Decontamination for decommissioning at Connecticut Yankee

Connecticut Yankee declared its intent to enter into the decommissioning phase of operation in December 1996. After a detailed ALARA evaluation, the site management team elected to perform a full system decontamination (FSD) prior to the removal of any primary components. The objective of this FSD was a reduction in dose rates from the primary systems by a factor of 15. It was estimated that a reduction of this magnitude would provide exposure savings of over 1000 person-Rem.

This FSD was conducted in a joint effort between CY decommissioning staff, Duke Engineering and Services and Siemens AG. This work was contracted through Siemens Power Corp. Siemens was selected as the chemical decontamination vendor because of its advanced chemical solvent technology and extensive experience in operational and decommissioning plants.

The decontamination approach developed for CY was a unique application which involved removal of the reactor core barrel and installation of a reactor nozzle dam system to direct the flow from one steam generator loop to the other while bypassing the reactor pressure vessel. The motive force for the decontamination was supplied by the residual heat removal pumps operated in the shutdown cooling mode. The reactor coolant pumps were not used during this decontamination. This arrangement allowed a relatively high flow rate (1500 gallons per minute) which was sufficient to allow all of the primary systems such as the reactor coolant piping, residual heat removal, and chemical volume control system to be decontaminated in a single application. The advantages to this approach were:

• Reduced schedule impact due to a single application that decontaminated the entire system.

• Reduced waste volume due to elimination of activity from the reactor pressure vessel.

• Removal of activated material from the reactor vessel was avoided.

Artifact Test Results

Before the start of the decontamination work, laboratory tests were conducted on original CY artifact piping in order to select the decontamination process. The tests were carried out in three stages. In all of the tests, the decontamination cycles were considerably shorter than necessary or usual for decontamination operations in a nuclear power plant.

It became clear in the course of project planning that a considerable amount of time and effort could be saved if chemical decontamination could be performed in the presence of residual boric acid (approximately 700 ppm). A further series of tests was therefore included in the laboratory test programme to demonstrate that the presence of boric acid in the decontaminating fluid would not adversely affect the decontamination results.

The tests were specified in detail in a test plan/procedure and approved by all decontamination team members. Tests were carried out in March 1998 at the Siemens laboratory in Erlangen.

Test stages

Pretests were conducted to demonstrate that conducting the time-consuming and cost-intensive process selection and flow test investigations would be warranted.

The process selection tests were aimed exclusively at investigating and comparing the effectiveness of the chemistry. While process parameters such as flow velocity, temperature, etc. may improve the decontamination efficiency of chemical processes, they must not be an essential prerequisite for the effectiveness of such a process. If the chemistry fails, the process is unsuitable. The process selection test indicated that the Siemens HP CORD D UV was the most effective chemical solvent for the oxide films on CY piping. The flow testing was performed using only the HP CORD D UV solvent.

The flow test was conducted under conditions similar to those existing at Connecticut Yankee. The aim of the flow test was to demonstrate that the decontamination results would not be adversely affected by fluid flow under realistic conditions at Connecticut Yankee and in the planned circuit configuration.

During performance of the tests, the amount of dissolved activity and the amount of dissolved cations contained in the decontamination solution were determined. The activity removed using the HP CORD D UV process, both with and without boric acid, was identical. The key data from artifact testing are given in the table.

Decontamination Approach

According to CY’s requirements, the decontamination was planned and executed using, to the maximum extent possible, installed plant equipment such as the residual heat removal pumps and the chemical volume control system ion exchange columns. In the preceding engineering and preparation phase, the necessary systems were modified and adapted for the decontamination application by CY. In parallel, Siemens specified and prepared the components of the external decontamination equipment AMDA (Automated Modular/Mobile Decontamination Appliance) to be used during the decontamination. The following flow diagram shows the concept for the Connecticut Yankee FSD.

The primary system was drained and refilled prior to the FSD. No further efforts were made to reduce the boric acid, which was 440 ppm. Decontamination of the primary system was performed excluding the reactor pressure vessel (RPV). This was achieved by the installation of specially designed nozzle dams in the RPV which permitted the routing of decontamination solution from loop to loop without wetting the RPV. To further save personnel dose, the decontamination solution was routed through the steam generator tubing instead of installing mechanical jumpers across the primary manways. The total effective surface area increased by a factor of two due to the I600 tubes (about 20% of steam generator tubing) coming in contact with decontamination solution. The essential nuclear power plant data are given in the table.

Recirculation of the process solution was performed by using one residual heat removal pump at a time, while continuous solvent regeneration was performed via the chemical volume control system ion exchange columns. The external AMDA modules (chemicals injection module, UV modules, pumps, heaters, measuring devices, etc) performed the task of pumping the purified decontamination solution back to the system and provided the means to adjust the process specific chemical parameters. This was the first application using normal plant equipment for a decontamination application, and also the first application using the residual heat removal pumps alone, without the reactor coolant pumps, to provide the necessary motive force.

Decontamination Results

The chemical decontamination was originally scheduled for 4 cycles of HP CORD D UV. However, due to difficulties associated with the operation of the chemical volume control ion exchange system, the decontamination was concluded after the completion of the second cycle. At the end of the second cycle, the average decontamination factor from a total of 36 measuring points was 15.9. The detailed evaluation for the individual systems showed that despite premature termination of the decontamination, excellent results were obtained in all primary and subsystems. A comparison of decontamination factor results obtained in the CY decontamination project shows a high reproducibility and high reliability of HP CORD D UV as a chemical decontamination process. Independent of the various system conditions (with and without bypass cleaning, high flow rate vs low flow rate, etc), comparable results were obtained in each of the systems.

Lessons Learned

During the decontamination process, it became apparent that the chemical volume control system ion exchange columns were not appropriate for a decontamination application due to the age and physical condition of the units. Problems encountered included difficulty in system operation due to valve operations and unexpected leaks in the system. These plant system difficulties prevented the decontamination from being performed according to plan. During the individual cycles it was not possible to operate the chemical volume control ion exchange system for up to several days. This disruption in solvent regeneration and activity removal did not have any influence on the overall solvent effectiveness, and results similar to the artifact flow test were obtained after two cycles were completed. It is apparent from the data review that this disruption in ion exchange flow did cause some minor precipitation of metals during the first CORD cycle. Once ion exchange capabilities were returned to service, this precipitation was later redissolved during the clean up phase. No local hot spots of activity were identified in the post decontamination surveys indicating that all precipitation that had formed had been redissolved. While the decontamination was planned for up to four cycles, the process was terminated after two cycles due to the achievement of an average DF value of 15, and due to continued ion exchange system difficulties. The overall impact of these problems was a delay in the project schedule.

Conclusions

The overall decontamination of Connecticut Yankee was a success which will result in estimated savings of over 1000 person Rem over the time period of the decommissioning of the station. The approach taken in this decontamination by utilising the plant residual heat removal pumps and bypassing the reactor vessel proved to be a valuable and efficient application method.