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TH E S L-1 REA CT O RThat experience eclipses all of the other experiences I had up there.

—Dr. George Voelz, NRTS Medical Director—

In the arctic tundra, a treeless plain north of the coniferforests of Canada and Alaska, plant liferelies on shallow bogs and a few inchesof top soil. Below, the soil is frozen allyear long, permanently unresponsive tothe spring thaw. Vast acreages of thetundra allow practically no life at all,the ground a dark and flinty mat ofstones.

This was the setting thatthe U.S. A r m ydescribed to theA rgonne Lab as thedestination for asmall nuclear powerplant. The Army hadin mind the DistantEarly Warning sys-tem, the DEW L i n e .Beginning in 1953,dozens of mannedradar stations ringedthe Arctic Circle, onconstant watch for aSoviet air invasion. T h eArmy regularly shipped dieselfuel to each station for heat and electrici-t y. This was costly and sometimes haz-ardous in such remote areas, and the

Army hoped to replace the diesel supplyline with nuclear power.

The vision was to package a powerplant in three or four pieces, fit theminto cargo planes or trucks, and havesoldiers assemble them in a few hours.Easy to operate, a plant would run atleast three years on one fuel loading.The plant need generate only a thou-

sand kilowatts, andwhen the mission

ended, the crewcould pack it upagain and shipit elsewhere.Would Argonnedesign a proto-type?1

Applying its BORAX experience,Argonne developed the project using aboiling water reactor concept. Thevirtue of the system was that steamfrom the boiling water powered the tur-bine directly, eliminating the weight andcomplexity of a secondary loop andheat exchanger equipment.2

With tundra permafrost conditions inmind, the Army wanted to test not onlythe reactor, but its building as well. T h eprototype, assigned to Idaho, was readyto build in 1957. The building shellwas a circular steel tank, a silo-likecylinder forty-eight feet high and aboutthirty-eight feet in diameter. It sat ondummy piers arranged in a circle. Inthe arctic the piers would hold the bot-tom of the silo two feet above the per-mafrost and leave airspace between the

floor and the ground.This would preventtransferring heat tothe permafrost.Despite its armoredappearance, the silowas not intended asa containment struc-ture. Both the NRT Sand potential arctic

destinations were sufficiently remote,and the power level of the reactor

Cutaway of SL-1 reactor and the control building.

Reactor floor is above shielding gravel. Fan floor is

above reactor floor.

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Pressure vessel for Stationary Low-Power Reactor has been placed during construction. Steel plate is

going up around the reactor building. Circa 1958.Argonne National Laboratory-West 103-4055

s u fficiently low, that the AEC deemedsuch a feature unnecessary.3

Inside, the plant was arranged like athree-layer cake. In the bottom third,native stone and gravel shielded thepressure vessel containing the reactor.The middle third was the operatinglevel, giving the crew access to the topof the reactor, the turbine generator, andcontrol rod machinery. On top, a “fanfloor” attic contained equipment to con-dense and cool the recirculating water.A weather-protected stairway snaked upthe side of the cylinder to connect theadjacent control building to the operat-ing floor level.4

After Argonne handed over the finishedplant to the Army’s operating contrac-tor, Combustion Engineering (CE), theArmy named the reactor StationaryLow-Power Reactor Number One, orSL-1. This name distinguished the reac-tor from a whole family of other smallreactors that the Army already had built

or was planning. Reactors were to be“stationary,” “portable,” or “mobile,”depending on the intended field appli-cation, and rated for “low,” “medium,”or “high” power.5

The IDO opened up an Army ReactorExperimental Area (AREA) about ahalf mile north of State Highway 20and ten miles east of Central Facilities.

The SL-1 was the first of three antici-pated Army experiments at the NRTS.The other two were intended to perfecta “mobile” reactor so miniaturized thatit would fit on a truck and move when-ever a field station moved. As of 1958all the reactors built in the UnitedStates had been cooled with water orliquid metal. But gas was a coolant pos-sibility and the Army chose to exploreit for mobile reactors. Ultimately, theArmy hoped ordinary air could be used,further simplifying the power plant andeliminating more weight. The Armycontracted Aerojet General Corporationto design a Gas Cooled ReactorExperiment (GCRE) and do for the gas-cooled concept what BORAX, SPERT,and TREAT had done for the other con-cepts: determine its safe operating para-meters and select the best fuel. Thatdone, the Army would use the remote-ness of the NRTS to field-test a proto-type for the Mobile Low-PowerReactor, or ML-1.6

The IDO reserved sites for the threereactors at half mile intervals along anaccess road it named Fillmore Avenue.The GCRE, a water-moderated reactorsituated in a “swimming pool” pitbelow the floor, went critical in 1959,and the ML-1 was expected to arrivesometime in 1961.7

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Above. Cadremen at SL-1 control panel. Left.

Operators make adjustments in GCRE reactor pit.INEEL 63-4454

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The Army planned to train its futureDEW Line crews at the SL-1, so CEran the reactor with a military crew.The Army and the Air Force senttrainees; the Navy, interested in thepotential of the SL-1 for Navy mis-sions, sent Seabees. The mixed“cadres” trained together and rotated toIdaho after several months of instruc-tion at Fort Belvoir, Virginia, where theArmy operated its prototype StationaryMedium-Power (SM-1) reactor eigh-teen miles from the White House.

The SL-1 went critical for the first timeon August 11, 1958, and produced itsfirst electricity two months later onOctober 24. Thereafter, the rhythm ofwork involved running the reactor forperiods ranging between one and sixweeks and then shutting down for train-ing in maintenance and repair or toinstall improvements. The first cadrehad been trained, tested, and certifiedby May 1959, and many others fol-lowed. CE’s Christmas routine was toshut down the reactor, celebrate theholiday, and then do annual mainte-nance tasks before the next start-up.8

Accordingly, on December 23, 1960,CE shut down the reactor. Crewsreturned on December 27, reporting towork in three shifts around the clock.Start-up was scheduled for January 4,1961. The men calibrated instrumentsand attended to the valves, pipes, andpump that circulated coolant waterthrough the reactor. To do part of thiswork, they lowered the water level inthe reactor about two feet.

The last task was to insert forty-fournew cobalt flux wires into the core forlater mapping of the reactor’s neutronflux. The January 3 day shift insertedthe flux wires. To gain access to thereactor core, they had moved out of theway several large concrete shieldingblocks that ringed the top of the reactor.Then they disconnected the control rodsfrom their drive mechanisms, these alsobeing in the way. It would fall to the 4p.m. shift to reconnect the control rodsto the drive mechanism.9

In miserably cold weather—the temper-ature was headed for seventeen degreesbelow zero that night—the three-mancrew arrived from Idaho Falls and setto work, the only workers at the SL-1area. As usual for night shifts, noguards were posted at the entry gate,which the day-shift cadremen hadlocked behind them as they left.

Reconnecting the drive mechanismsinvolved several steps, one of whichwas to lift the control rod—manually—about three inches out of the reactor. Atthe top of the control rod was a smallball. A “gripper” from the drive mecha-nism latched onto this ball, completingthe connection between the rod and therest of the mechanism and its motor.

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Above. Enclosed stairway

from SL-1 support building to

operating floor of reactor silo.

Left. The control rod drive

mechanism. Note cruciform

shape of control rod (bottom

of drawing).

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The control rods were not cylindricalin shape, as the name “rod” mighti m p l y. Rather they were cruciform,with four narrow fin-like projectionsthat moved up and down the length ofthe core in narrow sheaths. The fuelzone was nearly twenty-eight incheslong. Moving the rods three inches wassafe and not enough to invite a chainr e a c t i o n .1 0

In recent weeks, the control rods hadseemed to stick slightly, perhapsbecause other items in the reactor corehad bowed, invading clearance spacesand putting pressure on the sheath,crowding the slender fins of the controlrods. The three men, wearing their graycoveralls, were in the silo at 9 p.m.,two of them directly over or very closeto the top of the reactor, working withthe central control rod. At 9:01 some-thing happened, the reactor went“prompt critical” and blew up.

When the reactor went critical, itreleased so much heat energy in fourmilliseconds that it flashed the watersurrounding the fuel to steam. Thesteam, being of lower density than liq-uid water and thus a less effective mod-erator, quenched the nuclear reaction.The decay heat built up rapidly. Withno system operating to remove the heat,twenty percent of the fuel melted. Thesteam forced upwards the seven-footcolumn of still-liquid water above it.The water rushed through the two feetof air space. It slammed against the lidof the pressure vessel at a velocity of160 feet per second and 10,000 poundsper square inch exactly as if it were apiston—a water hammer. The entirevessel jumped nine feet into the air, hit

the ceiling, and thumped back intoplace, shearing all of its connections tothe piping and instrumentation systems.Iron pellets packed near the reactor asthermal insulation and radiation shield-ing scattered all over the floor. Thewater hammer expelled the control-rodshield plugs, water, fragments of fuel,fission products, and other metal fromthe top of the reactor, leaving openholes. The violence of the explosionkilled all three of the men.11

Two of them died instantly, one thrownsideways against a shielding block andthe other straight upwards, where oneof the shield plugs pinned his body tothe ceiling. The head wounds of thethird were fatal, but his pulse continuedfor another two hours. The blast blewshards of radioactive metal into theirbodies, making them a danger to thosewho soon would try to rescue andrecover them.12

It would take nearly two years of tena-cious inquiry to make plain what hadhappened in two seconds, and the longprocess of discovery began immediately.

At 9:01 p.m., the heat-sensitive firealarm above the SL-1 reactor radiosone long and two shorts—the code forthe SL-1 complex—at the security cen-ter and the three NRTS fire stations.Accident response procedures com-mence. A security force notifies theduty officer and sets off for SL-1. Thefiremen at Central, the closest to theArmy complex, grab a card detailingthe potential hazards at SL-1 andreview it as the fire engine rolls. Theduty officer tunes in to their conversa-tion. Nine minutes later, they arrive atthe locked gate. Equipped with keys, adetail covered by prior planning, thefiremen unlock the gate. They observeno fires, no apparent disturbance, and

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Scene at control station set up near junction of

Highway 20 and Fillmore Avenue.

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no one about in the freezing cold. Theynote a slight trail of steam coming fromthe reactor building, a typical sight inthe cold.13

The firemen enter the control buildingcarrying radiation detection instruments.They walk down the central corridor,calling, inspecting each room. In one,they see three lunch pails on a table.Finding nothing amiss, they movetoward the door at the bottom of thecovered stairway. The radiation indicatormoves sharply. They withdraw.

By 9:17 p.m., an HP from the MTRarrives and, together with a fireman,approaches the stairs. Each wears aScott Air-Pak, a 40-pound tank of airharnessed and buckled onto the backwith a hose to a face mask. It pumps airto the face, so that if the mask leaks, airis expelled, protecting the wearer fromdangerous gas or contaminated dust. Asthe rescuers start up the stairs, thedetectors read 25 R/hr; they withdraw.By now, the security detail has deter-mined that the men are nowhere else atthe NRTS. Other searchers find no onein the SL-1 support buildings. Thegrowing crowd of rescuers is forced toconclude that three cadremen are some-where inside the very quiet reactorbuilding.

In a few minutes, two more HPs arrive,dressed in fully protective clothing andequipped with Jordan Redectors, aninstrument able to detect gamma radia-tion up to 500 R/hr. One of themascends the stairs with two of the fire-men, a constant eye on the Jordan. A tthe top of the stairs, they discern damageinside, no men. The indicator needlepegs. The HP orders all to withdraw.

Meanwhile, in accordance with theemergency plan, CE and IDO authori-ties had been notified. Around 9:20p.m., John Horan, the IDO director ofHealth and Safety, answers the tele-phone at home and then tells his wife,“There’s been an accident.” He leavesfor his Idaho Falls office where he hasradio communication with the entireSite. As he passes the security desk, theguard tells him three people are miss-ing, but there is no fire.

Horan declares a “Class 1” emergency,meaning the incident is restricted to onelocation. IDO personnel in Idaho Fallsspeed to the site—the IDO duty officer,environmental HPs, the medical direc-tor and his assistant, others. The newsgoes to AEC Headquarters. The weath-er station reports the direction of thewind. The Radiological Assistance Plangoes into effect, activating the HPs inother complexes for response. The firedepartment sets up an operations trailer

at the intersection of Highway 20 andFillmore Avenue. Security forces pre-pare to block Highway 20 traffic, ifnecessary.14

The environmental HPs take detectorsinto the desert beyond the SL-1, look-ing for radioactivity on the sagebrushthat might have come from a cloud inthe light breeze. Others head forHighway 20 and collect samples there.Horan orders an aircraft and asks theaerial-monitoring team to stand by.Then he thinks better of an idea to sendthem on a night monitoring expedition.Heretofore, they had not practiced low-level night flights. They will fly at 6:30a.m. Meanwhile the ground surveyorsconclude that the public—and they—can travel safely on Highway 20.15

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HPs check Highway 20 for contamination on the

morning after the accident.

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Around 10:30 p.m., the CE supervisorand HP arrive at SL-1, equip them-selves, and enter the reactor buildingto look for the men. They find onebadly mutilated and clearly dead, andthey observe a small movement bya n o t h e r.

They withdraw, formulate a plan, andenter once more, this time with twoother military men and an IDO HP.Radiation is estimated at up to 1,000R/hr, lethal. The HPs will allow no oneinside the building for longer than oneminute, and they use stopwatches totime the rescuers and order them outwhen the minute is up. The five menrush up the stairway with a stretcher.They skid and slip on the water and themarble-like pellets littering the floor.Face masks fog up. Two men collectthe man for whom they have somehope of survival while the others try,but fail, to locate the third man.

The rescuers place the man in a couriervehicle and drive toward the intersec-tion with Highway 20. On the way,they meet an ambulance and transferhim. As they reach the intersection, theNRTS night nurse, Hazel Leisen,arrives in her car and enters the ambu-lance, hearing what proves to be theman’s last breath. She applies an oxy-gen resuscitator to no avail. Around11:00 p.m. the assistant medical direc-tor, Dr. John Spickard, pronounces theman dead.16

The accident was unprecedented. Thesewere the first reactor casualties in thefourteen-year history of the AEC.Emergency planning had not accountedfor an event quite like this one. Allthose involved in the recovery, there-fore, now confronted situations andmade decisions unlike any they hadfaced before.

One of the first was made by Dr.George Voelz, the medical director,who arrived a few minutes after hisassistant and considered what should bedone with the body. The dispensary atCentral was completely unsuited forreceiving a contaminated body. Thesmall facility had been designed for theliving; it consisted of one shower headinstalled under the dispensary’s base-ment stairway. It was useless for thissituation. Dr. Voelz recalled:

Because his level of radiation was sohigh, we had no place to put him. Weleft him in the ambulance until wecould figure out what we were going todo next. The ambulance was sitting outin the desert, amongst the sagebrush.We decided the first thing we would dowas see how much radioactive materialwe could remove by taking off theclothing. We wanted to get this donebefore the morning traffic came ontothe Site. It was about four o’clock inthe morning when we decided to try totake the clothing off under the lights ofa couple of automobiles. Because theradiation levels were 500 rads perhour, we decided to work outside at 20below zero in anti-C [anti-contamina -tion] coveralls...

With the moisture [released in theexplosion] and the cold temperatures,the [clothes] were just one solid chunkof ice, having sat out there most of thenight. The health physicists had givenus about a minute’s working time [toremove his coveralls]. But we hadanticipated this, and had some prettyheavy-duty tools that we could use onthese coveralls... They had a stopwatchon us, and we got the job done. Iremember we went a few seconds over.I think it was a minute and seventeenseconds.

That gives you an idea of how you haveto improvise when you get into accidentscenarios. We were able to get theclothing off, and we put him back in theambulance...17

Removing the clothes didn’t help.Radiation levels remained as before.They wrapped the body in blankets anddraped lead aprons to protect the dri-ver’s seat in the ambulance. Around6:30 a.m., the ambulance drove to theChem Plant and into a large enclosedreceiving bay. Near it a decontamina-tion room lined with stainless steelserved as a mortuary. Dr. Voelz had thebody submerged in alcohol and ice topreserve it until he could develop aplan.

Because the reactor had not been oper-ating, early speculation was that achemical explosion had caused theaccident. But of this no one could becertain. Radiation fields were extremelyhigh within the silo. If the explosionhadn’t killed the three men, the radia-tion would have.18

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In the daylight of January 4, Horandeliberately slowed the pace of recov-ery operations. The victims werebeyond saving, and it was now impera-tive to prevent further injuries. AllanJohnson told Horan, “John, I’m settingup a special account for the SL-1.Don’t hold back on anything youneed...it will be covered.” They decidedto document pictorially everything theycould about the accident and the subse-quent proceedings.19

We wanted the nation and the world tolearn what our experience was. It wasunique, and we wanted to make surethat people knew our successes and ourfailures. That is why three movies wereproduced. For the first one of the basicaccident, we took all the people that

were involved in the initial response,and they played in the movie the rolethat they actually did that night. Thatwas part of our effort to be as factualas we could and not have some actordo it.20

AEC Headquarters appointed an inves-tigating committee and a technicaladvisory committee. Most of themarrived on January 4, ready to begin.The JCAE sent one of its staff to pro-vide it with independent information assoon as possible. The national presssent reporters. AEC Headquarters staffarrived, and other AEC labs sent spe-cialists as well. Later, the investigatingcommittee complimented the IDO onits management of the crisis, but saidalso that the rapid arrival of so manyoutsiders, including themselves, hadbeen a disservice to the IDO and thecontractors as they undertook therecovery.21

IDO’s immediate priorities were todetermine what threat the accidentposed to public health, retrieve the othertwo men, and discover if the reactorwas stable or not. The airplane pilotsreported that the roof of the reactor wasintact, undamaged. A cloud of radioac-tive iodine-131 had drifted south towardAtomic City, but its dispersion and mix-ing with air had reduced its concenta-tion well below any health concerns.Ground surveys found that the onlyplace needing to be quarantined was theimmediate SL-1 yard, where the rescueattempt had tracked some contamina-tion. Beyond that, amounts above back-ground levels were negligible.22

A squad of six volunteers, all cadremen,spent January 4 rehearsing a plan toretrieve the body of the second man intwo-man relays. Someone else ran intothe silo to grab the logbook and a neu-tron detector. Crews decontaminated theambulance. The medical staff set up atemporary decontamination center at theGCRE building just up the road fromSL-1. The men who had retrieved thefirst body had not waited for specialgloves; they—and in particular theirhands—had to be washed clean of cont-amination, a process involving water,potassium permanganate, and plenty ofscrubbing. All who had been involvedreceived medical check-ups. The med-ical staff found no radiation injuries andhospitalized no one.2 3

That night, the cadremen retrieved theirsecond comrade. The film and radiationfoils in his dosimeter badge had beenblown away by the explosion. In lieu ofthose items, radiochemists examinedthe gold buckle of one victim’s watch-

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A crew prepares to collect a water sample from

within the SL-1 reactor building.

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band and a tiny screw in a pocket ciga-rette lighter. They found radioactivegold-198 and copper-64. Only neutroncapture could have created such iso-topes. Then they identified other fissionproducts and several isotopes of urani-um in the debris that had come out of

the reactor building with the men. Allof this evidence proved that the reactorhad gone critical.24

The third man finally was found. Hisposition directly above the reactor pre-sented a new hazard. Aside from theobvious difficulty of working in a highradiation field at an awkward location,physicists feared that if pieces of debrisnear him fell onto or into the reactorthrough the open shield-plug holes, thedisturbance might start a chain reaction.A photographer suited up and enteredthe room for one minute, taking asmany photographs as possible. With thehelp of the photos, a plan took shapefor the retrieval. Army volunteers froma special Chemical Radiological Unit atDugway Proving Ground wanted thepractical experience offered by thechallenge of removing the body. Thetwenty-four enlisted men and five offi-cers perfected a plan and rehearsedtheir moves on a full-scale mock-up of

the SL-1 erected at Central. Theyrigged a special net on the boom of acrane and positioned it to prevent thebody or anything else falling onto thereactor. Metal workers shielded thecrane operator’s cab. On January 9,eight men, paired in two-man relayslimited to sixty-five seconds inside thebuilding, recovered the body and low-ered it to the ground.25

The body joined the other two at theChem Plant. A team of health physicsspecialists and a forensic pathologistfrom Los Alamos conducted autopsies,improvising with long-handled instru-ments and other procedures to keepthemselves safe. They hoped that them e n ’s injuries might contribute someinsight into where they had been at themoment of the accident and what mighthave caused it. Most urg e n t l y, they man-aged to reduce the radiation fields ema-nating from each body to between oneand ten percent of the original levels.2 6

The subject of burial already had beenquietly controversial within the topranks of IDO and AEC management.A.R. Luedecke, AEC general manager,had proposed that the men be buriedtogether somewhere on the NRTS siteand a monument erected in their memo-ry. Johnson and other IDO officialsobjected strenuously, feeling that themen’s families deserved the right toconduct funeral rites of their ownchoosing and as naturally as possible.Their view prevailed. The IDO ordered

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Above. Recovery crews practiced at a mock-up, seen

here under construction, at Central Facilities Area.

Left. Crew from Dugway Proving Ground. INEEL 61-674

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three standard metal caskets from IdahoFalls and had them delivered to one ofthe NRTS shops. Craftsmen lined themwith lead. Extra lead wrap shieldedbody parts still heavily contaminated.Aside from protecting people whowould handle the caskets, the casketshad to meet radiological shipping stan-dards set by the Interstate CommerceCommission. Preparations complete,the respective military services tookpossession of thebodies and flewthem to the destina-tions requested bythe families. Ahealth physicistaccompanied eachto provide any nec-essary advice andconsultation.27

The retrieval of thethird body endedwhat came to beknown as PhaseOne of the SL-1r e c o v e r y. PhaseTwo lasted fromJanuary 10 through late April, the time ittook for CE to be certain that the reactorwas stable. No one entered the silo dur-ing those months. Operators dangledremotely controlled closed-circuit televi-sion and other equipment inside thereactor from the boom of a crane. CEfinally concluded that the core no longercontained any moderating water and thatthe reactor could not go critical again.

That done, it was time to clean up thesite and see if the reactor core couldreveal the cause of the accident. A f t e rPresident Kennedy canceled the nuclearairplane project in March, GE had people

available and offered to clean up thebuilding, decontaminate the site, andinvestigate. GE physicist Jay Kunze saidlater that the GE scientists considered theSL-1 job to be a choice assignment.BORAX and SPERT had failed to blowup. SL-1 should have been inherentlysafe. What had gone wrong? They wel-comed a chance to solve the puzzle. Butthe cleanup had to come first. Kunzec o n t i n u e d :

When IDO first asked GE if it could usethe Hot Shop to analyze the reactor, GEsaid, “It’s not available.” But after thecancellation, the word was, “The HotShop is available!” GE had about fivehundred people at the Site, eighty per -cent of whom were due to be terminat -ed, most of those to be transferred toother GE sites as a result of the cancel -lation of the Aircraft NuclearPropulsion Project. All of the profes -

sionals managed to find jobs with thisproject.

The SL-1 was a mess. It hadn’t beencleaned up at all. To clean it up, peoplehad to make short trips inside and dolimited tasks within a couple of minutesand then get out. Even though you suit -ed up, those couple of minutes wouldexpose you to your quarterly dose ofradiation, and you couldn’t go back in

for three months ordo any other workthat could potentiallyexpose you. Hundredsof people at GE,including those aboutto be transferred andmany workers fromother locations at theSite [and fromDugway], volunteeredto take their quarterlyradiation dose doingclean-up at the reac -tor. For many clean-up tasks, that was theonly way of handlingit. I don’t remember

anyone being particularly fearful of therisk.28

The time keepers were the HPs, whostood half-way down the stairway andbanged on metal when someone’svacuum-cleaning stint was over. ByNovember, the passage of time andremoval of debris had reduced radiationlevels. It was time to remove the reac-tor. Anticipating that the forty-miletruck ride to the TAN Hot Shop mightdisturb some of the evidence, the GEteam went underneath the reactor anddrilled several holes in the bottom ofthe pressure vessel. Through a special

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The stretcher rig. Crews practiced before the rig was

inserted into the reactor building to collect the body

of the third man.

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tube, they inserted a camera and tookpictures inside the core.

The GE team had for months beenexamining debris fragments and otherrecovered reactor parts in an attempt toacquire as much information as possi-ble before having to take the drasticaction of cutting the pipes connectingthe vessel to the rest of the plant’sequipment. Kunze recalled:

Our main concern was how to cut thoselarge pipes so that a crane could lift thevessel up thirteen feet and then out andonto a truck. At first we envisioned nomethod except to use manpower in theform of many welders with their pipe-cutting torches, taking turns cutting asmuch pipe as possible before receivingtheir allowable maximum radiationdose.

But we continued to play around withbasic physics ideas—and came acro s sthe idea that the water, ejected upwardby the nuclear steam explosion, hadt r a n s f e rred its momentum to the vessel,p e rhaps sufficiently to cause the vesselto break the pipes and be lifted. Our cal -culations indicated that the vessel mayhave risen enough to hit the ceilingimmediately following the brief nuclearexcursion. So we took a close look at thefan floor, that is, the ceiling of the re a c -tor room, and saw that certain smalldents matched up with the head of thevessel. Now we saw that there was noneed to worry about how to cut thepipes. Much personnel risk and engi -neering frustration had been eliminated,all the result of the nature of this stillsomewhat mysterious accident.

We examined the recovered central con -trol rod, plug, and housing mechanismcarefully in the Hot Shop. The assemblyhad been recovered essentially with therod in the “down” position. However,upon disassembly, scratch marks on therod extension and the inside of theguide tube clearly showed that theguide tube had collapsed, the result ofthe 10,000-pound water hammer pres -sure, and had seized the rod extensionat the 26 1/4 inch withdrawn position.

Subsequently, as the unit hit the ceiling,the extension rod was forced back downto nearly the zero withdrawn position.29

Scratch marks had been made on theway back down, confirming that the rodoriginally had been withdrawn 26 1/4inches. That the rod had been with-drawn and then jammed back downinto the reactor to its “safe” positionafter hitting the ceiling was a bizarrecoincidence. One finding had led to

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Schematic of SL-1 reactor.

another, and the team gradually recreat-ed the mysterious two seconds. Themechanical and material evidence,combined with the nuclear and chemi-cal evidence, forced them to believethat the central control rod had beenwithdrawn very rapidly. They built amock-up of the reactor vessel withidentically sheathed and weighted con-trol rods. In King Arthur fashion, menof lesser, similar, and greater strengthas the crew tried to lift the rod. Mostmanaged with little difficulty. The sci-entists questioned the cadremen: “Didyou know that the reactor would gocritical if the central control rod wereremoved?” Answer: “Of course! Weoften talked about what we would do ifwe were at a radar station and theRussians came. We’d yank it out.”30

On November 29, a large crane liftedthe reactor vessel out of the silo andonto a truck for the trip to the Hot Shopthe next day. Once it was inside thehuge remotely-operated laboratory, thescientists re-photographed the core andplugged the holes. They filled the ves-sel with water and confirmed that thereactor was quite subcritical; it hadgiven its one burst in the accident andthat was all. Then they examined everyinch of the vessel. They were particu-larly grateful that the flux wires hadbeen freshly installed, for they por-trayed the neutron flux uncompromisedby previous history.31

Thus the core and the mangled piecesof metal surrendered their story. In July1962, the GE investigators publishedtheir final report, observing that manualwithdrawal of the central control rodcould explain the accident: “No othermeans of withdrawing the rod has beenfound to be in accordance with the evi-dence.”32

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