Role of Chemistry in the Phenomena Occurring in Nuclear Power Plants Circuits

Reactor physics, thermal hydraulics or material sciences are mainly studied to understand phenomena occurring in nuclear power plants or to improve the performance of existing or future reactors: improvement of core performances, research of new materials (fuels, core, reactor pressure vessels, internal structures, …) and so on. Nevertheless, in the same way as these physical sciences, chemical sciences (solution chemistry, thermodynamics, kinetics, ...) can also contribute to these objectives. Firstly, to improve availability, safety and lifetime of existing reactors, the best chemical conditioning has to be used in the different nuclear reactor circuits (primary, secondary, tertiary and auxiliary circuits). In that way, solution pH (B-Li coordinated chemistry), redox (value and chemical choice), water treatment, on-line chemical automates have, for example, to be optimized. Secondly, chemistry allows understanding phenomena occurring in the different reactors circuits. In the primary circuit, the understanding of the contamination by corrosion products (Fe, Ni, Cr, …), activation products (58Co, 60Co, …), fission products and actinides is a crucial issue for reactor operation and design. The main processes involved in the contamination transfer are dissolution/precipitation, erosion/deposition, convection, purification, neutron activation, radioactive decrease. Consequently, the chemistry knowledge has a role to play in the same way as other sciences (nuclear physics, material science, thermohydraulics, …). In the secondary circuit, formation of concentrated media, which can lead to the tube fouling and sometimes to the tube support plates blockage of steam generators, is of major concern. These phenomena have for consequences a loss of thermal performance and efficiency of SGs. Chemistry, in addition to thermal hydraulics, can help to explain them and consequently to their mitigation by, for example, optimization of the secondary side chemical conditioning (pH level, amine choice, …). In the tertiary circuit, scale, corrosion, deposits and microbial growth affect the plant performance and can be partially controlled by a “good” chemistry. To understand the chemistry role or impact, an important R&D work is necessary. Indeed, only few data exist in the literature for the physico-chemical conditions met in a nuclear reactor. So these data have to be obtained either experimentally (solubility measurements, liquid-steam equilibrium study, solid solution interfacial flux measurements and so on) or by extrapolation (which implies the development or the use of theoretical models (for example, for temperature or ionic strength effects)). Then, chemical database have to be built and simulation codes (for example, to describe cold shutdowns or SGs tube fouling) have to be developed. These R&D studies are either home-made or realized in collaboration with many companies such as EDF, AREVA NP or GDF SUEZ. The goal of this paper is to present the CEA methodology used for these R&D studies and few applications of chemistry to the understanding of some phenomena occurring in water-cooled nuclear reactors

Preventing iron( II ) precipitation in aqueous systems using polyacrylic acid some molecular insights

International audienceWe present molecular dynamics simulations of aqueous iron(II) systems in the presence of polyacrylic acid (PAA) under the extreme conditions that take place in the secondary coolant circuit of a nuclear power plant. The aim of this work is to understand how the oligomer can prevent iron(II) deposits, and to provide molecular interpretation. We show how, to this end, not only the complexant ability is necessary, but also the chain length compared to iron(II) concentration. When the chain is long enough, a hyper-complexation phenomenon occurs that can explain the specific capacity of the polymer to prevent iron(II) precipitation

Simulations of corrosion product transfer with the PACTOLE V3.2 code

International audienceActivated corrosion products generate a radiation field in PWRs, which is the major contributor to the dose absorbed by nuclear power plant staff working during shutdown operations and maintenance. Therefore, a thorough understanding of the mechanisms that control the corrosion product transfers is of the highest importance. For about thirty years, the French strategy has been based on experiments in test loops representative of PWR conditions, on in-situ gamma spectrometry measurements of the PWR primary system contamination and on simulation code development. The simulation of corrosion product transfers in PWR primary circuits is a major challenge since it involves many physical and chemical phenomena including: corrosion, dissolution, precipitation, erosion, deposition, convection and activation. In addition to the intrinsic difficulty of multi-physics modelling, the primary systems present severe operating conditions (300°C, 150 bar, neutron flux, water velocity up to 15 m.s-1 and very low corrosion product concentrations). The purpose of the PACTOLE code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the contamination of the PWR primary system. The PACTOLE code allows researchers to analyse the corrosion product behaviour and to calculate the activity in the fluid and the surface activity in the primary system. Nowadays, the PACTOLE code is considered to be not only a tool for numerical simulations and predictions but also one that might combine and organize all new knowledge useful to progress on contamination caused by activated corrosion products. This paper presents the modelling implemented in the PACTOLE V3.2 code. In this version, the chemical aspect has been improved by coupling the PACTOLE code to a chemistry code. Comparisons with in-situ gamma spectrometry measurements of PWR primary systems and studies based on experimental feedback are shown

The OSCAR code package : A unique tool for simulating PWR contamination

International audienceUnderstanding the PWR primary circuit contamination by corrosion products, fission productsand actinides is a crucial issue for reactor operation and design. The main challenges aredecreasing the impact on personnel exposure to radiation, optimizing the plant operation,limiting the activity of the wastes produced during the reactor lifetime and preparingdecommissioning.In cooperation with EDF and AREVA NP, CEA has developed the OSCAR code package, aunique tool for simulating PWR contamination. The OSCAR package results from the mergingof two codes, which simulate PWR contamination by fission products and actinides (PROFIPcode) and by activated corrosion products (PACTOLE code).These two codes have been validated separately against an extensive set of data obtained over 40years from in-situ gamma spectrometry measurements, sampling and analysing campaigns ofprimary coolant, as well as experiments in test loops or experimental reactors, which arerepresentative of PWR conditions.In this paper, a new step is presented with the OSCAR code package, combining the features ofthe two codes and motivated by the fact that, wherever they originate from, the contaminationproducts are subject to the same severe conditions (300 °C, 150 bar, neutron flux, water velocityup to 15 m.s-1) and follow the same transport mechanisms in the primary circuit. The main processes involved are erosion/deposition, dissolution/precipitation, adsorption/desorption,convection, purification, neutron activation, radioactive decrease.The V1.1 version of the OSCAR package is qualified for fission products (Xe, Kr, I, Sr),actinides (U, Np, Pu, Am, Cm) and corrosion products (Ni, Fe, Co, Cr).This paper presents the different release modes (defective fuel rod release, fissile materialdissemination, material corrosion and release), then the processes which govern contaminationtransfer, and finally, we give examples of the comparison of the OSCAR package results withmeasurements in French PWR primary circuit obtained for representative radioisotopes : $^{133}$Xe,$^{90}$Sr, $^{58}$Co, $^{60}$Co. In particular, we focus on the main upgrades in the OSCAR simulations compared to thePROFIP and PACTOLE codes : adaptation of the MARGARET module to assess fission productrelease out of fuel pellets in a defective rod, adsorption/desorption model development forstrontium behaviour, multi-criteria calibration of input data which are not well known forcorrosion product simulation