JP3740570B2 - Recycling method for lead contaminated with radioactive materials - Google Patents
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Description
【０００１】
【発明の属する技術分野】
この発明は、放射線防護の目的のために使われ、放射性物質に表面を汚染された鉛から、放射性物質を取り除き、鉛を再び使用できるように再生する技術に関するものである。
【０００２】
【従来技術】
放射性遮蔽材鉛の在来の処理について述べる。
放射性物質を取り扱う施設、とりわけ、核燃料製造施設、放射性廃棄物取り扱い施設、放射性同位元素取り扱い施設、原子力発電所などでは、放射線の人体への被爆を防止するため放射線防護設備が使われている。
【０００３】
この放射線防護設備には放射線遮蔽材料が使われていて、この材料としては 水、プラスチックス複合材料、鉄、鉛、コンクリ−トなどが代表的である。
【０００４】
これらのうち鉛は密度が高く、放射線損傷が少なく、遮蔽厚さを他の材料と比べて薄くでき、成型加工も容易な上、入手しやすいのでγ線の遮蔽材料としてしばしば便利に用いられる。
【０００５】
一方、欠点として高温になる所には高温時の構造的強度不足のために、また融点（３２７℃）が比較的低く、耐熱材料としても使えないので、恒常的設備というよりは臨時的、仮設備的工事対策や作業対策、または実験設備などで使用される場合が多い。
【０００６】
この放射線遮蔽材料としての鉛の使われ方の特徴から、使用時に放射性物質に汚染される場合も多く、使用後は大概の場合片付け、撤収されることになる。
【０００７】
一度使用された遮蔽材の鉛を再度使用するには、放射性汚染による被曝防止のため、汚染の状況を計測記録して次回の使用を適正にするための管理や、放射性汚染された材料の管理保管が難しい。
【０００８】
この目的に使われる鉛はブロック状のいわゆる鉛レンガばかりでなく、シ−ト状で切断加工されたものや、粒状のもの、他の材料と複合して作られた物などがあり、片付けの時、切り裂かれたり、くしゃくしゃに丸められたり、異物が混在したりなど問題が多く、再使用を困難としている。
【０００９】
そのため臨時的作業の放射線防御に使われる鉛は、いわゆる新鉛が使われる場合が多く、未だその目的に使われてなく、汚染されてない、形状寸法のしっかりした鉛材である。
【００１０】
放射線遮蔽材としていったん使われた後の鉛の多くは、放射線汚染物としてそのまま保管にまわされている場合が多い。
【００１１】
この使用済み汚染鉛も原子力産業の初期には比較的問題にならなかったものが、次々に放射線汚染鉛が集積されてくると、貯蔵空間の確保と費用の問題、埋設処理する場合も費用が膨大になることが判明して、経済上の問題、資源の有効利用問題など、このまま放置しておくことは出来なくなってきた。
【００１２】
さらに、在来の汚染鉛の除染技術として、汚染された鉛からの汚染の除去には、もちろん一般的に使われる除染技術である化学洗浄や、研削、ブラスト研磨などの通常の金属表面処理的技術が適用され得ることは容易に理解できる。
【００１３】
またこれら一般的除染方法にはそれぞれ欠点がある。とくに複雑な形状の鉛や、繊維、プラスチックなどと複合されたものでは隙間に入った汚染物は除染できない。一方、上述したように放射線遮蔽に使用された後の鉛は形状的、寸法的にも毀損している場合が多く、複合材料では難しい。
【００１４】
そこで、再生するには一旦、通常の方法で溶解してインゴットに戻し、その後圧延などのような金属加工技術で再使用可能な形状に整形することを考える。
しかし、放射能汚染の核種にもよると思われるが、単なる再溶解では放射性汚染物の除去は十分にはできないのみならず、汚染放射性元素が鉛に合金拡散する心配がある。
合金溶解した放射性元素を取り除くことは更に厄介な鉛精練技術問題となるのである。勿論放射能汚染された鉛を通常の鉛精練工場に戻すことは、精練工程のすべての放射能汚染につながるので、不可能である。また、例え放射線管理区域内で処理する場合でもなるべく単純な方法、設備で、放射能汚染の設備装置への拡大を防止しなくてはならない。
【００１５】
従来、簡素かつ有効な、放射能汚染鉛の溶解回収技術がないため、上述したように放射能汚染鉛は再使用されること無く、貯蔵保管されているのである。
【００１６】
【課題を解決するための手段】
発明者は放射線防護用としては最も頻繁に使われている原子力発電所での汚染鉛の場合、発電用原子炉関連作業での放射線汚染は、主として原子炉冷却水の付着による汚染なので、原子炉冷却水中の放射性同位元素を想定した。
【００１７】
原子炉一次冷却水の測定例と放射性物質の半減期は表１のごとくである。
【表１】
水冷却型原子炉での一次冷却水が鉛に付着したとすると、冷却水中に含まれる放射性核種の中で半減期の長い 60Ｃｏ、137 Ｃｓが実際の汚染除去の対象となるので、これらの放射性元素を市販のいわゆる新鉛に付着させた模擬汚染試料を使い種々の条件で実証試験した。
【００１９】
模擬汚染試料は放射性塩化コバルト、放射性塩化セシウムの水溶液を、市販の鉛板の上に順次滴下し、さらに苛性ソ−ダ水溶液を滴下して中和し、放置乾燥して作った。試験片鉛は１個約１００ｇ，放射性物質はサンプル上に局在するがその線量は ５，０００〜７，０００Ｂｑ／個を示した。元の試験片の放射線量は極端に少ないため測定しなかった。
【００２０】
試験鉛の溶解はステンレス製はんだ溶解槽を使った。この槽の仕様は、るつぼ寸法５０φ×５０ｈ，電力容量２００Ｗ、温度制御装置付きであった。放射能量の測定はＧｅ半導体検出器によるγ線スペクトルによって行った。
【００２１】
なお当然のことであるが、鉛による衛生上の障害や、放射線障害、放射性物質の拡散を防止するために必要な設備、装置を完備したうえ、万全の注意を払って操作を行ったが、何としても放射性物質を取り扱う作業は種々な制約が多く、操作回数が自から少なく、操作条件を大幅に拡大することは困難であったが、科学的類推判断により本発明の構成を合理性の範囲内で設定した。
【００２２】
発明の実証に先立ち、汚染鉛試料を単純に溶解したのち放置冷却固化させ、表面の酸化物様のスラグを掻きとり、このスラグと金属鉛の放射線強度とを比較して調べたところ、全放射能の約８０％は酸化物に集まり、残り２０％は金属鉛に存在していた。汚染鉛は再溶解するだけでかなりの放射性汚染物が表面スラグに移行することが分かった。しかしこの程度の放射能除去では、汚染濃度がもともと低い場合を除き、再使用には適さない。
【００２３】
そこで発明者は酸化物様スラグに代表される汚染物の捕集を確実に行うために、少量の溶融塩を添加して鉛の溶融を行うことを思い付いた。
【００２４】
種々の溶融塩を添加して実験してみたところ、なるべく融点が低く、蒸気圧が高くなく、粘度が低く、しかも酸化性を持ち、放射性金属化合物を熱的に安定な複合酸化物にかえる塩基性組成が最適であることが分かった。
【００２５】
実証した鉛の融解量は毎回７００〜８００ｇ，複合溶融塩は毎回２０〜２５ｇ。その概略組成は酸化性浴成分として硝酸ナトリウム３５重量％、融点低下、塩基性成分として苛性ソ−ダ６５重量％とした。これらの塩は予め乾燥、予備溶解を行って使用した。この複合溶融塩は鉛の融点である３２７℃より低く、３００℃以下の温度で融解した。またこの複合塩の比重は融解状態で約２ｇ／ｍｌ、融解鉛の比重の数分の一と大差があるので容易に上層に分離した。
【００２６】
操作温度は塩と鉛が融解すれば頭書の目的は達成されるが、操作上の温度幅と塩の酸化力を強力に維持するために、４００℃とした。操作温度の上限は、塩の組成、塩と鉛の蒸気圧できまり、６００℃くらいまでが実用的である。
【００２７】
塩の組成は経済的理由からＮａ化合物を使用したが、Ｋ化合物、他のアルカリ化合物をなどで一部を置き換えたり、混合しても前述の効果は変わらない。
【００２８】
また複合溶融塩の技術で考察される、アルカリ土類化合物、ハロゲン化アルカリ、ハロゲン化アルカリ土類化合物を少量混合することは、溶融塩の粘度、融点を下げ、発明の効果の向上に役立つ場合がある。
【００２９】
上述のことから、この発明の放射性物質で汚染された鉛の再生方法は、融解した汚染鉛に融解した溶融塩を接触させて、鉛の放射性汚染物を取り除く方法であり、またその方法において、溶融塩が汚染鉛を融解処理する温度で融解する組成の、単独塩または複合塩であり、さらにその溶融塩が塩基性組成として、水酸化ナトリウム、アルカリ水酸化物、酸化アルカリを単独または組み合わせてなるものであり、さらにまた溶融塩が酸化性組成物として、硝酸ナトリウム、硝酸アルカリを単独または組み合わせてなるものである。
【００３０】
またこの発明の放射性物質で汚染された鉛の再生方法は、複合塩の組成が０〜２０％のハロゲン化アルカリ、または／およびアルカリ土類化合物であり、また前記融解した汚染鉛と溶融塩の接触が、機械攪拌、空気攪拌または気体による攪拌を、単独または組み合わせて行われ、さらに、該汚染鉛の融解が、あらかじめ融解させた溶融塩の中に、汚染鉛を温度が大きく下がらないように少量づつ投入して、還元性雰囲気とならないように酸化性雰囲気下で溶解することであり、さらにまた、融解した汚染鉛と、溶融塩を接触させる温度が、鉛および溶融塩の融解温度より高く、６００℃より低い温度とし、望ましくは３８０〜４５０℃とすることである。
【００３１】
さらに本発明の放射性物質で汚染された鉛の再生方法は、融解した汚染鉛と融解した溶融塩の接触を攪拌によって行い、その攪拌をやめて静置し、溶融塩を上層に分離し、その後分離された塩を排出する、この過程を１回以上、再度塩を添加して溶解し、複数回繰り返すことであり、また融解した溶融塩が、融解した汚染鉛と溶融塩の接触を行うための攪拌をやめて静置し、上層に分離した後排出した溶融塩であることと、及び融解した汚染鉛と溶融塩の接触を行うための攪拌をやめて静置し、上層に分離した後排出した溶融塩において、再び該接触に利用されない溶融塩を、冷却固化の後、水に溶解し固形物を濾過して除き、残りの濾液は中和して再濾過洗浄の後、液は希釈してイオン交換樹脂と接触させ、放射性物質を取り除くことであり、さらにのべるならば、融解した汚染鉛と融解した溶融塩を同槽内で攪拌して互いに接触させ、その後両者を比重差によって分離することである。
【００３２】
【発明の実施の形態】
実証試験の結果は顕著に効果的であった。上記の溶融塩組成物を使って放射線汚染鉛を溶解処理すると、上層部の溶融塩層に放射性物質の９５％以上が移行したが、移行率を確実に上げるためには更に種々の操作上の工夫が必要であることが分かった。
【００３３】
まず、溶融塩と溶融した鉛との接触を良くするために、十分に溶融塩と共に溶融鉛を撹拌することが重要であった。撹拌にはステンレスのプロペラ式撹拌機を使った。撹拌機の仕様は２０φステンレス羽根回転型、回転数１８０ｒｐｍとした。これは空気等のガスを底部に注入する方法、二つの容器の間を互いに移し替える方法等でも同様の効果が得られると考える。
【００３４】
次に、鉛の融解時に、表面に付着した放射性化合物の還元を防止するように操作することも大切であった。まず、予め塩と少量の非放射性鉛を融解し、操作温度以上に温度を上げておき、少量づつ汚染鉛を投入する。もちろんこの時できるだけ撹拌は続けながら行うのがよい。汚染鉛は使用された過程でしばしば油などの還元性物質が付着している場合があり、汚染金属元素を還元する恐れがあるので、このような場合は注意して、とくに少量づつ融解することが重要であった。
【００３５】
すべての処理材料である汚染鉛の投入融解が終了したら、１０〜２０分撹拌し鉛の操作温度以上に維持する。当然、添加された塩も融解状態である。その後、撹拌を止めると上層に比重差により溶融塩が集まる。放射性不純物を十分に吸収した溶融塩は固化する前に、溶解容器を傾けるか真空で吸い上げるなどして完全に排出する。この組成の溶融塩は反応後も極めて粘度が低く、融解に使ったステンレスるつぼや、撹拌器などへの付着量も少なく、金属と分離して排出する事はさほど難しくないが、塩を残さぬように注意することが重要であった。
【００３６】
最後に同一組成の塩を溶融した鉛に加え撹拌、融解させる。これは微量に残っている放射性物質を含む塩を取り除くためである。その後、融解した塩は上記同様排出する。必要によりこの操作を複数回繰り返すことはこの発明の要件の範囲を逸脱しない。
【００３７】
この操作で使われた塩は次回の処理に再び使用することもできる。とくに２段目以降に使われた塩は次回の１段目に使うに当たって何等問題はない。ただし、この複合塩は吸湿し易いので密封容器に保存するのがよい。
【００３８】
使用後、放射性元素を含んだ塩は、水で溶解、残滓を濾過分離して中和などの後、カチオン交換樹脂で処理して放射性廃棄物の量を低減させるプロセスに付すことが容易に可能であるのもこの発明の意義である。
【００３９】
【発明の効果】
もともと一般工業用ＪＩＳ−Ｈ−２１０５第３種（９９％以上）規格品地金は、製造ロットにより差があるが高い場合で１０Ｂｑ／ｇ程度の放射能を示す。工業用鉛地金試験片に上記の方法で放射性物質を数千Ｂｑ付着させ、この発明方法を使って処理した鉛の放射能の測定結果は２〜９Ｂｑ／ｇまで下がっていた。
【００４０】
次に処置された鉛を通常の方法で鋳造、圧延して、１ｍｍ厚みの鉛シ−トとし、放射線測定、比較を行ってみたが、有為差ある測定結果は得られなかった。これによって放射能が下がったのは、放射性元素が鉛の内部に合金化して取り込まれたのではないことが分かり、ほとんど全ての放射性汚染物は溶融塩に移行して取り除かれたことが分った。鉛シ−ト、鉛粒子として再使用する場合でも、本発明の方法によって処理された回収鉛は全く放射線被曝の心配なく、使用に付すことができる。
【００４１】
実際の操作では、経済的理由もあって、さらに大きな融解、鋳造、環境保全設備を使うことになるが、上述の実施例は大型設備に通常の技術でそのまま適用可能である。
【００４２】
溶解炉は、耐熱鋼、鋳鉄、耐火物製等の中から選択できる。その加熱方法は電熱、誘導加熱、燃焼加熱等の中から選択できる。炉の操作は傾注、下抜き、真空サイフオン式等の中から選択できる。炉の撹拌は耐熱鋼製の撹拌機、ガス吹き込みによる撹拌、溶解炉移し替えによる撹拌等から選択できる。鋳造は通常の方法で可能であるが、放射性物質の取扱いを考慮すると、連続自動鋳造方式が適当である。
【００４３】
当然、この実施例の溶解炉、鋳造設備、溶融塩受容設備等は、排気集塵設備を放射性粉塵を十分に考慮したものとしなければならないが、これは通常の放射性物質取扱い施設にかかわる技術で可能である。
【００４４】
この発明の方法によって、上述したように原子力発電所などの放射線取扱い施設で使用後汚染鉛として大量に保管されている鉛の再使用を、放射線被曝の危険無く進めることができ、管理保管や、低レベル放射性廃棄物埋設処理に要する経費の大幅な節減になり、鉛資源の有効活用と放射性物質取扱い施設の運用の経済化に大いに役立つことができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique used for the purpose of radiation protection, which removes radioactive material from lead contaminated with radioactive material and regenerates the lead so that it can be used again.
[0002]
[Prior art]
The conventional treatment of radioactive shielding material lead is described.
In facilities handling radioactive materials, especially nuclear fuel manufacturing facilities, radioactive waste handling facilities, radioactive isotope handling facilities, nuclear power plants, etc., radiation protection equipment is used to prevent radiation exposure to human bodies.
[0003]
Radiation shielding materials are used in this radiation protection equipment, and water, plastics composite material, iron, lead, concrete, etc. are representative.
[0004]
Of these, lead is often used as a γ-ray shielding material because it has high density, less radiation damage, can be made thinner than other materials, can be easily molded, and is easily available.
[0005]
On the other hand, the disadvantage is that in places where the temperature is high, due to insufficient structural strength at high temperatures, and because the melting point (327 ° C.) is relatively low, it cannot be used as a heat-resistant material. It is often used in equipment construction measures, work measures, or experimental equipment.
[0006]
Due to the characteristics of the use of lead as a radiation shielding material, it is often contaminated by radioactive substances during use, and after use, it is usually cleared and removed.
[0007]
In order to reuse lead of the shielding material once used, in order to prevent exposure due to radioactive contamination, management to measure and record the status of contamination to make it suitable for the next use, and management of radioactively contaminated material Storage is difficult.
[0008]
The lead used for this purpose is not only the so-called lead bricks in the form of blocks, but also those that have been cut and processed in the form of sheets, granules, and those made in combination with other materials. At times, there are many problems such as being torn, crumpled and crumpled, and it is difficult to reuse.
[0009]
Therefore, the lead used for radiation protection in temporary work is often so-called new lead, and it is a lead material with a solid shape and size that is not yet used for that purpose and is not contaminated.
[0010]
Most of the lead once used as a radiation shielding material is often stored as radiation contaminants.
[0011]
Although this used contaminated lead was not a relative problem in the early days of the nuclear industry, the accumulation of radiation-contaminated lead one after another has caused problems in securing storage space and costs, and in the case of landfill treatment. It became clear that it became enormous, and it has become impossible to leave it as it is, such as economic problems and effective use of resources.
[0012]
In addition, as a conventional decontamination technique for contaminated lead, the usual metal surface such as chemical cleaning, grinding, blasting, etc., which are commonly used decontamination techniques, is of course used to remove contamination from contaminated lead. It can be readily understood that processing techniques can be applied.
[0013]
Each of these general decontamination methods has its drawbacks. In particular, contaminants that enter the gap cannot be decontaminated with complex shapes such as lead, fibers, and plastics. On the other hand, as described above, lead after being used for radiation shielding is often damaged in terms of shape and dimensions, which is difficult in a composite material.
[0014]
Therefore, in order to recycle, it is considered that the material is once melted and returned to an ingot by a normal method, and then shaped into a reusable shape by a metal processing technique such as rolling.
However, although it seems to depend on the radionuclide nuclide, it is not only possible to remove the radioactive contaminants by mere remelting, but there is a concern that the contaminating radioactive elements may be alloyed into lead.
Removing the radioactive elements dissolved in the alloy becomes a more troublesome technical problem for lead scouring. Of course, returning radioactively contaminated lead to a normal lead smelting plant is impossible because it leads to all radioactive contamination in the scouring process. In addition, even when processing in a radiation control area, it is necessary to prevent the spread of radioactive contamination to equipment with simple methods and equipment as much as possible.
[0015]
Conventionally, since there is no simple and effective technique for dissolving and recovering radioactively contaminated lead, as described above, radioactively contaminated lead is stored and stored without being reused.
[0016]
[Means for Solving the Problems]
In the case of contaminated lead in nuclear power plants that are most frequently used for radiation protection, the inventor indicated that the radioactive contamination in power reactor related work is mainly due to the adhesion of reactor cooling water. Radioisotopes in cooling water were assumed.
[0017]
Table 1 shows the measurement examples of reactor primary cooling water and the half-life of radioactive materials.
[Table 1]
If the primary cooling water in a water-cooled nuclear reactor is attached to lead, 60 Co and 137 Cs, which have a long half-life among the radionuclides contained in the cooling water, are the targets for actual decontamination. Demonstration tests were conducted under various conditions using simulated contamination samples in which radioactive elements were attached to so-called new lead.
[0019]
A simulated contaminated sample was prepared by sequentially dropping an aqueous solution of radioactive cobalt chloride and radioactive cesium chloride onto a commercially available lead plate, further dropping a caustic soda aqueous solution to neutralize it, and drying it by standing. The specimen lead was about 100 g, and the radioactive substance was localized on the sample, but the dose was 5,000 to 7,000 Bq / piece. Since the radiation dose of the original specimen was extremely small, it was not measured.
[0020]
The test lead was melted using a stainless solder melting bath. The specifications of this tank were a crucible size of 50φ × 50h, a power capacity of 200 W, and a temperature control device. The amount of radioactivity was measured by a γ-ray spectrum by a Ge semiconductor detector.
[0021]
As a matter of course, the equipment and equipment necessary to prevent sanitary hazards due to lead, radiation hazards, and diffusion of radioactive materials were fully used, and the operation was carried out with utmost care. In any case, the work of handling radioactive materials has many restrictions, the number of operations is small, and it has been difficult to greatly expand the operating conditions. Set within range.
[0022]
Prior to the demonstration of the invention, the contaminated lead sample was simply dissolved and then allowed to cool and solidify, and the oxide-like slag on the surface was scraped, and the radiation intensity of this slag was compared with that of metallic lead. About 80% of the capacity was collected in the oxide, and the remaining 20% was in the metallic lead. It was found that the contaminated lead simply re-dissolved and a considerable amount of radioactive contaminants migrated to the surface slag. However, this level of radioactivity removal is not suitable for reuse unless the contamination concentration is originally low.
[0023]
Therefore, the inventor has come up with the idea of adding a small amount of molten salt to melt lead in order to reliably collect contaminants typified by oxide-like slag.
[0024]
Experiments with adding various molten salts revealed that the base has a melting point as low as possible, vapor pressure is not high, viscosity is low, and it has oxidizing properties, and converts a radioactive metal compound into a thermally stable composite oxide. The sex composition was found to be optimal.
[0025]
The amount of lead melt demonstrated is 700-800 g each time, and the composite molten salt is 20-25 g each time. The approximate composition was 35% by weight of sodium nitrate as an oxidizing bath component, a melting point reduction, and 65% by weight of caustic soda as a basic component. These salts were used after drying and preliminary dissolution. This composite molten salt melted at a temperature lower than 300 ° C., which is lower than the melting point of lead, 327 ° C. Further, the specific gravity of this composite salt was about 2 g / ml in the molten state, which was a fraction of the specific gravity of the molten lead, so it was easily separated into the upper layer.
[0026]
The operating temperature is 400 ° C. in order to maintain the operating temperature range and the oxidizing power of the salt, although the purpose of the headline is achieved if the salt and lead are melted. The upper limit of the operating temperature is determined by the composition of the salt and the vapor pressure of the salt and lead.
[0027]
Although the Na compound was used for the composition of the salt for economic reasons, the above effect is not changed even if a K compound, another alkaline compound, etc. are partially replaced or mixed.
[0028]
In addition, mixing small amounts of alkaline earth compounds, alkali halides, and halogenated alkaline earth compounds, which are considered in the technology of composite molten salts, can lower the viscosity and melting point of the molten salt and help improve the effects of the invention. There is.
[0029]
From the foregoing, the method of recycled lead contaminated with radioactive materials of the present invention, by contacting the molten salt melted into molten contaminated lead, a method removing radioactive contaminants lead, and in that way , a composition that melts at a temperature at which molten salt melts processing contaminated lead a solely salt or complex salt, as further its molten salt basic composition, alone sodium hydroxide, alkali hydroxide, an alkali oxide or in combination are those comprising, as a still further molten salt oxidizing composition, sodium nitrate, is made alone or in combination of nitric acid alkali.
[0030]
The method for regenerating lead contaminated with radioactive substances of the present invention is an alkali halide or / and alkaline earth compound having a composite salt composition of 0 to 20%, and the molten contaminated lead and molten salt are mixed. contact, mechanical stirring, stirring by air agitation or gas, conducted alone or in combination, further, the melting of the pollution lead, in the pre-thaw is not the molten salt temperature does not drop significantly contaminated lead and small amounts one Dzu turned as is to dissolve in an oxidizing atmosphere so as not to reducing atmosphere, furthermore, the contamination lead was melted, the temperature of contacting the molten salt, the melting of lead and molten salt The temperature is higher than the temperature and lower than 600 ° C., preferably 380 to 450 ° C.
[0031]
Furthermore, in the method for regenerating lead contaminated with radioactive substances of the present invention, the contact between the molten contaminated lead and the molten salt is carried out by stirring, the stirring is stopped, and the molten salt is separated into the upper layer and then separated. The process of discharging the dissolved salt is to add the salt once more, dissolve again, and repeat the process a plurality of times, and the molten salt is in contact with the molten contaminated lead and molten salt. Stop the stirring, leave it to stand, separate it into the upper layer, and then discharge the molten salt, and stop the stirring to bring the molten contaminated lead into contact with the molten salt, leave it at the upper layer, separate it into the upper layer, and then discharge it In the salt, the molten salt that is not used again for the contact is cooled and solidified, dissolved in water, and the solid matter is removed by filtration. The remaining filtrate is neutralized, washed again by filtration, and the solution is diluted to ion. Contact with replacement resin to remove radioactive material. If further described, the molten salt melt and the molten contaminated lead in contact with one another with stirring in the same tank, is that then separating them by density differences.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
The results of the demonstration test were remarkably effective. When radiation-contaminated lead was dissolved using the above molten salt composition, 95% or more of the radioactive material was transferred to the molten salt layer in the upper layer portion. It turns out that a device is necessary.
[0033]
First, in order to improve the contact between the molten salt and the molten lead, it was important to sufficiently stir the molten lead together with the molten salt. A stainless propeller type stirrer was used for stirring. The specifications of the agitator were a 20φ stainless steel blade rotating type and a rotation speed of 180 rpm. It is considered that the same effect can be obtained by a method of injecting a gas such as air into the bottom, a method of transferring between two containers, or the like.
[0034]
Then, during the thaw of lead, it was also important to operate so as to prevent the reduction of the radioactive compound attached to the surface. First, loosened beforehand salt and a small amount of non-radioactive lead melt, keep raising the temperature above the operating temperature, is charged little by little contamination lead. Of course, it is better to keep stirring as much as possible. Contamination lead may reducing substance such as often oils in the process used is attached, there is a possibility of reducing the contaminant metal elements, such if a note, in particular to small portions thaw It was important.
[0035]
When turned thawing pollution lead are all process material is completed, stirring is maintained to above the operating temperature of the lead 10 to 20 minutes. Of course, the added salt is also in a molten state. Thereafter, when stirring is stopped, the molten salt collects in the upper layer due to the specific gravity difference. Molten salt that has sufficiently absorbed radioactive impurities is completely discharged before it is solidified by tilting the dissolution vessel or sucking it up in a vacuum. After the molten salt of the composition reaction is extremely low viscosity, stainless crucible and using the thawing, the adhesion amount of the like agitator is small and although it is not very difficult to discharge separated from the metal, leaving a salt It was important to be careful.
[0036]
Finally stirring addition to lead to melt the salt of the same composition, thereby thaw. This is to remove the salt containing radioactive material remaining in a trace amount. Thereafter, thawing salt is the same emissions. Repeating this operation a plurality of times as necessary does not depart from the scope of the requirements of the present invention.
[0037]
The salt used in this operation can also be used again for the next treatment. In particular, the salt used after the second stage has no problem in the next stage. However, since this composite salt easily absorbs moisture, it should be stored in a sealed container.
[0038]
After use, salts containing radioactive elements can be easily dissolved in water, filtered and separated to neutralize the residue, and then treated with a cation exchange resin to reduce the amount of radioactive waste. This is also the significance of the present invention.
[0039]
【The invention's effect】
Originally, JIS-H-2105 type 3 (99% or more) standard product bullion for general industrial use shows a radioactivity of about 10 Bq / g when it is high although it varies depending on the production lot. Several thousand Bq of radioactive material was adhered to an industrial lead ingot test piece by the above-mentioned method, and the measurement result of the radioactivity of lead processed by using the method of the present invention was lowered to 2 to 9 Bq / g.
[0040]
Next, the treated lead was cast and rolled by a usual method to obtain a lead sheet having a thickness of 1 mm, and radiation measurement and comparison were performed. However, a measurement result having a significant difference was not obtained. The decrease in radioactivity was found to be due to the fact that radioactive elements were not alloyed and incorporated into the lead, indicating that almost all radioactive contaminants had been transferred to the molten salt and removed. It was. Even when it is reused as a lead sheet or lead particles, the recovered lead treated by the method of the present invention can be used without worrying about radiation exposure.
[0041]
In actual operation, there is also economic reasons, greater thaw, casting, but be using environmental protection facilities, the above-described embodiment is as applicable in conventional techniques to large-scale facilities.
[0042]
The melting furnace can be selected from heat-resistant steel, cast iron, refractory and the like. The heating method can be selected from electric heating, induction heating, combustion heating and the like. The operation of the furnace can be selected from decanting, undercutting, vacuum wall-on type, etc. Stirring of the furnace can be selected from a heat-resistant steel stirrer, stirring by blowing gas, stirring by transferring the melting furnace, and the like. Although casting can be performed by a normal method, a continuous automatic casting method is appropriate in consideration of handling of radioactive materials.
[0043]
Naturally, the melting furnace, casting equipment, molten salt receiving equipment, etc. of this embodiment must make the exhaust dust collection equipment sufficiently consider radioactive dust, but this is a technology related to ordinary radioactive material handling facilities. Is possible.
[0044]
By the method of the present invention, as described above, the reuse of lead stored in large quantities as contaminated lead after use in radiation handling facilities such as nuclear power plants can be promoted without risk of radiation exposure, This will greatly reduce the cost required for the disposal of low-level radioactive waste and can greatly contribute to the efficient use of lead resources and the economics of the operation of radioactive material handling facilities.
Claims (12)
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| JP2000399417A JP3740570B2 (en) | 2000-12-27 | 2000-12-27 | Recycling method for lead contaminated with radioactive materials |
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| JP6150278B2 (en) * | 2013-01-07 | 2017-06-21 | 国立研究開発法人物質・材料研究機構 | Cesium decontamination method |
| CN112553479B (en) * | 2020-12-04 | 2022-08-16 | 广东先导微电子科技有限公司 | Method for removing high-purity antimony surface pollutants |
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