Failure Knowledge Database - 100 Selected Cases

Criticality Accident at JCO Uranium Processing Plant【September 30th, 1999

Tokai, Ibaraki, Japan】

Mitsuo Kobayashi (Graduate School of Frontier Sciences, University of Tokyo)
Masamitsu Tamura (Graduate School of Frontier Sciences, University of Tokyo)

On September 30th in 1999, a cr iticality accident occurred at the T okai Factory of
JCO Co. w hich is a uranium fuel processing

facility located in thearea where thenuclear energy related facilities gathered in Ibaraki Prefecture. It became the w

orst

accident in the histor y of nuclea r ene rgy in Jap an, 150 p ersons, including threeworkers, were exposed to radioactive materials, and two of the three workers died.
The acciden t happened when highl y concentra ted (U 235 18.8%) uranium used forthe fast br eeder exp erimental r eactor "J oyo" was manu factured te mporally us ing
facilities for low concentrated uranium (U235 3-5%). JCO w as concentrating uranium

fuel, which is a p art of the fu el fabrication p rocess. Th e ac cident o ccurred d uring t heequalizing process, inwhich the equalizatio n of the p roducts of som e batches w as

executed. T his p rocess was almost the final stage of JC O op eration, it was op eratedusing an illegal procedure throughout the co mpany. According to the statement b y a
specialist, t he d irect c ause of th e accid ent is that "th ey inj ected a uranyl nit rate
solution, including uranium that w as over critical mass into the preci pitation drum
that was originally designed for the different purpose and was no t designed in the

shape that is required for criticality safety".
The critical state continued for several hour s without stopping immediately . As it
was the first time that a criticality acci dent oc curred i n Jap an, the G overnment

Countermeasure Headquarters, chaired by Mr. Obuchi who was the Prime Minister atthat time, the Accident

Countermeasure Headquarters, chaire d by the DirectorGeneral of Science and Technology Agency, and the Government Field Countermeasure
Headquarters were or ganized. The countermeasures for closing the crit ical state were
examined a t the Field Countermeasure H eadquarters, an d it was d ecided that thewater in the p recipitation d rum jac ket that caused the criti cal state should be d rawn
off. This is very dangerous work, because it is necessary to approach the precipitation
drum in whi ch radiation leaked. Volunteer employees of JC O worked under the th reatof de ath, and the critical state stoppe d at last after havin g continu ed for n early 20
hours.
The concentration operation was contract work with no profit for the company, and1

Failure Knowledge Database - 100 Selected Cases

moreover the work was comp licated. Therefore, essential safety countermeasures wereignored, and the comp any allowed too t houghtlessly illegal work methods, whichresulted in the occurrence of th e accident. The problems w ere th at not only theoccurrence of the accident but also a lack of prior preparation for c ountermeasures inthe event of a criticality accident and the fact that a basic part of nuclear energy policy
was left in the charge of a private enterprise and concrete operation procedures had notbeen checked by the government.
A large number of criticality accidents at nuclear fuel facilities have been reported.
Many of these criticality accidents happened in the U.S.A. and the former Soviet Union
during the initial stage of nuclear ener gy development incriticality accident at nuclear fuelthe 1950’ s to 1960's. A

facilit ies was suppos ed to be the accident

thathuman bein gs had ov ercome in the p ast. Th e impact of the criticality accident that
occurred in Japan was very strong as it wa s generally considered that “Japan is themost advanced nation in atomic energy where nuclear accidents are very few”.
This accident and the leak accident of metal sodium at th e prototype fast breeder
reactor “Monju” made a difficult state for continuing the nuclear energy policy of Japanas before.

Although there w ere some

excellent count ermeasures such as theord er ofevacuation of inhabitants living within 350 meters of the facilities by the village leader,the d ecision to organize the Gov ernment Countermeasure Headquarters beyond the2Failure Knowledge Database - 100 Selected Casessectionalism of each govern

ment department, and t he actions of the F ield

Countermeasure Headquarters tha t accurately instructed the m easures to stop t hecritical state, the overall impression of the countermeasures was a little confused.
The description of this paper will be limited to only technical aspects such as thecause, countermeasures and the b ack gr ound of th e acci dent, and n uclear energy
policies, medical cure of the victims, actual conditions of the rumor damage, and so on
will not be mentioned.
1. Event
To begin with, the manufacturing of uranium fuel for nuclea r energy generation is
simply exp lained. The manufacturing op eration consists o f three p rocesses. T he f irst
process is t he c onversion p rocess of p roducing uranium hexafluoride UF 6 gas fromuranium raw or e. Th e next p rocess p roduces a p owder f orm of t riuranium o ctoxide
U3O8 from the gas. The final process removes impurities and adjusts the concentration
by dissolving triuraniu m octoxide powder in nitric acid. In JCO, this final process wasundertaken. (although some reports indicated that JCO conducted the entire operationfrom the production of the uranium hexafluoride). The quantity of highly concentrated
uranium fu el to b e ma nufactured was sma ll because th ere was only one use r, t heexperimental react or J oyo. Th erefore th e hig hly c oncentrated urani um fu el w as
produced by using the f acilities installed in the conversion test building, since it could
not be processed in the production facilitie s designed for produc ing low concentration
uranium fuel.
The processing method

is as follows. T

o b egin with, t riuranium octoxide isdissolved in nitric acid. Impurities are extracted by the solv ent and then separated in
the precipitation drum. After that, the solution is made into solid purified triuranium
octoxide in the calcination furnace. The solid purified triuranium octoxide is thendissolved again in nitric acid in order to equalize the properties of the product, and it ismade into uranyl nitrate containing 18% U 235. After mixing and equalizing the productin some batch processes, the product is shipped as a liquid c ontaining 18% of U 235. The
criticality accident happened during the process of re-melting and equalization.
Like other processes, p ermission from th e Sc ience and Technology Agency isnecessary for the faciliti es an d operation methods for thisoperation w orking paper isfinal process. The formal

called the “ official manual”. However , JCO changed

facilities an d operation methods a ccording to the “JCO

company manual” with out

obtaining permission from the Science and Technology Agency, since the work involvedin the re-melting and equalization process took a lot of labor and time.

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Failure Knowledge Database - 100 Selected Cases

The p roduction op eration in whic h the acci dent occu rred was atemporary

operation t hat was started s everal m onths after the p revious run. T he p roduction
operation for the process of re-melting and equalization was always a te mporary work,
because the amount of the order for the highly concentrated product was so small. In
this operation, the facil ities and w orking methods were changed again because it was
still troublesome to carry out the operation even using the re-melting and equalizat ion
process in the JCO company manual. The comparison of these three processes is shownin Fig.2.

The final process is labeled as the “illegal

process at the accident” in the figure.

In the official manual, re-melting work is required to be done in the dissolution tower,
and the subdivision is required to b e done using a specially designed container . In t he
JCO company manual, the re-melting work was done using stainless steel buckets andthe subdivision was done usin g a n ormal stora ge t ower. T he st orage t ower was 3. 5m
height and 17.8 cm in radius, and the bottom valve used for supplying the liquid to thestainless steel bucket was mounted only 10 cm above the ground. The operation using
the storage tower was very difficult. Therefore, in the operation that led to the accident,
another change was made to use the stainless steel buckets and the precipitation drum.
Even worse, the p recipitation d rum was attached to a water jacket. The image of the4

Failure Knowledge Database - 100 Selected Cases

precipitation drum is shown in Fig. 3.

There were some absurd misunderstandings in the operation. It had been specified
that 2.4 kg of uranium should beprocessed in a single

batch from solidification to

re-melting in nitric acid. On ly for the equalizing operation using the storage tower, thevolume treated during equalizing, which had been called “a lot”, was seven batches. As
it was the fi rst operation to using the precipitation drum for equalizing, the operators
requested the p ermission to mix s even batche s from th e supervisor. The sup ervisor
consulted w ith a collea gue who h ad finish ed the master’s c ourse i n the nuclear

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Failure Knowledge Database - 100 Selected Cases

engineering department . As the colleague a ssured the supervisor that i t will be saf e,
the superviso r pe rmitted the work. Ho wever, the colleague had a m isunderstanding
about the concentration. He thought the c oncentration was about 5%, the level of the
low-concentration product that was generally manufactured, and m ade his e valuation
without doing any confirmation or special calculation. As a result, after several batchesof the nitri c acid soluti on were added to the drum , a crit ical state was caused, and theuranium in the drum started a runaway chai n reaction. It was as if a small nuclear
reactor suddenly appeared very n ear the workers. A report described that the accident
was like an exp losion of a miniature atomic bomb with a large e mission of radiation at
the beginni ng of the accident. The critical state continued without st opping, and theradiation kept leaking around the facilities.
After receiv ing the r esearch results for st opping the cr itical state fromtheGovernment Field Cou ntermeasure Headquarters, JCO deci ded to organize suicidesquads who had to wo rk ve ry ne ar the pre cipitation drum from which radiation kept
leaking. The suicide squads succeed ed in dr awing off the w ater from the precipitati on
drum jacket, and the critical state stopped. The complete end of the cri tical state wasassured by injecting a boric acid solutionof a neutron absorber into the precipitation

drum. The drawing-off of the jacket water helped the critical state stop because some of
the neutrons, which were created by nuclear fission, were rebounded by the water i nto
the p recipitation d rum and kep t t he nu clear fission oc curring. If th e shap e of the
precipitation drum had beennearer toa circle and if the temp

erature and/or

concentration conditions were more likely to cause a critical state, it would have beennecessary for JCO to destroy the precipitation drum itself or inject a boric acid solution
into the p recipitation d rum in order to st op the critical state. These two actions we remore d angerous than drawing-off work of t he water fr om the jac ket. Th e w ater
drawing-off was an effe ctive measure, because the rebounding of neutrons occurs evenin a water l ayer of only 2.5cm thick, and wh en the neutrons return into the drum, t heprobability of a collision with the atomic nucleus of U235 rises.
Besides, radiation kept leaking while the critical state conti nued. Three operators
who were working near the precipitation

drum were exposed to a he avy do se o fradiation; they had to be hosp italized, and in the end tw o op erators died from t heradiation exposure. In a ddition, employees of the facilities and about 150 neighboring
inhabitants were exposed to harmful radiatio n. The influe nce to the socie ty was also
large. For instance, the inhabitants living within a radius of 350m from the factorywere evacuated, and traffic regulation was executed within a radius of 10km.

6Failure Knowledge Database - 100 Selected Cases

2. Course
2.1

Course of the critical state and surrounding situation
At JCO, purification work of solid tr

iuranium octoxid e started in the

middle of

September, and it e nded on September 28th. The triuranium octoxide was dissolved innitric acid for purification on September 29th, and the work to produce a uranyl nitratesolution wa s started. At this time, stainl

ess steel buck ets were us ed instead of the

dissolution tower that was specified in the manual.
Equalizing of uranyl nitrate also started on September 29th. Four batches of theuranyl nitrate soluti on were injected into

the precipitation drum.

The quantity ofuranium in a 6.5 liters batch was 2.4 kg.
On September 30th, three morebatches of uranyl nitrate were added into theprecipitation drum. At 10:35, a criticality a ccident happened, and three o perators were
exposed to radiatio n. A t 10:43, a J CO employee made an emergency call to the f ire
station and requested an ambulance for patients who showed “the epilepsy symptoms ".
When the emergency team arrived at the site, they collected information, as they f elt
something s trange in th e surrounding state. Th rough th e collect ed information, they
determined that the patients with epilepsy symptoms had been exposed to radiation.
At 1 1:19, JCO made t he first rep ort t o th e S cience and Technology Agency (thegoverning agency for nuclear fuel processing at the time) and at 1 1:22, JCO made thereport to the Ibaraki Prefecture. The Tokai Village Office received a facsimile from JCO
at 11:34.
The ambulance to ok th e rad iation victims to National Mito hospital at 1

1:49.

They were transferred to the Natio nal Institute of R adiological Sciences in the C hiba,
Prefecture, and they arrived there at 15:23.
At 12:15, the Accident Countermeasure Headquarters wa s established in T okai
village. At 12:30, the Tokai village headman requested that the nearby inhabitants stayindoors. In addition at 15:00, he de cided to evacuate the inhabitants w ithin a 350m
radius of the facilities.
At 14:30, the Accident Countermeasure Headquarters was organized in the Scienceand Technology Agency. At 15:00, the Gove rnment Countermeasure Headquarters w asorganized. At the begi nning, the Director Ge neral of Sci ence and T echnology Agency
was its ch airman, bu t later the Pri me Mi nister b ecame the ch airman oftheHeadquarters.
At 22:30, t he Governor of Ibaraki Pref ecture made a recommendation that the
inhabitants within a 10km radius of the facilities stay indoors.
At around 2 2:30, the Government Field Countermeasure Headquarters decided to7

Failure Knowledge Database - 100 Selected Cases

draw off the water in the prec ipitation drum jacket. At 02:35, according to the decision,
two persons of JCO started the draw-off work as the first suicide squad. At 06:04 in thefollowing m orning, th e critical state ceas ed d ue to th e suc cessful d rawing-off of thedrum jacket water by the tenth JCO suicide squad, and the criticality stopped.
At 08:39, the injection of a bo ric acid solution i nto the precipitation drum finishedand the critical state ended at last.
At 16:30, the Governor of Ibaraki Prefecture cancelled the recommendation to stayindoors for inhabitant s in a 10

km radius . Afterwards , investigations and healthexaminations related to radioactive contamination were carried out.
2.2

History of high concentrated uranium fuel processing in JCO
JCO received a license to manufacture powder uranium with a 12% concentrationin November 1980, and received permission to manufacture liquid products with a 20%
concentration in June 1984. JCO manufactured the liquid products using the permitted
manufacturing method until around 1986.
Around 1993, JCO stopped using the dissolu tion tower for the re-melting process
and started to us e stainless steel bu ckets instead. As for th e equalizing operation, thefirst single batch liquid in a stainlesssteel drum was divide d into subdivisioncontainers and the n ext single batch liqui d was also di vided into t he subdivisioncontainers. The same operation was repeat ed, and the equalization was performed insuch a way. This method was the same as the licensed method.
In the operation around 1995, the stor age tower was used for theequalizing

process. It seem ed that this choic e was an attemp t to imp rove th e effic iency of w ork.
In the yea r of 1999 when the accident o ccurred, JCO began using the precipitation
drum instea d of the sto rage t ower in the eq ualizing process. This seemed t o be an
attempt to further improvement in efficiency.
The course of events surrounding the accident is described in detail in "The blue
flash, Tokai criticality accident doc ument" pub lished by Chuou Koron New Company
and edited by the Yomiuri Shimbun editorial office.
3. Cause
As d escribed in the ou tline, th e cause is tha t “they inje cted a u ranyl nitratesolution, including uranium that has overcritical mass, into the precipitation drum

that was originally designed for a different purpose and was not designed in the shapethat is required for criticality safety”. However, this is too ridiculous a reason from theviewpoint of a specialist on nuclear energy.

8Failure Knowledge Database - 100 Selected Cases

U235 is a radioactive material that generates three or four neutrons when it decays.
If more than one of these three or four neutrons causes another U235 atom to decay, thenumber of nuclear decay will increase rapidly in geometric progression. The generated
energy becomes enormous, and many neutrons are radiated. This is called a “criticalstate”. If the critical st ate is controlled, it can be us ed for nuclear power g eneration,and if it is not controll ed, it can become an atomic bomb or a disaster like Chernobyl or
Three Mile Island. These matters are all well understood at l east by persons related to
the nuclear power industry.
In order to reach the critical state, two cond itions are ne eded. First th e number ofU235 atoms must exceed a certain limit. Se
generated n eutrons with an atom

cond, the p robability of a collision ofof U 235 must be high. Considering that neutrons

escape from the s urface of th e fuel canister, a critical state is hardly generated wit h acontainer with a big surface area, in other words a slender container.
Even thoug h it was at the requ est of th e w orking operators, the ac cident was
triggered b y the d ecision of th e manager to change from a slender container to astumpy container with a small su

rface area , and to conduct equalizing work wi

thuranyl oxide with critical mass without any confirmation, or calculation, based on thewrong conviction that the concentration of the material was 5%, though actually it was
18%.
Basically, illegal changes were executed by the company , and the constitution of
the corporate must be a true cause of the accident.
1) The illegal work change in the company se emed to b e t he primary cause of theaccident. Although the permission of the governing legal authorities was required
for any changes concerning the facilitie s and operation methods consid ering thedanger in handling the nuclear reactor fu el, they changed the operation without
any permission. Though JCO recognized this illegality, it had hidden the fact.
2) The final c hange that led to the a ccident was the res ult of the op inion of anengineer wh o se emed no t to hav e a direct r esponsible to th e op eration. He said
that it was “maybe saf e” without confirmation of th e sit e and witho ut p roper
calculations. Based on his recommendation, the permissi on to use the d rum wasdecided by the manage r on th e l ower manag ement lev el, not th e t op of the
facilities, who did not have a right to de cide the change of operation manuals. It
is too careless a decision, which must not be made in any industry.
3) There was no consciousness of the possib ility of creating a critical state in thewhole facilities. It is necessary f or the nuclear industry to have the consciousness
from the beginning to the end, because a critical state can c ause a fatal accident.

9Failure Knowledge Database - 100 Selected Cases

Some people say that the p roposal from the site must be regarded as imp ortantbecause the imp rovement fro m th e sit e is t he bigg est power in J apaneseindustries. However, safety must b e prior t o other items. There must h ave beenboth lowering of morals and a decline the safety consciousness.
There ar e s ome argum ents that t he instructi ons and th e sup ervision of thegoverning legal authorities are the problems, which cause common use of the facilities
and existence of different kinds of facilities in the same building.
4.

Process of the cause elucidation
The injuries were of radiation burns, and it was confirmed t hat the neut ron beam

had sufficient strength to cause the burns. The causes will be clarified if the working
conditions at the accident place are shown by hearing investigation.
5.

Immediate action
1) An ambulance carried t he thr ee in jured p ersons to an e mergency hosp ital afte r
confirming the radiation damage.

After that, they were transported to the

National Institute of Radiological Sciences in Chiba City by helicopter.
2) The following regulations were carried out: evacuation of th e inhabitants within
a 350m radi us of the facilities, recomme ndation for the inhabitants wit hin a 10
km radius to stay indoors, stopping of va rious kind of local traffic facilities, andchanging of the source of water supply.
3) The critical state was st opped by drawing off water from the precipitation drum
jacket and by injecting a boric acid solution into the precipitation drum.
4)
6.

Decontamination work and so on.

Countermeasure
The way to p revent recurrence of t he ac cident is “to op erate th e manufacturing

process according to the prescribed work method at the pr escribed facilities”. Since oneof the causes of the acci dent was that the fac ilities were difficult to run, it is desirabl ethat the facilities be remodeled. However, a s nuclear energy is one of the mostdangerous technologies in the history
construction, and the operation ofof human civilization, the design, thethe fac ilities must b e executed wit h most carefu lattention a nd established techniq ues. Not only op erators but also engine ers andmanagement staff should realize again the basic of the nuclear industry.
“Back to the basis”; this is the only countermeasure to pr event recurrence of th e

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Failure Knowledge Database - 100 Selected Cases

accident.

(Other countermeasures, such as countermeasures for inhabitants are were

not described here.)
7. Knowledge
1)

A thoughtless change of work devices an d work methods may cause the danger.
Not only at JCO but a

lso at other pl aces, similar thoug htless chan ges arerepeated.
2)

Safety confirmation of a work method ch ange just before t he accident was done
easily by analogy witho ut any confi rmation of the situation or calculation. The
supervising officer gave permission for the wo rk m ethod change based o nly on
the analogy. Such a thoughtless action must be avoided, not only in the nuclear
energy ind ustry that hand les enor mously d angerous mat erials but also in allother industries.

3)

From the various reports, it appears th at JCO did not have the basic minimum
level of risk management and of crisis management, which means preparing for
the worst case before

an accident, taking countermeasures, and af

accident, rapidly tran smitting info rmation. It is important to not

ter theify thehighest-ranking person who is related to the accident without reconfirmation ordetailed information, if the accident is predicted to cause serious damage.
8.

Influence of the Failure
As for th e human damage, th ree w orkers w ere hospitalized for serious radiation

damage, an d two of the m subsequ ently d ied. T he t otal nu mber of p ersons exposed toradiation was 150. Furthermore, the inhabitants within a 3 50m radius were ordered toevacuate, and the inhabitants within a 10 km radius had to stay indoors.
As for the social influence, just after the accident, traffic was restricted such as the
JR train system, the expressways, and ot her traffic systems within a 10 km radius.
Also, facilities such as schools and day care centers were tempora rily closed. In themedium ter m, ru mors of radioactive cont amination caused sales of Ibaraki-ma

deagricultural products to decrease. Finally, the failure had a major effect on the national
nuclear energy policy.
Although the monetary damage to JCO fo r the compensation for the deaths andthe injured as well as for the economic damages caused by rumors was large, losses inboth the social status
cannot be measured.

11

and economic valu ation to both JC O and its parent comp any

Failure Knowledge Database - 100 Selected Cases

9. On the sideThis accid ent originate d from the careless ma nagement of a change. Ther e is achange of t he situation when something is modified, but this change of the situation
was not rec ognized. Unlike the acci dent of the bituminized material in the old Power
Reactor and Nuclear Fuel Development Corp., this failure was the result of problems
that could solve only by the techn ology of the nuclear industry. Moreover, the accident
appears to have occurred as a result ofa concession by anunskilled engineer tooperators who wanted working methods easier. There is n o doubt that proposals fromthe op erators such as “ Kaizen (imp rovement)” or “5S” are one of the s ources of th e
power sources of the Japanese enterprises. However, it must not be forgotten that i t isthe d uty of the engineers and the task for t he manage ment sup ervisors to av oid
theoretically dangerous situations and to make improvements in technical studies.
References
・Yomiuri Shimbun editorial office, “The blue flash - Document of T okai criticality

accident”, New Chuo Koron(2000)
・Eiji Oshi ma, “B lind spot of th e accid ent p revention countermeasure”, Saigai no
Kennkyu, 31. 43-49 (2000)
・Yoichi Uehara, “JCO accident, which safety engineer observed” Saigai no Kennkyu 32,
5-13 (2001)
・Mito district court, “ Defendant matters such as the Law for the

Regulations of

Nuclear Source Material, Nuclear Fuel Material and Reactors co ntravention

Lower

court main ruling information”, Mito district court home page (2003)
・Norio Watanabe. Hitoshi T amaogi, “Examples on the criticality accident in overseas
nuclear fuel facility” JAERI-Review 2000-006, Japan Atomic Energy Res. Inst. (2003)
・Kunihide Saso, “Uran ium process ing plan t criticality accident and human factor
J.Japan Soc. for Safety Engineering, 39, 326-333 (2000)

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