忘却からの帰還〜Atomic Age

核爆発の平和利用実験Plowshare Programの一部として、運河や港湾建設のための掘削実験Project Sedan (104 kt水爆)が、1962年7月6日に、ネバダ核実験場ユッカ平原で行われた(リヒタースケールM4.75の地震相当）。


[ Project Sedan Nuclear Test


クレーターの傍に設置された看板によれば、クレータの縁での爆発１時間後の放射線量は500R/hで、27日後には500mR/hまで低下し、167日後にはクレーター中心部でも35mR/hとなり、防護服は必要なくなっていた。この点では、悪くない結果だった。

しかし...

The Sedan shot resulted in a radioactive cloud that separated into two plumes, rising to 3.0 km and 4.9 km (10,000 ft and 16,000 ft). The two plumes headed northeast and then east in roughly parallel paths towards the Atlantic Ocean.[9] Nuclear fallout was dropped along the way, narrowly dispersed in a relatively small number of United States counties.[3] Detected radioactivity was especially high in eight counties in Iowa and one county each in Nebraska, South Dakota and Illinois. Most heavily affected counties were Howard, Mitchell and Worth counties in Iowa, as well as Washabaugh County in South Dakota, an area that has since been incorporated into Jackson County and is wholly within Pine Ridge Indian Reservation. These four counties measured maximum levels higher than 6,000 microcuries per square meter (220 MBq/m2).[10]
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Sedan's fallout contamination contributed a little under 7% to the total amount of radiation which fell on the U.S. population during all of the nuclear tests at NTS. Sedan's effects were similar to shot "George" of Operation Tumbler-Snapper, detonated on June 1, 1952, which also contributed about 7% to the total radioactive fallout. Uncertainty regarding exact amounts of exposure prevents knowing which of the two nuclear tests caused the most; George is listed as being the highest exposure and Sedan second highest by the United States Department of Health and Human Services, Centers for Disease Control and Prevention, and the National Cancer Institute.[12][13]

セダン爆発は放射性の雲を作り出し、その雲は2つのプリュームに分裂して、上空3km及び4.9kmに上昇した。2つのプリュームはおおよそ並行して北東及び東の方向、大西洋に向かって移動した。その経路上に沿って、狭い領域の、比較的限られた米国の郡に、放射性降下物を降らせた。アイオワ州の8郡と、ネブラスカ州とサウスダコタ州とイリノイ州の1郡ずつで、特に高い放射線量が検出された。もっとも高かったのはアイオワ州のホワード郡とミッチェル郡とワース郡と、サウスダコタ州のワシントン郡（現在はジャクソン郡に含まれ、パインリッジ・インディアン居住区を含む）である。これらの4郡では、最大220 MBq/m2が観測された。
...
セダンの放射性物質は、ネバダ実験場から米国内にお放出された放射性降下物の7%弱を占める。セダンの影響は、1952年6月1日のオペレーションタンブラースナッパーのジョージ爆発（同じく7%を占める）と同様だった。正確な被曝量がわからないため、どちらが最大かはわからない。米国保健局とCDCとNCIは、ジョージは最大被曝量に、セダンは第2位に位置付けている。

[3] United States Nuclear Tests; July 1945 through September 1992, DOE/NV--209-REV 15 December 2000, p. xv.
[9] The Utah Democratic Progressive Caucus. Press Releases. Nuclear Testing Packet
[10] Miller, Richard L. (2002). U.S. Atlas of Nuclear Fallout, 1951-1970. 1 (Abridged General Reader ed.). Two Sixty Press. p. 340. ISBN 1-881043-13-4. Retrieved 2008-07-19.
[12] Report on the Feasibility of a Study of the Health Consequences to the American Population from Nuclear Weapons Tests Conducted by the United States and Other Nations, Vol 1. Technical Report. Department of Health and Human Services, Centers for Disease Control and Prevention, and the National Cancer Institute. May 2005
[13] Top Ten Contributors to Population Exposure Figure 17. Appendix to Report on the Feasibility of a Study of the Health Consequences to the American Population from Nuclear Weapons Tests Conducted by the United States and Other Nations, Vol 1. Technical Report. Department of Health and Human Services, Centers for Disease Control and Prevention, and the National Cancer Institute. May 2005 ]

[ wikipedia:Sedan (nuclear test) ]

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0:27 Sedan, 100 kt nuclear cratering detnation, recently executed as part of the Atomic Energy Commission Plowshare program.
0:37 Plowshare envisions many uses for nuclear explosive, including applications in nuclear excavation, mining, production of useful isotope, and power.
0:48 Nuclear excavation is however the most straightforward application and the nearest to practical using.
0:55 The chairman of the Atomic Energy Commission has recently established within the Plowshare program, a research goal to determine the feasibility of nuclear excavation within approximately the next five years.
1:08 Project Sedan was the first step in the implementation of this program.
1:14 The basic objective of Sedan was to determine the cratering effect of deeply buried, high-yield nuclear explosion.
1:23 Previous experience with 1 kt or smaller explosive has shown that as a nuclear explosion is buried deeper and deeper, crater size steadily increases.
1:38 Until the depth of burial reaches for which crater dimensions are maximum.
1:45 Below the depth, crater size decrease.
1;51 Also as the explosion is buried deeper and deeper, the fraction of the radioactivity released to the atmosphere become less and less.
2:06 Thus, it is possible to choose a depth-first that will give almost maximum crater size with the release of only a few percent of the radioactivity.
2:17 Using these concepts the depth of burst chosen for sedan was 635 feet.
2:26 The predicted greater diameter range from 1200 to 1500 feet, and the depth from 182 315 feet
2:36 the release of radioactivity was expected to be less than four percent.
2:40 The explosive used for sedan was a 100 kiloton thermonuclear device in which less than thirty percent of the total energy came from fission.
2:53 Since only a few percent of this fission fraction escape to the atmosphere, less than two kilotons of radioactivity was released by Sedan.
3:04 The site of sedan was the Atomic Energy Commission Nevada test site, located in southern nevada 70 miles northwest of Las Vegas
3:15 The specific location chosen for Sedan, was at the north end of Yucca valley near the site of two earlier nuclear cratering detonations and much high-explosive cratering work.
3:39 Sedan was detonated at 10am July 6, 1962.
3:41 This is a view taken from the top of a 1500 foot tower, located six miles from Ground Zero.
3:49 These sequences are being shown at about 1/3 normal speed.
4:02 The base surge seen forming here, continue to grow until it covered an area about five miles in diameter.
4:11 The main cloud rose to a height of 12,000 feet above the desert floor.
4:33 This close-up view was taken from a ground station three miles from Ground Zero.
4:40 The dome rose to a height of 290 feet before it vented at three seconds.
4:58 This long view gives a better picture of the mass of material moved by the explosion, and the early stages of cloud formation
5:14 A very large fraction of the radioactivity that escaped from Sedan, was mixed with the fall back in the crater area.
5:23 Thus the cloud carried only a small fraction of the radioactivity.
5:26 The area covered by the base surge is clearly discernible in this aerial view by the pattern of light color dust.
5:38 The crater is about 1,200 feet in diameter and 320 feet deep.
5:47 The lip ranges in height from 20 to 100 feet.
5:55 The predictions and the results compare favorably.
6:03 The fallout pattern from Sedan extended almost due north from the crater area.
6:10 Shown here are the 0.5 R and 3.9 R Isodose lines.
6:17 These correspond to the total dose of person would receive assuming continuous residence
6:25 The highest dose received by any off-site resident was 0.2 R.
6:32 These isodose lines represent the dose one would receive assuming entry into the area one week after the shot and continuous residence there after.
これらの等被曝線量線は、実験一週間後に立ち入って、そのままとどまった場合の被曝量を示している。
6:45 Research is being done on nuclear explosives with so little radioactivity that excavation project will be possible with no off-site radiation problem
6:57 Large-scale excavation projects such as harbors and canals require the use of explosives in the range from 100 kiloton to over a Megaton.
7:10 Sedan has been a major step forward in the nuclear excavation program by extending our cratering experience into this high yield range

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