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LnsALAMOS SCIENTIFIC LABORATORY

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NUCLEAR-CRITICAL ACCIDENT AT THE LOS

SCIENTIFIC LABORATORY ON DECEMBER 30. 1958

REFERENCE

TAKEN FROM THIS ROC

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LEGAL NOTICE

This report was prepared as an account of Government sponsored work. Neither the United States, nor the

Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty or representation, express

or implied, with respect to the accuracy, completeness,

or usefulness of the information contained in this report,

or that the use of any information, apparatus, method, or

process disclosed in this report may not infringe privately

owned rights; or

B. Assumes any liabilities with respect to the use

of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this

report.

As used in the above, ‘@erson acting on behalf of the

Commission” includes any employee or contractor of the

Commission to the extent that such employee or contractor prepares, handles or distributes, or provides access

to, any information pursuant to his employment or contract with the Commission.

Printed in USA. Price 50 cents. Available from the

Office of Technical Services

U. S. Department of Commerce

Washington 25, D. C.

.

.

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LAMS-2293

HEALTH AND SAFETY

(TID-4500, 14th cd. )

LOSALAMOS SCIENTIFICLABORATORY

OF THE UNIVERSITYOF CALIFORNIA LOS ALAMOS NEW MEXICO

REPORT WRITTEN February 1959

REPORT DISTRIBUTED February 20, 1959

NUCLEAR-CRITICAL ACCIDENT AT THE LOS ALAMOS

SCIENTIFIC LABORATORY ON DECEMBER 30, 1958

by

H. C. Paxton

R. D. Baker

W. J. Maraman

Roy Reider

Contract W-7405-ENG. 36 with the U. S. Atomic Energy Commission

_ . .

c,

~. This report is an informal account of the findings of a Los Ala- ~$

.-m mos Scientific Laboratory committee appointedto investigate the o—~—.- b circumstances surrounding the nuclear-critical accident that oc-

;E: curred at the Laboratory in December of 1958. It has been preJ===== _——0 pared to distribute the information as promptly as possible and

<_

z— o presents the facts known at tbe time of writing.

~-; ”--

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4
ABSTRACT

P

The nuclear-critical accident that occurred in the plutonium processing plant at the Los Alamos Scientific Laboratory on December 30, 1958, resulted from an unusual and

complex set of circumstances. Reconstruction of the steps

that preceded the accident and analysis of the materials

involved give a reasonably specific picture of the conditions

at the time of the radiation burst.

ACKNOWLEDGMENTS

The authors wish to acknowledge the assistance of the

following in the preparation of this report: P. C. Fisher,

of Group N-2; Joseph Leary and Eldon Christensen, of

CMB-ll; and J. N. P. Lawrence, of H-1.

.

3
.

.

.

9

.

.
.

-.

.

INTRODUCTION

On Tuesday, December 30, 1958, at 4:35 p.m., a nuclear-critical accident which resulted in the fatal radiation injury of a Laboratory employee

occurred in the plutonium recovery operation at DP Site of the Los Alamos

Scientific Laboratory. An official investigation of the circumstances surrounding the incident was conducted, and this report summarizes the findings

concerning the events leading to the accident, the situation at the time of the

critical burst, the removal and analysis of the solution that caused the burst,

and the steps being undertaken to prevent a recurrence.

OPERATIONAL BACKGROUND

The layout of the site at which the accident occurred and the specific

area involved (Rooms 213-218) are shown in Fig. 1. The part of the plutonium recovery operation involved in the accident was the tri-n-butyl phosphate (TBP) solvent-extraction process used to concentrate and puri~ plutonium from so-called ?~leanl’ residues, typically, solutions containing less than

O.1 g/liter of plutonium and traces of americium. A simplified flowsheet for

the process is given in Fig. 2, and Fig. 3 shows the corresponding layout of

equipment in the plant. Figures 4 to 9 delineate the steps involved in the

procedures shown in Fig. 2.

Slag, crucible (from lWetal Preparation), and other lean residues are

appropriately dissolved, and the resulting solution is clarified by filtration.

The removed solids are treated separately by the method shown in Fig. 4.

The filtrate is contacted in columns with TBP carried in organic solvent to

extract the plutonium, then the plutonium is back-extracted into an aqueous

phase that is concentrated by evaporation. This operation is shown in Fig.

5. The concentrated product, which contains plutonium at a few grams per

liter, is further purified by a TBP extraction in a mixer-settler operation

(Fig. 6) and the product material removed for metal preparation.

The raffinate from the column extraction operation contains some residual plutonium and small amounts of americium. After chemical treatment

of this solution (which causes the formation of a suspension of finely divided

silicates), the plutonium and americium are removed by batch-contacting with

5
6

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O** *#l >“

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Isolutions

Solvent

4

Stripped

Purification Solvent Extraction

Raffinate I

Solvent Raffinate

I*

for Recycle Solvent Extraction

I

I

I Strip

Solutione

+

I

Purification I

I

Stripped

Solvent

Recycled

I

I

+

Raffinate

to Waate

Treatment

I

Fig. 2 Schematic flowsheet for the recovery of plutonium from lean residue.

7

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—--— UC(

8

.

.

.
.

Slag and Crucible

f

Nitric Acid

Aluminum Nitrate

I Leached with I

Dissolution Iv Caustic 9

Caustic

Gas Scrubber Discarded

r—Solids

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Fig. 4 Flowsheet showing dissolution process,

.

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.

.

F

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Solvent

+ Extraction Column

Solvent

Nitric Acid ~

Solvent

Nitric Acid

+

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b (stripping)

I Column No. 2

I

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Solvent Extraction

Strip Back Extraction

● (Stripping)

Column No. 1

Kerosene

Scrub Cohunn

1

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Aqueous +

I

Solvent Strip

Solvent Purification b Concentration

Purification

Fig. 5 Flowsheet solvent-extraction process.

10
.

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Strip from Extraction

columns

I

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Content ration

1

Solids

Dissolution Flowsheet Filtration

1

I

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Feed Stabilization

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Solids

Dissolution Flowsheet

I

Filtration

I

Solvent

Concentration

Batch Back Solvent Filtration

Extraction

Aqueous

+

Oxalic Acid

‘1 Precipitation

%

I Ignition to Oxide I

+

I

+ Residue

Filtration 4

Dissolution

Fig. 6 Flowsheet showing purification process.

.

“

11
TBP (Fig. 7). These are back-extracted into an aqueous strip solution, which

is concentrated by evaporation. The concentrate is stored for filtration and

future processing back through the columns. Continued recycle of this material ultimately allows the separation of an americium product solution.

The actions of heat, nitric acid, and alpha particles emitted by plutonium

on TBP result in its hydrolysis into mono- and dibutyl phosphates, both of

which form plutonium-rich compounds. Treatment of the solvent with caustic

precipitates these plutonium-containing materials, forming an emulsion. The

caustic is acidified, which leaves the hydrolysis products precipitated, and

the slurry is recycled through the filtration and column-extraction system.

After acidification, the solvent is acceptable for re-use. This process is

shown in Fig. 8.

Dilute aqueous streams are concentrated by evaporation for recycle

(Fig. 9).

EVENTS LEADING TO THE ACCIDENT

At the time of the accident, a physical inventory was in progress in the

TBP-solvent-extraction plant. The normal flow into this area was interrupted,

and residual materials in all process vessels were to be evaluated for plutonium content. This requires filtration of solutions, supplemented by thorough cleaning of vessels, and analysis of the clarified solutions and removed

solids .

Reconstruction of significant events indicates that plutonium-rich solids,

which normally would have been handled separately, were washed from two

other vessels into a large vessel that contained dilute aqueous and organic

solutions. After removal of most of the aqueous solution from this vessel,

the remaining -40 gallons of material was transferred to the stainless steel

solvent-treating tank in Room 218 in which the accident occurred (Figs. 10

and 11). This tank already contained about 80 gallons of caustic-stabilized

aqueous-organic emulsion that had resulted from the second step in the precipitation of TBP hydrolysis products.

Material transfers leading up to the accident were as follows (refer to

Fig. 3 for container locations).

10 Dilute Strip Tank: Fifty gallons of water had been previously added

and agitated to attempt to dissolve an estimated 20 lb of solids in the bottom

of this tank. This slurry was transferred into the Current Dilute Strip Tank,

and the remaining solids were flushed into that tank with a 15-gallon stream

of water.

2. Evaporator No. 1: Out of a 110-gallon batch of concentrated dilute

strip and peroxtde filtrate, 100 gallons was transferred to Filter Feed Tank

No. 2 for filtration. (This is the capacity of the tank. ) The remaining 10

.

b

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—

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A

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Aqueous

Back Extraction

w

Aqueous I

1

Pnrffication IQ=-@==

Solvent

J 4

esidue Nitrio Acid

Dissolution Flowsheet ~ Filtration Acidification

Aqueous

Separation J

Separation

Concentration

foilowed by

Solvent

Filtration and Solvent

I

Extraction

Concentration 1 v

Aqueous Extraction

Systems

Aqueous

I

Solvent Emulsion Sodium Hydroxide

●

✎

✎

EST‘~4 Itric Residue

Acidification Acid Filtration Dissolution Flowsheet

Solvent Solvent plus

Aqueous

Extraction Aqueous

Separation

Systems

Concentration

Solvent

Acidification

I_EGE-1

Fig. 8 Flowsheet showing solvent-purification process.

.

. 14
.

A.* . 

h

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—

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Fig. 10 218 Solvent Treating Tank.

16

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17
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gallons of slurry was transferred to the Current Dilute Strip Tank.

3. Current Dilute Strip Tank: In addition to the above solutions this

tank contained an estimated 320 gallons of dilute strip, 100 gallons of acidified

caustic from solvent treating, 15 gallons of solvent residual from phase separation of dilute strip, and 2 gallons of a special recovery solution containing

4gof Puo

4. 218 Solvent Treating Tank A 145-gallon batch of treated solvent

containing 2.9 g of Pu was transferred from this tank into Raffinate Tank

No. 1.

5. Storage Tank No. 2: This tank contained an 80-gallon batch of an

emulsion resulting from the second treatment of a batch of solvent. This

material was transferred into the 218 Solvent Treating Tank.

6. Current Dilute Strip Tank: From prior experience it is believed

that the solutions in and added to this tank contained

Solution

Dilute Strip

Acidified Caustic

Solvent

Special Recovery Solution

Rinse Water

Evaporator No. 1 Solution

Total

Volume, gal

320

100

15

2

65

10

512

Plutonium, g

30

40

1

4

10

12

97

The quantity of plutonium in the TBP hydrolysis product solids transferred

from each of the tanks cannot be estimated individually; however, the total

added is now known to have been 3.4 kg. A 308-gallon batch of aqueous solution, containing 181.2 g of Pu, was transferred into Precipitation Tank No.

2, and 165 gallons, with 50 g of Pu, was transferred into Evaporator No. 3.

The balance of aqueous solution and solvent was transferred into the 218

Solvent Treating Tank. A 13-gallon lot of concentrated nitric acid was used

to slurry residual solids from this vessel into the solvent-treating tank.

The addition of solutions and accompanying solids to the Current Dilute

Strip Tank is accomplished through a bottom inlet, and each transfer is

followed by a vigorous a.ir-spargfng of the tank contents. This treatment allowed extraction of the plutonium-laden hydrolysis products into the acidified .

.

18
solvent layer on top of the solution. Subsequent transfer of this plutoniumrich solvent and the nitric acid slurry of residual solids in the bottom of

the tank into the 218 Solvent Treating Tank (Figs. 10, 11, and 12) acidified

the emulsion, freeing additional solvent for further extraction of any remaining hydrolysis products into the combined organic phase. As in the case of

the Current Dilute Strip Tank, mixing and extraction resulted from the airsparging of this material through the bottom inlet of this tank. Upon cessation of the air-sparging the solvent phase separated into an 8-in. layer on

top of the aqueous which was subcritical at this time. This thickness was

determined by evaluation of the solutions in the tank after the accident.

Phase separation took about 2-1/2 minutes.

The standard operating procedures require that slurries and solids

residual in the various processing tanks be separately transferred from each

tank to the Filter Feed Tank and into the filtration area, where the solids

are removed, sampled for analysis, and processed in geometrically-favorable

equipment. When solids cannot be transferred from a tank in the form of a

slurry, the tank is opened and the solids are mechanically removed. The

mixing of slurries with TBP is to be avoided, not only for purposes of criticality control, but also from the standpoint of acceptable processing procedure. The contacting of slurries with TBP generally results in a very stable

emulsion caused by the silicates present.

Although filtration procedures had been performed on the solutions and

slurries resulting from the first treatment of the material transferred into

the 218 Solvent Treating Tank from Storage Tank No. 2, they were not carried out on the slurries transferred from Evaporator No. 1 and the Dilute

Strip Tank into the Current Dilute Strip Tank, and from that vessel into the

solvent-treating tank. It is concluded that the major source of plutonium

was the solids from these tanks.

THE ACCIDENT ,

The situation in the 225-gallon, 38-in. -diameter, solvent-treating tank

immediately before the accident is believed to be as illustrated in Fig. 12:

87.4 gallons of aqueous solution contained 40 g of Pu, and on top, a 42.2-

gallon layer of solvent contained 3.27 kg of Pu. Solids (containing 60 g of

Pu) were suspended in both the aqueous and organic phases and at the interface. Few solids settled to the bottom of the tank. Estimates based on

Fig. 13, taking into account the tank diameter and effects of neutron !@oisons, ?’

indicate that the 8-in. -thick solvent layer was barely subcritical at the plutonium concentration of 20 g/liter (the estimated critical thickness is 8-1/4

inches, and the actual configuration was roughly 5 dollars subcritical).

.

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organic phases before stirrer was activated.

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A chemical operator started the motor driving the mechanical agitator

in the solvent-treating tank. There was a “blue flash” and a muffled report.

The operator, who was looking into a sight glass at the top of the tank

(Figs. 10 and 12), was knocked off the two-step ladder on which he stood.

Though the shock displaced the tank about 3/8 inch at its supports, the tank

was not ruptured and no plutonium escaped. The operator apparently turned

the stirrer motor off, then on again (this time noticing a rumbling sound),

ran out a nearby exterior door, and called that he was “burning up. ” A

second operator, some 40 feet away in an adjoining room, saw a reflection

of the light on the walls (Illike a photo-flash??), heard the report, went to

help, and was joined by a third operator. Returning to lead the first man

to a shower (on request, apparently as the result of suspecting an acid burn),

they passed the tank, and the second operator turned off the stirrer motor.

The burst activated a radiation alarm located 175 feet away in a nearby building. TMS unit, connected to an indicating instrument, was set to

sound at 10 mr/hr. The entire plant was evacuated.

THE BURST

The initial action of the stirrer in the solvent-treating tank forced

aqueous solution up along the wall, displacing the outer portion of the solvent

layer and thickening the central portion. An average increase in solventlayer thickness of about O.4 inch, corresponding to an average radius decrease of 1/2 inch, could account for the obviously supercritical configuration. The burst was certainly terminated by the violent disturbance that it

generated, and continuation of the stirring diluted the plutonium beyond the

point at which a critical reaction could recur.

None of the gamma-sensitive recording detectors within range of the

accident showed an ideal trace. They suggest, however, that there was but

a single burst -- see, for example, the record reproduced in Fig. 14.

Radiochemical analyses for Mogg (concentrated mostly in the aqueous

phase) indicate that the total number of fissions in the burst was about

1.5 x 1017. This value may be subject to refinement when analyses for other

fission products are complete.

EXPOSURES

.

Results of radiation monitoring in the vicinity of the solvent-treating

tank after the accident are summarized in Table 1, and Table 2 lists radiation dosages received by personnel at the plant.

The chemical operator at the solvent-treating tank went into deep shock

22

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Fig. 14 Trace of slow-acting recorder (Point No. 4) located 160 ft from tank.

23
.

.

.

TABLE 1

RESULTS OF lUIDIATION MONITORING OF ROOM 218

I

Direct Sight

Distance from

Date Time r/hr Tank, ft

12/30/58 4:40 p.m. 20 25

5:10 p.m. 45 7.5

5:15 p.m. 0.2 110

5:35 p.m. >50 4.5

7:30 p.m. 12 1.5

9:45 porno 0.1 25

12/31/58

\

1/1/59

8:20 a.m. 0.05 25

1:40 p.m. 10 Contact

3:55 p.m. 7.5

10:01 p.m. 5 Contact

9:00 a.m. 2 Contact

TABLE 2

DOSAGE ESTIMATE FOR OPERATING PERSONNEL

Location of Chemical Operators* Dosage, rem

Against 218 Solvent Treating Tank 12,000 * 50%

Behind Feed Tank No. 3 (43 ft) 134

In purification area (35 ft) 53

In other areas (>75 ft from supercritical burst) <4

*See Fig. 3.

24

.

r
within 15 minutes of exposure. He regained consciousness about 6 hours

later and remained rational and comfortable until nearly the time of his

death, 3:15 a.m. on January 1. The operators who went to his assistance

show no physical effect of exposure other than typical changes in blood

count in the case of the man who arrived first and switched off the stirrer

motor. Both have continued on regular duty. The fact that they received

predominantly gamma dosage in@cates, of course, that there was no succeeding burst while they were near the tank.

SOLUTION REMOVAL AND ANALYSIS

Immediately after the incident, a bank of ten 6- and 5-in. -diameter

cylinders was fabricated and set up about 100 feet from the point of the accident (see Figs. 3, 15, and 16). This storage array was connected to a

transfer line that leads from the bottom outlet of the solvent-treating tank.

As the disposition of plutonium in the tank was then unknown, opening of the

bottom outlet valve was a suspect operation and was therefore accomplished

from behind a temporary shield by means of a 10-foot extension handle.

The solution was vacuum-transferred into the bank of cylinders without complication on January 1, 1959. The solution level within the cylinders was

followed by gamma-sensing instruments. Maxfmum reading at contact was

about 1 r/hr, and indications along the wall of the solvent-treating tank immediately after emptying were less than 50 mr/hr.

Successive 9-liter lots of solution were removed from the storage cylinders, and each batch was sampled for chemical analysis. Results of analysis are summarized in Table 3. Not only was the bulk of the plutonium

found to be in the organic solvent phase, but a concentration of Zrgs in a

suspension at the aqueous-organic interface suggested that the situation was

the same at the time of the accident. Furthermore, the chemical form in

which the plutonium occurred in the solvent was consistent with the hypothesis that it had been complexed with TBP hydrolysis products.

Additional evidence that the plutonium was concentrated in the solvent

layer when the burst occurred is given by the long-lived gamma activity of

the stainless steel baffle (type-347) that was located about 6 inches within

the wall of the solvent-treating tank (Fig. 12). Scanning indicated a peak in

activity about 1 inch below the mid-plane of the solvent layer.

NUCLEAR-SAFETY CONTROL

A nuclear-criticality review of the entire plutonium plant had been

completed about a month before the accident. Changes in equipment

25
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26
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TABLE 3

EVALUATION OF l?u IN THE GEOMETRICALLY-FAVORABLE TANKS

FROM THE 218 SOLVENT TREATING TANK

Aqueous

Volume, 1 Pu, g

Organic

Volume, 1 Pu, g

Solids

Pu, g

1.33

1.09

9.89

7.20

0

1.50

0.76

18.3

7.4

0

47.5

Tank No.

1

2

3

4

5

6

7

8

9

10

63.5

67.2

43.5

0

0

68.0

54.0

34.7

0

0

7.54

8.80

3.38

0

0

8.02

7.07

2.43

0

0

0

0

15.8

56.2

0

0

0

25.0

63.0

0

0

0

362

1129

0

0

0

518

1263

0

.

.

Total 330.9 37.2 160.0 3272

Avg. cone., g/1 0.112 20.4

Estimated solids not removed from Tank Nos. 3 and 8: (2,000 g)(O.01) %

2ogof Pu

Material Volume, 1 Pu, kg

organic 160 3.27

Aqueous 331 0.04

Solids 0.06

Total z 3.37

.

28
,

.

.

u

associated with a hopefully-improved solvent-extraction process were planned

to reduce dependence upon procedural control of batch sizes for nuclearsafety purposes, although this type of control had been used successfully

during the 7-1/2 years that the plant was in operation. Installation of the

new equipment was scheduled for June and July 1959. Depending upon the

efficiency of the new extraction process, the possibilityy of eliminating the

succeeding raffinate solvent-extraction and concentration stages (Fig. 2) was

to be explored.

Because primary dependence had to be placed on procedural controls for

a while longer, they were bolstered by re-emphasis and improved solidssampling methods. Other measures undertaken as a result of the review

were replacement of a product vessel for concentrated solution by one of

safer geometry, review of operating procedures, improvement of the nuclearsafety training of personnel, design and installation of gamma-sensing radiation alarms, and updating of evacuation procedures.

The accident occurred in a process that was believed to be relatively

safe because of small throughput. Long-term holdups of material, however,

resulted from the requirement that certain plutonium accounts be balanced

before material in other accounts could be processed. So, even here, proced~ral control was recognized as a safety requirement.

Since the accident, of course, there has been further detailed review of

the entire plutonium plant. As a result, additional safety precautions have

been adopted to prevent recurrence of an accident, and work on those already

planned has been accelerated. Following is an outline of the steps being

undertaken as a result of the review prior to the accident and the investigation following it:

1. Geometrically-favorable dissolvers and feed tanks for filtration and

solvent extraction have been designed, and it is expected that the equipment

will be installed before operations are resumed.

2. Supplemental transfer lines, such as those required for emergency

procedures, will be blocked to minimize the opportunity for abnormal interchanges.

3. Cadmium nitrate solution has been placed in vent tanks and vacuumbuffer tanks to protect against criticality in case of accidentally introduced

plutonium solutions. The use of both soluble and fixed neutron poisons in

large process equipment is being studied, although unequal distribution between phases in the one case, and plutonium deposition on surfaces in the

other, present difficulties.

4. Neutron detectors are being calibrated and tested for indicating abnormal deposits of plutonium.

5. Measurements to give nuclear-safety data for hydrogen to plutonium

ratios in the region of 10 to 1 are being planned. A computational program

29
.

.

is needed to interpolate between experimental points.

6. Gamma-sensing alarms are to be located in all areas where a criticality-accident potential exists. Equipment was temporarily installed at the

site where the accident occurred, and a permanent installation has been designed which will be in operation by July 1, 1959.

7. Coupled with alarm devices, emergency procedures have been established and employees trained to follow them. As part of the training program, practice actions for emergency situations will be conducted at reasonable intervals. Physical changes in buildings, processes, gates, and fences

will require a review and re -evaluation of evacuation procedures.

8. Although nuclear criticality is admittedly a complex and sophisticated

subject, it is recognized that certain general safety fundamentals exist which

can be taught to employees at the technician and operator level, e.g. , the

nature and consequences of criticality accidents. That such training is not

scare tactics has been repeatedly demonstrated in the explosives industry,

where violent demonstrations have been used effectively. Where an experi -

mental criticality safety facility exists, it might well be used to show employees graphically the development of ionizing radiation from critical assemblies. It is planned to use the Pajarito laboratory of LASL group N-2

for this purpose.

30

.

*

i-
.

APPENDIX

PLUTONIUM CRITICALITY DATA

Criticality data for homogeneous plutonium-water systems are summarized

in Figs. A. 1 and A.2. * Critical masses and critical volumes are given for

bare spheres and water-reflected spheres. Deduced from these values are

the corre spending critical radii of infinite, cylinders and critical thicknesses

of infinite slabs shown in Figs. A.3 and 13 of the text. For interconversion

between critical spheres (radius rs) and cylinders (height hc, radius rc), the

following relation, which has been checked in detail for U235 solutions, is

consistent with the limited plutonium solution data:

where 6s and rSc are given in Fig. A.4.

Figure A.5 is a guide for the use of cadmium in aqueous plutonium solutions as a secondary criticality safeguard. Any other strong thermal poison

may be substituted by adjusting its

indicated for cadmium. The curve

tion

macroscopic cross section to equal that

of Fig. A.5 was computed from the rela-

(

.2:+ $%:2

km =

/) s

= 1,

-th , “res “th)” /i -b

a a ‘al ‘s

where Zth and ~~hre~are thermal macroscopic cross sections for fission and

total absorption, Xf and Z~s are the corresponding resonance integrals,

and zs is the macroscopic scattering cross section of hydrogen.

*Solution data are from F. E. Kruesi, J. O. Erkman, and D. D. Lanning,

Hanford Report HW-245 14, May 1952 (classified).

31
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