Aqueous, in situ primary circuit decontamination19 June 2014
A technology that removes activity and corrosion products from inner pipe and vessel surfaces yielded overall decontamination factors of 80-90 at Unterweser and Neckarwestheim 1, and factors as high as 150 for steam generator tubing. By Christian Topf, Luis Sempere-Belda, Kai Tscheschlok and Klaus Reuschle
Germany was confronted with the decontamination and decommissioning (D&D) issue when the 13th amendment to the German Atomic Energy Act was passed in the backlash following the Fukushima accident. With it, Germany's nuclear programme reversed almost overnight from life extension to immediate and permanent shut down. This caused a major overhaul in the utilities' operational and financial strategies, forcing them to re-examine the cost-effectiveness of their D&D programmes.
All existing decommissioning strategies were considered, but immediate decommissioning after the operational phase soon emerged as the favoured option for all the country's utilities. The experience accumulated in D&D in Europe over the last few decades, especially in Germany, has established full system decontamination (FSD) - simultaneousdecontamination of the complete primary circuit and auxiliary systems - as the key element of immediate decommissioning. It is considered to be state-of-the-art best practice. It has been used in recent decommissioning projects and is planned for projects in preparation.
Over the last few decades AREVA has performed FSD for return-to-service and decommissioning at 18 reactors worldwide - some immediately or several years after final shutdown. The complexity of such FSD projects always requires a flexible approach regarding process chemistry and control to achieve the best possible result. The multi-cycle CORD concept (chemical-oxidation-reduction-decontamination), with its dynamic process control and flexible application times, makes best use of the chemicals employed, helping minimise the waste generated during decontamination.
FSDs at Unterweser in 2012 and Neckarwestheim 1 in 2013 were the first such applications in Germany in the aftermath of the Fukushima event and the German decision to phase out nuclear.
Engineering and chemistry concept
The key to a successful FSD application is symbiosis between system operation and the chemical concept.
The first and vital step is to clearly define the main systems in the decontamination area. To achieve the best possible decontamination the area should include all the systems that are in contact with primary coolant during normal plant operation. A variety of auxiliary systems (for example reactor coolant storage system, waste water treatment system and the nuclear exhaust system) support the application.
For PWRs the typical decontamination area is the primary circuit, including the RPV with internals, pressurising system, residual heat removal system (RHR), chemical volume control system (CVCS), and reactor water clean-up system. For BWRs, it includes the reactor vessel, recirculation system (depending on the design of the plant), RHR system, and reactor water clean-up (RWCU) system.
To make sure the decontamination solution reaches all parts of the system, the number of dead legs (runs of piping ending in a termination) are minimised by making temporary connections to them with hoses and pipes. A flushing/rinsing programme at the end of decontamination helps to flush the remaining dead legs.
Figure 2 shows the schematic drawings of the decontamination area of the FSDs per-formed recently at Unterweser (2012, left side) and Neckarwestheim 1 (2013, right side).
The detailed engineering approach for these plants was designed in close collaboration between AREVA and the utilities. The basic technical data of the plants are listed in Table 1.
The system was adapted and the technical modes of operation were substantially developed and configured by experienced plant operational personnel in cooperation with AREVA engineering and chemical personnel. Plant operators controlled the plant systems during the FSD.
The main reactor coolant pumps and the RHR system pumps were used to recirculate the solution in the decontamination area, and the thermal energy liberated by operating them supplied the necessary process heat. The negative pressure suction head to run the coolant pumps during FSD mode was achieved by using one of the RHR accumulators as a pressurizer. Based on the different main coolant pump designs (see Table 3) the operational pressure during FSD differed from around 21 bar (~300 psi) to 26 bar (~375 psi). The plant pressurizer was operated in solid mode to obtain the highest possible decontamination on the complete surface. To ensure exchange of the decontamination solution the main and auxiliary spray lines were operated either in parallel or alternately during the FSD.
Different approaches at Unterweser and Neckarwestheim
Different approaches used the plants' RHR systems for process temperature control. At Unterweser temperature was regulated using all four RHR redundancies. At Neckarwestheim 1 RHR system 30 was excluded from the decontamination area. Within the regulatory framework RHR system 30 had to stand by under normal conditions at any time because it provides redundancy for the spent fuel pool cooling system, so temperature was regulated using only the two other RHR 10/20 systems.
For both applications proprietary decontamination equipment Automated Modular Decontamination Appliance (AMDA®) was used with the plant´s own systems during FSD. This allowed AREVA's radwaste volume-reducing technologies to be applied, such as UV decomposition of the decontamination chemicals. AMDA was also used for sampling during the application, for detailed process control and mechanical filtration of the decontamination solution. AMDA equipment, which can include pumps, heaters, ion exchange columns, filters, UV reactors, sampling modules, monitoring systems, automation and remote controls and chemical injection equipment, was connected in each case to two different RHR systems (see Figure 3).
The CVCS and RWCU systems in Neckarwestheim were part of the decontamination area and were operated under normal operating conditions and temperature restrictions. In Unterweser the sensitive centrifugal charging pumps in the CVCS were replaced by an external pump to ensure undisturbed operation during FSD application but remaining in standby mode for redundancy. In contrast to Neckarwestheim, the Unterweser RWCU system supported the clean-up of the decontamination solution during certain phases of the application. The increase in the cleaning rate reduced treatment time and supported process control during the FSD operation.
To have maximum decontamination effect at minimum material impact, a five-cycle chemical oxidation reduction decontamination (CORD) UV chemical decontamination concept using permanganic acid (HP) was developed by AREVA and the plants. Generally speaking, the formulation is adjusted to the characteristics of the oxide present. The main process variables are volume, surface, construction materials, oxide characteristics, operating history, process temperature, ion exchange rate and chemical concentration.
The entire process takes just one fill of water. Each cycle is divided into the following steps:
- Oxidation with HP
- Reduction of HP with decontamination chemical
- UV decomposition of decontamination chemicals and clean-up.
Oxidation prepares the oxide layers for their removal, but does not mobilize large amounts of activity and corrosion products itself. The mobilization of activity and oxide dissolution take place during the decontamination step. The water is not flushed. The dissolved corrosion products and activity are removed via ion exchangers and the water is reused in the next cycle.
Bypass purification is performed during the decontamination step to fix the dissolved activity and corrosion products on ion exchange resins (Co-60 is the dominant nuclide in recently shut-down plants, and the main cause of dose). After the decontamination step the ultraviolet decomposition of the remaining decontamination chemicals takes place in-situ. The decontamination chemicals are decomposed to water and carbon dioxide while the remaining activity and corrosion products are being purified. Through this procedure, the system water reaches a purity that is close to demineralised water at the end of each cycle, so the water can be reused in the next cycle.
The HP CORD UV process does not require a predetermined number of cycles to be performed. The number of cycles is varied depending on the decontamination tasks and targets.
The cycle duration depends on the speed of dissolution of deposits, the total amount of deposits present, and the speed of removal of dissolved species from solution. During application the decontamination solution is analysed in the hot lab and the progress of the process is determined through analyses of corrosion products and radionuclides in solution.
The decontamination targets for Unterweser and Neckarwestheim assumed immediate decommissioning, but maintained the possibility of a return to service. They were:
- Minimizing the total activity inventory
- Reducing dose rates at plant systems, especially the primary circuit with its heavy components, to facilitate further handling
- Minimizing ambient dose rates to lower future personnel doses (ALARA)
- Avoiding a significant shift of the gamma-to-alpha ratio (which can have consequences in later stages of decommissioning operations, particularly concerning health physics and radiation protection monitoring).
Based on these targets, five HP CORD UV cycles were applied in both PWRs. During on-site application AREVA's chemical experts provided 24-hour supervision.
Table 2 summarises the results. The decontamination factors are the ratio of the contact dose rates before and after FSD at specific measuring points.
Based on these results and the total surface of the decontamination area, the average oxide layer removed is 4.3 µm. Considering the amount of corrosion products removed and the total surface of the decontamination area, the average oxide layer in Unterweser is 4.9 µm. During the commissioning phase and in the course of the FSD applications at Unterweser and Neckarwestheim the primary coolant was permanently filtered. In the Unterweser plant 9x1012 Bq of activity (Co-60) was additionally removed by filtration.
A number of measuring points were identified in the decontamination area to determine the average decontamination factor. In Unterweser (a four-loop PWR) 83 measuring points were used, and in Neckarwestheim (a three-loop PWR) 66 were used. Before FSD, at Unterweser 100% of the points in the decontamination area had dose rates above 0.5 mSv/h (0.05 Rem), while at in Neckarwestheim the distribution was about 90%. The initial dose rates in the primary systems differed significantly, from an average of 5 mSv/h (0.5 Rem) in Unterweser to 630 µSv/h (0.063 Rem) in GKN 1.
The contact dose rates in the primary systems after FSD were significantly lower, even in the area of plugged tubes. They averaged 30 µSv/h (0.003 Rem) in both plants. The distribution of dose rates in the decontamination area of both plants shifted significantly, with 90% of the measuring points lower than 0.1 mSv/h (0.01 Rem).
AREVA's HP CORD UV concept, adjusted to the needs of these plants, resulted in a very high decontamination while keeping all systems operational. Extensive inspection programmes after treatment revealed no detrimental effects on the material. Technically, the plants could return to operation.
Performing the decontamination right after the operational phase proved to be a great advantage. The key to success was involving the plant's own personnel during all phases of the decontamination. Their detailed system knowledge, expertise, and extended operational experience, together with the close and open cooperation with AREVA's decontamination personnel, produced the best result.
About the authors
Dr. Christian Topf, Luis Sempere-Belda, AREVA GmbH, Abteilung Chemistry Services, Paul-Gossen-Str. 100, 91054 Erlangen; Kai Tscheschlok , E.ON Kernkraft GmbH, Kernkraftwerk Unterweser, Abteilung TMT, Dedesdorfer Str. 2, 26935 Stadland. Klaus Reuschle, EnBW Kernkraft GmbH, Kernkraftwerk Neckarwestheim, Im Steinbruch, 74382 Neckarwestheim, Germany