stopa chain reaction, (ii) cool down a fissile fuel and(iii) confine radioisotopes) are essential for safety
securement. In the accident of Fukushima Daiichi,
since the cooling of fuel rod due to decay heat wasinsufficient, radioisotopes such as 131I, 137Cs and soon were released in the environment. Therefore,
we have reviewed safety analysis in a fusion reactor,
paying much attention to the decay heat, andre-considered safety aspect on tritium in this report.
2. Safety analysis of a fusion reactor
2.1 Decay heat
The decay heat has been analyzed for the fusion
reactor Slim-CS with PF=3GW. In Fig. 1 thetemporal evolutions of the decay heat are shown for
Out-Board (OB) blanket, In-Board (IB) blanket,Divertor and Radiation shield.

Fig. 1 Temporal evolution of the decay heat
Just after the shutdown of the reactor the total decay
heat is roughly 54 MW, which is about 1.8% of thefusion power in operation. The decay heat at theOB blanket is quite large, and the dominantradioisotope is 56Mn(T1/2=2.58 hours), which isoriginating from a ferritic steel F82H. At one day
after the shutdown, the decay heat of the divertor
becomes larger than other components. This isbecause the contribution from the tungsten decay
heat becomes dominant. The main radioisotope is187
W (T1/2=1 day).
The tungsten is a firstcandidate for the first wall and divertor at theDEMO, since the tungsten has many advantages

such as high melting temperature, low sputtering
yield and no long-life radwaste. However, fromthe viewpoint of the decay heat just after theshutdown we have to pay careful attention to thetungsten. At one month after the shutdown thedecay heat becomes around 0.1% of the fusion
power in operation.
2.2 Safety analysis of a fusion reactor
In 1990’s intensive study (so-called SEAFP and
SEAL activities) has been conducted in Europe.
Figure 2 is a typical result of the time evolutions foreach component in the case of LOCA at the PPCS
reactor, where the decay heat at the blanket
transfers to the outside of the cryostat throughconduction, radiation and convection.
The
maximum temperature was limited around 1200 C.
Here we should notice that in Fig. 2 a neutron wallloading was relatively low (i.e., 2MW/m2). The
maximum temperature strongly depends on theneutron flux. This is because the decay heat justafter the shutdown is proportional to the neutron
flux, not to the neutron fluence. Since the shortlife time radioisotope such as a few days half-life
contributes to the decay heat, the amount of theseshort life time radioisotopes saturate during a long
period operation.

Fig. 2 Temperature behavior in the case ofLOCA.

3. Safety aspect on tritium
When we discuss reassurance of nuclear plant,
comparison of the hazard potential related to total
amount of vulnerable radioisotopes might be useful.
Here we have compared hazard potential of tritiumin fusion plant with that of 131I in Light Water
Reactor (LWR). Since a total amount of tritium
depends on the plant design in a fusion reactor, weassumed a tritium of 1 kg. Comparison result issummarized in Table 1. Roughly speaking, thetotal amount of radioactive isotope (A) was oneorder smaller in fusion reactor than that in fission
reactor. While, the maximum permissible density
in the air (B) is quite different between tritium and131I; i.e., about 500 times in tritium as large as in 131I
because of difference of beta-ray energy. Based onthese values, let us define the hazard potential as a
total amount of radioisotope (A) divided by themaximum permissible density in the air (B); i.e.,
A/B value. I believe this hazard potential might
become a kind of reassurance index. If so, wecould say that the hazard potential of 1 kg tritium infusion reactor is smaller by a factor of 6800 thanthat of 131I in fission reactor.
By the way, IAEA and OECD/NEA defines theinternational nuclear and radiological event scale,from Level 1 to Level 7. In this INES report, 131I
equivalent values are defined for various
radioisotopes, and the 131I equivalent value oftritium is evaluated to be 1/50. If we use thisvalue, the hazard potential of fusion plant is around1/680 as small as that of LWR.
Table 1. Comparison of hazard potential between
fusion plant and LWR.

4. Future issues
Recently safety research was activated in theframework of BA activity in Japan, by paying
attention to following aspects;(a) difference between ITER and DEMO,(b) passive and active safety functions/tools,(c) code development and V&V experiments,(d) iteration between safety and design.
In addition to these engineering researches,
activities related to social understanding and public
acceptance might be strongly expected for thefusion energy development.
Reference[1] Report on Characteristics of Fusion Energy andSafety/Security Issues for Fusion Reactor,NIFS-MEMO-63 (2013).