General outline of the decommissioning of the prototype fast breeder reactor Monju

高速増殖原型炉もんじゅ廃止措置の全体概要

小林 秀治; 成瀬 恵次; 平子 一仁; 澤崎 浩昌; 後藤 健博; 小幡 行史; 松井 一晃

Kobayashi, Hideharu; Naruse, Keiji; Hirako, Kazuhito; Sawazaki, Hiromasa; Goto, Takehiro; Obata, Ikuhito; Matsui, Kazuaki

高速増殖原型炉もんじゅは、ナトリウムを冷却材とする高速炉として日本で初めて廃止措置を実施する。この廃止措置は、2018年から開始され、約30年を経て、4段階の工程により完了する予定である。廃止措置第1段階は、「燃料体取出し期間」として2次系ナトリウムの抜取り及び炉心内の燃料体の燃料池への移動・貯蔵を行うとともに、建屋・設備等の汚染分布に関する評価を行う。廃止措置第2段階においては、「解体準備期間」としてナトリウム機器の解体準備のために炉心内のしゃへい体等の燃料池への移動・貯蔵、ナトリウムの搬出、水・蒸気系発電設備の解体撤去及び引き続き建屋・設備等の汚染分布に関する評価を行う。廃止措置第3段階は、「廃止措置期間I」としてナトリウム機器の解体撤去、使用済み燃料の搬出及び引き続き水・蒸気系発電設備の解体撤去を行い、最終第4段階「廃止措置期間II」において管理区域の解除と建屋等の解体撤去を実施する。本稿においてはもんじゅの廃止措置の全体概要ともんじゅ廃止措置第1段階の実績並びに現在取り組んでいる廃止措置第2段階の詳細について述べる。

The prototype fast breeder reactor MONJU is a loop-type sodium-cooled nuclear reactor to be decommissioned in Japan. Japan Atomic Energy Agency (JAEA) started decommissioning of MONJU in 2018, and will complete four phases of the decommissioning in about 30 years. The first Phase is the Fuel Assembly Removal Period, during which secondary sodium was drained, the fuel assemblies in the reactor core were removed and put in the storage in the spent fuel pool, and the distribution of contamination in buildings, components, equipment, etc. was evaluated. In the second Phase, the Preparation for dismantling period, the neutron shields in the reactor core will be moved and put in the storage in the spent fuel pool in preparation for the dismantling of the sodium equipment, the transport of sodium, and the dismantling and removal of the water and steam power generation system. We also continue to assess the distribution of contamination in buildings, components, equipments, etc. The third Phase is the Decommissioning Period I, which includes the dismantling of sodium equipment, the transport of spent fuel, and the removal of the water and steam power generation equipment. The final Phase is the Decommissioning Period II. The radiation controlled area will be freed and all the buildings will be dismantled and removed. This paper provides an overview of MONJU decommissioning, the results of its First Phase, and details of the second Phase, which is currently underway.