First Criticality

Tales from the Nuclear Age:

 Copyright © 2009 by Charles Glassmire

(Stories as true as my memory recalls)

_________________________________________________________________

Sept. 12, 2009

First Criticality

          I was visiting the Los Alamos Scientific Laboratory (LASL) in New Mexico, on the NERVA program, along with my friend and Nuclear Engineer Dave. We were to examine radioactive samples of NERVA (Nuclear Engine for Rocket Vehicle Applications) fuel in the gigantic “Hot Cells” at Los Alamos. The samples were to be examined under microscope, then polished and photographed as examples of NERVA fuel material after it had been irradiated with high doses of neutrons. This would show how the fuel metal structure in the Nuclear Rocket Engine would behave after a period of operation in space. LASL had one of the few radiation qualified Metallographic facilities to handle the hot material.

          The Los Alamos Scientific Laboratory is nestled high in the Sangre de Cristo (Blood of Christ) Mountains of New Mexico. It is a desolate yet beautiful area which covers the Pajarito Plateau, featuring tree-filled canyons and parched high altitude scrub growth typical of the high plains of that state, and abounding in wildlife. It was first established in strict secrecy during World War II. Back then it was identified only by its government code name “Site Y”. Groves named it “the Manhattan Engineering District”, (MED) because there weren’t any operations in Manhattan. It later became known as The Manhattan Project.

          The government facility had been purchased from the locals and was chosen because it was very remote and access through the mountains was limited. It was created from the lands surrounding a private boy’s school and a hunting lodge (known as “The Ranch”) owned by Site Y’s first Scientific Director, Dr. J. Robert Oppenheimer. After he was appointed by General Groves, “Oppie” moved his family into the Ranch, and rumor has it the Bomb was designed at breakfasts around the Ranch’s big wooden dining table. Preliminary sketches were actually done on a napkin, which is preserved in the LASL bomb museum.

          The mission of the Manhattan Project was to research, design and then build the first Atomic Bombs for the United States, which was then involved in defeating the Axis Powers (Germany, Japan, Italy) in open combat. Originally, the lab was built by an Army Colonel (later General), Leslie Groves. Groves was used to running big projects and was fresh off an assignment charged with building an odd shaped building known as the Pentagon. The Army was in charge of Site Y because scientists were worried that Nazi Germany was hard at work building their own version of the bomb. Letters from Albert Einstein and Leo Szilárd directed to President Roosevelt noted the fact that the element Uranium could be used to create a “Chain Reaction” with the release of enormous amounts of energy. Einstein/ Szilárd said this could probably be used to create a new kind of gigantically powerful “bomb”. Albert Einstein really didn’t know anything about nuclear fission, but the scientists knew his name was important enough to be credible. The letter certainly got the president’s attention.

          Ultimately, the lab was to employ over 130,000 army and civilians, and was to cost over $2 billion dollars ($24 billion in today’s dollars).  The Manhattan District involved over 30 different locations around the United States. It was tasked to develop the first two Atomic Bombs (code named “Little Boy” and “Fat Man”) which ushered the modern world into the Nuclear Age.

          On the day before Dave and I began work, we were being driven about Los Alamos by engineer friends who lived in the area and wanted to show us around. I was riding in the back seat while the driver narrated information about what we were seeing. Much of the area was heavily forested and interspersed with laboratory buildings and crude Army barracks, given various code number designations. As we drove down a hill in Omega canyon, the driver pointed to a forested area to the right of the car and said

 “Of course you know about the first criticality incident. There’s where it occurred in a wooden shack back in the woods there. We can’t go in there; it’s fenced off now and it’s still rather hot…”

           I had no idea what he was talking about, and quickly asked for details as we continued driving on down the road. It turns out the wooden shed he casually pointed out had developed a rather ominous history. Here is the story he told:

(Current declassified accounts of this incident now differ somewhat, but this is what I was told in 1965 by the nuclear engineer who lived near Los Alamos ) :

           The time was 1945, and valuable data still was needed on the criticality of Plutonium, the element used in the Fat Man bomb. “Criticality” was a term implying how much Plutonium was needed to cause a nuclear explosion. Calculations had been made but nobody really knew. Plutonium was a new element, only discovered in 1941, and very little had been made and refined. Thus, it was precious and expensive stuff, and not much was available for measurement on bomb grade material.

          So a researcher was charged with determining the criticality measurements on the isotope, and was given enough in the form of two half spheres (later estimated to be about 6 Kg of Pu239) to experiment with.

          He was assigned a rickety wooden shack in the woods and some measuring equipment, so that any accidental contamination might not turn into a serious expensive cleanup. (You did things this way because there was a war on.) It was his job to confirm experimentally exactly how much Pu would create an atomic explosion, and how close the two half-spheres had to come to initiate a fission chain reaction. Knowing this, one could estimate the neutron “cross section” for Pu239. (i.e. how big of a target did the Plutonium 239 atom make when being hit by a neutron. This number then gave the probability of a fission event).

          I found it amusing that neutron cross sections of fissile nuclei were measured in a unit called “Barns”. More Barns meant the nucleus made a bigger target for incident neutrons. The cross section data was collected into a series of graphs for each nucleus, and the data was put into a book which was called the “Barn Book”. This book was colored green, we used it all the time in our work, and the book had a line drawing of a farmer’s barn on the cover!

          Now bringing two halves of a nuclear bomb together was not a terribly wise experiment to do, but lots of people risked their lives under the duress of wartime. When one has assembled enough enriched material (Plutonium 239 or Uranium 235) to produce a nuclear chain reaction, simply accidentally bringing that amount together in the nearby space can produce a nuclear chain reaction and release of energy into the surrounding air. This is called a “Criticality Event”. It is not a bomb explosion, but lots of energy, fission products and lethal radiation is released. It can be deadly to anyone nearby. It is even possible to occur in a modern laboratory when two lab technicians are transferring enriched material from one room to another. Should they pass close by in the hallway, carrying enough material, the two masses can interact with emitted neutrons to “flash” into a critical event chain reaction.

          So, personnel from the Health Physics department are employed to track the movements of all radioisotope materials in a laboratory. The amount in a room is weighed and measured, and recorded by the Health Physics people. One cannot transfer material without the permission of the isotope control folks, who will record how much is moving, where it is going to, whether the destination is “overloaded” and who else is in transit in that local area. This insures no persons will accidentally pass in the hallways and go critical.

          But these controls had not yet been developed in the early days of the Manhattan Project. The Criticality Event was an unknown effect no one had ever seen occur, and there was little experience of this in the short history of atomic research.

          So our researcher had mounted the two half spheres of a bomb core on a lathe bed, and, accompanied by the clicking of Geiger counters and neutron detectors which measured emitted radiation, he was slowly cranking the two Plutonium halves of an Atomic Bomb closer and closer together, and recording the data in a little notebook, to see what would happen…

(To be continued)

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One Response to “First Criticality”

  1. Sharon Lippincott Says:

    Chuck, your continuing saga is eloquently told. This episode is a blast from my past. I grew up in Los Alamos, listening to these local legends. Those were the days! Looking back on that time and place through the eyes of an adult, in the light of later conversations I’ve had with some of the early scientists and their offspring, it appears that in spite of everything, a certain youthful optimism pervaded the culture of The Hill, as well as the science.

    Having been gone for nearly half a century, I can’t speak for the culture today, but the Hill itself looks like a chemo patient, with bald slopes rising behind the town, scarred forever by the deliberately set Cerro Grande firestorm of 2000. That whole area, once inhabited by people who knew the earth as Sacred Mother. “Modern Man” has dumped toxic chemicals, causing mutations and damage to the ecology and care-less-ly allowed a searing inferno to render the mountainsides sterile for ages.

    I weep for the land and grieve the passing of the days of innocence.

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