There were four huge exhaust fans up on the second floor, designed to keep the first floor at negative pressure. They blew the air into a concrete tunnel and up a smokestack, 142 feet high. To keep any plutonium dust particles from making their way into the atmosphere, the air from the first floor passed through a massive bank of HEPA filters on the second floor before exiting the building. The filter bank ran the entire length of the floor, over 200 feet long, made of 620 paper filters. Each filter was a foot thick and presented four square feet to the airflow. The fans, pulling 200,000 cubic feet of air per minute at high speed, sucked fresh air into the blazing fire and sent the flames up to the second floor, where by 10:28 P.M. they set fire to the paper filter elements. As was exactly the case with Windscale, thoughtful consideration for the men who were fighting the fire was keeping it going and spreading it.
The blowers should have automatically shut down by now because of the heat buildup in the filter bank, but the fire-detection equipment had been disabled earlier because it was always going off and disrupting production. Fire was obviously being sucked into the air vents in the corner of Room 180. Owen was still apprehensive about causing a criticality in the room, but by now Vandegriff was willing to allow the fire department to hose it down with water. He suggested fog nozzles on the inch-and-a-half hose that the firemen had already laid, and told them not to aim at the glove boxes. The flames went down almost immediately.
Eminger and Vandegriff rushed to the second floor to see if the filters were on fire. The filters had not been changed in four years, and there was a large buildup of finely divided plutonium detritus from all the machining. Just as they opened the door, the dust exploded, knocking Vandegriff to the floor and Eminger back through the double doors. The filtering function was destroyed, and plutonium dust was forced to where it would not go if left on its own, up the stack and into the air over Colorado, by the four powerful blowers. The blast was intense enough to dislodge the lead cap on top of the smokestack. By 10:40 P.M., the remains of the filter bank were engulfed in flame, and at 11:10 P.M. the electrical power failed, finally turning the blowers off. By 11:28 the fire was officially extinguished.
Building 771 was back in business by December 1, 1957, but Room 180 would not be decontaminated until April 1960. The cleanup and replacement of the filter bank cost $818,000. Of the plutonium known to be in the building, 18.3 pounds of it could not be found.[165]
That was the small fire. The big one started in connected Buildings 776–777 on Sunday, May 11, 1969. Mother’s Day.
Small, controllable plutonium fires had become a way of life at Rocky Flats. The fire trucks had been called to hundreds of fires since Building 771 was smoked back in ’57. The worst small fire broke out in 1966, when workers were trying to unclog a drain. In that barely mentionable incident 400 people were contaminated, with most of them inhaling plutonium smoke. Countless fires were smothered in the day-to-day work without calling the fire department or mentioning it to superiors.
Plutonium is a very strange element, and some of its characteristics are not understood. It has seven allotopes, each with a different crystal structure, density, and internal energy, and it can switch from one state to another very quickly, depending on temperature, pressure, or surrounding chemistry. This makes a billet of plutonium difficult to machine, as the simple act of peeling off shavings in a lathe can cause an allotropic change as it sits clamped in the chuck. Its machining characteristic can shift from that of cast iron to that of polyethylene, and at the same time its size can change. You can safely hold a billet in the palm of your hand, but only if its mass and even more importantly its shape does not encourage it to start fissioning at an exponentially increasing rate. The inert blob of metal can become deadly just because you picked it up, using the hydrogen in the structure of your hand as a moderator and reflecting thermalized neutrons back into it and making it go supercritical. The ignition temperature of plutonium has never been established. In some form, it can burst into white-hot flame sitting in a freezer.
Unfortunately, the frequent plutonium fires did not make everyone wary of this bad-behaving material. The effect was just the opposite. Everyone in the plant, starting with the top management, became convinced, at least subconsciously, that a plutonium fire was easy to control and was no big problem. Starting in 1965, there were more than 7,000 pounds of plutonium in Building 776–777 at a given time, and it was not given a second thought. There was a list of dangerous procedures, equipment configurations, and building materials at Rocky Flats that converged on Mother’s Day 1969.
First, there was the Benelex. It was a type of synthetic wallboard, no longer made, that resembled Masonite. It had a high density, it could be put together with glue, and unfortunately it was flammable. It was used to make boxes to hold plutonium, shielding for entire glove-box lines, and even the walls of fabrication rooms. Compared to other versatile materials, it was inexpensive. In 1968, 1.17 million pounds of flammable Benelex and Plexiglas were added to Building 776–777.[166]
The walls and the glove boxes were flammable, and to top that the hangers on the overhead conveyor belt, used to hook heavy parts and take them to another building, were made of magnesium. Magnesium cut down on the weight, but if it caught fire, it would burn white-hot, like a flare.
Another problem was the cleanup of the machine tools, which made a lot of plutonium chips, scraps, and even dust. These remnants were always oily from the coolant that was sprayed on the plutonium parts as they were shaved down into shape on the lathes and milling machine. When this stuff started building up under the machine, it was gathered, put in a can with a lid, and sent by conveyer to Room 134. Here it was supposed to be degreased using carbon tetrachloride and then pressed into a rough briquette, three inches in diameter and an inch thick, weighing about 3.3 pounds. The degreasing was unofficially dropped from the procedure, as the carbon tetrachloride treatment would too often result in a fire or an annoying explosion. When the press squeezed down on the non-degreased plutonium debris, oil flowed onto the floor, taking with it little pieces of plutonium. Workers sopped it up off the floor using rags.
On that Mother’s Day in ’69, late in the morning, a heap of oily, plutonium-enriched rags beneath the briquette press spontaneously caught fire. There was nobody at work on the plutonium line that day. The Benelex glove box above the burning rags had a ventilation fan, feeding air into the big filters on the second floor. It pulled the hot air from the fire into the box, where there was a can holding a briquette. Someone had neglected to put the lid on the can. The plutonium briquette caught fire, and it burned white hot. The Benelex glove box started smoldering. It lighted some more briquettes. At this point, the fire alarms should have been blaring and automatically calling the fire department, but the detection equipment had been removed to make room for all the new Benelex shielding. More plutonium ignited. The Plexiglass windows and the rubber gloves caught fire, flaming up, and this left the arm-holes open. Air rushed into the glove boxes and fanned the burning plutonium. The fire moved down the north-south conveyer line, away from the connected building 777, taking everything that would burn.
At 2:27 P.M., the heat sensors in the building triggered, alarms sounded, and the fire department rushed to the scene. The firemen found the north plutonium foundry in Building 776 fully engaged. Captain Wayne Jesser ordered a man to discharge a hand-held carbon dioxide extinguisher at the fire to try to scare it while he rolled a fifty-pound extinguisher to the east end and emptied it into the flames. The fire was not impressed. Aware of all the dangers of using water on a plutonium fire, the risk of a hydrogen explosion from oxygen being pulled out of the water and a possible criticality from the moderation effect, he could see no choice. At 2:34 P.M., he ordered his men to deploy the fire hoses and wet it down.
165
Dow Chemical’s report of this accident was classified SECRET until 1993. The investigation found that somewhere between 1.8 ounces and 1.1 pounds (50 to 500 grams) of plutonium made it up the smokestack and landed somewhere in Colorado. No trace of it has ever shown up in radiation surveys of the surrounding land. Note that the safety exposure limit for a worker at Rocky Flats was 0.0000005 grams of plutonium.
166
Why Benelex? It turns out, Benelex is an excellent neutron shield, and in the 1960s it was used extensively in nuclear research for shielding neutron collimators and interferometers. A component of Benelex is wood fibers, and harvested wood in the U.S. grew up soaked with borax, a wood preservative and insect-damage preventative. Plutonium-239 can spontaneously fission on occasion and send neutrons careering through the room, and the contaminant plutonium-240 is particularly apt to do this. When hit by neutrons of any speed, these particles are slowed to thermal speed by the hydrogen in the cellulose wood fibers and are summarily absorbed by the boron-10 remnants from the borax treatment. The classified purpose of using Benelex was to kill any neutron activity in the building, preventing the plutonium pieces in the building from cross-connecting by neutron flight and causing the building to become one enormous nuclear reactor, running uncontrolled with people inside it. On Mother’s Day 1969 there were 7,641 pounds of plutonium in the building. The first power reactors in the world, the plutonium production reactors at the Hanford Site, used Masonite, a similar material, for neutron shielding beginning in 1944.