Tales from the Nuclear Age:
copyright © 2009 by Charles Glassmire
Nov. 26, 2009
The Demon Core
(Thanks to author Martin Zeilig for some of the following events):
By May 21, 1946, it had been nine months since the Japanese surrender. World War II had finally ended with the atomic bombing of Nagasaki. There was still much unknown about the properties of Plutonium, and new modifications of the Fat Man bomb design were being investigated. Atomic scientist Louis Slotin was working with his replacement, Alvin Graves, in testing the critical properties of a new geometry for a plutonium bomb core.
Born in 1910, Slotin was the eldest of three children to Orthodox Jewish parents who had fled the pogroms in Czarist Russia and settled in Winnipeg Canada. He showed exceptional promise as a young student, and entered the University of Manitoba at age 16. He earned the Master of Science degree in 1933, and then went to advanced study at Kings College, London England, where he was awarded a Doctorate in Radiobiology. After duties at Oak Ridge, he arrived in Los Alamos, New Mexico to work on the Manhattan Project in December of 1944.
The particular bomb core being worked on this day had acquired a growing “reputation” in the Los Alamos community. It was the same one which had killed Slotin’s assistant, Harry K. Daghlian, in a particularly nasty criticality accident. Nine months earlier on August 21, 1945, it had given Daghlian a large radiation dose; causing him to suffer a lingering death over a period of twenty-one days. (See my “LA-1” blog entry earlier). In the late evening hours at the local bars, some were whispering this core was evil, and were even calling it “The Demon Core”.
In July of that year, Slotin and Daghlian had worked together assembling the original bomb for the Trinity Shot in the New Mexico desert. This was the only bomb test before the combat drop, and the test verified the calculations for the plutonium design of the Fat Man weapon. Slotin still prized the hand written receipt a U.S. Army Officer had given him when he delivered the assembled Trinity device to the Army testing grounds. He called it the culmination of a two billion dollar development project. Afterward he was to receive the nickname of “Chief Armourer of the United States”. The two scientists were of a remaining few who had the hands-on experience to actually put together an atomic weapon. Now Slotin was almost the last, and he was training a replacement.
Since the end of the conflict, Los Alamos was emptying out. Scientists were returning to the research berths they had before the war and their conscription into the Manhattan District for bomb work. There was a growing awareness of a lack of experienced personnel as the original designers departed, one by one.
Slotin was looking forward to returning to his research at the University of Chicago. When there, he had gotten involved with cyclotron separation processes and creating radioisotopes for medical use, and became aware of the creation of the first nuclear “pile” under the stands of the squash courts at the university. Enrico Fermi, designer of that project, recruited Slotin for further work at Oak Ridge and later on, at Los Alamos. Recently, Fermi had become alarmed when he discovered the dangerous testing Slotin was performing on bomb cores at Los Alamos, and in an ironic foreshadowing, he warned Slotin
“You won’t last a year if you keep doing those experiments…”
Slotin was rumored disappointed at not being cleared to attend the Tinian Island combat air drop against the Japanese (he was Canadian and his clearance hadn’t come through in time). Although he finally was slated to attend the Operation Crossroads Bikini tests in the Marshalls, he had later seemed somewhat disillusioned and expressed in a letter
“I have become involved in the Navy tests, much to my disgust. The reason for this is that I am one of the few people left here who are experienced bomb putter-togetherers…”
So on this Tuesday afternoon in May, Slotin was working in the 49 room of the secret Omega Site laboratory, testing a new core geometry and demonstrating procedures to his replacement. The 6 Kg sphere of bomb grade Plutonium was surrounded by two half spheres of Beryllium, which was a neutron reflector. The Beryllium covering would bounce stray neutrons back into the Plutonium core, and thus increase the strength of the chain reaction. The Be cladding was divided into two half spheres, and Slotin was gradually lowering the top half sphere closer and closer to the core cradled in the bottom half-sphere of Beryllium. As the two halves got closer, more radiation was released and the chain reaction approached full criticality.
For safety purposes, two shims had been created, to rest in-between the two half spheres, so they could not nestle tightly around the Plutonium, and produce a super critical event. Slotin had removed the two shims from the experiment, and was now manually holding the two halves apart by wedging a screwdriver blade in between the two halves with his right hand. He was closing the gap microinch by microinch, by holding the top half with his left hand, and twisting the screwdriver blade to allow the pieces to approach each other; he watched the radiation count increase as he explained to Alvin.
There were six others present in the room, standing at various distances from the experiment table. They were watching this rather irregular procedure quite closely, since Slotin was violating safety rules by removing the shims. He was generally accorded to be the resident expert on core handling procedures, and he believed in hands-on testing.
Author Martin Zeilig describes what happed next, at 3:20 P.M. on Tuesday afternoon 21 May 1946:
“Then, in that fatal moment, the screwdriver slipped. The halves of the beryllium sphere clapped together, and the Plutonium went supercritical…It happened in an instant. A sudden blue glow momentarily enveloped the room before evaporating. In that moment, as the Geiger counters clicked wildly, scientist Louis Slotin knew that he had received a lethal dose of gamma and neutron radiation from the core of the plutonium bomb he was testing…”
Slotin immediately jerked his left hand away, throwing the top half sphere to the floor, and stopping the supercritical chain reaction. It was too late. It was later calculated he was exposed to an instantaneous lethal radiation dose of 1,000 rads. (500 rads will kill 50% of those exposed.) Kline stood four feet away from the core and received about 100 rads. Graves was a bit closer receiving 166 rads.
The observers described a feeling of heat in the room. Slotin said he experienced a sour taste in his mouth and burning sensations in his left hand, which had rested on the top of the assembly. All ran outside, and Slotin immediately vomited. This is the first symptom of the Acute Radiation Syndrome. It is an intense gastrointestinal reaction to the radiation. All ran to the gate and shouted for the guard to open and sound the emergency whistle. Slotin was taken to the hospital.
Later on, Slotin called his parents, saying he was in an accident and they might come down to be with him. Hearing of the incident, General Leslie Groves, commander of the Manhattan District, dispatched a U.S. Army DC-3 to Canada, and flew the parents to Los Alamos and Slotin’s bedside.
Slotin apologized to Alvin Graves, saying he was sorry for “getting you into this…” and allowing that he (Slotin) had a 50% chance of dying. Multiple transfusions proved useless. He died on 30 May as further symptoms of the radiation syndrome appeared: severe diarrhea, kidney collapse, swollen hands, redness of the body, massive blisters on hands and arms, hugh white cell blood count, paralysis of intestinal activity, gangrene and collapse of all body functions.
Israel and Sonia Slotin flew the body home from Santa Fe. At the funeral, a letter from General Groves thanked the scientist for his
“… bravery and quick action which saved the lives of seven co-workers…”
Three of the seven others in the room died early, probably from radiation complications. The Demon Core had claimed its last victims. All further hand testing or assembly of critical materials was prohibited at Los Alamos. The Demon Core was used as the bomb core for the ABLE test detonation of the Crossroads series of nuclear weapons tests in the South Pacific, thus scattering the atomic heritage of this core to the four winds …
(to be continued)