Radiological incident occurred in the fuel element auxiliary pool, involving Co-60 withdrawn from an adjuster unit
Tuesday, July 22, 2003
On July 1st 2003, a B(U) type transport package, loaded with Co-60 having a total activity of 11.47 PBq, was submerged in the Embalse NPP fuel transfer pool. The goal was to withdraw its internal grid having 47 zircaloy pencils (with 16 slugs of Co-60 each one), and then take the empty package out of the pool in order to carry out inspection and maintenance operations on it. Such task was necessary because the package had been recently repaired and it is a normal procedure to perform a complete cycle of charge and discharge of radioactive material in order to verify the fulfilment of the water tightness test, before and after the heating cycle.
The incident started when withdrawing the supposed empty package from the pool. According to normal procedures, a radiation protection officer was supervising the activity when a dose rate of 0.1 Sv/h was measured while the package was still 1 meter under water. This activity rate was also detected by area monitors in the pool building and the high level alarm was activated. After these indications, withdrawal was immediately interrupted and the package was returned to the pool bottom.
Later on, it was detected that 3 slugs were still inside the package (radiation source detected) while another 6 slugs had fallen out of it and were laying at the pool bottom.
It was determined that during the grid handling inside the pool, one of the pencils lost its seal so that 9 out of 16 slugs came out of it. Three slugs were found inside the package while the other 6 fell out, reaching the pool bottom.
This incident is considered serious due to the radiological risk associated with loss of confinement of the sealed source called pencil. Such risk should be avoided not only during package loading and unloading, but also during its transport and periodic tests prior to its expedition. An investigation to determine root causes of this event was thus initiated.