報告書番号

382024

タイトル（和文）

フォールアウト（放射性降下物）を利用したプルトニウムの地中移動速度の検討

タイトル（英文）

EVALUATION OF THE MIGRATING VELOCITY OF FALLOUT PLUTONIUN IN AN UNSATURATED ZONE

概要 （図表や脚注は「報告書全文」に掲載しております）

放射性廃棄物の陸地貯蔵あるいは地層処分時の安全評価の決定核種の一としてプルトニウムを挙げることができる。安全評価上地下水の移動速度のほかに重要因子として放射性核種の土壤・岩盤等への分配係数や地中での核種の移動遅れを表わす遅延係数を考えなければならない。特に実環境条件下でのプルトニウムの分配係数・遅延係数の実測結果は，今迄に報告された例がない。本研究では，フォールアウト起源が明白な長崎原爆のフォールアウトプルトニウムの３６年間の地中移動分布を調査することにより分配係数・遅延係数の評価を行った。（１）Ｐｕの総吸着量の９０，９５，９７％までが地表から１０，２０，３０ｃｍまでの土壤に保持されていた。（２）降水の浸透速度は，１００ｃｍ／ｙと推定された。（３）西山土壤（シルト係粘土ローム）に対するＰｕの遅延係数は約４００，分配係数は約２２０（ｍｌ／ｇ）であった。

概要 （英文）

THE ATOMIC BOMB FATMAN WHICH CONSISTED OF PLUTONIUM AS A FISSILE MATERIAL WAS EXPLODED AT THE HIGHT OF 500M ABOVE URAKAMI IN NAGASAKI-CITY AT 11:00 A.M. ON AUGUST 9 IN 1945.IN A HALF HOUR AFTER THE DETONATION,A LARGE AMOUNT OF FISSION PRODUCTS AND UNFISSIONED PLUTONIUM IN THE FORM OF FALLOUT FELL DOWN ON THE GROUND SURFACE ACCOMPANYING WITH RAIN AT THE NISHIYAMA DISTRICT,LOCATED AT THE WEST SUBURB OF NAGASAKI-CITY,AT 3KM FROM THE HYPOCENTER.IN THIS WORK,WE FOCUSED GREAT EMPHASIS ON THE FOLLOWING MEASUREMENTS AT THE NISHIYAMA DISTRICT:(1)DETERMINATION OF THE ORIGINATION OF THE ACCUMULATED PLUTONIUM ON THE SURFACE SOILS,(2)THE VERTICAL DISTRIBTION OF PLUTONIUM IN THE GROUND DETERMINING THE MIGRATING VELOCITY OF IT IN THE DEEP DIRECTION,AND(3)THE TRITIUM CONCENTRATION OF SOIL WATER IN AN UNSATURATED ZONE TO DETERMINETHE INFILTRATING VELOCITY OF THE RAIN WATER. (1)PLUTONIUM SURVEY ON THE GROUND SURFACE AT THE NISHIYAMA DISTRICT. WE CONCLUDED THAT MORE THAN90% OF PLUTONIUM ACCUMULATED IN THE NISHIYAMA SOIL WAS DERIVED FROM THE EXPLOSION OF THE ATOMIC BOMB IN 1945. (2)VERTICAL DISTRIBUTION OF PLUTONIUM IN THE GROUND OF THE NISHIYAMA DISTRICT. WE CONFIRMED THAT THE POSITION OF THE MIGRATION FRONT FOR 90% OF THE TOTAL RETAINED PLUTONIUM AND FOR 97% MOVED DOWN TO 10CM AND 30CM DEPTH DURING 36YRS,RESPECTIVELY. (3)THE TRITIUM CONCENTRATION OF SOIL WATER IN THE UNSATURATED ZONE. WE MEASURED THE TRITIUM CONCENTRATION OF SOIL WATER TO DETERMINE THE INFILTRATING VELOCITY OF RAIN WATER IN LOAM LAYER. INFILTRATION RATE WAS PREDICTED ABOUT 1.0M/YR BASED ON THE TRITIUM PROFILE. (4)EVALUATION OF PLUTONIUM RETARDATION FACTOR(RE) AND DISTRIBUTION COEFFICIENT(KD) FOR THE NISHIYAMA SOIL. RE AND KD WERE EVALUATED 400 AND 220(ML/G)FROM THE FIELD DATA,RESPECTIVELY.

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