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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.

我々には新しいタイプの原子炉が必要になるだろう.

我が国にとって原子炉は必要不可欠のものだ.特に関西電力は供給量の50%を原子力でまかなっている.しかしながら,フクシマ後は既存の原子炉を減らして行かざるを得ないだろう.電力不足はおそらく,これからくる少子化と移民受入の絶望的状況を鑑みると,問題にはならない.問題は別のところにある.

我が国の稼働中の原子炉は全て軽水炉だ.軽水炉は濃縮ウラン(これはウラン235を自然界よりも多めにウラン238と混ぜ合わせたものと言える)を燃料として使う.核分裂性のウラン235は低速な熱中性子を吸収して核分裂し,より小さな原子(セシウムやヨウ素など)に分裂する.この過程でウラン235は高速中性子を放出するが,高速中性子は原子炉内の水分子によって速度を落とし,熱中性子へと変化する.

核分裂にほとんど寄与しないウラン238は熱中性子を吸収しないが,高速中性子をたまに吸収して,プルトニウム239へと変化する.原子炉内には高速中性子もいるし,核燃料の大半がウラン238であるから,我が国の原子炉はプルトニウム239を生産し続けていることになる.

熱中性子がプルトニウム239にぶつかると,プルトニウム239はほとんど単にプルトニウム240になる.一方,高速中性子がプルトニウム239にぶつかると,プルトニウム239は核分裂を起こし,なおかつ高速中性子を放出する.つまり,十分な量があれば臨界に達する.

我が国にはプルトニウムを燃料とする原子炉は無い(あったけど壊れている).そこで我が国では,濃縮ウランを作った後の残りのウラン238(劣化ウラン)と核燃料廃棄物から取り出したプルトニウム239をよく混ぜ合わせて,濃縮ウラン用に設計された軽水炉でだましだまし使い始めている.この方法がうまくいくのかは,まだわからない.

プルトニウム239はウラン235よりも半減期が短く(つまり放射能がより強く),かつ人類のタイムスケールから言えば極めて長く持つので,出来ることなら使い切ってしまうか,あるいは100,000年程度安全な場所に保管しておくしかない.

プルトニウム239を本気で使い切るには,どうしても専用の原子炉が必要なのである.

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