Cause of and Responses to Leak of Radioactive Liquid Waste in Basement 2 Level, Auxiliary Building, Hamaoka Nuclear Power Station Reactor No. 3 (Radiation Controlled Area)
February 25, 2010
Chubu Electric Power Co.,Inc.
Chubu Electric has studied the causes of and implemented countermeasures in response to a leak of radioactive liquid waste that occurred on December 1, 2009 in Hamaoka Nuclear Power Station Reactor No. 3 (boiling water reactor, rated output 1,100 MW)*1.
The results of this review have now been compiled and we are announcing them here.
These results were reported to the Nuclear and Industrial Safety Agency, today.
The Nuclear and Industrial Safety Agency has judged that the incident represents a violation of the relevant safety regulations*2, and has today issued instructions to Chubu Electric.
In accordance with these instructions, Chubu Electric is implementing appropriate responses and working to ensure that the incident does not reoccur.
1 Inspection and investigation results
(1) Results of inspection and survey of equipment
An internal inspection was conducted of the areas in which the leak occurred, the wastewater pit, the wastewater pipes, the high-conductivity liquid waste sump tank (B) ("sump tank" below), to which the liquid waste was being sent, and the concentrated liquid waste storage tank (C) ("storage tank" below), from which the liquid waste was being sent. These inspections showed that insoluble substances such as rust were widely spread over the interior of the wastewater pipes, and had also accumulated on the bottom of the sump tank (B). The wastewater pipe outlet in the sump tank was submerged under the accumulated material. In addition, the concentration of insoluble substances in the wastewater in storage tank (C) was higher than the concentration measured prior to discharge. The concentration was so high that transfer to sump tank (B) was not possible. (Reference 1)
No foreign matter was found in the interior of the wastewater pipes that would cause a blockage of the pipes.
(2) Results of Study of Wastewater Inspection Process
As a result of the malfunctioning of the plastic solidification equipment ("solidification equipment" below) in 2005, from 2006 the wastewater in the storage tank has been completely discharged using the method followed in the present case. Because of this, the processes associated with wastewater discharge at that time and currently in use were investigated. As a result of this investigation, it was discovered that conformance of the procedure to the technical standards required for wastewater pipes in response to the properties of the wastewater*3 had not been verified, and that there had been insufficient checking of sedimentation of insoluble substances in the wastewater in storage tank (C) when the wastewater was discharged in the present incident.
2 Probable causes
The following factors are believed to have caused the leak of radioactive waste liquid. (Reference 2)
<1> Insoluble substances in the wastewater in storage tank (C) settled, and wastewater with a high concentration of insoluble substances remained in the bottom of the tank.
<2> This residual wastewater should have been sent to the solidification equipment, but was discharged through the wastewater pipes, and the insoluble substances accumulated in sump tank (B).
<3> Because the wastewater pipe outlet in sump tank (B) was submerged in insoluble substances, the flow rate of wastewater in the pipe was low, resulting in further accumulation of the insoluble substances in the wastewater, and leakage of wastewater from the wastewater pit.
3 Measures to prevent reoccurrence
(1) Measures to prevent wastewater leaks
The wastewater pipes from storage tanks (A), (B), and (C) to sump tank (B) are used solely for the purpose of discharging cleaning water from the storage tanks, and are not used for the discharge of wastewater, as in the present case.
Given this, the following measures have been put in place in the areas of operation and equipment.
<1> A rule has been established that when a storage tank is inspected., the wastewater in that storage tank must be sent to the solidification equipment or stored in another storage tank using a special temporary facility. Measures to prevent leaks while the wastewater is being sent from the tank, will be reflected in operational guidelines. (Operational measure)
In addition, operational guidelines will state that when the cleaning water in the storage tank is discharged into sump tank (B) via the wastewater pipes, that cleaning water must have been sufficiently diluted. (Operational guideline)
<2> The wastewater valve in the wastewater pipe located upstream from the wastewater pit will be kept locked and the wastewater pipe will be isolated between the wastewater valve and the wastewater pit, preventing normal use. The isolated section of pipe will be plugged in order to prevent leaks. (Operational and equipment measure)
Other tanks that contain wastewater with a high concentration of insoluble substances, like the tank from which the present leak occurred, will be inspected and the same measures will be taken as necessary.
(2) Measures related to the inspection process
In consideration of the fact that liquid waste was discharged without verification of the conformance of the procedure to the technical standards required for wastewater pipes in response to the properties of the wastewater, the question of why the systematic verification of the decision-making process failed to function will be subjected to a root cause analysis ("RCA" below), and measures will be put in place after the factors necessary to prevent a reoccurrence are identified.
4 Receipt of instruction document from the Nuclear and Industrial Safety Agency regarding the violation of safety regulations
Safety regulations stipulate that the wastewater in the storage tank must be solidified in a special container using the solidification equipment, and stored in the solidified waste storage building.
The present incident, in which wastewater with a high concentration of insoluble substances, which should have been sent to the solidification equipment, was discharged into the wastewater pipes, is contrary to the method of treatment of wastewater stipulated by the safety regulations. Because the wastewater was sent to the high-conductivity liquid waste treatment system using the wastewater pipes, the Nuclear and Industrial Safety Agency has issued a notification that Chubu Electric violated the relevant safety regulations, was negligent in observing conformance with technical standards, and failed to comply with legal requirements. Chubu Electric has been instructed to report on the findings of its RCA and the measures put in place to prevent a reoccurrence of the incident.
The present violation of the safety regulations falls under the category of Operating Data Table 2-14*4.
Based on these instructions from the Nuclear and Industrial Safety Agency, Chubu Electric will formulate the necessary measures to prevent a reoccurrence of the incident based on the results of the RCA discussed in Section 3 (2), and will implement these measures in an appropriate manner.
*1 On December 1, 2009, in order to conduct an inspection of storage tank (C) on basement level 2 of the auxiliary building of Hamaoka Nuclear Power Station Reactor No.3 (within the radiation controlled zone), liquid waste in the storage tank was discharged into sump tank (B), via the wastewater pit, from the discharge outlet at the bottom of the tank. The liquid waste backed up, and leaked from the wastewater pit and three other wastewater pits connected by the wastewater pipes. Liquid waste containing radioactive substances leaked over an area of approximately 1.7 m2 on the floor of basement level 2 of the auxiliary building (within the radiation controlled zone).
*2 The safety regulations for nuclear facilities are government-certified regulations that specify guidelines that must be observed by the operators of nuclear power plants for the safe operation of the plants, based on Article 37, Paragraph 1, of the Nuclear Reactor Regulation Law.
*3 The relevant technical standard is the Ministerial Ordinance to Provide for Technical Specification of Nuclear Generation Facilities.
*4 Operating Data Table (Information List that mutually agreed between regional government and Chubu) 2-14 applies when there is an irregularity in relation to criteria for judgment of inspections, and when suggestions have been made on the basis of a safety inspection.