Double decontamination30 January 2018
Germany’s Grafenrheinfeld is the only reactor worldwide to perform two full system decontaminations during its lifetime. Christian Topf, Sigrid Schütz and Christian Volkmann explain how the technique has led to significant dose savings￼
During the last two decades, Germany’s nuclear industry has had to accept fundamental changes in political conditions. At the beginning of the new century, Germany’s social democratic- green government under chancellor Gerhard Schröder decided on a final retreat from nuclear power by 2022. In 2006 the new German government under Chancellor Angela Merkel started to discuss a roll-back of the nuclear phaseout, with life extensions for all German nuclear plants, to bridge the time until renewable energy could reliably take over.
At the end of 2010, a life extension averaging 12 years for German reactors was decided by the German government. However, during 2011, as a direct consequence of the events in Fukushima, Germany’s nuclear programme reversed almost overnight from life extension to immediate and permanent shutdown. With the implementation of the 13th amendment to the German Atomic Energy Act (Atomgesetz, AtG), eight German nuclear plants lost their operational licences and had to be shut down. A final shutdown date for the nine remaining reactors was set at the end of 2022.
During earlier life extension discussions, the German utilities agreed to invest in their nuclear facilities and complete extensive safety-related technical improvements.
PreussenElektra (formerly E.On Kernkraft GmbH) decided at an early stage that a full system decontamination (FSD) would be a key part of refurbishment activities at its plants – Grafenrheinfeld (KKG) and Unterweser (KKU) – following the ALARA principle. AREVA was contracted to plan and perform the decontamination in preparation for the extensive maintenance work planned for during the 2010 refuelling outage, followed by a specific passivation treatment during restart of the plant.
Based on FSD field experience and the lessons gained in Grafenrheinfeld in 2010, and the results achieved from FSDs
at Stade, Obrigheim, Unterweser and Neckarwestheim 1, in 2014 PreussenElektra contracted AREVA to plan and perform a second FSD at Grafenrheinfeld after the final shutdown of the plant. The reactor, which had started exporting to the grid in 1981, was permanently shut down in June 2015 after 34 years of operation.
The FSD was performed in the second half of 2016. That made Grafenrheinfeld, a KWU- type third-generation four-loop PWR with a net electrical power of 1345MWe, the first and only nuclear power plant worldwide to perform an FSD twice during its lifetime.
Dose reduction concept
Dose rates at nuclear plants increase during operation as the activity inventory builds up. The activity buildup is influenced by the construction materials, past and present water chemistries, and the operating history of the plant. Depending on these factors, concrete actions to reduce the overall radiation exposure may become necessary before maintenance activities as the dose levels increase.
AREVA has developed a Concept for Sustainable Dose Reduction (CSDR) in operating BWRs and PWRs, a programme of joint corrective measures to minimise dose levels and to keep them low for continued operation. The measures can be applied in plants of all constructors and designs.
CSDR is achieved through the coordinated application of proven technologies, including:
- Full system decontamination to minimise the existing activity inventory
- Operation in protective layer setup mode to form a new, stable and contamination-free protective oxide layer on the bare metal system surfaces
- Using advanced water chemistry to minimise corrosion and metal release including – but not limited to – optimised chemical parameters, injection of depleted zinc oxide (DZO), improved shutdown process, and particle avoidance and removal.
At Grafenrheinfeld AREVA’s FSD involved the simultaneous decontamination of the complete primary circuit and auxiliary systems. Planning and application were the results of the close collaboration between KKG and AREVA, which was monitored by the independent technical consultancy and German Technical Inspection Association, TÜV.
The FSD was applied during the 2010 outage along with AREVA’s CSD, forming a stable non-contaminated protective oxide layer and the introduction of an advanced water chemistry including the injection of depleted zinc oxide (DZO).
Take one at Grafenrheinfeld
AREVA’s Chemical Oxidation Reduction Decontamination (CORD®) was applied for the FSD at Grafenrheinfeld. CORD comprises multi-cycle, regenerative chemical decontamination processes. One of the major advantages of CORD is the ability to tailor the process to the site requirements and customer needs. This goes hand in hand with low waste generation.
The principle of chemical decontamination with HP CORD UV is shown in Figure 1.
The HP CORD UV process has been applied in operating reactors worldwide without any detrimental effects on the treated material of construction.
In the decontamination, the plant’s systems were used, enhanced by AREVA’s Automated Modular Decontamination Appliance (AMDA®) for chemical process control. That included sampling and injection of chemicals, accelerating the overall project duration by flexible and efficient cleaning via ion exchange resin and mechanical filtration and UV decomposition of the decontamination chemicals to minimise the total radioactive waste volume.
The decontamination area with all nuclear plant systems involved including AMDA connections is shown in Figure 2.
In 2010 three cycles of the HP CORD UV were applied. The residual heat removal (RHR) system 10 was excluded from the decontamination area for the FSD application due to the regulatory framework. The RHR system 10 had to stand by under normal conditions at any time because it provided redundancy for the spent fuel pool cooling system. The decontamination of RHR system 10 was performed after FSD was finalised, as a single system decontamination. The plant systems were operated by plant personnel, while the AMDA was operated by AREVA.
The average decontamination factor (DF) for the decontamination area was 60.5. Average contact dose rates at the steam generator shell, the main loops, the pressuriser and connected piping, were below 0.1mSv/h.
Visual inspections directly after FSD showed that the decontaminated surfaces were as clean as they were before the initial startup of the plant in 1981. These findings were consistent with the general experience gathered with the AREVA decontamination process HP CORD® UV in previous applications at Stade or Obrigheim.
The achieved dose rate values post decontamination were so low that it was assumed that their contribution to recontamination could be neglected.
After completion of FSD and further outage works the Grafenrheinfeld was prepared for restart. During startup, the plant was operated sub-critically with strict control of the chemical parameters of the primary circuit. A new, very stable protective oxide layer was introduced on the system surfaces, preventing the incorporation of activated products into the oxide layers during further operation. The protective layer formed was further consolidated through the posterior operation at constant basic load and all chemical parameters were monitored and adjusted – if necessary – during the following one hundred days of operation.
The injection of zinc and other modified water chemistry measures were included in this mode of operation to assist the formation of the protective layer. The water chemistry measures were:
- Plant status: core loaded, sub-critical hot
- Deoxygenation during heat-up: O2 < 0.1 ppm at 170°C
- Start of coolant hydrogenation after disconnection of (RHR) system at 170°C (Target H2 > 2ppm at 260°C)
- Lithium concentration: 6.3±0.3ppm Li at 170°C
- Zinc concentration: 5-15ppb Zn at 170°C.
Operational dose development
During the planning and engineering phase of the project, the recontamination level of the plant had to be estimated. As a first baseline, the operating experience and data from the first outages after the start of commercial operation were used as a reference case for estimating post FSD trends and values.
Loop dose rates in the first three outages after commissioning Grafenrheinfeld were in the range of 2mSv/h (see Figure 4). The long-term zinc reduction effect at mature plants from experience is in the field of 50%. Converting this factor to Grafenrheinfeld, expected loop dose rates would be within the range of 1mSv/h as a first expectation.
For further estimation of dose rate trends post FSD, a second set of data from a sister plant was used. At this plant zinc injection began two days after first criticality. This plant was operated with modified B/Li chemistry, pH 6.9-7.4. In spite of the rather high stellite inventory compared to Grafenrheinfeld, loop dose rates in the first few outages were in the range of 0.2mSv/h.
The main difference between the startup data and the situation at Grafenrheinfeld post FSD was the remaining crud inventory from the fuel assemblies, the in-core instrumentation and the control rods – including guide assemblies and drive rods – which had been removed before decontamination.
The crud on fuel rods was sampled and analysed to estimate the influence of the crud on these component surfaces on the recontamination of Grafenrheinfeld after FSD. Crud weights and activities of the samples were extrapolated to the total surface of the fuel. In a second step, it was assumed that this crud is completely dissolved in the reactor coolant during subsequent plant operation and homogeneously deposited on out-of-core surfaces. Based on the resulting surface gamma activity concentrations, it was concluded that the contribution of the crud on removed parts to Co-60 and Co-58 activities
is one to two orders of magnitude lower than the values measured before FSD. The operator consequently decided to reinstall these components with the given pre FSD condition.
Dose rates at the main loops of 0.2-1mSv/h post FSD were deemed to be realistic by the utility and AREVA.
Before FSD, surface activity in the loops had been dominated by Co-60, which is typical for older generation Siemens PWRs. The Co-60 surface activity in the first post FSD outage was five to ten times lower than the pre FSD values.
Co-60 recontamination post FSD was effectively prevented by the water chemistry measures. Gamma scans revealed that Co-58 had become a significant dose rate source post FSD, due to the Co-60 inventory removed by the decontamination. This contribution of Co-58 was expected, and the measured contact dose rates at the loops ranged between 0.2 and 0.5mSv/h.
The average contact dose rate of 0.8mSv/h for all measuring points at the main loops in the first outage post FSD confirmed the theoretical approach during the planning phase. This value was less than half of the dose rate level measured in the first outage of the plant in 1983 and less than 30% of the dose rate level at the main loops in 2010 (see Figure 4).
The recontamination development during the following five years of post FSD operation was significantly lower than in the years after first criticality in 1981. The average dose rate increase of 0.050mSv/year developed almost linearly in the following five years of operation, reaching 1mSv/h at the end of the operational lifetime in 2015 (see Figure 4).
The job-related personal doses or cumulative radiation exposure (CRE) for comparable outage activities in high dose rate areas decreased by a factor of three.
The FSD resulted in dose savings of 3500mSv in the FSD outage and 1300mSv in the post-FSD outage after CSDR. In total, 6400mSv of personnel dose was saved during the following five years of operation (see Figure 5).
During the first cycle of FSD, considerably more corrosion products and activity than expected was released from the inner surfaces of the decontamination area.
Activity spread into low-flow areas and dead legs, resulting in high area dose rates in the plant. An extensive flushing programme in cooperation between PreussenElektra and AREVA was established to mitigate the impact on further FSD operations and the upcoming outage works.
This situation affected the performance schedule and increased the amount of waste from the original estimates.
As a result of this unexpected development, Grafenrheinfeld PreussenElektra and AREVA had an extensive exchange on lessons learned to prevent such incidents in later applications.
Detailed discussion resulted in a significant development and change in the chemical and technical concept for FSD performance. The changes were applied in the subsequent decontamination at Unterweser in 2011.
Continuously challenging and adapting FSD to the specific plant requirements led to outstanding results for FSD applications at Neckarwestheim 1 in 2013, Isar 1 in 2015 and Krümmel in 2016, and finally at Grafenrheinfeld again in 2016.
Take two at Grafenrheinfeld
PreussenElektra decided not to perform another outage in 2015 and to cease operation of Grafenrheinfeld in the middle of 2015.
Based on the FSD performed in 2010 and the experience of FSD during the non-operational phase in Germany, PreussenElektra and AREVA began planning for FSD at the beginning of 2015 with a target date in the second half of 2016.
Although the general concept approach for 2016 did not differ from that of 2010, some changes were needed.
Whereas in 2010 the application was strongly influenced by the timeframe of the outage, the targets for FSD 2016 were focused on the most effective decontamination of the plant and specific components to be prepared for future decommissioning works.
Based on this, the conceptual engineering and chemical approach strongly focused on integrating as many plant systems
as possible to minimise contact and ambient dose rates within the plant in the most efficient way. It also considered minimisation of radioactive waste created by the application.
One of the primary targets was to optimise the treatment of the recuperative heat exchanger (RECU) as a single component, to improve the decontamination outcome. A specific treatment concept was engineered and planned by Grafenrheinfeld and AREVA and was applied within the overall FSD application.
As in 2010, the operation of the nuclear plant systems during the FSD application was performed by the plant personnel, while the AMDA equipment was operated by AREVA. The general data of the decontamination area in Grafenrheinfeld 2016 are similar to 2010 differing in the involvement of the RHR systems TH10 and 40, as both systems are necessary to retain the fuel pool cooling.
The chemical concept was based on the results and lessons learned from the decontamination in 2010 and the data collected during the successive operational phase, especially focusing on the effect of the passivation step and the zinc injection. The oxide layer on the system surfaces would be thinner but more compact.
Grafenrheinfeld and AREVA agreed to perform four cycles of the HP CORD UV. Within the overall FSD performance, the recuperative heat exchanger was treated with a preservative oxidation phase to maximise decontamination.
In total 2.8x1013 Bq of activity – corresponding to Co-60 – and a total of 190.2kg of corrosion products were removed from the decontamination area and placed on 7.8m3 of ion exchange resin beds (see Table 2).
As expected, the most of the zinc (3.2kg or 70%) was removed during the first cycle. During each of the following three cycles, between 0.4 and 0.6kg of zinc was removed.
The ambient dose rate at the component location was reduced by a factor of 14 resulting in the current ambient dose rate of only 9μSv/h.
The influence of the special treatment of the RECU is obvious. In 2010 the average dose rate at the measuring points after three cycles (see Figure 6) was 1.8mSv/h, an average decontamination factor of 12.5. After the 2016 application, the average dose rate at these measuring points was 11μSv/h, an average decontamination factor of 155 (see Table 3).
Germany’s Grafenrheinfeld is the only reactor worldwide to perform two full system decontaminations during its operational lifetime.
The decontamination for life extension in 2010 followed the concept of sustainable dose reduction and resulted in significant dose rate savings during the plant’s subsequent six-year operational life. The FSD during the non-operational phase in 2016 focused on the preparation of the plant for decommissioning. The good results for dose reduction in
the 2016 application, especially for the recuperative heat exchanger, supports the planning and execution of decommissioning at Grafenrheinfeld.
About the authors:
Michael Fischer, Luis Sempere Belda, and Christian Topf (firstname.lastname@example.org) work in the Chemistry Services Department at AREVA GmbH. Sigrid Schütz, Gerhard Grob
Volker Berger and Ralph Oster work at Kernkraftwerk Grafenrheinfeld for PreussenElektra.
Christian Volkmann is with ESG Engineering Services GmbH