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The Only Way to Finish 239Pu Off Is To Use It / プルトニウム239は使わなければ無くならない

In this article I argue that we must have a new kind of nuclear reactor in Japan.

Nuclear reactors are indispensable part of society of Japan. In Kansai region, where I live in, 50% of electricity is generated by nuclear reactors, and we cannot cut off as much as half of current power consumption.

Nevertheless, we experienced Fukushima and are under strong pressure to reduce nuclear reactors gradually —- lack of electricity won’t be a problem because demand for electric power will be decreasing due to rapid fall of the population of Japan —- another problem remains.

All of nuclear reactors in Japan, that are light water reactors, are designed to use enriched uranium, that is, mixture of 235U and 238U. In the reactors 235U, which are fissile, capture low-speed neutron and break in several lighter radioactive atoms like cesium and iodine. In this process 235U also produce high-speed neutrons. These high-speed neutrons slow down while traveling in the water that fills the nuclear reactors, and become low-speed neutrons.

The atoms of 238U, which are not fissile in a contrast, don’t capture the low-speed neutrons, but only capture the high-speed neutrons. The high-speed neutrons change 238U to 239Pu, plutonium, which are fissile. Since not every neutrons in the reactor speeds down, and 238U is the major part of the nuclear fuel, the nuclear reactors in Japan are generating small amount of 239Pu as a part of nuclear waste.

When the low-speed neutron hits 239Pu, it just turn 239Pu to 240Pu, and cause no critical. On the other hand, the high-speed neutron burns 239Pu and triggers critical if there are enough amount of 239Pu.

We do not have plutonium reactor currently in Japan. We used to have one but is broken and doesn’t work any more. As a second choice, we mix the plutonium and depleted uranium and put them in the conventional reactors for generating powers. Remember that there are high-speed and low-speed neutrons in the light water reactor, the 239Pu atoms capture high-speed neutrons and change to lighter elements, while the other 239Pu atoms capture low-speed neutrons and change to 240Pu.

The 239Pu has shorter half life than 235U, and much shorter half life than 238U. This means 239Pu is more radioactive than 235U; and still the half life of 239Pu is longer than human’s life. If we didn’t put 239Pu in a reactor, we must put them in a safe and secure place and keep them for about 100,000 years.

The only way to clean up 239Pu on the earth is using it. For this purpose, we need plutonium reactor.









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