Editing Nuclear weapon design (section)

===Light pipes===
An important tool for test analysis was the diagnostic light pipe. A probe inside a test device could transmit information by heating a plate of metal to incandescence, an event that could be recorded by instruments located at the far end of a long, very straight pipe.{{Citation needed|date=June 2021}}

The picture below shows the Shrimp device, detonated on March 1, 1954, at Bikini, as the [[Castle Bravo]] test. Its 15-megaton explosion was the largest ever by the United States. The silhouette of a man is shown for scale. The device is supported from below, at the ends. The pipes going into the shot cab ceiling, which appear to be supports, are actually diagnostic light pipes. The eight pipes at the right end (1) sent information about the detonation of the primary. Two in the middle (2) marked the time when X-rays from the primary reached the radiation channel around the secondary. The last two pipes (3) noted the time radiation reached the far end of the radiation channel, the difference between (2) and (3) being the radiation transit time for the channel.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume IV, pp. 211–212, 284.</ref>
[[File:Castle Bravo Shrimp composite.png|600 px|centre]]

From the shot cab, the pipes turned horizontally and traveled {{convert|7500|ft|km|abbr=on|order=flip}} along a causeway built on the Bikini reef to a remote-controlled data collection bunker on Namu Island.{{Citation needed|date=June 2021}}

While x-rays would normally travel at the speed of light through a low-density material like the plastic foam channel filler between (2) and (3), the intensity of radiation from the exploding primary creates a relatively opaque radiation front in the channel filler, which acts like a slow-moving logjam to retard the passage of [[radiant energy]]. While the secondary is being compressed via radiation-induced ablation, neutrons from the primary catch up with the x-rays, penetrate into the secondary, and start breeding tritium via the third reaction noted in the first section above. This [[lithium-6|<sup>6</sup>Li]] + n reaction is exothermic, producing 5 MeV per event. The spark plug has not yet been compressed and thus remains subcritical, so no significant fission or fusion takes place as a result. If enough neutrons arrive before implosion of the secondary is complete, though, the crucial temperature differential between the outer and inner parts of the secondary can be degraded, potentially causing the secondary to fail to ignite. The first Livermore-designed thermonuclear weapon, the Morgenstern device, failed in this manner when it was tested as [[Castle Koon]] on April 7, 1954. The primary ignited, but the secondary, preheated by the primary's neutron wave, suffered what was termed as an ''inefficient detonation'';<ref name="swordsIV">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=IV |date=1995 |url=https://www.uscoldwar.com/ |access-date=2016-05-20 |url-status=live |archive-url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |archive-date=2016-12-30}}</ref>{{rp|165}} thus, a weapon with a predicted one-megaton yield produced only 110 kilotons, of which merely 10 kt were attributed to fusion.<ref name="swordsIII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=III |date=1995 |url=https://www.uscoldwar.com/ |access-date=2016-05-20 |url-status=live |archive-url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |archive-date=2016-12-30}}</ref>{{rp|316}}

These timing effects, and any problems they cause, are measured by light-pipe data. The mathematical simulations which they calibrate are called radiation flow hydrodynamics codes, or channel codes. They are used to predict the effect of future design modifications.{{Citation needed|date=June 2021}}

It is not clear from the public record how successful the Shrimp light pipes were. The unmanned data bunker was far enough back to remain outside the mile-wide crater, but the 15-megaton blast, two and a half times as powerful as expected, breached the bunker by blowing its 20-ton door off the hinges and across the inside of the bunker. (The nearest people were {{convert|20|mi|km|order=flip}} farther away, in a bunker that survived intact.)<ref>Dr. John C. Clark, as told to Robert Cahn, "We Were Trapped by Radioactive Fallout", ''The Saturday Evening Post'', July 20, 1957, pp. 17–19, 69–71.</ref>