Radwaste | Treatment
Spinning Khmelnitsky9 May 2011
A new liquid radwaste treatment system at Ukraine’s Khmelnitsky nuclear power plant removes solid particles from stored radioactive liquid waste through a centrifugation process. The system removes 99% of suspended particles, reduces the radioactivity of wastewater by 60%, and dries solids in drums to a residual humidity below 30%.
The EUR2.4 million project has been financed by the European Commission (Aidco), and was carried out on site under supervision of Italian radwaste agency Sogin, in consortium with Iberdrola. The plant was designed by Ansaldo Nucleare.
Before the installation of the centrifuge facility, the liquid radioactive waste (LRW) at the Khmelnitsky plant was decanted in settling tanks, with accumulation of solids on tank bottom. Sedimentation of solids occurred in the drain pit tank (BPTV), in the sump tank (BO) and in the drain water tanks (BTV). Drain water was filtered through the bulk mechanical filter. This process was characterized by the parameters in Table 1 (see attachment).
The use of this technique during 20 years of operation at Khmelnitsky Nuclear Power plant has saturated more than 75% of the available storage tanks, limiting future plant operation. The alternatives were to install further sedimentation tanks to increase capacity, or to select new technology.
Choosing a new treatment system
In order to investigate and compare possible alternatives, the following main goals were identified:
- Achieving the necessary degree of purification
- Minimizing final waste volumes
- Assuring long-term safety of final waste
- Minimizing the personnel exposure during operation and maintenance
The principal new technology selection factors considered were:
- The quantity of incoming waste
- Ability to return the treated water into the process cycle
- Simplicity in technology, operation and maintenance
The main alternatives considered were:
- Flotation water clarification
- Water clarification in a layer of suspended sediment
- Precoat filtration
- Reverse osmosis
- Water clarification through centrifugal force field.
After a comparative analysis, the last alternative was chosen. It is effective for particles of dimensions in the range 5µm-1000µm. This technology is expected to fill drums with radioactive waste which can be classified as solid (that is, with limited residual humidity). Final conditioning will demand a solid radioactive waste treatment facility to be built. The drums to be used shall assure all safety requirements for short-term storage. Post-treatment wastewater will go to an evaporator (outside project scope).
The three main design goals – simplicity in equipment, operation and maintenance, high efficiency of purification, easy accessibility for maintenance activities – led to the adoption of a couple of centrifuges, the first one with role of ‘decanter’, the second with role of ‘separator’. Figure 2 presents the system as built.
The selection of the main contractor Ansaldo was performed by an open tender, in the frame of the European Commission TACIS programme (now replaced by the Instrument for Nuclear Safety Cooperation programme). The contract kick-off meeting occurred in the first half of 2004.
An important limitation for the project has been the available space in the existing building for LRW treatment, where three rooms were available, each one at a different elevation (Figure 3).
The system is mainly composed of:
- Waste tank and feed subsystem, including sampling system, located in the upper floor room
- Decanter and separator at floor below,
- Drum handling subsystem, with dryer and glovebox for operator at ground floor. Empty drums are transported from the upper to the ground floor through a passing tube. In regular operation, 4-6 drums/week would be filled, dried, sampled, closed and sent back to upper floor through the same tube, which has a relatively large diameter to prevent any possible blockage.
After 18 months to finalize design, factory acceptance tests took place in Italy at the end of 2005 and beginning of 2006. After delivery and customs clearance, site activities started in the second half of 2006.
Erection and commissioning of the system were delayed by several difficulties linked to the following main factors:
- Presence of several different technologies and sub-suppliers in a new system
- Use of equipment not strictly designed for use in nuclear environment (such as separator and decanter), with consequent need of minor adjustments
- Employment of a local company for installation and erection, with the consequential need of coordination which far exceeded the estimated effort
- Differences in rules, habits, practices and approaches between designer and end user.
Commissioning was completed by the end of 2007. Training of personnel was initially performed under supplier supervision, then thoroughly revisited by Sogin within the European Commission project. It was then enlarged in terms of scope, extended to maintenance activities, and to prepare teachers of future site personnel. In this second phase an amount of training equivalent to 150 person/weeks has been performed, including in-depth training on the control system software environment, which allowed plant experts to perform fine tuning during trial operation.
After completion of in-depth personnel training and execution of further on-site tests, the system was released for trial operations in April 2010.
The main results of trial operations are shown in Table 2. Measured results in input and output water showed reduction of particles from 3919 mg/dm3 to 28 mg/dm3 (a reduction factor of 47.1). It reduced activity from 9598x107 Bq/dm3 to 3518x107 Bq/dm3 (a reduction factor of 2.73).
After trial operation was completed, the data have been transferred to a scientific institute for review and preparation of a final safety analysis report, which is necessary to achieve authorization to regular operation.
In conclusion, a new centrifugation system has proved to be able to reach high efficiency in water purification, achieving a significant reduction in waste water specific activity, and has produced drums which may be transported to temporary storage in agreement with applicable standards.
It is ready to start regular operation, preventing further installation of 300 new sedimentation tanks, with financial savings and dose reductions to plant operators.
FilesTables 1&2 and Figures 1&2