報告書番号

T29

タイトル（和文）

高レベル廃棄物地層処分の人工バリアシステムの安全評価

タイトル（英文）

SAFETY ANALYSIS OF ENGINEERED BARRIER IN GEOLOGICAL DISPOSAL OF HIGH-LEVEL WASTE

概要 （図表や脚注は「報告書全文」に掲載しております）

高レベル放射性廃棄物の深地層処分の安全性を評価するため，人工バリア長期シュミレーションコードを開発し，それを用いて人工バリアを構成するガラス固化体，オーバーパック，緩衝材の持つ核種保持機能を明らかにするため，安全評価解析を実施した。主な成果は次の通りである。（１）オーバーパック（ＣＲＡＮＰによる解析）深部地下の還元状態において炭素鋼の全面腐食に対する寿命は，２５ｃｍ厚のオーバーパックでは１万年以上と推定された。しかし，現状の知見では信頼性のある評価は困難であるので，ここでは千年とした。（２）ガラス固化体（ＳＴＲＡＧ４による解析）ガラス固化体の溶解寿命は１００万年以上になるとの結果が得られた。これは，オーバーパックの寿命の間に固化体周辺温度が地温まで低下するためである。ガラス固化体の割れによる表面積の増加は，Ｃｓ－１３５のような可溶性核種の放出を増加させるが，ＴＲＵのよ

概要 （英文）

THIS REPORT PRESENTS PERFORMANCE ANALYSES FOR AN ENGINEERED BARRIER FOR HIGH-LEVEL RADIOACTIVE WASTE DISPOSAL. THE ANALYSES WERE PERFORMED BY THE CRNP/CRANP,STRAG,AND GESPER/RAPRAN CODESFOR OVERPACKING,WASTE GLASS AND BUFFER MATERIAL,RESPECTIVELY. SOME RESULTS ARE SHOWN BELOW. (1)CORROSION LIFE OVERPACKING CRNP/CRANP INDICATED THAT THE GENERAL CORROSION LIFE OF 25-CM CARBON STEEL IS MORE THAN 10,000 YEARS. ALTHOUGH OUR ESTIMATE IS STILL ROUGH BECAUSE OF LIMITS ON CURRENT KNOWLEDGE ABOUT PITTING CORROSION,THE LIFE CAULD BE 1000 YEARS. (2)DISSOLUTION OF WASTE GLASS STRAG4 CALCULARED THAT THE TIME FOR COMPLETE DISSOLUTION OF WASTE GLASS IS MORE THAN 1MILLION YEARS. THE REASON FOR SUCH A HIGH DISSOLUTION RESISTANCE IS MAINLY DUE TO BOTH THE HIGH SILICA CONRENT IN BENTONITE PORE WATER AND THE RESULTING SLOW IRREVERSIBLE DISSOLUTION RATE FAVORED BY A LOW UNDERGROUND TEMPERATURE OF 30 . INCREASES IN THE GLASS SURFACE AREA CAUSED BY SPLITTING INTO SMALL PARTS INCREASE DISSOLUTION OF SOLUBLE NUCLIDES LIKE CS-135 BUT NOT THAT OF INSOLUBLE NUCLIDES LIKE AM-243,NP-237,ETC. (3)NUCLIDE MIGRATION IN BUFFER MATERIAL GESPER INDICATED THAT THERE ARE LARGE DIFFERENCE IN MIGRATION OF NP-237 BETWEEN THE OXIDISED AND REDUCED CONDITIONS. THE BREAKTHROUGH TIME IN THE FORMER CONDITION IS FASTER BY SEVERAL ORDERS OF MAGNITUDE THAN THAT IN THE LATTER CONDITION. RAPRAN SHOWED THAT THE DOMINANT NUCLIDES ARE AM-243,AND LONG-LIVED FISSION PRODUCTS OF CS-135,AND SE-79. (4)NUCLIDE MIGRATION IN GEOSPHERE COMPARATIVE CALCULATIONS WERE PERFORMED TO INDICATE THE PERFORMANCE OF THE ENGINEERED BARRIER SYSTEM. THE NORMALIZED HAZARD(FLUX DIVIDED BY ALL)AT THE EDGE OF A GEOSPHERE WITH A MIGRATION LENGTH OF 500 M WAS CALCULATED FOR A SYSTEM HAVING NO ENGINEERED SYSTEM AND HAVING A REGULAR SYSTEM. THE RESULTS INDICATE THAT THE PEAK HAZARD FOR NP-237,CS-135,TC-99,AND SE-79 IS REDUCED BY FACTORS OF 10-6,10-3,AND 10-5,RESPECTIVELY,BECAUSE OF THE PERFORMANCE OF THE REGULARENGINEERED BARRIER.

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