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Vol.1 No.3 previous GA 8 - 9 - 10 - AA 10 - NT 14 - 15 - 16 - 17 - 18 next Vol.2 No.1
General Articles
Vol.1, No.4, GA9
 

Regarding the "Casebook of Troubles
At Rokkasho Reprocessing Plant"

 
Haruki AOYAGI and Takeyoshi KIMURA
Japan Nuclear Fuel Limited
 
 

1. Introduction

Rokkasho Reprocessing Plant is a facility where uranium and plutonium are recovered from spent fuel generated at Japan's nuclear power reactors. For the reprocessing of spent fuel, there are already track records at AREVA NC in France and JAEA (Japan Atomic Energy Agency) in Japan, and on the basis of their experience, Japan Nuclear Fuel Ltd. constructed the first commercial reprocessing facility in Japan, and commenced a series of tests from 2001.

In implementing uranium tests at Rokkasho Reprocessing Plant, a study has been conducted regarding concept of the public announcement of accidents and troubles. It has been decided to promptly make public announcement even for the minor ones, and at the same time, to announce widely regarding the actual operating situation of the reprocessing facilities.

Following up on this decision, we examined information regarding accidents and troubles which occurred previously at our predecessors' plants, and selected in advance such incidents which may likely occur during uranium tests, and finally compiled a "casebook" of 130 troubles, etc. Then, while implementing the test, we continually added new cases, and try to make the contents as substantial as possible. Here we explain this "Casebook of Troubles".

2. Outline of Rokkasho Reprocessing Plant

Rokkasho Reprocessing Plant extracts and recovers uranium and plutonium through treating spent fuel by chemical agents (nitric acid, organic solvents, etc.). Storage of recovered nuclear materials is handled in a stable, solid body. The plant is designed in the way that its consequence does not impact the entire operation in case of a stop of a process by installing a part of equipment and systems redundantly and placing buffer vessels (storage vessels) in between each process. Outline of the process is shown in Figure 1.

EJAMGA1-4-GA9_Fig.1_Outline_of_Rokkasho_Reprocessing_Plant
Figure 1 Outline of Rokkasho Reprocessing Plant

3. Tests carried out at Reprocessing Plant

At Rokkasho Reprocessing Plant, an operation test has been carried out step by step, in conditions resembling the actual commercial operation. Contents of Tests are shown below.

During "Water Test", tests have been carried out by using water and steam, confirmation of movement and performance of each equipment and system has been carried out, and the transfer capacity of transfer equipment and stirring capacity of stirrers have been also confirmed.

During "Chemical Test", tests have been carried out by using chemical agents such as nitric acid as follows, movement and performance of each equipment and system has been carried out, movement and performance of density meter、flow meter and etc. have been confirmed. At the same time, it has been also confirmed that safety functions such as the effectiveness of diluting agent for fire/explosion prevention.

During "Uranium Test", by using uranium which is the principal ingredient in spent fuel treated at the time of commercial operation, it has been confirmed that performance of equipment and units which can be confirmed only by nuclear material, and also that equipment and process possess confinement and criticality safety functions related to each process of shearing/dissolution, separation, purification, and denitration, etc.

During "Active Test", by using the actual spent fuel, it has been confirmed that the safety function of reprocessing process and the performance of equipment/units relative to the treatment of Plutonium and fission products which could not be confirmed during the preceding tests. So far, confirmation of the safety function and the performance of equipment and units have been almost completed, while at present, the recovery activities of troubles occurred in HALW Vitrification Facility are being carried out.

Test Steps
[Water Test] [Chemical Test] [Uranium Test] [Active Test] [Commercial Operation]
Started in 2001   Started in 2002   Started in 2004   Started in 2006   Start scheduled in 2010

4. Regarding compilation of the Casebook of troubles likely to occur at Rokkasho Reprocessing Plant

At Rokkasho Reprocessing Plant, efforts are made in design, construction and operation test to improve safety by well taking into account of the know-how obtained from accident/trouble information involved at the predecessors' plants in and out of the country, and of their countermeasure experiences.

At the basic design stage, the lessons from the main accidents and troubles which have previously occurred have been reflected in safety design, and confirmation for its validity has been made during safety examination.

Still after the basic design stage, with regard to the JCO criticality accident, re-confirmation for protective measures for similar accidents and safety confirmation have been made at each timely opportunity, and all possible measures for nuclear safety have been taken.

Further, from 1990 onward, efforts are being made to obtain information relative to accidents and troubles from the view point of safety operation as a commercial facility. Concretely, by establishing technical tie-up with the predecessors' reprocessing plants, mainly their accident/trouble information and operation information are continuously obtained, and such information is properly reflected on unit improvement as well as in operation procedures and maintenance procedures. (Refer to Figure 2)

EJAMGA1-4-GA9_Fig.2_Reflection_of_past_operation_accidents_and_troubles_to_RRP
Figure 2 Reflection of past operation accidents and troubles to RRP

On the other hand, we should note the fact that, even at the predecessors' plants rich in experiences, it is very difficult to completely eliminate troubles to zero in reality. With this notion, we considered that it should be very important to study of the nature of possible troubles and approaching the countermeasures to them before the start of uranium tests involved with the handling of nuclear material. With this respect, we have evaluated the impact which trouble cases at the predecessors' plants may cause to our environment, on the basis of information we have obtained. Since the countermeasure at the occurrence of trouble depends heavily on the magnitude of the particular trouble, we have made our evaluation taking note of the International Evaluation Scale for Nuclear Facilities, etc. (hereinafter referred to as "INES"). Until now, almost all of the troubles are very minor troubles of either level 0, or out of scope of evaluation. (Figure 3)

EJAMGA1-4-GA9_Fig.3_Trouble_Incidents_at_the_predecessors_plants
Figure 3 Trouble Incidents at the predecessors' plants

Since there are numerous equipment and units at Rokkasho Reprocessing Plant, troubles such as malfunctions were well within the scope of our assumption, so we have exhaustively put in order the content and the method of treatment for troubles of INES Level 0 or below, which were possible to occur during Uranium Test.

Regarding the contents of these troubles, they have been compiled in a form of a trouble casebook so that that the residents in the local community of the Reprocessing Plant may better understand the actual status of the Reprocessing Plant, and as well as for the purpose of a right presentation of information. In this way, it has been decided to make the contents public.

The composition of its contents include outline of the incident, measures at the time of occurrence, and impact on nuclear safety. They are internally being used as a manual for coping with troubles, and also as an education material.

In preparing the Casebook, cases at the predecessors' plants were used as reference, and 130 cases have been organized first in taking into account of facility’s structure, materials, etc. handled at Rokkasho Reprocessing Plant during Uranium Test, and up to 190 cases have been added during the course of Uranium Test.

Later, addition of cases and supplementing of insufficient explanations have been done to enrich the contents by making use of experience at the test operation. At the start of Active Test, 229 cases have been put in order.

5. Scope and Classification of the "Casebook of Troubles"

Here are explanations in the way of thinking behind editing and some concrete examples of cases introduced in the "Casebook of Troubles".

5.1 Scope of cases chosen

On the basis of information of troubles occurred in the predecessors' plants, minor equipment malfunctions which are difficult to avoid have been selected including those which have occurred during operation, such incidents which are expected to occur at Reprocessing plant, as damage to the shearing machine blade, or clogging of piping.

5.2 Classification of cases

On the basis of accidents and troubles information of the predecessors' plants, we classified them according to the nature of incidents, and as a result, we classified expected troubles of Rokkasho Reprocessing Plant into 9 categories as follows: "clogging・accumulation (45cases)","Leakage (21cases)", "malfunction of equipment (63cases)", "malfunction of measurement/control systems (25cases)", "abnormality of power sources (7cases)", "contamination (23cases)", “breakage (16cases)", "fire (2cases)", "others (27case)".

5.3 Format of the "Casebook of Troubles"

The format of one incident per page is provided to put the following items, an explanatory diagram, an outline of the incident, impact caused by the incident and the method of its recovery, the INES classification level, information category which shows the grave or non-grave nature of the incident, and the classification according to 5.2 above is arranged by facility (refer to attached information).

Attached 1: Potential troubles likely to occur at Reprocessing Plant and their countermeasures (No.1-15)
[Case: "Clogging In Instrumentation Piping of Dissolution Feed Vessel"]

EJAM1-4-GA9-Attached1-Potential_troubles_likely_to_occur_at_Reprocessing_Plant_and_their_countermeasures

Attached 2: Potential Troubles likely to occur at Reprocessing Plant and their countermeasures (No. 2-22)
[Case: "Leakage of High Active Liquid Waste in Vitrification Cell"]

EJAM1-4-GA9-Attached2-Troubles_likely_to_occur_at_Reprocessing_Plant_and_their_countermeasures

Attached 3: Potential troubles likely to occur at Reprocessing Plant and their countermeasure (No.3-02)
[Case: "Fuel Assembly Gets Stuck in the Tilting Crane While pushing In"]

EJAM1-4-GA9-Attached3-Potential_troubles_likely_to_occur_at_Reprocessing_Plant_and_their_countermeasure.png

When a trouble occurs, what is the impact outside the plant, how is the impact on safety, what is the impact on production management, and what is the impact on other processes, - all these points are answered and indicated. Further, an explanatory diagram is given in order to help understanding of that incident.

First, regarding the impact outside the plant, we can confirm that there is no impact due to discharge of radioactive material outside the plant by checking out that the confinement system in the building or the room installed the troubled equipment is functioning properly. This item is evaluated in advance and is so described.

Regarding safety, upon careful examination of an equipment status the description of whether or not there is impact on safety is mentioned.

Further, in case of occurrence of minor equipment breakdown, securing of safety and recovery is attempted in accordance with the pre-defined procedures, in the same time their contents have been also described. Concretely, this means that when any equipment is in trouble, a judgment is made first on whether or not an operational problem exists in the unit with the equipment concerned. In case there is no impact on the unit, it becomes possible to recover the equipment which caused the trouble while continuing operation. Also, even in the case that there is problem over operation of the unit, it is possible to recover the equipment which causes trouble by switching system if there are multiple systems. In the case that the equipment which caused the trouble has one system only, the recovery should be done by stopping that equipment. Therefore, there is the case that the impact extends over a wide range of the unit. The sphere of this impact is determined based on how much buffer capacity remains in the equipment up-stream and down-stream. The length of stoppage in this case can be prolonged, and it becomes necessary to stop wider range of the unit.

On the basis of the ideas mentioned in the above, the recovery patterns in the "Casebook of Troubles" are classified into following five cases and described accordingly:

  1. recovery while continuing operation of the equipment concerned by cleaning and adjustment,
  2. switch to the reserve equipment, and repair the equipment in trouble while operating the reserve in case of breakage of an equipment which has a reserve,
  3. stop only the equipment concerned, and repair the stopped equipment while continuing to operate the up-stream and down-stream equipment,
  4. stop the whole unit including the broken-down equipment, and repair the broken-down equipment while continuing to operate the up-stream and down-stream equipment, and
  5. stop all units which are impacted by the broken-down equipment, and repair the broken-down equipment.

Finally, when such troubles as mentioned in the above occur, accidents and troubles including minor ones are all made public including the information and announcement category which indicate the gravity of the incident.

The basis of information category and announcement category in case of occurrence of trouble are shown in Table l.

Table 1 Information and Announcement Category

Information Category Announcement Category
Category A Information (Inform immediately) statutory incidents based on Article 19-16 of the rules relative to spent fuel for reprocessing business, incidents which may have social influence prompt press release, regardless of weekend and holiday
Incidents similar to above though not urgent basically, same as above, but in case of occurrence at night, basically, press release on next morning
Category B Information (Inform promptly) incidents which could become A category due to evolution of the incident basically, announced on web-site next afternoon, (however, in case next day is holiday, on the nearest workday)
Category C Information (For reference) minor malfunction, etc. which need to be specially informed same as above
O
t
h
e
r
s
Operational info ・ daily info. very minor equipment malfunction, etc. which occurs during normal operation announced on web-site next afternoon (in case holiday, on the nearest work day)
Operational info. ・ monthly info. ・clogging or stoppage of equipment which cannot be prevented despite normal operation or maintenance
・non-conformity, main maintenance/repair tasks
summarized monthly, and place on web-site

6. Conclusion

In spite of preventive efforts towards trouble and accident, there is the fact regrettably that it is impossible to eradicate troubles and malfunctions completely. For this reason, we contemplated compiling in advance a casebook of concrete countermeasures against potential troubles. This contributes not only to make safe and prompt action for countermeasures possible, but also to promote understanding of the reprocessing plant for the residents in the local community as well as giving them sense of assurance and security.

We intend continuously to strive towards adding new cases to the "Casebook of Troubles" and making it more comprehensive and complete.