Helium chemistry for very high temperature reactors

高温ガス炉ヘリウム化学

坂場 成昭 ; 濱本 真平  ; 竹田 陽一*

Sakaba, Nariaki; Hamamoto, Shimpei; Takeda, Yoichi*

超高温ガス炉(VHTR)の1次冷却材であるヘリウムガス中に含まれる化学的不純物は、1次系機器材料の構造劣化,熱物性劣化等に影響し、その寿命に影響を与える。従来、冷却材中の化学的不純物は、炉心に使用される黒鉛の酸化を抑制する観点から、可能な限り化学的不純物を除去する方針とし、上限値が管理されてきた。しかし、この方法では高温材料の熱物性劣化あるいは構造劣化を制御することができず、材料に対して過剰な裕度を持たせた設計が必要であった。そこで、本報では、供用中の高温ガス炉HTTRの実験値をリファレンスとして使用し、解析により高温材料ハステロイXRの熱物性劣化をもたらす炭素析出及び強度劣化をもたらす脱炭を制御可能な化学的不純物制限の考え方を提案した。また、原子力機構が設計した超高温ガス炉GTHTR300Cについて、炉心黒鉛の酸化抑制,高温材料ハステロイXRの熱物性劣化抑制及び強度劣化抑制を可能とする化学的不純物濃度の範囲を示した。提案する制限値を逸脱した場合には、一酸化炭素を付加することにより強度劣化をもたらさない安定領域に導くことができることを見いだし、その必要濃度が10ppmであることを明らかにした。本報は高温ガス炉ヘリウム化学の研究成果として、VHTR商用炉の経済性向上につながることが期待できる。

Lifetime extension of high-temperature equipment such as the intermediate heat exchanger of high-temperature gas-cooled reactors (HTGRs) is important from the economical point of view. Since the replacing cost will cause the increasing of the running cost, it is important to reduce replacing times of the high-cost primary equipment during assumed reactor lifetime. In the past, helium chemistry has been controlled by the passive chemistry control technology in which chemical impurity in the coolant helium is removed as low concentration as possible, as does Japan's HTTR. Although the lifetime of high-temperature equipment almost depends upon the chemistry conditions in the coolant helium, it is necessary to establish an active chemistry control technology to maintain adequate chemical conditions. In this study, carbon deposition which could occur at the surface of the heat transfer tubes of the intermediate heat exchanger and decarburization of the high-temperature material of Hastelloy XR used at the heat transfer tubes were evaluated by referring the actual chemistry data obtained by the HTTR. The chemical equilibrium study contributed to clarify the algorism of the chemistry behaviours to be controlled. The created algorism is planned to be added to the instrumentation system of the helium purification systems. In addition, the chemical composition to be maintained during the reactor operation was proposed by evaluating not only core graphite oxidation but also carbon deposition and decarburization. It was identified when the chemical composition could not keep adequately, injection of 10 ppm carbon monoxide could effectively control the chemical composition to the designated stable area where the high-temperature materials could keep their structural integrity beyond the assumed duration. The proposed active chemistry control technology is expected to contribute economically to the purification systems of the future very high-temperature reactors.