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Los Alamos Criticality Accident, December 30, 1958

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Los Alamos Criticality Accident, December 30, 1958 rP,rti;;foffl&;he l involved in critical ntI 1 accident I soltitions Stripped y&3; cnl>ront o.+rnr+;nn Purification@ F-1 IIGII~ '~~~~~ Jccident) u 1-y I“ n c: :7 tqH A-Filter feed tank Super- B-C-Evaporators ‘Y rods, D-Dilute strip tank audible I Plutonium- ! 4 E-Precipitation tank circuit,s, F-Solvent treatina tank nagnets :rvisorvs G-Current dilute itrip tank -P--l/-I H-Raffinate tank J-Nitric acid station release K-Feed tank 'spring. I-Storage tank Iriving FIG, 1. FLOW SHEET for plant in which accidental criticality FIG. 2. PLANT LAYOUT shows relation of different processing oreas and tanks set to receive near-critical liquids ‘,red by ,c.urred. Portion involved is at right -ication, can be he tank Qection ving the Los AIamos Criticality Accident, )ved re. removal .a meas- December 30, -1958 and the By HUGH C. PAXTON, R. D. BAKER, W. J. MARAMAN, and ROY REIDER its were dent or Lbe Alamos Scienti~%c Laboratory, Los Alamos, New Mezico )f heavy hat was srsonnel EDITORS’ NOTE: A sudden criticality in The resulting solution, containing plutonium was transferred to the stainless-steel solvent- ere peo- Dprocessing vessel at the LOS Alamos ptuto- at a few grams per liter, then is fed back treating tank in which the accident occurred he time. nium-processingplant occurred on Decem- into an earlier recovery stage. (Fig. 3). This tank already contained about 80 gal of caustic-stabilized aqueous- lccident ber30. As a resdt a technician died 36 hr These processes are complicated by hy- drolysis of TBP into mono- and dibutyl organic emulsion that had resulted from later (NU, Feb. ‘69, p. aI). NUCLEONICS phosphates from the actions of heat, nitric the second step in the precipitation of TBP presentshere a report of the ojicial investiga- acid and alpha-particle radiation. Both hydrolysis products. In addition, 13 gal ,n octo- tion of the accident. The report tells that reactor hydrolysis products tend to form tightly of concentrated nitric acid was used to wash the accident occurred during an inventory -;embly) bound plutonium complexes, and periodi- solids from the bottom of the large vessel : “Boris in which normal plant$ow was interrupted. cally they must be precipitated from the into the solvent-treating tank. Transfers percriti- ‘Ln unusual amount of plutonium had col- TBP-solvent mixture. These plutonium- into the bottom of the tank were followed by air-sparging, which mixed the contents, +nity of lrcted in a proiessing tank. When a rich solids are filtered and sent to another so that the emulsion was broken by stirrer was actuated an aqueous solution at part of the plant for processing in small mge 154 acidification and plutonium originally in /he bottom of the tank flowed up around batches. Figure 1 is a simplified flowsheet for the residue-recovery operation, and the solids was extracted into the solvent. the plutonium-rich organic phase above it, Fig. 2 shows the corresponding layout of Phases then separated; about 2.5 min was producing criticality. equipment. required. The resulting situation in the 225-gal, Tam NUCLEARCRITICAL ACCIDENT resulted At the time of the accident a physical 3%in.-diameter, solvent-treating tank im- from an unusual and complex set of circum- inventory was in progress in the TBP- mediately before the accident is now be- stances. Reconstruction of the steps that solvent-extraction plant. The normal in- lieved to be as illustrated in Fig. 4: 87.4 gal preceded it and analyses of the materials flow to this area was interrupted, and of aqueous solution contained 40 gm of involved now give a reasonably specific residual materials in all process vessels plutonium, and on top, a 42.2-gal layer of Picture of the conditions at the time of the were to be evaluated for plutonium content. solvent contained 3.27 kg of plutonium. radiation burst. This requires filtration of solutions supple- mented by thorough cleaning of vessels Solids (containing 60 gm of plutonium) OPerational Background and analyses of clarified solutions and were suspended in both the aqueous and removed solids. organic phases and at the interface; few The portion of the plant in which the solids settled to the bottom of the tank. accident took place is used for concentration Before the Accident Estimates based on Fig. 5, taking into and purification of “lean” residues from a account the tank diameter and effects of Plutonium recovery operation. Typically, Reconstruction of significant events indi- neutron poisons, indicate that the S-in.- solutions contain less than 0.1 gm/liter of cates that plutonium-rich solids, which thick solvent layer was barely subcritical Plutonium and traces of americium. The normally would have been handled sepa- at the plutonium concentration of 20 gm/ piutonium and americium are separated by rately, were washed from two other vessels liter (the estimated critical thickness is We*ted batch extraction into tri-n-butyl into a large vessel that contained dilute S$i inches, and the actual configuration lows and Phosphate (TBP) carried in organic solvent, aqueous and organic solutions. After re- was roughly 5 dollars subcritical). r is for f”llomed by back extraction into an aqueous moval of most of the aqueous solution from Phase that is concentrated by evaporation. this vessel, the remaining 40 gal of material Continued on next page got’@ “O’* ‘7, NO. 4 - April, 1959 107 The Accident TAME 1-4’~ in Withdrawal Tanks’ A chemical operator started. the motor driving the stirrer in the solvent-treating - tank. There was a “blue f&h” and a Aqueous solution Organic sol&m m&led report. The operator, who was - -.-- looking into a sight glass at the top of the tank, (Fig. 4) was knocked off the two-step Sol&$ ladder on which he stood. The operator Tmk no. Volutne (liters) Pu (sm) Vohme (liters) pu (VN apparently turned the stirrer motor off and ----- then on again (this time noticing a rum- 1 63.5 7.54 0 0 1% bling sound), ran out a nearby exterior 2 67.2 8.80 0 0 1.09 door and called that he was “burning up.” 43.5 3.38 15.8 362 w 9.89 Though the shock displaced the tank about 0 56.2 4 0 1129 7.2c g in. at its supports, the tank was not 5 0 0 0 0 ruptured and no plutonium escaped. 0 6 68.0 8.02 0 0 The tank in which the accident occurred 1.56 7 7.07 0 was not bolted to the floor. Tangential 54.0 0 0.76 shift of one of the supporting legs was appar- 8t 34.7 2.43 25.0 518 la.3 63.0 ent because the initial position was outlined 9 0 0 1263 7.4 by the latest coat of floor paint. Appar- 10 0 0 0 0 0 ently the tank pivoted about the opposite TOTALS 330.9 37.2 iso. icEi 5 foot. Average concentrat~ions (gmjliter) We cannot be certain that there was no 0.112 20.4 following low-order burst when the stirrer Estimated solids not removed from tanks 3 and 8, and other solids was turned on the second time, but it seems 60 unlikely. No accompanying blue flash was reported, and it is doubtful that the few sec- * Tanks shown in Fig. 6 are numbered successively from bottom to top. The nearer op, onds presumed to have elapsed while the are Tanks l-5; the farther ones, Tanks 6-10. They were filled from bottom Upward, stirrer was off would have been sufficient to re-establish a potentially critical situation. Radiation-detector traces are consistent with a single-burst hypothesis, but do not exclude the possibility of two closely spaced photoflash”), heard the report, went to the tank in which the accident Occurred,th pulses. The stirrer rumbles when turned help, and was joined by a third operator. second operator turned off the stirrer mote, on under normal conditions; it appears that They joined the first operator who had by The burst activated a radiation alar, degree of rumble could easily have been mis- this time run out of a nearby door. ill1 located 17.5 ft away in a nearby buildin, judged. We will never know for sure. three men returned past the accident tank (This unit was connected to an indicatia At the time of the burst a second operator to lead the injured operator to a shower. inntrument and set to sound at 10 mr/h some 40 ft away in an adjoining room saw a By this time he suspected that he was suffer- After a report of the alarm, the entire plan reflection of the light on the walls (“like a ing from an acid&burn. 4s the three passed Continued on pap. 16 ,! i 42.2 galsOl@nf 87.4 gal w@ yFloor line FIG. 4, RECONSTRUCflON OF SITUATION at time of OCcidsd Id@ FIG. 3. ACCIDEN?’ TANK, shown at left, was moved g in. by indicates tank had barely subcritical layer of organic fl shock that occurred when stirrer was turned on above aqueous layer April, 19S9 - NW-EC mos Accident Special Committee Senior Chemists, This article starts on page 107 Reviews Report Physicists, H/ Pu23gAtomic Ratio A technical report* describing the circumstances of the Los Metallurgists Alamos criticality accident has and Engineers for been made available by the Atomic Energy Commission. It has been written by the authors of this FUELELEMENT i article and can be got at the Office of Technical Services, Department of Commerce, Washington 25, DEVELOPMENT D.
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