Auxiliary system contamination in French PWRs

International audienceIn order to study the impact of design and operating parameters on French PWR contamination, the CEA, by request of EDF, has been performing in-situ gamma spectrometry measurements using the so-called EMECC device for 37 years. These measurements have allowed researchers to characterize contamination levels accurately (surface activities by radionuclide), mainly in primary systems, but also in some reactor auxiliary systems such as the Chemical and Volume Control System (CVCS), the Residual Heat Removal System (RHRS), the Nuclear Sampling System (NSS) or the Fuel Pool Cooling and Cleanup System (FPCS). Based on EMECC campaigns performed in different standardized plant series, this paper shows that the contamination of nuclear auxiliary systems presents differences in comparison to the contamination of the primary system:-The Co-58 and Co-60 deposited activities on the CVCS, RHRS, NSS and FPCS are generally lower. On the other hand, the Ag-110m contamination is higher with a considerable recontamination resulting during cold shutdowns.-The dose rates around the auxiliary systems are generally higher.-The Co-58 contribution and especially the Ag-110m contribution are both higher. For auxiliary systems that do not operate for several months, the Co-60 contribution emerges as the major one.-The nuclear auxiliary systems are the only circuits that allow us to measure the Cr-51 surface activities.These characteristics are due to different design and operating conditions, particularly the following : the presence of heat exchangers and lower fluid temperature, different flow rates, thinner thickness of pipes and of exchanger shells, the presence of filtering systems and operations during a short period. A final observation is that, the peculiar behavior of Ag-110m inside the NSS generally results in a poor estimation of the Ag-110m volume activity in the primary coolant

EDF strategy for the primary coolant pH

International audienceEDF has experienced different boron-Lithium coordination curves into the primary coolant of nuclear power plants. The applied chemistries have led to the target pH of 7.0 and 7.2 at 300°C for the middle of the fuel cycles. But different strategies have been tested for the beginning and the end of cycle giving a larger range of pH. Since the load following implementation for EDF plants, it is difficult to keep a constant pH and it is more reasonable to consider that it moves around the target value. This paper also draws the influence of the pH evolution on the primary-secondary leaks for the oldest plants with 600 MA steam generator tubes. Actually, the demonstrated effect of the primary coolant effect on crack oxide solubility explains the level of the weak and stabilised leaks. This leads to propose a specific B/Li coordination to mitigate operation difficulties. This analysis may help chemists to choose the suitable pH for different plant families according to the various objectives

Low primary system contamination levels in some French PWRs

International audiencePWRs equipped with Steam Generator tubing in a nickel-based alloy (alloy 600 or 690) are generally characterized by higher Co-58 peaks during cold shutdowns (order of magnitude of 100 GBq/m3) and higher Co-58 deposited activities (several GBq/m²) compared to PWRs equipped with SG tubing in an iron-based alloy (alloy 800 or stainless steel). However, the trends are different for the Steam Generator Replacement (SGR) with alloy 690 carried out in the last decade, in particular for the SAINT-LAURENT-B1 PWR: after an increase in the Co-58 contamination just after the SGR, the contamination on the primary system decreases and reaches a very low level several cycles after the SGR. The Co-58 peak during cold shutdowns follows the same variation, it drops by a factor of 10 to 100. Similar levels are also measured in the last French operating PWR generation (N4). The key factor of these low Co-58 deposited and released activities is the surface finish of alloy 690TT SG tubing by means of an improved manufacturing process. The nickel release is low enough to limit the presence of metallic nickel or nickel oxide, which tend to form deposits on fuel elements. In consequence, nickel deposits on fuel elements and then 58Co volume and surface activities are reduced

Chemical degassing on EDF units - Feed back experience and method

International audienceThe chemical degassing feasibility of all French units RCS during outages is usually performed on the four 1500 MWe French units since July 2004 and is planned for 2007 on the 34 units of 900 MWe and the 20 units of 1300 MWe units. It helps to an optimized shutdown planning management, especially in case of unplanned shutdowns, and helps to limit the thermal stress corrosion of RHR at temperature higher than 120°C by avoiding temperature step extension after bubble collapsing to remove hydrogen increase. Chemical degassing is performed at temperature equal or lower than 80°C, by hydrogen peroxide injection if 3 < H2dissolved < 35 cm3/kg. H2O2 calculation can be stoechiometric or increased by 20 % depending on the SG tubes material and the full power operation duration before the shutdown. Purification flow is adapted to each design type units characteristics to easily manage a hypothetical early oxygenation. Early oxygenation management consists in forecasting the dilution of the VCT (Volume Control Tank) gaseous phase by nitrogen to avoid dangerous H2 /O2 gaseous mixture and to obtain oxygenation criteria on VCT. Three kinds of dilution can be used: nitrogen burping with high levels amplitude, continuous nitrogen flushing or discontinuous flushing with a maximal constant VCT level but pressure variation

Complete analyse of zinc injection impact at BUGEY 2&4

International audienceZinc injection at BUGEY 2 and 4 provides a distinctive experience in the world, which can be used for a better understanding of the role of the zinc in the primary coolant and its impact on the radiation fields, fuel and wastes. BUGEY Unit 2 and Unit 4 are two EDF 900 MWe PWRs. The alloy 600 MA Steam Generators of BUGEY 4 have been replaced by alloy 690 TT SGs at the end of cycle 23. EDF implemented the Zinc injection for the first time in France at BUGEY 2 Cycle 22 four years ago in order to reduce the dose rates. In 2005, the zinc injection was also initiated at BUGEY 4 Cycle 23, during the previous cycle of the steam generator replacement. The gamma-spectrometry measurements performed at BUGEY on the steam generator tubing and primary pipes for several cycles (before and after the zinc injection) seem to show that the zinc incorporation on the oxide layers is not immediate or uniform in the different portions of the circuits and the dose rate reduction may be influenced by different factors. The interest of BUGEY feedback is the fact that unlike other units in the world, the zinc injection is the only chemistry modification introduced in the primary coolant: other actions supposed to have a positive effect on radiation fields (pH, lithium concentration increase, fuel cleaning) have not been applied. Thereby the actual benefit due the zinc injection can be evaluated for the units similar to those of BUGEY (with alloy 600 and 690). This paper gives an overview of the experience gathered at BUGEY during the first cycles of zinc injection and presents the future EDF strategy concerning zinc injection

Corrosion product transfer in French PWRs during shutdown

International audiencePhysicochemical conditions significantly vary during cold shutdown in French PWRs: power and temperature decrease, chemical conditions are modified (pH decrease and change from reducing to oxidizing conditions). These changes involve corrosion product releases in the primary water (dissolving and crud bursts) which can be seen by a considerable corrosion product increase, from 2 to 3 orders of magnitude, in the activity concentrations (about a hundred GBq/t of 58 Co, about one GBq/t of 60 Co…) and in the chemical species quantity (several kilograms of nickel, several hundred grams of chromium…). The 58 Co peak activities which are lower than the French PWR average are generally due to delayed or incomplete oxygenations or unscheduled shutdowns with oxygenation during operating cycles. An unscheduled shutdown with oxygenation in the last six months of an operating cycle must reduce the 58 Co activity peak during refueling shutdown, more especially as the unscheduled shutdown occurs near the end of the cycle. Thus, when the shutdown happens in the three months prior to the end of the cycle, it is very likely that the 58 Co peak during the refueling shutdown will be two times lower than the expected peak. The large quantity of corrosion products released during shutdown mainly come from the dissolution of the incore deposits due to primary system oxygenation, in particular the dissolution of nickel metal or nickel oxide, leading to the dissolution of the 58 Co derived from the nickel sites in the crud. Thus, the total 58 Co activity released during oxygenation is equal to, or even higher than, the out-of-core total surface activity. Therefore, the cold shutdown procedure must prevent the transfer of this considerable activity from the in-core to the out-ofcore surfaces. Deposited activity measurements carried out in French PWR primary circuits have never shown any visible contamination reduction during cold shutdown. On the contrary, recontamination can occur because of a temperature plateau or an oxygenation at a temperature over 80°C. That is the reason why: • Primary circuit oxygenation at 80°C, • Rapid temperature decrease and • Improvements on primary fluid purification, during cold shutdowns, have, up to this day, been the main points in the EDF strategy to reach the required dosimetric goals (ALARA), all the while optimizing unit outage schedules (availability). This paper thus proposes a description of corrosion product behaviour during French PWR shutdowns in order to optimise cold shutdown procedures

Study of various chemical species behaviour for contamination risk

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lnfluence of major PWR radiochemical pollutants on dose rates and dosimetry - lmportance of efficiency of retention techniques

International audienc

Status of primary system Contamination in French PWRs

International audienceThe reduction of collective dose is an important objective for all nuclear plant operators. Since the beginning of French PWR commissioning, the French strategy has been to study the impact of design and operating parameters. With the codes and the test loops, the third support to these studies is the measurement campaigns of contamination in PWRs.Since 1971, the deposited activities in PWR primary systems have been measured by gamma spectrometry, deposited activities providing more information than dose rates. These gamma spectrometry measurements with the EMECC system represent about 250 measurement campaigns in France and abroad. This paper presents the status of primary system contamination in French PWRs and some of the main lessons learnt from the EMECC campaign analyses.The primary system 60Co contamination in French PWRs has a tendency to increase for several cycles and then to be constant or even to decrease, whereas 58Co contamination tends to decrease on primary coolant piping and to slightly increase on SG tubing. Thanks to the improvements of French PWR design and operation (pH300°C of 7.2, Zry spacer grids, 690TT Steam generator tubing with a low cobalt content…), the dose rate indices tend to decrease with PWR generations and operating cycles