Possibilities of Applying Nuclear Fusion Technologies to the Development of Nuclear Weapons in Japan
Tadahiro Katsuta
In this article, I want to talk about the history of the possibilities of applying nuclear fusion technologies to the development of nuclear weapons in Japan to date. Some people are concerned about nuclear weapons ambitions in Japan because the Japanese Government strongly promotes nuclear power for commercial use. In fact, Japan has 52 nuclear power plants, which makes the country number 3 in the world, and has about 5.4 t of plutonium. Moreover, a new re-processing power plant and a fast breeder reactor is are under construction. And yet the Government insists that all these facilities are only for commercial use and they quote the IAEA’s findings that Japan’s nuclear policy is restricted to peaceful uses.
Obviously, nuclear armament is not only linked to plutonium production technologies. For example, a nuclear reactions database created for physical studies may also be used to develop nuclear armament technology. Therefore, from the view of socially responsible scientists, it is important to monitor whether the technology is being abused by politicians or policymakers. In the following, I will discuss nuclear fusion research and development (R&D) technology in Japan since the 1950’s. In this context, some basic physics of nuclear fusion is given in the appendix for readers who are not familiar with this field.
Nuclear Fusion and Nuclear Weapons
| Laser energy [MJ] | Gain of nuclear fusion power | Pulse energy of nuclear fusion [MJ] | Pulse repetition (laser) [Hz] | Electric power [MWe] | |
| Fast ignition | ~0.3 | ~80 | ~20 | 3 | 20 |
| system | 12 | 80 | |||
| 0.6 | 150 | 90 | 3.3 (10) | 100 | |
| 1.0 | 200 | 200 | 3 (15) | 240 | |
| Spark ignition | 2 | 100 | 200 | 3 (15) | 240 |
| system | 4 | 100~150 | 400~600 | ~3 (6) | ~600 |
Inertial Confinement Fusion
The Institute of Laser Engineering at Osaka University has worked hard at realizing Inertial Confinement Fusion (ICF) since the 1970’s. Although most of the research focused on a spark ignition system, a fast ignition system was developed in 2002, and it has continuously produced good results to date. This was a breakthrough in ICF research. In order to create the high pressure confinement and pellet fuel heating, the spark ignition system uses only one type of laser, whereas the fast ignition system uses two kinds of lasers for each target. As a result, the latter can operate with smaller power lasers, and proponents of this technology expect this will allow construction of a small and economically competitive reactor for commercial use in the future. The table above shows a comparison of the two systems.
What is the explosion power that can be obtained from this nuclear fusion system? When 1 kt of TNT = 4.2x1012 J, 200 MJ of pulsed nuclear fusion energy is equal to 50 kg of TNT. Accordingly, this energy is about 1/20,000 as compared to a 1 kt nuclear bomb, i.e. it would create a relatively small explosion.
However, is this power “small” for us? Let us assume a situation where a human body us exposed to the resulting neutrons. The nuclear power reaction of this system is shown as follows,
D + T → α + n + 17.6 MeV
In this reaction, D is deuterium, T is tritium, α is a helium nucleus, and n is a neutron. In this case, 7x1019 neutrons per shot are obtained as 1 eV = 1.6x10-19 J. The absorbed dose at a distance of 100 m is calculated as follows (1 Gy=1 J/kg, a mean free pass of neutrons in water is 90 kg/m2):
| 7 × 1019 [neutrons/s] × 14.1 [MeV] × 1.6 × 10-13 [J/MeV] | = 1.4 [Gy] |
| 4π · (100 [m])2;·90 [kg/m2] |
Then a dose equivalent H is represented by the following equation (quality factor is 10, correction factor is 1):
H = 14 [Gy] · 10 · 1 = 140 [Sv]
According to the International Commission for Radiation Protection (ICRP) recommendations, this exposure causes serious damage to the nerves and can lead to death of even those people who are 100 m away from the explosion.
Magnetic Confinement Fusion
The JT-60 of the Japan Atomic Energy Research Institute is one of the largest experimental tokamak plasma devices in the world. It achieved world record 1.25 for the Q-value (ratio of output to input) in 1998.
For Magnetic Confinement Fusion (MCF), tritium is needed to sustain the nuclear reaction. Tritium production and tritium handling systems accordingly cause concern that MCF could be abused for military purposes. Although MCF devices like the JT-60 are not used to conduct deuterium-tritium reaction experiments, the Hydrogen Isotope Research Center at Toyama University has been conducting tritium handling studies since the 1980’s.
The International Thermonuclear Experimental Reactor (ITER), the operation start of which is planned for approximately 2013, will use 2.8 kg of tritium (the equivalent to 1018 Bq). According to the plan, the tritium will be bred from lithium in the blanket that surrounds the vacuum vessel and neutrons from the confined plasma. These reactions are as follows,
D + T → 4HE + n
: Nuclear fusion reaction
n + 6Li → 4HE + T
: Tritium breeding reaction
n + 7Li → 4HE + T + n
: Tritium breeding reaction
9Be + n → 2n + 24He
: Neutron doubled reaction
Rokkasho village in Aomori prefecture, where nuclear cycle facilities and the U.S. Misawa Air Base are located, is the candidate site for the ITER. The Japanese government never laid open why they decided on this site despite the fact that there are better-suited sites like Tokai village in Ibaraki prefecture.
Science and Technology in Japan and Recent Political Developments
There is little doubt that the studies mentioned above investigate fusion for commercial uses, but generally it is very difficult to distinguish between R&D studies for peaceful and those for military uses. Therefore, these technologies need to be safeguarded well and the facilities need to be supervised by a third party if we are to apply the “precautionary principle” to nuclear research as is already state-of-the-art in the field of environmental studies. Although the energy released by fast ignition systems is much smaller than needed for a nuclear bomb, the technology enables laboratory-scale and reproducible experiments. Furthermore, because we talk about a large-scale simulation study and with linear characteristics (i.e. the experiment keeps proportion even when it is scaled up), it has some similarities with subcritical nuclear tests.
The same applies to tritium production and handling technology. Even though scientists not might have dual-use thoughts, we need to keep an eye on R&D in this field.
Energy policy is one example of the ambivalent policy of the Japanese government. According to the long-term energy outlook released by the Ministry of Economy, Trade, and Industry, demand for electrical electricity will saturate in the 2020s. Furthermore, with the partial liberalization of the electrical power market soon to be continuing, it is economically unwise to build additional nuclear plants, even when it comes to light water reactors. In addition, fuel cell and renewable technologies have high potential in Japan and citizens want these power types. In this situation, the Government cannot convince us that there is need to promote nuclear fusion for power production.
Those scientists and engineers who are not concerned with the dualuse potential of a technology can easily be used as fig leaves by politicians with dual-use intentions.
In 2003, a Japanese magazine conducted an interesting survey on the opinions of the young-generation members of the Japanese parliament, the Diet. The results show that over 64% responded that Japan should amend Article 9 of its Constitution (see Fig. 1). 14% thought that Japan might in the future possess nuclear weapons. Japan declared the abandonment of war in Article 9 of its Constitution, and any revision of this part of the Constitution has been a taboo in the past. However, as the relations with North Korea have been worsening, people are changing their mind. Half of those who responded to the poll said that the current diplomacy on North Korea was insufficient and indulgent. It seems that the rise of a new generation who does not know war has created a shift from “peaceful diplomacy based on disarmament” to “national security based on armament.”
Conclusions
This article briefly discussed the possibilities of applying nuclear fusion technologies to the development of nuclear weapons. It is difficult to come to any conclusions on this issue. Certainly, in Japan, the technologies examined here have so far not been directly used for military purposes. However, this does not automatically mean that the Government of Japan will exclude the possibility to possess nuclear armament also in the future. Therefore, we need to follow this issue closely.
Appendix
Whereas nuclear fusion at a high temperature or electricity production based on such technology has not yet been achieved, the required conditions for nuclear fusion reactions have been determined theoretically (Lawson Criterion). In the case of a DT reaction, it requires not only the specific temperature that causes nuclear fusion, but the output energy must be at least equal to the input energy. To sustain a nuclear fusion reaction, adequate time τ [s] and density n [cm-3] must be maintained.
Lawson Criterion:
T = 100 million K (≈10 keV) and nτ > 1014 [s cm-3]
or
nτT = 1017 [s cm-3 eV]
According to the above equations, two approaches to nuclear fusion are possible: the longer time approach or the higher density approach. The former is called Magnetic Confinement Fusion (MCF), and the latter is called Inertial Confinement Fusion (ICF) (see Fig. 2). (In the sun, the confinement energy comes from its own gravity. As for the hydrogen bomb, high density and temperature are created by the energy of previously ignited atomic bomb.)
MCF does not require high density but a longer reaction time : ν∼1014 cm-3 and τ∼1 s each. This approach needs a magnetic field for confinement, which is why it is named MCF.
ICF requires high density but not a long reaction time : n∼1026 cm-3 and τ∼10-12 s. When a high-power laser beam fires on the fuel pellet, the pellet is compressed by the counterreaction of the high-temperature plasma’s expansion. The name “Inertial Confinement” comes from the fact that the expansion needs certain time – i.e. the pellet must be kept inert for a certain period.
