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Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Sat, 09-02-1991
ERF description: Fuelling machine failed to unlock and remove the shield plug. The shield was manually removed by draining the fuel channel. During the operation, no person was exposed to high radiation fields.
Event Description:
The shield plug is inserted in both ends (outlet and inlet) of a fuel channel and has to be removed for fuelling operations. Its function is to reduce the dose rates at the channel ends and to direct the coolant flow in the coolant tubes. The sticking of shield plug would prevent fuelling of particular fuel channel. If a shield plug remains stuck for an extended duration there is a possibility of fuel damage (or even failure) due to a burnup beyond permissible level as a result of prolonged stay in the core.
On 20th April, 1990, the north shield plug in the reactor channel K-05 got stuck during plant run-up and the fuelling machine failed to un-lock and remove the shield plug inspite of two attempts. The plant was shut down on 23 September, 1990, and another attempt was made to unlock and remove the shield plug with the maximum permissible increased torque, which remained unsuccessful. Finally, it was decided to drain the channel and remove the plug manually by detaching the liner which contained it. This operation was successful after which the reactor headers were filled and the cooling was restored to normal. During this operation no person was overexposed.
Safety Implications:
The sticking of shield plug has no bearing on the functions vital to the safety of a plant such as:
1. Safe reactor shutdown; 2. Cooling of fuel; 3. Confinement of radioactive products.
In fact the shield plug can not be placed in the category of safety systems required to maintain the integrity of the barriers against radioactive release (i.e. Emergency Injection System, Containment Sprays and Filters, etc.). Moreover its failure does not pose any threat to the functioning of such systems in case of an event (as that of Emergency Injection in case of LOCA).

BASIS FOR RATING: The philosophy of INES for an event to be classified under the Defence-in-depth criterion is to identify that whether an initiating event has actually occurred or a particular safety system has been degraded which was designed in order to limit the consequences of that event (postulated initiator in this case) to an acceptable level.
In this particular case neither an initiator has actually occurred nor an event (even the fuel damage or failure) can be postulated in which there could be any role of the shield plug in limiting its consequences.
On the basis of above, the event under consideration has no serious safety implications and, therefore, may be rated as below scale on INES.
DIFFICULTIES IN RATING:
ERF rating date: 01-01-1991
ERF contact person: Mr. N. Maqbool
ERF address: P.O. Box 1114, Islamabad, Pakistan
ERF affiliation: Pakistan Atomic Energy Commission, Directorate of Nuclear Safety and, Radiation Protection

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