JP2004294393A

JP2004294393A - Chemical decontamination method

Info

Publication number: JP2004294393A
Application number: JP2003090639A
Authority: JP
Inventors: Hiromi Aoi; 井洋美青; Ichiro Inami; 見一郎稲; Yutaka Uruma; 間裕閏; Motoyoshi Nakagami; 神元順仲; Kazuhiro Kani; 児和広可
Original assignee: Toshiba Corp; Chubu Electric Power Co Inc
Current assignee: Toshiba Corp; Chubu Electric Power Co Inc
Priority date: 2003-03-28
Filing date: 2003-03-28
Publication date: 2004-10-21
Anticipated expiration: 2023-03-28
Also published as: JP4167920B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a chemical decontamination method which chemically decontaminates placed components and pipes using only chemical decontamination equipments, maintaining material integrity, and maintains suppression effects of radiaoctives adhesion for a few cycles (operation cycles). <P>SOLUTION: The decontamination method is concerned with chemical removal of radioactives adhered on components and pipes of nuclear power plant by repeating oxidation and reduction by turns. After repeating the decontamination by oxidation and reduction by turns a few times, oxidation process by oxidizing gas is applied finally. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は原子力発電所において被ばく低減、材料健全性維持を目的とする一次冷却水系機器、配管表面への化学除染方法に関する。
【０００２】
【従来の技術】
図１５は、沸騰水型原子炉の一次冷却水系の概略構成を示す図であり、原子炉圧力容器１内には炉心２が配設されており、上記原子炉圧力容器１内で発生した蒸気は主蒸気管３を経て図示しない蒸気タービンに送られる。蒸気タービンに送給されそこで仕事を行った蒸気は復水器で復水された後、給水管４を経て原子炉圧力容器１内に環流される。上記原子炉圧力容器１には、炉心２に冷却材を強制的に送り込むために原子炉冷却材再循環系５が設けられている。すなわち、上記炉心２を取り囲むように配設された炉心シュラウド（図示せず）と原子炉圧力容器１との間の環状部内にジェットポンプ６が設けられており、原子炉圧力容器１内から取り出され再循環ポンプ７により昇圧された冷却材を、ジェットポンプ６のノズルから高速で噴出させ、この高速噴出流体により冷却材を炉心２の下部に強制的に送り込むようにしてある。
【０００３】
上記原子炉冷却材再循環系５には、再循環ポンプ７の吸込み側から残留熱除去系８が分岐され、さらに上記残留熱除去系８から炉水浄化系９が分岐されている。上記残留熱除去系８に導出された冷却材は熱交換器１０において冷却された後原子炉冷却材再循環系５等に環流され、また炉水浄化系９に導出された冷却材は熱交換器１１及び濾過脱塩器１２等を経て浄化された後、給水管４において給水中に混合される。
【０００４】
ところで、上記原子炉冷却材再循環系５等を含む一次冷却水系においては、特に上述のように原子炉圧力容器１内の炉水が循環するので、一次冷却水系機器、配管表面等の酸化皮膜中には炉水中における放射能が取り込まれ、これが被ばく線源となることがある。そこで、この酸化被膜を化学除染を行うことにより取り除き、機器点検や工事時における被ばくを低減することが行われている。しかし、除染後に露出した金属表面が再び放射能を含む高温水にさらされると、その金属表面に新たな放射能を取り込んだ酸化被膜が生成される。
【０００５】
金属面への放射能付着を抑制するには、予め電解研磨（例えば特許文献１参照）や表面酸化処理した配管（例えば特許文献２参照）を用いるような方策がとられている。
【０００６】
また、化学除染後に除染液中に過酸化水素等の酸化剤を添加して表面に酸化被膜を生成させ放射能再付着を抑制することも提案されている（例えば特許文献３参照）。
【０００７】
【特許文献１】
特開平８−６２３８４号公報
【特許文献２】
特開平９−４３３９３号公報
【特許文献３】
特開２０００−１２１７９１号公報
【０００８】
【発明が解決しようとする課題】
しかしながら、前もって電解研磨を行ったり前酸化処理を施すには一旦据え付けられた機器、配管に対しては適用が難しい。また、除染液中に酸化剤を添加する方法も供用中の機器、配管に対しては材料健全性上、濃度が制約されるため効果が期待できない上、余計な付帯設備や薬品類を必要とする場合が多い。
【０００９】
本発明は、このような点に鑑み、化学除染設備のみを用いて据え付け機器、配管の材料健全性を維持しつつ化学除染後、数サイクル（運転サイクル）に渡り放射能付着抑制効果が持続する化学除染方法を提供することを目的とする。
【００１０】
【課題を解決するための手段】
請求項１に係る発明は、原子力発電プラントの機器、配管に付着した放射能を酸化と還元を交互に繰り返すことにより化学的に除去する除染方法において、最終の工程を酸化工程とすることを特徴とする。
【００１１】
請求項２に係る発明は、請求項１に係る発明において、化学除染対象表面が鉄酸化物で覆われている場合には、上記化学除染をまず還元工程から始め、次いで酸化工程、還元工程を繰り返した後、最終の工程を酸化工程とすることを特徴とする。
【００１２】
請求項３に係る発明は、請求項１に係る発明において、化学除染対象表面がクロム含有量が多い酸化被膜で覆われている場合には、上記化学除染をまず酸化工程から始め、次いで還元工程を行いこのサイクルを繰り返した後、最終の工程を酸化工程とすることを特徴とする。
【００１３】
請求項４に係る発明は、請求項１に係る発明において、上記最終酸化工程は原子力発電プラントの機器、配管における炭素鋼配管部分だけを隔離し、その炭素鋼配管部分のみに実施することを特徴とする。
【００１４】
また、請求項５に係る発明は、請求項１乃至４のいずれかに係る発明において、前記最終酸化工程においては、配管内の水を抜いた後、乾燥雰囲気中に酸化性気体を供給することを特徴とする。
【００１５】
請求項６に係る発明は、請求項１乃至４のいずれかに係る発明において、前記最終酸化工程において、酸化性気体を処理対象配管内に流通させ、処理対象表面に常時流れが生ずるようにしたことを特徴とする。
【００１６】
請求項７に係る発明は、請求項６に係る発明において、前記最終酸化工程において、処理対象配管内に酸化性気体を注入するとともに、処理対象配管内に注入された水を加熱して飽和水蒸気を生成させるようにしたことを特徴とする。
【００１７】
請求項８に係る発明は、請求項７に係る発明において、前記最終酸化処理において、処理対象配管内の飽和水蒸気を冷却して凝結させ、処理対象面に濡れ面を形成するようにしたことを特徴とする。
【００１８】
さらに、請求項９に係る発明は、請求項１に係る発明において、前記最終酸化処理において、機器、配管等炭素鋼表面に対して１８０℃以上の乾燥条件下で酸化処理を施すことを特徴とする。
【００１９】
請求項１０に係る発明は、請求項９に係る発明において、前記最終酸化処理において、炭素鋼表面を１８０℃以上に加熱した乾燥状態で酸化処理することを特徴とする。
【００２０】
【発明の実施の形態】
以下添付図面を参照して本発明の実施の形態について説明する。
【００２１】
図１は、特に本発明の化学除染の対象部となる原子炉冷却材再循環系５及び残留熱除去系８等を示す図であり、沸騰水型原子炉のこのような一次冷却水系における機器、配管に対する除染を行う場合には、上記除染対象部である原子炉冷却材再循環系５、残留熱除去系８及び炉水浄化系９内に水張りを行い昇温した後、シュウ酸等の還元剤を注入し、その酸化剤により除染対象部の上層の鉄酸化物を還元溶解して還元除染を行い、次いで還元剤を分解する。次にクロム酸化物を溶解するための酸化剤による酸化工程を行った後、そのまま還元工程に移行する。これにより酸化剤は過剰の還元剤により分解されて水張り昇温直後と同じ還元工程となる。この酸化、還元工程をさらに繰り返し、配管内表面等の酸化被膜を除去する。このようにして化学除染が十分に行われてステンレス鋼金属面が露出した状態となったら、上記除染対象部内の水を抜いた後、オゾン発生器等の酸化性気体供給源２０から供給される例えばオゾンや酸素等の酸化性気体を、仮設循環ポンプ２１により残留熱除去系８の戻り弁２２及びライザ管ノズル部２３から原子炉冷却材再循環系５内に送給し、炉水浄化系入口弁２４、炉水浄化系ボトムドレンライン入口弁２５、及び残留熱除去系入口弁２６等から排出し、上記除染対象部に対する酸化性気体による酸化工程を１回だけ実施する。その後、酸化剤分解、廃液浄化、及び水のドレンを行う。図２にそのフローチャートを示す。
【００２２】
しかして、この一連の化学除染工程により、除染後の沸騰水型原子炉の一次冷却水系の弁、ポンプ機器、配管の接液部表面全体が酸化され、安定な酸化皮膜が形成される。
【００２３】
図３は、加圧水型原子炉一次冷却水系機器、配管に対する除染シーケンスのフローチャートである。沸騰水型原子炉の一次冷却水系機器、配管は通常の運転状態において酸化雰囲気であるが、加圧水型原子炉の一次冷却水系機器、配管は通常の運転状態において還元雰囲気であり、加圧水型原子力プラントのように一次系構成材料がニッケル基合金とステンレスの鋼の場合、クロム含有量の多い酸化被膜が化学除染対象表面を覆っている。そこで、昇温後、まずクロム酸化物を溶解するための酸化剤による酸化工程を行い、酸化終了後シュウ酸などの還元剤を注入する。これにより酸化剤は過剰の還元剤により分解されそのまま還元工程に移行する。還元剤による鉄酸化物の酸溶解、還元溶解後、還元剤を分解する。この酸化、還元工程をさらに繰り返し、配管内表面の酸化被膜を除去する。化学除染が十分に行われステンレス鋼金属面が露出した状態となる。この状態で沸騰水型原子炉の一次冷却水系の場合と同様にさらに酸化工程を１回実施する。
【００２４】
しかして、この場合にも上記一連の化学除染工程により除染後の一次系弁、ポンプ等機器、配管のステンレス鋼やニッケル基合金表面全体が酸化され、安定な酸化被膜が形成される。
【００２５】
図４は、沸騰水型原子炉の冷却材再循環系５から分岐した炭素鋼配管である残留熱除去系８の配管および炉水浄化系９の配管に対してのみ処理を行うようにしたものである。すなわち、原子炉冷却水再循環系５における再循環ポンプ７の入口垂直配管より取り合いノズルプラグ２７を残留熱除去系８の炭素鋼配管分岐部に挿入し、その取り合いノズルプラグ２７の端部を仮設循環ポンプ２１に接続する。また、残留熱除去系８の戻り配管には第一止め弁に片側閉止プラグ２８を取り付け、仮設循環ポンプ２１と接続する。一方、炉水浄化系入口弁２４、炉水浄化系ボトムドレンライン入口弁２５、及び残留熱除去系入口弁２６に酸化性気体供給源２０を接続し、これにより酸化性気体供給源２０から酸化性気体を、残留熱除去系８および炉水浄化系９の炭素鋼配管に対して流通させ、残留熱除去系および炉水浄化系の応力腐食割れのない炭素鋼配管部分のみの最終酸化処理を実施する。
【００２６】
また、この循環ループ構成によって原子炉再循環系ステンレス配管の化学除染時流路が阻害されることは図から明らかであるが、原子炉再循環ポンプ入口垂直配管からの取り合い部でステンレス鋼の原子炉再循環系配管、弁、ポンプからの取り合いノズルと本発明の分岐部ノズルプラグの取り合いとを両方取り出すようにあらかじめ施工しておくことで、一連の化学除染工程の途中での配管工事が不要となる。
【００２７】
この最終酸化処理においては、１８０℃以上に炭素鋼表面を加熱した乾燥条件下という、ステンレス鋼では応力腐食割れに対する健全性の確認を要する酸化条件で酸化処理を行うことにより、炭素鋼表面のみに腐食抑制に有効な酸化被膜を短時間で生成することができる。
【００２８】
図５は、これらの処理における酸化皮膜量の変化を示している。すなわち、図５から判るように、１８０℃空気酸化処理、または１８０℃オゾンガス処理を５時間施すことにより酸化皮膜を形成した場合、その後の浸漬時における酸化皮膜の成長量は、いずれも空気酸化処理やオゾンガス処理を行わない未処理の場合より下回っており、予備的な酸化皮膜を付与することによって供用時における腐食を抑制することができることを示している。この場合、腐食抑制に有効な酸化皮膜は放射能の付着抑制に有効であることと同義である。また、この処理を乾燥状態で行うと一層効果的であることはあきらかである。なお、この酸化処理時におけるオゾンガス濃度は１ｖｏｌ％であった。
【００２９】
さらに酸化処理温度を高くした場合の放射能付着量を図６に示す。６００℃までは未処理の場合より放射能付着量相対値が下回っており、６００℃までは放射能付着抑制効果があることを確認できた。酸化処理温度としては４００℃が最も放射能付着抑制効果が高いが、１８０℃から６００℃までの酸化処理温度で放射能付着抑制効果を得ることができる。
【００３０】
次に図７は、原子炉冷却材再循環系５の化学除染方法を示す図であり、残留熱除去系８の入口部フランジ２９と出口部フランジ３０とをそれぞれ閉じるとともに、酸化性気体供給源２０から酸化性気体、例えばオゾン、酸素などが再循環系入口ノズル３１、及び再循環ポンプ７の前後の除染座３２、３３から原子炉冷却材再循環系５内に導入され、均一性が維持されており、酸化性気体注入ライン３８中の酸化性気体はライザ管ノズル部３４を経て、クーラ３５で除湿され分解器３６を経て仮設循環ポンプ３７を介して排気される。通常、化学除染系統は水やシュウ酸等の化学薬品または浄化された水で水封されている。化学除染の最終酸化工程において、上記酸化性気体の導入により再循環系配管、弁ポンプのステンレス鋼表面全体が酸化され、安定な酸化皮膜が形成される。しかも、仮設循環ポンプ２１において連続的に処理対象表面上に上記酸化性気体を注入することにより常時流れを生ずることで安定した高濃度の酸化性気体の供給が可能となる
酸化被膜を形成するのに酸化性気体としてオゾンを使用する場合、純水中に溶解する濃度は溶液の温度に依存し、気液平衡状態にある飽和溶解オゾン濃度を算出する式として、
【数１】

ここで、Ｃ_Ｌ：飽和イオン濃度
Ｃ_ｇ：送気ガス中のオゾン濃度
の式が提唱されている。（１）式に基づいた気相オゾン濃度と平衡にある液相中オゾン濃度との関係の例を図８に示す。図８に示すように例えば気相オゾン濃度が１００ｍｇ／Ｌの場合、液相中のオゾン濃度は、
２５℃：２５．６ｐｐｍ
４０℃：１９．７ｐｐｍ
５０℃：１７．２ｐｐｍ
６０℃：１５．４ｐｐｍ
７０℃：１４．０ｐｐｍ
８０℃：１２．９ｐｐｍ
となっており低温ほどオゾンは溶解しやすい。しかし酸化膜形成は温度が高いほど効果的である。この相反する条件の最適範囲として液相温度は２５℃〜８０℃が好ましい。
【００３１】
また、図７に示す化学除染後の最終工程において、弁のドレンライン等を用いて系統内の水抜きを行い系統内を気体雰囲気とする。その後、オゾンや酸素の酸化性気体を２５℃〜８０℃の系統内に供給すると、系統内のステンレス鋼や炭素鋼の表面に残存している水にオゾンが溶解し、ステンレス鋼や炭素鋼の表面に酸化被膜が形成される。
【００３２】
また、酸化性気体、特にオゾンは自己分解するため濃度を維持することが困難である。この実施の形態では、図７に示すように複数の部分より酸化性気体を供給しているので、系統内全体で酸化性気体の濃度を均一に維持することができ、系統内全体で均一な酸化被膜の形成を行うことができる。
【００３３】
図９は、上記酸化処理によりステンレス鋼（ＳＵＳ３０４）の表面に酸化被膜を設けたものと酸化被膜を設けないものとの放射能付着量の違いを示す図であり、通常炉水中に暴露した場合、酸化被膜を設けたものにおいては放射能付着量が大幅に少なくなっていることがわかる。
【００３４】
そこで、さらに酸化処理によってステンレス鋼や炭素鋼に付与した酸化皮膜が放射能の付着に対して有効に機能することを確認するために放射能付着試験をおこなった。試験材としてステンレス鋼ではオーステナイト系ステンレス３１６鋼を、炭素鋼としてＳＴＳ４１０を選択した。
【００３５】
ステンレス鋼に対しては酸化性気体を注入するノズルとこれを排気するノズルとを設けた金属容器（容積２００ミリリットル）内に短冊状（２０ｍｍ×５０ｍｍ×厚さ０．３ｍｍ）の試験体を装荷し、容器を外部ヒータで加熱して温度を８０℃に保持した。試験ではまず、注入ノズルから乾燥窒素ガスを吹き込み、容器内の空気を置換した後、酸化性気体として約１０ｖｏｌ％（残り酸素ガス）のオゾンガスを毎分１０ミリリットルで供給した。処理時間は２時間とした。炭素鋼の酸化処理も同じ装置、同条件、同形状の試験片を用いて行い、相違点は設定温度を１８０℃にしたことのみである。
【００３６】
これら酸化処理を施した試験体をオートクレーブ内に装荷し、２８５℃、８Ｍｐａの高温高圧水を通水させた。供給水中には放射能としてＣｏ−６０を０．０２Ｂｑ／ミリリットルの濃度で含有させ溶存酸素２００ｐｐｂ、溶存水素１０ｐｐｂで５００時間の放射能付着試験を行った。試験終了後、各試験体に付着した放射能量を未処理材と比較した表が図１０である。ステンレス鋼においては未処理材の約６０％、炭素鋼においては約４０％の付着量となりいずれの鋼材でも酸化処理の有効性が確認された。
【００３７】
図１１は１８０℃以上の乾燥条件下で最終酸化を行う形態を示す図である。酸化性気体注入ライン３８よりオゾンや酸素などの酸化性気体を注入しながら、外部ヒータ４６により除染対象物を加熱することにより、さらに緻密な酸化皮膜の生長が助長される。
【００３８】
これまでに説明した実施の形態においては、ステンレス鋼や炭素鋼の表面に水が残存している場合について説明したが、ステンレス鋼や炭素鋼表面が乾燥している場合には、全ての実施の形態において酸化性気体とともに水蒸気を供給することで同様な効果を得ることができる。特に図１１に示す実施の形態においては、酸化性気体とともに水を供給し、ヒータ４０によりこの水を加熱することにより配管内に飽和水蒸気を発生させ、水蒸気を酸化性気体とともに供給した場合と同様な効果を得ることができる。
【００３９】
次に、図１１に示す実施の形態において、さらに強制的に水蒸気を凝結させ、酸化被膜生成を促進させる実施の形態を図１２に示す。この実施の形態は、酸化性気体注入ライン３８内に水を注入するようにしたものにおいて、さらに原子炉冷却材再循環系５の上部垂直管に外部クーラ４１を装着して、下部で生成した水蒸気を凝結させるようにしたものである。この場合、酸化性気体を溶かし込んだ水は垂直壁面を濡らしながら流下し、その間に原子炉冷却材再循環系５の配管内に酸化被膜を生成する。このように処理対象表面に水蒸気の凝結を起こすことにより酸化性気体を含む水滴との接触を図ることでステンレス表面への安定した高濃度の酸化性気体の供給が可能となる。一方、流下し下部に溜まった水は再びヒータ４０で加熱させ水蒸気として配管内を満たす。
【００４０】
図１３は、沸騰水型原子炉再循環系から分岐した炭素鋼配管である残留熱除去系配管および炉水浄化系配管に対して処理を行った図４の実施の形態の変形例である。化学除染により配管内表面の酸化被膜は除去され、炭素鋼金属面が露出した状態となった後、水抜き後、化学除染時の除染ループとの取り合い点、図１３の例では弁２４、２５、２６、２２から酸化性気体と加熱乾燥空気を混合させ残留熱除去系８の配管および炉水浄化系９の配管に供給している。余剰の酸化性気体はクーラ３５で除湿され分解器３６を経て仮設循環ポンプ３７で排気される。しかして、この方法により、除染後の炭素鋼配管、弁の鋼表面全体が酸化され、安定な酸化被膜が形成される。
【００４１】
図１４は同様の効果をねらった炭素鋼表面へ酸化被膜を生成する他の実施の形態を示す図である。この実施の形態では加熱乾燥空気ではなく、酸化性気体たとえばオゾン、酸素もしくはオゾンと酸素の混合気を吹き込みつつ酸化膜を生成すべき箇所に外部ヒータ４２、４３を施すことで所定温度条件例えば１２０℃にする。
【００４２】
【発明の効果】
本発明によれば化学除染装置構成に付帯設備や特別な薬品類を加えることなく、据え付けられた状態で化学除染後の機器、配管の金属表面に、安定した酸化被膜が生成でき、その後のプラント運転中において放射能の取り込みが抑制でき、被ばく低減を図ることができる。
【図面の簡単な説明】
【図１】特に本発明に係る化学除染の対象部となる原子炉冷却材再循環系及び残留熱除去系に対する化学除染方法の一実施の形態を示す系統図。
【図２】本発明に係る化学除染方法のプロセスを示すフローチャート。
【図３】本発明に係る化学除染方法の他のプロセスを示すフローチャート。
【図４】炭素鋼配管部分のみの化学除染を行うようにした化学除染方法を示す系統図。
【図５】本発明に係る炭素鋼への放射能付着抑制効果を示す実験結果の図。
【図６】酸化処理温度を高くした場合の放射能付着量を示す図。
【図７】本発明に係る化学除染方法の他の実施の形態を示す系統図。
【図８】純水中のオゾン溶解濃度の温度依存性を示す図。
【図９】本発明に係るステンレス鋼への放射能付着抑制効果を示す概念図。
【図１０】本発明に係るステンレス鋼および炭素鋼への放射能付着抑制効果を示す実験結果の図。
【図１１】本発明に係る化学除染方法の他の実施の形態を示す系統図。
【図１２】本発明に係る化学除染方法のさらに他の実施の形態を示す系統図。
【図１３】本発明に係る化学除染方法の他の実施の形態を示す系統図。
【図１４】本発明に係る化学除染方法の他の実施の形態を示す系統図。
【図１５】沸騰水型原子炉の一次冷却水系の概略構成を示す図。
【符号の説明】
１原子炉圧力容器
２炉心
５原子炉冷却材再循環系
６ジェットポンプ
７再循環ポンプ
８残留熱除去系
９炉水浄化系
２０酸化性気体供給源
２１仮設循環ポンプ
２２戻り弁
２３ライザ管ノズル部
２４炉水浄化系入口弁
２５炉水浄化系ボトムドレインライン入口弁
２６残留熱除去系入口弁
２７ノズルプラグ
２８片側閉止弁
３１再循環系入口ノズル
３２、３３除染座
３４ライザ管ノズル部
３８酸化性気体注入ライン
４０ヒータ
４１外部クーラ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a primary cooling water system device and a method for chemically decontaminating piping surfaces for the purpose of reducing exposure and maintaining material integrity in a nuclear power plant.
[0002]
[Prior art]
FIG. 15 is a diagram showing a schematic configuration of a primary cooling water system of a boiling water reactor, in which a reactor core 2 is disposed in a reactor pressure vessel 1, and steam generated in the reactor pressure vessel 1 is provided. Is sent through a main steam pipe 3 to a steam turbine (not shown). The steam fed to the steam turbine and performing the work there is condensed by the condenser, and then returned to the reactor pressure vessel 1 through the water supply pipe 4. The reactor pressure vessel 1 is provided with a reactor coolant recirculation system 5 for forcibly sending coolant to the reactor core 2. That is, a jet pump 6 is provided in an annular portion between a reactor core shroud (not shown) disposed so as to surround the reactor core 2 and the reactor pressure vessel 1, and is taken out of the reactor pressure vessel 1. The coolant pressurized by the recirculation pump 7 is ejected at a high speed from the nozzle of the jet pump 6, and the coolant is forcibly fed into the lower part of the core 2 by the high-speed ejected fluid.
[0003]
In the reactor coolant recirculation system 5, a residual heat removal system 8 is branched from a suction side of a recirculation pump 7, and a reactor water purification system 9 is further branched from the residual heat removal system 8. The coolant led out to the residual heat removal system 8 is cooled in the heat exchanger 10 and then returned to the reactor coolant recirculation system 5 and the like, and the coolant led out to the reactor water purification system 9 is subjected to heat exchange. After being purified through the filter 11 and the filter / desalter 12 and the like, it is mixed into the water supply in the water supply pipe 4.
[0004]
By the way, in the primary cooling water system including the reactor coolant recirculation system 5 and the like, since the reactor water in the reactor pressure vessel 1 circulates as described above, the oxide film on the primary cooling water system equipment, piping surface, etc. In some cases, radioactivity in reactor water is taken in, and this may be a radiation source. Therefore, it has been practiced to remove the oxide film by performing chemical decontamination, thereby reducing exposure during equipment inspection and construction. However, when the metal surface exposed after decontamination is exposed again to high-temperature water containing radioactivity, an oxide film incorporating new radioactivity is generated on the metal surface.
[0005]
In order to suppress the adhesion of radioactivity to the metal surface, a measure is taken to use electrolytic polishing (for example, see Patent Document 1) or a surface oxidized pipe (for example, see Patent Document 2) in advance.
[0006]
It has also been proposed to add an oxidizing agent such as hydrogen peroxide to the decontamination solution after chemical decontamination to form an oxide film on the surface and to suppress reattachment of radioactivity (for example, see Patent Document 3).
[0007]
[Patent Document 1]
JP-A-8-62384 [Patent Document 2]
Japanese Patent Application Laid-Open No. 9-43393 [Patent Document 3]
Japanese Patent Application Laid-Open No. 2000-121791
[Problems to be solved by the invention]
However, it is difficult to apply electropolishing or pre-oxidation treatment to equipment and piping once installed. In addition, the method of adding an oxidizing agent to the decontamination solution is not expected to be effective because the concentration of the equipment and pipes in service is limited due to the integrity of the material, and additional auxiliary equipment and chemicals are required. In many cases.
[0009]
In view of the above, the present invention has an effect of suppressing radioactive adhesion over several cycles (operation cycle) after chemical decontamination while maintaining the soundness of installation equipment and piping using only chemical decontamination equipment. It is intended to provide a sustainable chemical decontamination method.
[0010]
[Means for Solving the Problems]
The invention according to claim 1 is a decontamination method for chemically removing radioactivity attached to equipment and piping of a nuclear power plant by alternately repeating oxidation and reduction, wherein the final step is an oxidation step. Features.
[0011]
In the invention according to claim 2, in the invention according to claim 1, when the surface to be chemically decontaminated is covered with iron oxide, the chemical decontamination is first started from a reduction step, and then oxidized and reduced. After repeating the steps, the final step is an oxidation step.
[0012]
The invention according to claim 3 is the invention according to claim 1, wherein when the surface to be chemically decontaminated is covered with an oxide film having a high chromium content, the chemical decontamination is first started from an oxidation step, and then After the reduction step is performed and this cycle is repeated, the final step is an oxidation step.
[0013]
The invention according to claim 4 is characterized in that, in the invention according to claim 1, the final oxidation step is performed only on the carbon steel pipe part in the equipment and piping of the nuclear power plant, and only on the carbon steel pipe part. And
[0014]
According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, in the final oxidation step, an oxidizing gas is supplied into a dry atmosphere after draining water in the pipe. It is characterized by.
[0015]
In the invention according to claim 6, in the invention according to any one of claims 1 to 4, in the final oxidation step, an oxidizing gas is caused to flow through the pipe to be processed, so that a flow always occurs on the surface to be processed. It is characterized by the following.
[0016]
In the invention according to claim 7, in the invention according to claim 6, in the final oxidation step, an oxidizing gas is injected into the pipe to be processed, and the water injected into the pipe to be processed is heated to obtain saturated steam. Is generated.
[0017]
The invention according to claim 8 is the invention according to claim 7, wherein in the final oxidation treatment, the saturated steam in the pipe to be treated is cooled and condensed to form a wet surface on the surface to be treated. Features.
[0018]
Further, the invention according to claim 9 is characterized in that, in the invention according to claim 1, in the final oxidation treatment, the oxidation treatment is performed on carbon steel surfaces such as equipment and piping under a drying condition of 180 ° C. or more. I do.
[0019]
According to a tenth aspect of the present invention, in the invention according to the ninth aspect, in the final oxidation treatment, the carbon steel surface is oxidized in a dry state heated to 180 ° C. or more.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0021]
FIG. 1 is a diagram showing a reactor coolant recirculation system 5 and a residual heat removal system 8 and the like, which are the target parts of the chemical decontamination of the present invention, particularly in such a primary cooling water system of a boiling water reactor. When decontaminating equipment and piping, water is filled in the reactor coolant recirculation system 5, the residual heat removal system 8, and the reactor water purification system 9, which are the decontamination target parts, and the temperature is increased. A reducing agent such as an acid is injected, the iron oxide in the upper layer of the decontamination target is reduced and dissolved by the oxidizing agent to perform reduction decontamination, and then the reducing agent is decomposed. Next, after performing an oxidation step using an oxidizing agent for dissolving the chromium oxide, the process directly proceeds to a reduction step. As a result, the oxidizing agent is decomposed by the excess reducing agent, and the same reducing step as that immediately after the temperature rise with water filling is performed. The oxidation and reduction steps are further repeated to remove an oxide film on the inner surface of the pipe. When the stainless steel metal surface is exposed by the sufficient chemical decontamination in this way, the water in the decontamination target is drained and then supplied from an oxidizing gas supply source 20 such as an ozone generator. An oxidizing gas such as ozone or oxygen is supplied from a return valve 22 of the residual heat removal system 8 and a riser pipe nozzle portion 23 into the reactor coolant recirculation system 5 by a temporary circulation pump 21, and the reactor water The effluent is discharged from the purifying system inlet valve 24, the reactor water purifying system bottom drain line inlet valve 25, the residual heat removing system inlet valve 26, and the like, and the oxidizing gas is oxidized to the decontamination target portion only once. Thereafter, oxidant decomposition, waste liquid purification, and water drainage are performed. FIG. 2 shows the flowchart.
[0022]
By this series of chemical decontamination processes, the entire surface of the liquid-contacting parts of the primary cooling water system of the boiling water reactor after decontamination, pump equipment, and piping are oxidized, and a stable oxide film is formed. .
[0023]
FIG. 3 is a flowchart of a decontamination sequence for the pressurized water reactor primary cooling water system equipment and piping. The primary cooling water system equipment and piping of a boiling water reactor are in an oxidizing atmosphere under normal operating conditions, while the primary cooling water system equipment and piping of a pressurized water reactor are in a reducing atmosphere under normal operating conditions. When the primary constituent material is a nickel-based alloy and stainless steel as described above, an oxide film having a high chromium content covers the surface to be chemically decontaminated. Therefore, after the temperature is raised, first, an oxidizing step using an oxidizing agent for dissolving the chromium oxide is performed, and after the oxidation is completed, a reducing agent such as oxalic acid is injected. As a result, the oxidizing agent is decomposed by the excess reducing agent and proceeds to the reduction step as it is. After the acid dissolution and reduction dissolution of the iron oxide by the reducing agent, the reducing agent is decomposed. The oxidation and reduction steps are further repeated to remove the oxide film on the inner surface of the pipe. The chemical decontamination is sufficiently performed and the stainless steel metal surface is exposed. In this state, the oxidation step is further performed once as in the case of the primary cooling water system of the boiling water reactor.
[0024]
In this case, too, the entire surface of the stainless steel or nickel-based alloy of the primary system valves, pumps and other equipment and piping after the decontamination is oxidized by the above series of chemical decontamination steps, and a stable oxide film is formed.
[0025]
FIG. 4 shows a configuration in which the processing is performed only on the pipes of the residual heat removal system 8 and the pipes of the reactor water purification system 9 which are carbon steel pipes branched from the coolant recirculation system 5 of the boiling water reactor. It is. That is, the fitting nozzle plug 27 is inserted into the branch portion of the carbon steel pipe of the residual heat removal system 8 from the vertical pipe at the inlet of the recirculation pump 7 in the reactor cooling water recirculation system 5, and the end of the fitting nozzle plug 27 is temporarily provided. Connect to circulation pump 21. Further, a one-side closing plug 28 is attached to the first stop valve on the return pipe of the residual heat removing system 8, and is connected to the temporary circulation pump 21. On the other hand, the oxidizing gas supply source 20 is connected to the reactor water purification system inlet valve 24, the reactor water purification system bottom drain line inlet valve 25, and the residual heat removal system inlet valve 26. The oxidizing gas is passed through the carbon steel pipes of the residual heat removal system 8 and the reactor water purification system 9, and the final oxidation treatment is performed only on the carbon steel pipe portions having no stress corrosion cracking of the residual heat removal system and the reactor water purification system. carry out.
[0026]
It is also clear from the figure that this circulation loop configuration hinders the flow path during chemical decontamination of the stainless steel piping of the reactor recirculation system. Piping work in the middle of a series of chemical decontamination steps can be performed by performing in advance so that both the connection nozzles from the furnace recirculation system piping, valves, and pumps and the connection of the branch nozzle plug of the present invention are taken out. It becomes unnecessary.
[0027]
In this final oxidation treatment, only the carbon steel surface is oxidized by performing the oxidation treatment under a drying condition in which the carbon steel surface is heated to 180 ° C. or higher, which requires confirmation of soundness against stress corrosion cracking in stainless steel. An oxide film effective for suppressing corrosion can be generated in a short time.
[0028]
FIG. 5 shows the change in the amount of oxide film in these treatments. That is, as can be seen from FIG. 5, when the oxide film is formed by performing the air oxidation treatment at 180 ° C. or the ozone gas treatment at 180 ° C. for 5 hours, the growth amount of the oxide film at the time of the subsequent immersion is both air oxidation treatment. This is lower than the case where no treatment was performed without ozone gas treatment, indicating that corrosion during operation can be suppressed by providing a preliminary oxide film. In this case, an oxide film that is effective in suppressing corrosion is synonymous with being effective in suppressing the adhesion of radioactivity. It is apparent that this treatment is more effective when performed in a dry state. The ozone gas concentration during this oxidation treatment was 1 vol%.
[0029]
FIG. 6 shows the amount of radioactive deposition when the oxidation treatment temperature was further increased. Up to 600 ° C., the relative value of the amount of radioactive deposition was lower than in the case of untreated, and it was confirmed that up to 600 ° C., there was an effect of suppressing radioactive deposition. As the oxidation treatment temperature, 400 ° C. has the highest effect of suppressing radioactive adhesion. However, the effect of suppressing radioactive adhesion can be obtained at an oxidation treatment temperature of 180 ° C. to 600 ° C.
[0030]
Next, FIG. 7 is a view showing a chemical decontamination method of the reactor coolant recirculation system 5, in which the inlet flange 29 and the outlet flange 30 of the residual heat removal system 8 are closed, and the oxidizing gas is supplied. An oxidizing gas, for example, ozone or oxygen, is introduced from the source 20 into the reactor coolant recirculation system 5 from the recirculation system inlet nozzle 31 and from the decontamination seats 32 and 33 before and after the recirculation pump 7 to obtain uniformity. Is maintained, and the oxidizing gas in the oxidizing gas injection line 38 passes through the riser pipe nozzle 34, is dehumidified in the cooler 35, is exhausted through the decomposer 36, and passes through the temporary circulation pump 37. Usually, the chemical decontamination system is sealed with water or a chemical such as oxalic acid or purified water. In the final oxidation step of chemical decontamination, the introduction of the oxidizing gas oxidizes the entire surface of the stainless steel of the recirculation system piping and valve pump to form a stable oxide film. In addition, the temporary circulating pump 21 continuously injects the oxidizing gas onto the surface to be treated, thereby constantly generating a flow to form an oxide film capable of supplying a stable high-concentration oxidizing gas. When using ozone as an oxidizing gas, the concentration dissolved in pure water depends on the temperature of the solution, and as an equation for calculating the saturated dissolved ozone concentration in a gas-liquid equilibrium state,
(Equation 1)

Here, C _L: saturated ion concentration C _g: wherein the ozone concentration in the insufflation gas have been proposed. FIG. 8 shows an example of the relationship between the gaseous phase ozone concentration and the ozone concentration in the liquid phase at equilibrium based on the equation (1). As shown in FIG. 8, for example, when the gas phase ozone concentration is 100 mg / L, the ozone concentration in the liquid phase is:
25 ° C: 25.6 ppm
40 ° C: 19.7 ppm
50 ° C .: 17.2 ppm
60 ° C: 15.4 ppm
70 ° C: 14.0 ppm
80 ° C: 12.9 ppm
Ozone is easily dissolved at lower temperatures. However, formation of an oxide film is more effective as the temperature is higher. As the optimal range of the contradictory conditions, the liquidus temperature is preferably 25 ° C to 80 ° C.
[0031]
Further, in the final step after the chemical decontamination shown in FIG. 7, water is drained from the system using a drain line of a valve or the like, so that the system has a gas atmosphere. Then, when oxidizing gas of ozone or oxygen is supplied into the system at 25 ° C. to 80 ° C., ozone is dissolved in water remaining on the surface of stainless steel or carbon steel in the system, and stainless steel or carbon steel is dissolved. An oxide film is formed on the surface.
[0032]
Further, it is difficult to maintain the concentration of the oxidizing gas, particularly ozone, because it is self-decomposed. In this embodiment, since the oxidizing gas is supplied from a plurality of portions as shown in FIG. 7, the concentration of the oxidizing gas can be kept uniform throughout the system, and the uniformity can be maintained throughout the system. An oxide film can be formed.
[0033]
FIG. 9 is a graph showing the difference in the amount of radioactive deposition between the case where the surface of stainless steel (SUS304) is provided with an oxide film and the case where the oxide film is not provided by the above oxidation treatment. It can be seen that in the case where the oxide film was provided, the amount of radioactivity attached was significantly reduced.
[0034]
Therefore, a radioactivity adhesion test was performed to confirm that the oxide film applied to the stainless steel or carbon steel by the oxidation treatment functions effectively for the adhesion of radioactivity. As a test material, austenitic stainless steel 316 steel was selected as stainless steel, and STS410 was selected as carbon steel.
[0035]
For stainless steel, a strip (20 mm x 50 mm x 0.3 mm thick) test specimen is loaded into a metal container (200 ml capacity) provided with a nozzle for injecting an oxidizing gas and a nozzle for exhausting the same. Then, the container was heated with an external heater to maintain the temperature at 80 ° C. In the test, first, dry nitrogen gas was blown from the injection nozzle to replace the air in the container, and then about 10 vol% (remaining oxygen gas) of ozone gas was supplied as an oxidizing gas at 10 ml / min. The processing time was 2 hours. The oxidation treatment of the carbon steel was also performed using the same apparatus, the same conditions and the same shape of the test piece, and the only difference was that the set temperature was set to 180 ° C.
[0036]
These oxidized test specimens were loaded into an autoclave, and high-temperature, high-pressure water of 285 ° C. and 8 Mpa was passed through. Co-60 was contained as radioactivity in the feed water at a concentration of 0.02 Bq / milliliter, and a radioactivity adhesion test was performed for 500 hours with 200 ppb of dissolved oxygen and 10 ppb of dissolved hydrogen. FIG. 10 shows a table comparing the amount of radioactivity attached to each specimen with the untreated material after the end of the test. In the case of stainless steel, the amount of adhesion was about 60% of that of the untreated material, and in the case of carbon steel, the amount of adhesion was about 40%.
[0037]
FIG. 11 is a diagram showing an embodiment in which final oxidation is performed under a drying condition of 180 ° C. or more. By heating the object to be decontaminated by the external heater 46 while injecting an oxidizing gas such as ozone or oxygen from the oxidizing gas injection line 38, the growth of a denser oxide film is promoted.
[0038]
In the embodiment described so far, the case where water remains on the surface of stainless steel or carbon steel has been described. However, when the surface of stainless steel or carbon steel is dry, The same effect can be obtained by supplying water vapor together with the oxidizing gas in the embodiment. Particularly, in the embodiment shown in FIG. 11, water is supplied together with the oxidizing gas, and the water is heated by the heater 40 to generate saturated steam in the piping, and the same as when the steam is supplied together with the oxidizing gas. Effects can be obtained.
[0039]
Next, FIG. 12 shows an embodiment in which water vapor is forcibly condensed to promote the formation of an oxide film in the embodiment shown in FIG. In this embodiment, water is injected into the oxidizing gas injection line 38, and an external cooler 41 is attached to the upper vertical pipe of the reactor coolant recirculation system 5 to generate water at the lower part. The steam is condensed. In this case, the water in which the oxidizing gas is dissolved flows down while wetting the vertical wall surface, and during that time, an oxide film is formed in the piping of the reactor coolant recirculation system 5. In this way, by causing water vapor to condense on the surface to be treated, and by contacting with water droplets containing an oxidizing gas, it is possible to supply a stable high-concentration oxidizing gas to the stainless steel surface. On the other hand, the water flowing down and accumulated in the lower portion is heated again by the heater 40 to fill the inside of the pipe as steam.
[0040]
FIG. 13 is a modification of the embodiment of FIG. 4 in which processing is performed on a residual heat removal system pipe and a reactor water purification system pipe, which are carbon steel pipes branched from the boiling water reactor recirculation system. The oxide film on the inner surface of the pipe is removed by chemical decontamination, and after the carbon steel metal surface is exposed, after draining, the connection point with the decontamination loop at the time of chemical decontamination, the valve in the example of FIG. The oxidizing gas and the heated dry air are mixed from 24, 25, 26, and 22 and supplied to the piping of the residual heat removal system 8 and the piping of the reactor water purification system 9. Excess oxidizing gas is dehumidified by a cooler 35 and exhausted by a temporary circulation pump 37 through a decomposer 36. Thus, by this method, the entire steel surface of the carbon steel pipe and valve after decontamination is oxidized, and a stable oxide film is formed.
[0041]
FIG. 14 is a view showing another embodiment in which an oxide film is formed on the surface of carbon steel with the same effect. In this embodiment, the

external heaters

42 and 43 are applied to portions where an oxide film is to be formed while blowing an oxidizing gas, such as ozone, oxygen or a mixture of ozone and oxygen, instead of heating and drying air. Temperature.
[0042]
【The invention's effect】
According to the present invention, without adding ancillary equipment or special chemicals to the chemical decontamination apparatus configuration, the equipment after chemical decontamination in the installed state, a stable oxide film can be formed on the metal surface of the pipe, Incorporation of radioactivity can be suppressed during operation of the plant, and exposure can be reduced.
[Brief description of the drawings]
FIG. 1 is a system diagram showing an embodiment of a chemical decontamination method for a reactor coolant recirculation system and a residual heat removal system, which are the target parts of the chemical decontamination according to the present invention.
FIG. 2 is a flowchart showing a process of a chemical decontamination method according to the present invention.
FIG. 3 is a flowchart showing another process of the chemical decontamination method according to the present invention.
FIG. 4 is a system diagram showing a chemical decontamination method in which only the carbon steel piping is chemically decontaminated.
FIG. 5 is a view of an experimental result showing the effect of suppressing the adhesion of radioactivity to carbon steel according to the present invention.
FIG. 6 is a graph showing the amount of radioactive deposition when the oxidation treatment temperature is increased.
FIG. 7 is a system diagram showing another embodiment of the chemical decontamination method according to the present invention.
FIG. 8 is a diagram showing the temperature dependence of the concentration of dissolved ozone in pure water.
FIG. 9 is a conceptual diagram showing the effect of suppressing the adhesion of radioactivity to stainless steel according to the present invention.
FIG. 10 is a view of an experimental result showing the effect of suppressing the adhesion of radioactivity to stainless steel and carbon steel according to the present invention.
FIG. 11 is a system diagram showing another embodiment of the chemical decontamination method according to the present invention.
FIG. 12 is a system diagram showing still another embodiment of the chemical decontamination method according to the present invention.
FIG. 13 is a system diagram showing another embodiment of the chemical decontamination method according to the present invention.
FIG. 14 is a system diagram showing another embodiment of the chemical decontamination method according to the present invention.
FIG. 15 is a diagram showing a schematic configuration of a primary cooling water system of a boiling water reactor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Reactor pressure vessel 2 Reactor core 5 Reactor coolant recirculation system 6 Jet pump 7 Recirculation pump 8 Residual heat removal system 9 Reactor water purification system 20 Oxidizing gas supply source 21 Temporary circulation pump 22 Return valve 23 Riser pipe nozzle 24 Reactor water purification system inlet valve 25 Reactor water purification system bottom drain line inlet valve 26 Residual heat removal system inlet valve 27 Nozzle plug 28 One-side shut-off valve 31 Recirculation

system inlet nozzle

32, 33 Decontamination seat 34 Riser tube nozzle 38 Oxidation Gas injection line 40 Heater 41 External cooler

Claims

A chemical decontamination method in which the final step is an oxidation step in a decontamination method for chemically removing radioactivity attached to equipment and piping of a nuclear power plant by alternately repeating oxidation and reduction.

When the surface to be chemically decontaminated is covered with iron oxide, the above chemical decontamination is first started from the reduction step, and then the oxidation step and the reduction step are repeated. The chemical decontamination method according to claim 1, characterized in that:

If the surface to be chemically decontaminated is covered with an oxide film with a high chromium content, the above chemical decontamination starts with an oxidation step, then performs a reduction step and repeats this cycle. The chemical decontamination method according to claim 1, wherein the method is a step.

2. The chemical decontamination method according to claim 1, wherein the final oxidation step is performed only on the carbon steel pipe part of the equipment and pipes of the nuclear power plant, and is performed only on the carbon steel pipe part.

5. The chemical decontamination method according to claim 1, wherein in the final oxidation step, an oxidizing gas is supplied into a dry atmosphere after draining water in the pipe. 6.

The chemical decontamination method according to any one of claims 1 to 4, wherein in the final oxidation step, an oxidizing gas is circulated in a pipe to be processed, and a flow always occurs on a surface to be processed. .

The oxidizing gas is injected into the pipe to be treated in the final oxidation step, and the water injected into the pipe to be treated is heated to generate saturated steam. Chemical decontamination method.

8. The chemical decontamination method according to claim 7, wherein in the final oxidation treatment, the saturated steam in the piping to be treated is cooled and condensed to form a wet surface on the surface to be treated.

2. The chemical decontamination method according to claim 1, wherein in the final oxidation treatment, the surface of the carbon steel such as equipment and piping is subjected to oxidation treatment under a drying condition of 180 ° C. or more.

The chemical decontamination method according to claim 9, wherein in the final oxidation treatment, the carbon steel surface is subjected to an oxidation treatment in a dry state heated to 180 ° C or higher.