A new method for decontaminating28 January 2000
The Swedish 835 MW BWR, Ringhals 1, has used a new concept for decontamination of control rod drive mechanism components as part of an extensive modernisation of the CRDM servicing workshop
Contamination occurs on the control rod drive mechanism (CRDM) components in three forms: loose particles and particles bound either in a semi-hard oxide layer or in a harder oxide layer. The composition of the corrosion contamination varies due to plant specific water chemistry and temperature.
CRDM handling and servicing is closely associated with personnel radiation exposure and air-born contamination. Reducing the amount of oxide can potentially help in achieving low surface radiation as well as low collective exposure, and a thorough decontamination prevents the spread of loose radioactive particles in working areas. However, experience from BWR plants shows that conventional methods, like ultrasonic cleaning and ethanol wiping, are not capable of removing the hard coatings of oxide.
The new integrated CRDM decontamination system used at Ringhals 1, known as DDT-2000, was developed by Sweden-based Fagerstrom Industrikonsult. It comprises a computerised high pressure system that reaches pressures of 7250 psi/14 gpm, (500 bar/54 l/min). Extensive laboratory testing and evaluation lies behind the optimisation of the washing parameters such as relative nozzle velocity, rotating speed, nozzle distance to object, pressure, flow, impact and grinding efficiency. The very high water velocity results in direct noise from nozzles at a level of 120 dBA. The system is therefore efficiently insulated to produce a noise level outside the machine that comply with requirements of labour welfare legislation.
Default values are pre-programmed for optimal automatic decontamination of each individual CRDM component, but the operator is permitted to adjust parameters such as nozzle velocity and washing time in the set up menu. An operators’ panel displays each stage of the process and indicates the actual progress of each sequence.
Vertical or horizontal handling of the CRDM components is possible. Automatic, vertically adjustable supports ensure that the CRDM components are correctly located. A fibre optic system indicates which one of the different parts is loaded, and its location, and the system automatically sets the appropriate cleaning programme. Positioning of the CRDM parts is automatically controlled when the components are rotated during external decontamination. Two rotating jets perform external decontamination, while four rotating jets perform the internal decontamination.
Per Fagerstrom, president of Fagerstrom Industrikonsult,* explains the concept behind the system: “The key features for the design were rational logistics regarding CRDM-handling, as well as proven man-machine interface and ergonomics. We reached our goals to develop a system to efficiently remove oxide bound radioactive particles and residual graphite lubricant without affecting the base metal. Together with high demands of mechanical quality and reliability we believe that the Deco-system is designed to meet and exceed the requirements of the modern nuclear industry”.
Statistics from a CRDM-overhaul at Ringhals 1 in 1999 indicate a decrease of collective dose for the entire service campaign by roughly a factor of two. The average decontamination factor for typical runs of all CRDM items was d/f=1.5. Regarding surface contamination, smear sampling displays an average decontamination factor of d/f=15 – a final contamination level of 200-800 kBq/m2.