Control rod

Japanese: 制御棒 - せいぎょぼう

A rod that is inserted into and removed from the core to adjust the amount of neutrons generated in the reactor in order to effectively control the nuclear fission chain reaction in the reactor. It is made of materials that absorb thermal neutrons well, such as boron and cadmium, and is coated with stainless steel, aluminum, etc. If it is inserted deep into the core, neutrons are absorbed by the rod, suppressing the nuclear fission reaction in the reactor and reducing the output, and if it is pulled out, the reaction is promoted and the output of the reactor increases. In addition to coarse and fine adjustment, there are safety control rods that immediately stop the reaction in the event of an accident. In the major accident at the Fukushima Daiichi Nuclear Power Plant in March 2011, the reactor was shut down by the control rod and the critical state was stopped, but the fission products continued to decay in the nuclear fuel, which continued to emit heat, and there was no way to stop this, so water to cool it had to be circulated. However, the pumps used for this purpose lost power due to the tsunami and stopped working. The nuclear fuel in the reactor and spent fuel pool could not be cooled, and the critical situation continued with the release of radioactive materials. On December 16, 2011, Prime Minister Noda announced at the Nuclear Emergency Response Headquarters that the conditions for a "cold shutdown" had been met and that the accident had been brought under control. However, there is still molten nuclear fuel in the reactors, and they are still relying on temporary equipment to cool it. The abnormal situation remains unchanged, with the normal cooling system not functioning.
→Related ArticleMeltdown

Source : Heibonsha Encyclopedia About MyPedia Information

Japanese:

原子炉内の核分裂連鎖反応をうまくコントロールするために，炉心に出し入れして炉内で発生する中性子の量を調整する棒。ホウ素，カドミウムなど熱中性子をよく吸収する物質でつくり，ステンレス鋼，アルミニウム等で被覆する。炉心に深く挿入すれば中性子が棒に吸収されて炉内の核分裂反応が抑制され，出力が下がり，外へ引き出せば反応が促進されて原子炉の出力が上がる。粗調整用，微調整用のほか，事故の際直ちに反応を停止させる安全用の制御棒がある。2011年３月の福島第一原発の大事故では，制御棒で原子炉を緊急停止し臨界状態は止められたが，核燃料の中では核分裂生成物の崩壊が続いて熱を出し続け，これを止める手だてはなく，冷やす水を循環させなければならない。しかしそのためのポンプなどが津波で電源を喪失したため動かなくなった。原子炉や使用済み燃料プールにある核燃料を冷やすことができず，放射性物質の放出が続く重大な局面が続いた。11年12月16日，野田首相は原子力災害対策本部で，〈冷温停止〉状態の条件を満たしたと事故収束を宣言した。しかし炉には溶けた核燃料があり，その冷却は仮設の設備に頼っている状況が現在も続いている。通常の冷却システムが機能しない異常な状態に変わりはない。
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