Let's start with a short modern history of steel metallurgy.
From 1856 through the 1950's, steel was mostly manufactured using the Bessemer process. The Bessemer process was the first large-scale and inexpensive means of producing consistent, high-quality steel.
With the Bessemer process, ingots of pig iron, loaded with impurities like carbon and silica, would be melted in a Bessemer converter. Air would then be blown through the molten iron. Oxygen in the air would rapidly reduce or "burn off" these impurities. Timing on shutting off the air was based on the appearance of the flame exiting the mouth of the converter. After the blow, the required amounts of carbon and other alloy metals (chrome, vanadium, tungsten, etc) would be added to produce the desired type of steel.
Below, a Bessemer converter
The Bessemer process was replaced with the Basic Oxygen Steelmaking (BOS) process, which was invented in 1948. With this process, Calcium Oxide or Dolomite (which are basic or alkaline) are added as flux to the molten iron, and pure oxygen is blown into the vessel. This produces a higher grade of steel much more rapidly than the Bessemer process, and with even less expense.
This is about as deep as we need to go discussing steel-making for now.
Now let's discuss the atomic age. Beginning with the Trinity test July 16, 1945, the world has had man-made radioactive particles raining down on it.
Below: Trinity, the first nuclear detonation.
Followed in short order by many more tests. Below: Operation Crossroads "Baker" test in 1946.
Below: The largest pure fission explosion. Ivy "King" with a 500 kiloton yield. 1952
In 1954 the first thermonuclear (fusion) weapon was detonated. Ivy "Mike", a 10.4 Megaton blast.
The largest nuclear weapon the US ever detonated was Castle "Bravo" at 15 Megatons, also in 1954. The blast was 2.5 times larger than expected, due to an unanticipated fusion reaction in the lithium 7 isotope, which had been assumed to be inert in the fusion reaction. Most of the instrumentation designed to gather data on this weapon was destroyed in the blast. Additionally the crew of a Japanese fishing boat was sickened by the fallout, and 15 islands were contaminated, and the inhabitants were forced to relocate.
Below: The Soviet Union created the largest man-made thermonuclear explosion ever, detonating the Tsar Bomba in 1961. The blast was the equivalent of 50 Megatons of TNT.
Over 2100 successful nuclear tests were conducted. If fizzled tests, poor-yield tests, and zero-yield tests (testing for safety) are taken into account, nearly 2500 nuclear weapons have been fired, mostly by the US.
In 1963, world anthropogenic background radiation levels peaked at 0.15 milliSieverts, or 15 milliRem/Year. In that year, the Nuclear Test Ban Treaty went into effect, and since that time, anthropogenic background levels have fallen to 0.005 milliSieverts, or 0.5 milliRem/year.
This brings us back to the manufacture of steel. One process, the Bessemer process, uses air to reduce the pig iron, while the other process (Basic Oxygen) uses pure oxygen. Both are derived from ambient air. Since 1945, we have been contaminating the world's air with man-made radioactive particles, and these particles have become embedded in steel. Additionally, several Cobalt-60 sources have managed to make their way into the steel-making process, further contaminating the steel.
Low-background steel is steel that was manufactured before the Trinity test in July of 1945, and thus uncontaminated. The primary source of low-background steel is the German High Seas Fleet that was surrendered and intentionally scuttled as part of the terms of surrender at at the end of WW I
Low-background steel is very important, but in a slightly unusual way.
People who work in an environment where internal contamination is possible are required to undergo whole-body scans. I have worked in such an environment, and have been scanned. This is done at the beginning of employment, quarterly, and then at the end of employment. Turns out I have a little bit of internal contamination in the form of Cesium-137, a byproduct of nuclear fission.
To detect this requires a very sensitive scintillation detector, and extremely low background radiation levels. Your body acts as a shield, which prevents much of the radiation emitted by your internal contamination from reaching the detector. If there is noise in the detector from high-background steel, it will mask the tiny bit of radiation that needs to be measured.
Below, a whole-body counter. You lie on a mat, and over about 30 minutes, your entire body slowly moves underneath a plate-sized radiation detector, just inches above you. It has a panic button for claustrophobic types. These counters are in all sorts of places - Uranium mining, power plants, fuel manufacturing, weapons labs, plutonium and uranium processing facilities.
In experimental physics, there are neutrino detectors buried deep underground to shield them from all external sources of radiation, increasing their sensitivity. Low-background steel is the only possible steel that could be used for these experiments.
A few examples of these are HERE and HERE, HERE,
and HERE
No comments:
Post a Comment