Olkiluoto cleans up with centrifugal force

Olkiluoto’s discharge of radioactivity into the sea has amounted to about 6-7% of the annual level allowed by the authorities. It has been calculated that the discharge in the future will be only some 1% of the authorised level when the waste water project, which is part of the modernisation programme underway at the plant, is fully implemented.

With the Alfa Laval decanter and separator installed in the liquid waste treatment system of the plant, the quality of the water discharged into the sea can be improved by more than 80%.

The waste water project manager, Pekka Nousiainen, says that the new technology removes practically all suspended solids; but radioactivity in ionic form and dissolved chemical impurities will still demand cleansing based on ion exchange. However, the load on this system and the total amount of waste generated is reduced significantly.

SEPARATION OF SOLID PARTICLES

The activity in the water is largely associated with the solid particles which can be effectively separated by centrifugal force in the new decanter centrifuge and disc stack separator combination.

The decanter, which has medium G-force, removes the coarse particles in the waste water to be treated. The CHPX 510 separator has a G-force of approx 8800 and is then able to separate particles as small as 0.1 mm. The capacity of the decanter-separator-unit is 5 m3/h.

After cleaning the waste water mechanically with the Alfa Laval system, the resulting product along with all other types of waste water at the plant, with the exception of laundry water, will undergo a chemical cleansing by ion exchange before being released to the sea.

Waste water project storage tanks will be constructed at both Olkiluoto units. The 1500 m3 tanks, clad in stainless steel, are used to store reactor pool water and condensation pool water during annual refuelling and service work.

In the past some of this water was pumped to sea after filtration, but now all of it can be cleaned and used to refill the reactors and the condensation tanks.

The decanter model used for nuclear power plants is the type KWNX 416S. The machine is fitted with special seals and is operated under a light vacuum to prevent radioactive dust spreading outside the equipment. There are also other features built into the machine which facilitate the cleaning, eg CIP-wash with low speed cleaning and a very high specification of finish of surfaces which are in contact with the medium. The decanter also has a built-in centripetal pump (paring disc) to pump out the clarified liquid.

The equipment is is easily operated by remote control.

Section manager Thomas Söderlund from Alfa Laval Oy says that the decanter design is a further development of the equipment used in the food industry. “It is technically possible to cleanse just about any kind of waste water, but this project was a very difficult one due to the high requirements from the nuclear power industry.”

TRAINING

The contract also included training of staff. About 30 people from the operating and maintenance side participated in a course on how to run and service the equipment. Olkiluoto staff are now able to take care of many basic service assignments, such as changing the seals and the oil. Further advanced training is continuing in Sweden at Alfa Laval’s Training Institute in Tumba.

ON TIME DELIVERY

Pekka Nousiainen said that the delivery went according to plan – “any time we needed any help, the representative of Alfa Laval arrived in no time at all. The equipment has functioned well and has become a reference – Swedish colleagues have visited us to see our new equipment.”