Case Ditails

Scenario 100 Selected Cases (PDF)
Case Name Fire and explosion caused due to an abnormal reaction at treatment facilities to solidify radioactive wastes
Pictograph
Date March 11, 1997
Place Tokai, Ibaragi,Japan
Location Nuclear-fuel re-processing factory
Overview On March 11th, 1997. A fire occurred at facilities of the old Power Reactor and Nuclear Fuel Development Corporation for confining low-level radioactivity wastes in asphalt, and an explosion occurred about 10 hours later. The temperature of the bituminized solid rose during an experiment of reducing the charge rate per hour to the extruder. During natural cooling, an oxidation reaction was caused and it ignited. Furthermore, as fire extinguishing was insufficient, the asphalt smoldered, light gas was emitted, and this resulted in an explosion.
Incident A fire occurred at treatment facilities for radioactive wastes, and an explosion also occurred due to the poor response. Low-level radioactive waste fluid was being solidified with asphalt. During filling up drums, an unusual chemical reaction occurred and the drums caught fire. After being filled, the drums were left temporarily in a filling room. The fire was extinguished by spraying water. As the fire extinguishing check was insufficient, an explosion occurred about 10 hours later, and radioactivity leaked. Workers were exposed to radiation at the time of the fire and explosion.
Processing Consumption and usage
Substance Low-level radioactive waste
Asphalt
Type of Accident Fire, explosion, environmental pollution
Sequence About 10:06 on March 11th, 1997. At treatment facilities for radioactive wastes, drums that were filled with asphalt and placed indoors caught fire.
About 10:12. A worker sprayed water for about one minute, and informed the control room that the fire had been extinguished. The radiation and dust monitors in the building indicated rising levels, and workers evacuated from the building.
10:20-10:30. The filter of a ventilator clogged and the exit damper was closed. An attempt was made to start the ventilator manually. However, there was no exhaust function although the fan had started.
20:02. Combustible gas indoors exploded.
The building was damaged and radioactivity leaked.
About 20:40. The space radiological dosage rate rose temporarily at one monitoring point in the facilities.
After 21:00, the dosage rate returned to within the limits of normal fluctuations.
Cause The Cause of the fire.
During natural cooling of the treated solid, as a result of heat accumulating during a slow chemical reaction in a drum, the temperature of the treated solid rose locally. It is assumed that the reaction of nitrate/nitrite salt with asphalt progressed rapidly. The cause of the slow reaction is that the start temperature for natural cooling was high. The temperature rose because the amount of asphalt supplied to the extruder was reduced by twenty percent. This rise is characteristic of an extruder that kneads asphalt and wastes. The appearance of the extruder is shown in Fig2. Radioactive wastes were supplied with nitrate/nitrite salt.
Cause of explosion.
As fire extinguishing with water was carried out for a short time, although the fire appeared to have been extinguished, the insides of the drums were smoldering. Therefore, reactions between an bituminized solid and nitrate progressed, and a flammable substance was emitted from the asphalt. As ventilation was stopped by the fire, the filling room and an adjoining room were filled with flammable material. A combustible gas-air mixture was formed. And it is assumed that the high-temperature bituminized solid ignited again.
Countermeasures The treatment method using extruder tends to cause an exothermic reaction. Moreover, as neither waste fluid nor asphalt is always of the same composition, it is very difficult to control an exothermic reaction. Therefore, this method is not suitable, and other processing methods are desirable.
Knowledge Comment For nuclear safety, the consideration of safety of not only main component technology for a radioactive leak, but all component technology for chemical safety is necessary.
Even if a material is not radioactive, a serious accident such as a radioactive leak can be caused by an explosion and a fire resulting from a chemical substance. If various chemical substances are used, it is desirable to acquire knowledge about their potential danger, and handle them properly. Views from various special fields of study are required to assure safety.
Background 1. In the treatment system, an oxidizing substance and a flammable substance are mixed at a high temperature. Although it is appropriate from the viewpoint of preventing a radioactive leak, from the viewpoint of chemical safety, the potential danger is high.
2. The operating conditions were changed without carrying out an investigation or an examination of the characteristics of the machine to be used. As the extruder was a unique machine with a fixed capacity, the charge rate should not have been changed easily. No action was taken although the temperature of the distilled material from the extruder was rising.
3. In spite of having been specified as eight minutes or more, fire extinguishing with water was stopped after one minute. The check to see if the fire had been extinguished was perfunctory. Furthermore, the plant staff was not in charge of operations, and all of the work was left to a subcontractor. So, there was a problem with the management. The nature of the Power Reactor and Nuclear Fuel Development Corporation at that time is
speculated to be the main cause.
Incidental Discussion It is said that an accident cannot happen easily at core facilities or an area that has been thoroughly studied, and tends to happen at peripheral areas and areas that have not been thoroughly studied.
This accident is a typical example.
Reason for Adding to DB Example of fire and explosion caused due to a small change of operating conditions
Scenario
Primary Scenario Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Organizational Problems, Inflexible Management Structure, Insufficient Education/Training, Ignorance, Insufficient Knowledge, Stuck-up, Planning and Design, Poor Planning, Poor Obsevation Point of Operation, Malicious Act, Rule Violation, Safety Rule Violation, Bad Event, Chemical Phenomenon, Abnormal Reaction, Secondary Damage, External Damage, Fire/Explosion, Bodily Harm, Injury, Loss to Organization, Social Loss, Unification of Organization and Stop the Experiment, Loss to Organization, Economic Loss
Sources Power Reactor and Nuclear Fuel Development Corp., Outline of Bituminization facility fire and explosion, (1997).
Fire and explosion investigation committee of Tokai Nuclear Fuel Reprocessing Plant bituminization facilities. Fire and explosion in bituminization facilities at Power Reactor and Nuclear Fuel Development Corp. Tokai Nuclear Fuel Reprocessing Plant. (1997)
The Ibaraki Pref. Living environment division. Nuclear safety countermeasures section, Survey result on fire and explosion at bituminization facilities.(1998)
Power Reactor and Nuclear Fuel Development Corp., Examination of the cause of fire and explosion. (1997).
Number of Injuries 37
Physical Damage Major damage to an asphalt filling room (R152). Scattering of a shielding wall, a removable roof, and a door. Serious damage to a maintenance air lock room (A235). Damage to equipment, windows, doors walls, etc.
Consequences About 30 minutes after the fire occurred, all members were directed to evacuate. Radioactivity was detected at the exhaust dust monitors of the exhaust port of the 1st attached exhaust pipe, E and Z equipment. Moreover, there were measurement data of radioactivity with suspended dust in the air sampled in the vicinity of the treatment facilities and Oarai area.
Multimedia Files Fig2.Extruder figure
Field Chemicals and Plants
Author ITAGAKI, Haruhiko (Japan National Institute of Occupational Safety and Health)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
Scenario 100 Selected Cases (PDF)