Failure Knowledge Database - 100 Selected Cases

Fire and Explosion at Bituminization Facility in Power Reactor and Nuclear Fuel
Development Corp.
【March 11th, 1997

Tokai, Ibaraki, Japan 】

Haruhiko Itagaki (National Institute of Industrial Safety)
Mitsuo Kobayashi (Graduate School of New Frontier Sciences, University of Tokyo)
Masamitsu Tamura (Graduate School of New Frontier Sciences, University of Tokyo)

At around 10:06 on March 1 1th, 1997, at th e Japan Nuclear Cycle

Development

Institute T okai Nuclear Fuel R eprocessing Pl ant (forme rly the Power R eactor andNuclear Fuel Devel opment Corp.), a fire occu rred during the filling of a drum can at
the bituminization facilities of the plant.

At the facilities, a treat

radioactive waste from the reprocessing p lant in which

ed solutionofthe radioact ivity level wascomparatively low was mixed with asphalt, heated and dehydrated by an extruder. The
fire was extinguished by spraying

water. However , at around 20:04 after about ten

hours had passed, an explosion and fire occurred in the extr uder room that was left as
it was after the previous fire, where the filled drum can had been left. It seems that thecause of the fire is that the coolingafter the fire extinguis hing was insufficient. As aresult of th e explosion, the build ing and equip ment wer e damaged, and ther e wa s aleakage ofradioactive materials.

However, no actual harm resulted because

maximum l evel of radiation to which thethepublic and the workers were e xposed wasmuch lower than the annual dose equivalent determined by the government.
The operation that led to the ac

cident was not a nor

mal op eration. It wad

conducted fo r the purp ose o f de veloping the technology for im proving the vo lume
reduction rate in order to reduce the quantity of bituminized material.
Reprocessing means a p rocess to manufacture the uranium and plutonium fuel byprocessing of the spent nuclear fuel from the nuclear power plant.
1. Event
Bituminization facilities perform solidification treatment of radioactive liquid wastethat arises f rom a reprocessing plant in wh ich the radioactivity level is comparatively
low by mixing the radioactive liquid waste with asphalt.
The ext ruder room and the drum

can f illing room, which comprises

the cent ral

part of the f acilities, are shown in Fig.1 as the place where the fire occurred, and t heoutline of the treatment process is shown in Fig.2.

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Failure Knowledge Database - 100 Selected Cases

The facilities receive various kinds of radioactive waste. The waste liquids received
by the facilities at the time of the accident were a phosphoric acid waste liquid from the2

Failure Knowledge Database - 100 Selected Cases

waste solvent processi ng te chnology develop ment facilitie s, and a low radioactivity
concentrated waste liquid from th e waste disposal site. T he concentrated waste liquid
was stored in a waste li quid receivi ng tank since the begin ning of the op eration th at
led to the a ccident, and a small qu antity of ph osphoric aci d waste liquid was pou red
into the storage tank during the operation.
The received waste liqu id was trans ferred to the reactor, and its pH wa s adjusted
and the ins oluble tr eatment of cesium, st rontium and iodi ne in the w aste liquid was
executed. The treated waste liquid was fed to the extruder via the supply drum. In the
extruder, th e waste liquid p assed through tw o treatment p rocesses: d ehydration andmixing with asphalt. The outline of the extruder is shown in Fig.3.

In the extruder, raw materials asphalt and the waste liquid were fed and extruded
in the downstream direction by the rotation of the screw shaft, and at the same ti me
the materials wer e h eated by shea ring str ess, causing the moisture in the feed toevaporate. The condensers, which were mounted at the top of the extruder, condensed
and rem oved the st eam. In th e latt er p art of t he extruder, the mixing of th e asp haltwith nitrate and other materials in the waste liquid was carried out, the asphalt

andnitrate/nitrite mixture was discharged in a molten state fr om th e outlet at the final
stage, and the mixture was filled i nto the dr um cans on t he turntable. After natural

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Failure Knowledge Database - 100 Selected Cases

cooling, the filled drum cans were capped in the filling room , and they were stored in
the storage cell of the bituminized material storage facilities.
In the schedule from Fe bruary 7th to April 25th, s ome t est runs were planned.
On March 11th, a t est of the improvement of the volume reduction rate was e xecuted.
In the test, the fe ed rate of th e wa ste liquid to the ext ruder was d ecreased and t heevaporation rate of the moisture in the extruder was increased. Thus the volume of thegenerated bituminized material was d ecreased for th e sa me fe ed rat e of th e wa ste
liquid.
The test was performed by lowering the waste liquid feed ra te first from 200l/hr to
180l/hr, and then to 160l/hr . The supply rate of asphalt was lowered in proportion to
the waste li quid feed ra te. This was the first t est in which the waste li quid feed ra te
was lowered to 160l/hr for low radiation

concentration waste liquid including

phosphoric acid waste liquid.
At around 10:06 on March 1

1th, during thetest run, a fire oc curred at thebituminization facilities. A pillar of fire about 2m in height rose from a drum can filledwith the sol idified material in the f illing ro om, and immediately after t hat, pillars offlames rose from many nearby drum cans. Upon receiving the direction to extinguish
the fire using water spray from a member of the staff of the Power Reactor and Nuclear
Fuel De velopment Cor p., at around 10:12, the subcontra
sprayed water int o t he fillingroom forabout onector workers at the si

minute. Since the firetewasextinguished at around 10:13, the water spray was stopped.
As a result of this fire, the radioactiv

e material that h ad been sealed in thebituminized material spread inside this building and into t he adjoining buildings, a nd
a rise in the radiation level was confirmed by the alarm. However, the abnormally highlevel of radiation was not observed outside the building by environmental monitoringat that time.
The ventila tion system of the facilities

stopped working after the fire, and theventilation of the facilities became impossible.
At around 20:04, about ten

hours after th e f ire, an explosion oc curred at thefacilities. As a result of this explosion, th e radioactive material was discharged to theenvironment from openings such as windows of the building.
The order t o evacuate the facilities was

issued about thi rty minutes after the

explosion in order to av oid exposure of the wo rkers to radiation, and all field workers
were evacuated. The confirmation of the l evel of radiation exposure was conducted on
the 129 workers who were in the building wher e the accident occurred, near the fire, orat the explosion. A very low level of 134Cs and 137Cs were detected in 37 workers, but all

4

Failure Knowledge Database - 100 Selected Casesof the detected values were under the national safety standard.
Finally, an off-limit zone and a restricted zone were established in the building inorder to regulate the entrance to the rad ioactive c ontamination zone, and outsid e thebuilding, the areas around the bituminization facilities were temporarily designated ascontrolled areas.
2. Course
On March 6th, the waste liquid feed rate to the extruder was lowered to 160l/hr.
At around 10:06 on March 1 1th, a worker found a light from the filling room, andhe confirmed through t he peep-hole that a pilla r of fire rose from one filled drum can.
After less than a minute, the worker observed that all of the drum cans began to burn.
The automatic fire alarm rang at 10:10.
At around 10:12, the worker started to exti nguish the fire by spraying water . He
stopped spraying water at ar ound 10:13, because he could not see any fire through t hepeep-hole.
At around 1 0:13, dust monitors st arted ringing. In a ddition, at around 10:23 theventilator stopped as the outlet damper remained closed.
At around 20:04, the explosion occurred.
3.

Cause
The main c ourse of th e fire is asfollows. A s a result of

the d ifference of th eproperties of the waste liquid and the low ering of the feed rates of asp halt and waste
liquid, the temperature of the mix

ture of the asphalt and the nitrat e/nitrite at t heoutlet of the extruder rose. During the natural air-cooling stage of th e process, a sl ow
exothermic reaction was generated, which caused heat to a ccumulate in the dru m can,and then a runaway reaction, in which the exothermic reaction between nitrate/nitrite
and asphalt progressed rapidly, was induced. By the exothermic reaction, the local heataccumulation at the center of the drum was accelerated, and the temperature became
high, causing the asphalt to d ecompose. Finally a combustion reaction between air andthe asphalt decomposition material was generated, and the fire occurred.
The bituminization treatment was carried out through the following method. First,
asphalt, wh ich is a mixture of combusti ble organic substances incl

uding varioushydrocarbons, and the r adioactive waste liquid to be treated, which cont ains oxidizing
materials such as nitrate and nitrite, werecharged toget her to the extruder . In the

extruder, the two charged materials were dehydrated and mixed at a high temperature.
Then they were filled i nto drum cans and we re solidified b y natural cooling. However,5

Failure Knowledge Database - 100 Selected Cases

as the oxidizers and the combustible materials were left together at a high temperature,
there was a high possibility of causing an exothermic reaction. In addition, as both t heasphalt and the radioactive waste liquid are mixtur es of many kin ds of ch emical

substances, control of the thermo-chemical reactions is very difficult.
Therefore, while the bitumini zation processin g system is generally considered an
appropriate method from the vi ewpoint of p reventing the l eakage of ra dioactivity, thepotential d anger is larg e fr om th e viewp oint of ch emical safety , s o that sufficie nt
knowledge regarding ch emical characteristics of the bituminization processing syst em
is required f or ensuring the safety of the treatment. A basic fact or l eading to the theaccident might be the insufficient consideration of the chemical safety.
The cause of the explosion seemed to be the insufficiency of the extinguishment of
the first fire. Although as a g eneral rule, water spraying sh ould be cont inued for overeight minutes, the spra ying was st opped afte r only on e minute because the fire could
no longer be seen. Therefore, cooling of the inside of the drum was insufficient, and theflammable materials were f ormed from th e bituminized material. The first fire caused
the cell vent ing system t o stop, s o the fla mmable materials a ccumulated in the fillingroom and leaked to the adjoining room. A combustible gas-air mixture was formed bymixing with the air from the v
gas-air mixture was ignited by thentilation system of the facilities. The combustiblee heat ca used by an exother mic r eaction of t hebituminized material i n the drum canon the turntable, and flames reached theextruder room via the duct causing the explosion there.
4.

Process of cause elucidation
It appeared certain tha t the dru m, in which t he first infl ammation occurred byheat generation was filled with bituminized ma terial of a certain batch during the test
operation period from the conditions of the combustion and the estimated combustion
occurrence time. In addition, it was estimated that the waste liquid of the batch or theoperating conditions related to the treatment of the batch affected t o the accident.

From the p roperties of the waste liquid , it was estimated that the reactivity with theasphalt was higher tha n that of the conven tional waste liquid; this was also supposed
to be on e of the causes of the accid ent. However, the effe ct of the p roperty was d enied
as a result of the sup porting experiment. Beside the high t emperature of the extr uderoutlet was regarded as another cause of the accident.
The temperature rise was estimated with a high probability from the observationsof the worker, the indications of the thermometer, and so on. Moreover, an even larger
heat gen eration was p redicted to h ave oc curred as a result of changin g the fe ed r ate

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Failure Knowledge Database - 100 Selected Cases

and the d ifference in the p roperty of the was te liquid , a nd it wasproven by t hesupporting experiment.
5. Immedia te action
Spraying of water at the first fire was the only immediate action that was executedduring the whole chain of ev ents. Measur es such as th e medical examination ofradiation exp osure an d evacuatio n of the w orkers fr om the build ing wer e ta ken,because it was an accident related to nuclear energy.
6. Countermeasure1) Innovations of the management system of the Power R eactor and Nuclear Fuel
Development Corp: In addition to the techni cal problems of the bituminized material,
there were some problems in the management system of th e Power Reactor a nd
Nuclear Fu el Dev elopment Corp.

According to the research

report by Ibaraki

Prefecture, “About p roblems of the safety management sy stem and d irection of t he
improvements”, there were ma ny points concerning of th e constitution of the Power
Reactor an d Nuclear

Fuel D evelopment C orp. itse lf from the viewpo ints o f thefollowing: prevention of the occurrence of the accident, prevention of its escalation, andinformation transfer transmission at the ti me of the accid ent. Here, s ome points arequoted from the problems concerning the prevention of the accident:
a. There

was insuffi cient shari ng of

the techni cal information because theorganization of the P ower R eactor an d Nucl ear Fu el D evelopment C orp. had
become too rigid.
b. In the operational control aspect , as the Environment Facility Department tha t
was in charge of it was isolated from th e site, the department paid little attention
to the problems at the site. The op erational control of the solidification disposing
facilities was left to thesubcontractor only, as ment ioned above, and the Power

Reactor and Nucle ar Fue l De velopment Corp. did no t m ake any atte mpt to grasp
the operating state.
c. Regarding the s ecurity management system , concern for education and training of
the subco ntractor’s e mployees was insufficient, the ap proval method s in making
changes to the operation plan from the conventional procedure were indefinite, theevaluations of the safety at changes of the operation plan were not sufficient.
2) The bituminization method cannot be re garded as a chemically saf ety method as
described in the “cause” section. It is nece ssary to take sufficient countermeasures f or
the facilities and operation acquiring accurate knowledge of chemical safety.

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Failure Knowledge Database - 100 Selected Cases

7. Knowledge
1)

For ensuring the nuclear safety , it is necessary to consider safety from theviewpoint o f not only t he l eakage of rad ioactivity, whic h is the main el emental
technology, but also chemical safety of all other elemental technologies.
If an explosion and fire occurs

as a re sult of problems related to chemi

cal

substances and so on, not only damage from the explosion and fire but also damage
from the leakage of radioactivity is caused, and the accident could become a majordisaster.
2)

Various chemical substances are us

ed in nuclear power fa

cilities. The

understanding of the potential risk of the chemical substances is a basis of chemicalsafety, and the appropriate handling must be done based on this understanding.
3) Considering the siz e of the drum ca n, only the surface will be cooled by about one
minute of c ooling fr om the surfac e, and the cooling of the insid e is i mpossible. I ttakes time t o cool t o the inside. Also, he at accumulation ca nnot be avoided if there
is some heat generation in a container with a si ze above the critical threshold. This
fact can be understood if it is considered that heat generation is proportional to thevolume of the container, which is p roportional to the "cube of the length of a sid e",
and the coo ling is p roportional to t he surface area, which is p roportional to the"square of the length of a side". This fact is also applied to t he accident occurred in
Mie P refecture, whe re a fire a nd exp losion o ccurred in alar ge tank o frefuse-derived fuel (R DF) that was used for p roducing electric p ower by making
solid fuel from the garbage.
8. Influence of failure
1) As for the physical damage, the b ituminization facility buildings were severely
damaged.
2) Although 37 persons where exposed to radioactive material, the level of exposure
was low in all cases, and there was no actual harm.
3) The biggest damage must be the l oss of social confidence in the safety of nuclear
energy.
4) The bituminization facilities were stopped.
9. On the side
The extruder is a rotating machine that is generally used in the pelletizi ng processof synthetic resin such as polyolefin. However it is a rather special machi ne, so it must

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Failure Knowledge Database - 100 Selected Cases

be used with care. Due to its

characteristics, a change of outp ut is performed bychanging th e operation time not by changing th e fe ed rat e t o th e machine. Wh en t he
feed rate is lowered, it is usual to decrease the rotation frequency of the screw after thetests are repeated, because flow conditions in th e extruder change when the flow ratechanges.
Moreover, it may have b een a bad choice from the viewpoint of the chemical safety
to have used the extruder for mixing nitrate/nitrate and asphalt because its control for
safety is very difficult.
In the twen ty-first century, large accidents appear to occur frequently. The issues
raised in 1) of the c ountermeasure section should be generally true not only for thePower Reactor and N uclear Fuel Development Corp. but also f or the entire indust rial
world in our country.
References
・ Nuclear Safety Count ermeasure Section, Living Environment Division, Ibaraki
Prefecture Government, “Surv ey result on f ire and ex plosion acci dent atbituminization body disposing facility - Safety management system and directionof the improvement” (1998)
・ Power Reactor and Nuclear Fuel Development Corp. "Examination of the cause of
the fire and explosion" (1997)
・ Fire and e xplosion accide nt inve stigation committee on bituminized material
disposing facility at T okai Nuclear Fuel Reprocessing Plant, “Fire and explosion
accident in t he bituminization facility at Tokai Nuclear Fuel Processing Plant inPower Reactor and Nuclear Fuel Development Corp” (1997)
・ Power R eactor and Nu clear Fu el Development C orp. “Th e outlin e of fire andexplosion accident in the bituminization facility” (1997 )・ Power Reac tor and N uclear Fuel Deve lopment Co rp., “Fire andexplosionaccident in the bituminization facilit y at Tokai Nuclear Fuel Reprocessing Plant”
Kindai Shobo, extra ed. July 2000, 129-133. (2000)

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