Plant Lifetime Extension Research Articles

Real-Time Nanogravimetric Monitoring of Corrosion in Radioactive Decontamination Systems

ABSTRACTMonitoring and understanding of corrosion on nuclear sites is key to safe asset management (predicting plant life, assessing efficacy of corrosion inhibitors for plant lifetime extension) and supporting informed choice of decontamination methods for steels due for decommissioning. Recent advances in Quartz Crystal Nanobalance (QCN) technology offer a means to monitor corrosion in-situ in radiologically harsh environments, in real time and with high sensitivity.Oxalic acid has been widely used in nuclear facilities as a corrosion inhibitor for carbon steels and as a decontamination cleaning agent due to its ability to remove rust from the surface of ferritic metals and alloys. As an exemplar system for decontamination, the corrosion behavior of pure iron samples in 1 wt% oxalic acid solution has, for the first time, been measured in simulated radioactive environments in real time, in situ using the QCN. Results show that the QCN is a promising tool for studying the efficacy of oxalic acid decontamination agent formulations with the sensor providing useful mechanistic information regarding decontaminating agent mode-of-action derived from the mass change (gain or loss) in real time when a quartz crystal is immersed in oxalic acid solution in simulated and non–simulated radioactive environments.

Real Time Nanogravimetric Monitoring of Corrosion for Nuclear Decommissioning

Monitoring and understanding of corrosion on nuclear sites plays a key role in safe asset management (predicting plant life, assessing efficacy of corrosion inhibitors for plant lifetime extension) and supporting informed choice of decontamination methods for steels due for decommissioning. Recent advances in Quartz Crystal Nanobalance (QCN) technology offer a means to monitor corrosion in-situ in radiologically harsh environments, in real time and with high sensitivity. Oxalic acid has been widely used in nuclear plants and installations as a corrosion inhibitor for carbon steels and as a decontamination cleaning agent due to its ability to remove rust from the surface of ferritic metals and alloys. As an exemplar system for decontamination, the corrosion behavior of mild carbon steel and pure iron samples in 1 wt% to 8wt% oxalic acid solutions have, for the first time, been measured and compared in real time and in situ using the QCN. Corrosion rates measured using the QCN are found to agree with those obtained using corrosion current (iCORR) measurements, with the added advantages of: (i) real time and potentially in situ and higher sensitivity measurement; (ii) reduced uncertainty in the conversion of the QCN measured frequency change to a mass change-based corrosion rate compared to the conversion of the icorr measured by LSV with Tafel extrapolation to similar; (iii) the provision of mechanistic insights into the action of oxalic acid on Fe-rich steels.

Real-Time Nanogravimetric Monitoring of Corrosion in Radioactive Environments

Monitoring and understanding of corrosion on nuclear sites plays a key role in safe asset management (predicting plant life, assessing efficacy of corrosion inhibitors for plant lifetime extension) and supporting informed choice of decontamination methods for steels due for decommissioning. Recent advances in Quartz Crystal Nanobalance technology offer a means to monitor corrosion in situ in radiologically harsh environments, in real time and with high sensitivity. The QCN measures minute changes in frequency of a quartz crystal resonator with weight gain/loss. Using the Sauerbrey equation, the drop in frequency observed during corrosion testing can be converted to an instantaneous corrosion rate with nanogram sensitivity. Initial experiments will concentrate on determining corrosion rates in acids and complexants used in chemical decontamination processes, particularly methods involving the commonly used cleaning agents nitric acid and oxalic acid (e.g. the CORD-UV process). Oxalic acid is currently being studied as an Enhanced Chemical Cleaning (ECC) decontamination agent in the decommissioning of high level waste (HLW) storage tanks at the Hanford and Savannah River Sites (SRS) in the US. The tanks are comprised of low carbon steel; thus the decontamination process must be carefully monitored to avoid over-aggressive decontamination that may result in a loss of asset structural integrity. To avoid this, the oxalic acid concentration being used has been reduced to 1 wt% from the 4-8 wt% range typically used during decontamination campaigns. This has the additional advantage of avoiding downstream precipitation of oxalic acid’s sparing solubility of Na salt, an outcome that presents process engineering and secondary waste management issues in sodium rich environments However, the efficacy of the ECC and the corrosion behaviour of steels at low oxalic acid concentrations is not widely studied and knowledge gaps remain. These gaps afford an ideal opportunity for achieving the twin objectives of device development whilst providing new insights into the behavior of a hitherto unstudied corrosion vulnerable system. Work to date has concentrated establishing the validity of using pure iron as a low carbon steel surrogate, so allowing the use of less resource intensive iron-based QCN crystals (compared to high unit price steel crystals) for corrosion monitoring during oxalic acid-driven decontamination. Cyclic voltammetric studies of mild carbon steel and iron electrodes reveal a commonality of behavior in 1, 2 and 8 wt% (not shown). In summary: as the applied potential increases in the range -1 to +1 V, iron(II) generated at the electrode surface at ~-0.5 V reacts with solution oxalate to form an insoluble passivating Fe(II) oxalate layer. This persists until +0.4 V whereupon Fe(II) oxalate is oxidized to soluble Fe(III) oxalate, a process that continues until repassivation at ~0.75 V. Studies using the QCN on Fe electrodes support this interpretation. During the forward scan of the cyclic voltammassogram of Figure 1 a mass increase associated with the formation of the Fe(II) oxalate layer is observed at -0.4V whilst a mass loss, associated with layer oxidation to soluble Fe(III) oxalate, is seen at 0.5V. Total mass change in either direction is 18718 ng cm-2. The evolution of the chemical composition during the formation and subsequent removal of this protective oxalate layer, the presence of which has ramifications for decontamination efficiency, is currently the subject of in situ Raman spectroscopy experiments in our laboratory. Figure 1: Cyclic voltammassogram of an iron crystal in 1wt% oxalic acid solution. Figure 1

The Power Distribution and Neutron Fluence Measurements and Calculations in the VVER-1000 Mock-Up on the LR-0 Research Reactor

Abstract The power density distribution in a reactor has significant influence on core structures and pressure vessel mechanical resistance, as well as on the physical characteristics of nuclear fuel. This quantity also has an effect on the leakage neutron and photon field. This issue has become of increasing importance, as it touches on actual questions of the VVER nuclear power plant life time extension. This paper shows the comparison of calculated and experimentally determined pin by pin power distributions. The calculation has been performed with deterministic and Monte Carlo approaches. This quantity is accompanied by the neutron and photon flux density calculation and measurements at different points of the light water zero-power (LR-0) research reactor mock-up core, reactor built-in component (core barrel), and reactor pressure vessel and model. The effect of the different data libraries used for calculation is discussed.

FISA-2009 Conference on Euratom Research and Training Activities: Nuclear Fission – Past, Present and Future (Generation-II, -III and -IV + Partitioning and Transmutation)

This paper is an introduction to the research and training activities carried out under the Euratom 7th Framework Programme (FP7, 2007–2011) in the field of nuclear fission science and technology, covering in particular nuclear systems and safety, and including innovative reactor systems and partitioning and transmutation. It is based on the more than 40 invited lectures that were delivered by Euratom project coordinators and keynote speakers at the FISA-2009 Conference ( FISA, 2009), organised by the European Commission DG Research, 22–24 June 2009, Prague, Czech Republic. The Euratom programme must be considered in the context of current and future nuclear technology and the respective research effort: • Generation-II (i.e. yesterday, NPP construction 1970–2000): safety and reliability of nuclear facilities and energy independence in order to ensure security of supply worldwide; • Generation-III (i.e. today, construction 2000–2040+): continuous improvement of safety and reliability, and increased industrial competitiveness in a growing energy market; • Generation-IV (i.e. tomorrow, construction from 2040) for increased sustainability though optimal utilisation of natural resources and waste minimisation, and increased proliferation resistance. Consequently, the focus of the lectures devoted to Generation-II and -III is on the major scientific challenges and technological developments needed to guarantee safety and reliability, in particular issues associated with plant lifetime extension and operation. The focus of the lectures devoted to Generation-IV is on the design objectives and associated research issues that have been agreed upon internationally, in particular the ambitious criteria and technology goals established at the international level by the Generation-IV International Forum (GIF). In the future, electricity must continue to be produced competitively, and in addition high temperature process heat may also be required, while exploiting a maximum of fissile and fertile material and recycling all actinides, both safely and reliably. Scientific viability studies and technological performance tests for each Generation-IV system are now being carried out in many European Union (EU) Member States, in collaboration with other laboratories worldwide as part of the inter-governmental GIF agreement. The ultimate phase of commercial deployment will be from 2040, but no one can predict accurately when industry and investors will make firm, often difficult decisions regarding the construction of these very innovative Generation-IV systems. However, to be deployed commercially, it must first be demonstrated that Generation-IV technology can be a beneficial, responsible and sustainable response to the long-term challenges faced by society to establish a low-carbon economy.

Coke Oven Carbon Deposits Growth and Their Burning Off

Burning off is the main method of carbon deposit removal from coke oven chambers. Optimisation of this operation is of a great interest both from the point of view of current production loss reduction and of coke oven plant lifetime extension. In this study the deposits originating from different parts of chambers have been characterised by chemical analysis, optical microscopy, XRD, Raman spectroscopy and mercury porosimetry. This investigation has shown that deposits contain two carbonaceous phases: the char from pyrolysed carry over coal particles and the pyrolytic carbon from raw gas cracking. Furthermore, the monitoring method for carbon deposits growth in coke oven chamber is presented. The statistical analysis of the visual inspection data has been a useful tool in the evaluation of the rate of deposit growth in individual coke oven chambers. The combustion kinetics of carbon deposits were studied by thermogravimetry under non-isothermal conditions with natural air draft and under isothermal conditions at controlled air flow rate. It was found that the reactivity of carbon deposits is in-between metallurgical coke and electrographite reactivity. At the temperatures below 700 degrees C, under laboratory conditions, the combustion rate (on the basis of external surface) is controlled by surface reaction at higher temperature and activation energy is around is 170 kJ/mol. At temperature higher than 700 degrees C combustion is controlled by gas film diffusion. Modelling of combustion in thermobalance experiments and in industrial coke oven chamber was carried out on the basis of measurement of kinetics using commercial CFD software.

Composition and magnetic character of nanometre-size Cu precipitates in reactor pressure vessel steels: Implications for nuclear power plant lifetime extension

The sensitivity of positrons to nanometre-size Cu precipitates in Fe alloys has enabled us to apply a novel spin-polarized element-specific method to probe the composition and magnetic character of the precipitates responsible for reactor pressure vessel (RPV) steel embrittlement. The results clearly show that the precipitates are non-magnetic and place an upper limit of about 10at% on their Fe content. The practical implication of this result is that the Cu precipitate contribution to RPV steel embrittlement saturates is not expected to contribute further during lifetime extension. Our study demonstrates that polarized positrons can be used as a powerful probe of the magnetic character of nanoscale materials, even those embedded in a strongly magnetic host.

A Modification of the Inner and Outer Core for Reactor Pressure Vessel Lifetime Extension

The feasibility of nuclear power plant lifetime extension was examined by reducing the fast neutron fluence at the reactor pressure vessel (RPV) and relieving irradiation embrittlement of materials, and thus ensuring enough structural integrity beyond the design lifetime. Two fluence reduction options, peripheral assembly replacement and additional shield installation in the outer core structures, were applied to the Kori Unit-1 reactor, and the fluence reduction effect was carefully analyzed. For an accurate estimate of the neutron fluence at the RPV and a reasonable description of the modified peripheral assemblies, a full-scope explicit modeling of a Monte Carlo simulation was employed in all calculations throughout this study. The Kori Unit-1 cycle-16 core was modeled on a three-dimensional representation by using the MCNP4B code, and the fluence distribution was estimated at the inner wall beltline around the circumferential weld of the RPV. On the basis of fracture toughness requirements of the RPV, the two modified cases were predicted to have an additional life of 7 to 10 effective full-power years. Throughout the core nuclear characteristics analyses, it was confirmed that the critical peaking factors for safe reactor operation were satisfied with the design limits.

A Fast Neutron Fluence Reduction Strategy for Pressure Vessel Lifetime Extension

The feasibility of nuclear power plant lifetime extension was examined by the evaluation of fast neutron (>1 MeV) fluence at the reactor pressure vessel (RPV). A fluence reduction option, additional shields installation in outer core structures, was applied to the Kori Unit 1 reactor, and its fluence reduction effect was carefully analyzed. Full-scope explicit modeling of Monte Carlo simulation with MCNP4A code was employed in the fast neutron fluence calculations. An optimized design option was found from various choices in geometry and material for shield structure. It is expected that the magnitude of fast neutron fluence would be reduced by 39% at the circumferential weld of the RPV. On the basis of the criterion of pressurized thermal shock requirement of the RPV, it is predicted to obtain the additional life of 4.4 effective full power years. It was also investigated that the nuclear characteristics and thermal hydraulic factors at the internal core were negligibly influenced by the installation of additional shield structure.