Corrosion products coolant

Water as coolant in a nuclear reactor is rendered radioactive by neutron irradiation of corrosion products of materials used in reactor constmction. Key nucHdes and the half-Hves in addition to cobalt-60 are nickel-63 [13981 -37-8] (100 yr), niobium-94 [14681-63-1] (2.4 x 10 yr), and nickel-59 [14336-70-0] (7.6 x lO" yr). Occasionally small leaks in fuel rods allow fission products to enter the cooling water. Cleanup of the water results in LLW. Another source of waste is the residue from appHcations of radionucHdes in medical diagnosis, treatment, research, and industry. Many of these radionucHdes are produced in nuclear reactors, especially in Canada. 

When antifreeze becomes unsuitable for use, either because of depletion of inhibitors, presence of corrosion products or corrosive ions, or degradation of the fluid, recycling and reuse of the antifreeze, rather than disposal, may be considered. Although ethylene glycol is readily biodegraded in typical municipal waste treatment faciHties, antifreeze disposal becomes problematic because the coolant may contain hazardous quantities of heavy metals picked up from the cooling system. Recycling may be economically preferred over coolant disposal and reduces the concern for environmental impact. 

Normal mill coolant pH was near 5. The upset caused large amounts of iron corrosion products to be swept into the coolant. Settling of iron oxides and hydroxides fouled many mill components. 

When corrosion products are deposited on the fuel surfaces, they are activated by neutron capture. Some of the most prominent of these activities are 55Fe, 63Ni, 60Co, 54Mn, 58Co, and 59Fe. These radionuclides will then be found in the reactor coolant. 

Radioactivity transport in reactor coolant circuits involves both surface corrosion and deposition. Several XPS studies(8,9) of reactor boiler alloys have been reported which show the very strong effect of coolant chemistry on the films deposited. The chemistry of corrosion products precipitated on ZrO and Al O surfaces has been studied using XPS.ly More recently, chemical decontamination of radioactive boiler circuits has been assisted by XPS analysis of the surface-active decontaminating agent.(1 ) Surface oxidation in gas-cooled reactor circuits has also been investigated. AES has been used to follow the CC>2 oxidation of a chromium steel(H) and some pure metals. (12)... 

The initial surface composition of boiler tubing, prior to its installation will have an important impact on the amount and type of activated corrosion products in an aqueous reactor coolant. Consequently, the type of thermal pre-treatment the tubing undergoes, for example, for mechanical stress release,will affect the surface oxide film, and ultimately, the corrosion behavior. This particular work has been directed toward characterization of surface oxide films which form on Inconel 600 (nominal composition 77% Ni, 16% Cr, 7% Fe, — a tradename of Inco Metals Ltd., Toronto Canada) and Incoloy 800 (nominal composition 31% Ni, 19% Cr, 48% Fe 2% other, — a tradename of Inco Metals Ltd., Toronto, Canada) heated to temperatures of 500-600°C for periods of up to 1 minute in flowing argon. These are conditions equivalent to those experi enced by CANDU(CANadian Deuterium Uranium)ractor boiler hairpins during in situ stress relief. 

Flow sheets of the chemical plant were prepared with a pre-design of the main equipment. For the capital cost assessment, the factored estimate has been chosen because it considers characteristics of the process corrosive products, high temperature (up to 850°C for Section II) and high pressure (up to 50 bar for the helium coolant). The factored method is essentially based upon charts and formulas developed over 30 years in the petroleum and chemical industries in France (Chauvel, 2000). It consists of estimating costs of basic equipment (generally carbon steel) and correcting them for materials factors. 

The primary motivation for predicting the electrochemical properties of the coolant circuits of water-cooled nuclear power reactors has been that of explaining and predicting tenacious operating problems that include SCC and CF, mass transport of corrosion products and subsequent fouling of heat transfer surfaces, activity transport due to the movement of neutron-activated radionuclides from the core to out-of-core surfaces that are not shielded, and, in the case of PWRs, the axial offset anomaly (AOA). This latter phenomenon results from the deposition of boron... 

Boiler tubes are often cleaned with EDTA or NTA solutions to remove both CaC03 scale and corrosion products. In pressurized heavy water nuclear power reactors, radioactive corrosion deposits (in effect, magnetite in which some of the Fe has been replaced by radioactive Co ) can be removed from the coolant water circuits with an aqueous mixtiu e of oxalic and citric acids (both good chelators for Fe " ") and EDTA. In home laundry operations, bloodstains on clothing can be removed by treatment with oxalic acid, which takes up the iron from the hemoglobin (Section 8.2) as Fe(ox)3 . By the same token, oxalates axe toxic when taken internally, as are many other complexing agents. For example, EDTA is used as a means... 

Rodliffe, R.S. and Means, F.A., 1979, Factors Governing Particulate Corrosion Product Adhesion to Surfaces in Water Reactor Coolant Circuits. Report CEGB RD/B/N4525. 

An important area where improved reactor-system design and operation have been achieved is in the control of all aspects of coolant chemistry. The major development here has been the identification of the factors controlling movement of corrosion products by the coolant into the reactor core where they are activated, and the subsequent deposition of these radioactive species on out-reactor components causing radiation fields that may interfere with maintenance work during shutdowns. In commercial CANDU reactors the fields from such long-lived radioactivity have been controlled successfully to low values (17). 

The water chemistry of CANDU reactors embraces control of corrosion and corrosion-product transport in the coolant system, control of radiolytic decomposition of the moderator (51) and control of the concentration of soluble neutron absorbers used to adjust reactivity and control of boiler-water chemistry to minimize tube corrosion (52). The major chemical engineering effort has dealt with coolant technology and I will confine this review to that aspect of water chemistry. 

The important chemical processes which can occur in the coolant are radiolytic decomposition to produce oxygen, corrosion of the system materials, dissolution of the metal oxides so formed, deposition of corrosion products on the system surfaces, and transport of radioactive nuclides generated within deposits on the fuel sheaths. The major sys-... 

Activity transport effects can be minimized by selecting materials with a low cobalt content and by rigid adherence to chemical specifications for the coolant. Because of the important role of corrosion product particles in this transport, filtration has been studied extensively as a means of reducing the rate of growth of radiation fields. High flows are needed to be effective and therefore the filters must operate at full coolant temperature. Two types of filter which have proved successful in pilot tests at the NPD reactor are a deep bed of graphite particles and a bed of steel balls in an electromagnetic field (61). 

Heavy fuel deposits were expected in boiling systems, and therefore the initial studies of deposition and activity transport for power reactors concentrated on the CANDU-BLW concept until the fields at Douglas Point became a concern. The deposit thickness was proportional to iron concentration in the coolant and to the square of the heat flux (69) deposition was reversible and quickly reached a steady value set by the local conditions. The corrosion products initially deposit by hydrodynamic and electrostatic effects then boiling accelerates deposition by drawing water and its contained iron into the deposit to replace the steam that leaves. Local alkalinity gradients within the deposit determine whether iron crystallizes to cement the deposit or dissolves to weaken it, and erosion processes then define the equilibrium thickness (70), This model works well in explaining deposition under boiling conditions. 

Corrosion mechanisms can become very damaging if not controlled. They are identified in Module 2, Properties of Metals. High corrosion resistance is desirable in reactor systems because low corrosion resistance leads to increased production of corrosion products that may be transported through the core. These products become irradiated and contaminate the entire system. This contamination contributes to high radiation levels after shutdown. For these reasons, corrosion resistant materials are specially chosen for use in the primary and secondary coolant systems. 

Water purity is of extreme importance since even with concentrations of metallic or halide ions as low as 0.1 ppm, the corrosion of certain metals can become excessive. In addition, if corrosion prodncts are allowed to accumulate on heat transfer surfaces, on fuel elements, or in small flow passages, overheating, lowered efficiency, and actual storage and actual breakdown of the reactor may result Another consequence of the accumulation of corrosion products is a high level of radioactivity in the coolant water, which is in most cases highly undesirable. 

It is essential for steady and safe operation of a sodium cooled fast reactor to limit the coolant and cover gas impurities to prevent corrosion of reactor component materials and to reduce radiation dose by corrosion products (CPs). Therefore, impurity concentrations of both coolant sodium and cover gas argon were measured during the duty cycle operation and annual inspection period. The sodium impurity data include oxygen, hydrogen, nitrogen, chloride, tritium, metal elements and radioactive ° Ag, Na, Xs. The cover gas impurity data include O2, N2, CO, CO2, H2, CH4, He, H and radioactive xenon and krypton isotopes. 

Methods of control of fission and corrosion products activity in the coolant and on the primary piping walls have been developed. 

From the results achieved in the DECODE project, it can be concluded that the accumulation of cationic stainless steel corrosion products in the electrolyte membrane can be suppressed by proper design of the MEA, particularly by avoiding any contact of free electrolyte membrane with liquid water originating from the coolant or from condensate accumulated in the active area. Furthermore contact of corrosion inducing contaminants such as for example chloride ions with the metallic bipolar plates must be prevented. 

The interior surfaces of the vessel are typically cladded with austenitic stainless steel. Cladding is used to prevent general corrosion by coolant and to minimize the build-up of corrosion products in the reactor coolant system. The cladding is applied in one or two layers by single-wire, multiwire or strip-cladding techniques. 

Another possibility is to develop a direct gas turbine system at high temperature and pressure to increase the efficiency. Since helium has small cross section for neutron capture and is also chemically inactive, no induced radioactivity and no corrosion products can be expected in the coolant. Furthermore, the oxidative microparticle fuel coated with layers such as pyrolytic carbon or silicon carbide is expected to prevent the release of the fission gas into the coolant. Eventually the HTGR is suited to adopt the direct cycle, i.e., the gas turbine cycle, which allows building a more compact electric-power facility than the indirect cycle with a steam turbine system. While the thermal efficiency of electricity generation by steam turbine is about 40%, the gas turbine cycle can increase the efficiency up to ca. 50%. 

Particulate fouling may be defined es the accumulation of particles from liquid or gaseous suspensions onto heat transfer surfaces. Particulate fouling occurs in a wide range of situations. In liquid streams the best known example is probably that of corrosion products In boiler waters and reactor coolants. In... 

In addition, chemical and radiochemical analyses are performed to determine boron concentration, fission and corrosion product activity, crud concentration, dissolved gas and corrosion product concentrations, chloride concentrations, coolant ph, conductivity of the reactor coolant, and noncondensible gas concentration in the pressurizer. 

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