Shutdown corrosion products

In the determination of the mass of the deposited corrosion products there is still the question of whether or not the shutdown corrosion product spiking (see... 

Table 2. Corrosion Products Contributing to PWR Shutdown Radiation Levels ...

Continuous releases of concentrated HjS streams must be segregated in a separate flare system to limit the extent of fouling and plugging problems. Releases of HjS such as diversion of sour gas product to flares during shutdown or upset of a downstream sulfur recovery unit are considered to be continuous, but safety valve releases are not included in this category. However, if a special HjS flare system is provided for continuous releases, the concentrated HjS safety valve releases should be tied into it rather than into the regular flare system. Due to the nature of HjS one should plan on frequent inspection and flushing of HjS flares to remove scale and corrosion products. 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

The fuel filters were then partially blocked by rust and corrosion products carried by the water. Below-freezing temperatures possibly initiated the formation of ice within fuel filters and in fuel lines containing water. The combination of ice and corrosion products blocked fuel flow through vehicle fuel lines and filters. Gasoline flow to the engine was halted and engine shutdown followed. 

The primary incentive for returning condensate is to recover its heat value. The first condensate that returns after a shutdown will have very little recoverable heat value and will contain higher levels of unwanted corrosion products. However, the level of corrosion products will probably be higher than normal even when hot condensate starts returning to the boiler house. 

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). 

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. 

Accumulation of undesirable corrosion products on heat exchanger tubing and pipelines decrease the heat transfer efficiency and reduce the pumping capacity. Soluble corrosion products can contaminate a system, and decontamination of the system results in additional cost. An example of this is the expensive shutdowns of nuclear reactors during the decontamination process. 

Stable emulsion formation] [surfactants present] /contamination by particulates example, products of [corrosion products, see Section 1.3], amphoteric precipitates of aluminum or iron/pH far from the 2pc/contamination by polymers/tem-perature change/decrease in electrolyte concentration/the dispersed phase does not preferentially wet the materials of construction/coalescence -promoter mal-fimctioning/improper cleaning during shutdown/[rag buildup]. ... 

In evaluating fuel rod failures from the presence of these fission products in the primary coolant, it has to be considered that some of them are also formed by neutron activation of metallic core materials, e. g. Zr/ Nb, Mo/ Tc, Sb. Determining which of these two production mechanisms is responsible for the coolant activity of the isotopes just mentioned is often difficult. In some cases, the shutdown spiking can be used to identify their origin from failed fuel rods however, in most cases this effect is not unequivocal since the radioactive corrosion products also show such a spiking (see Section 4.4.). 

During shutdown of a PWR plant and, somewhat less pronounced, also during its startup, a strong increase in the concentrations of corrosion products and the associated radionuclides in the primary coolant is observed, an effect which will be discussed in more detail in Section 4.4.3.3. This process causes a dissemination of the radionuclides over the entire primary circuit and, as a possible consequence, results in increased radiation dose rates in the area surrounding it. For this reason, the origin of these radionuclides is of interest, with their possible source being the resuspension of activated corrosion products previously deposited either inside or... 

Some observations made at operating PWR plants have suggested the idea of influencing the primary circuit dose rates by appropriate modalities of the reactor shutdown procedures, e. g. in the course of refuelling, either to achieve minimum corrosion product redistribution during the shutdown spiking or to achieve a certain decontamination of the out-of-core surfaces. In this context, different procedures carried out at PWR plants have been reported however, when evaluating the results obtained one has to bear in mind that in some of the reported procedures it is not possible to correlate cause and effect properly therefore, the possibility of pure coincidence cannot always be ruled out. 

Amey, M, D. H, Johnson, P. A. V. Improved chemistry Key to reduce occupational radiation exposure in water-cooled reactors. Nucler Europe Worldscan X, 34-36 (1990) Anthoni, S., Ridoux, P, Chevalier, C. Evaluation of corrosion products in a pressurized water reactor during shutdown. Proc. 5. BNES Conf. Water Chemistry of Nuclear Reactor Systems. Bournemouth, UK, 1989, Vol. 2, p. 35-39 Antipov, S. A., Dranenko, V. V., Konkov, V. F., Sokolov, I. B., Khaikovskij, A. A. Influence of fuel clad surface on performance under operational and accident conditions. Report... 

Jarnstrom, R. T. Corrosion product behaviour in Loviisa NPP primary coolant and measures taken to lower the radiation levels by modified startup and shutdown procedures. IAEA Internat. Symp. Water Chemistry and Corrosion Problems of Nuclear Reactor Systems and Components, Vienna 1982, Paper IAEA-SM-264/8 James, D. W. Problem assessment of discrete radioactive particles. Report EPRI NP-5969... 

The changes in the ambient conditions that occur in the course of a shutdown of the plant facilitate the release of zinc from the iron oxide fuel deposits, resulting in a Zn spiking similar in magnitude to that of the other corrosion product radionuclides. 

Protection of Austenitic Stainless Steel from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment Collection and Identification of Corrosion Products Initial Conditioning of Cooling Water Equipment On-Line Monitoring of Cooling Waters... 

Other common transition metal corrosion products typically monitored at various sites within the plant include iron, copper, nickel, zinc, and chromium. More than 80% of BWR plants analyze for iron, nickel, copper, and zinc in reactor water, and nearly all of the BWR plants determine these metals in feed water. In addition, zinc is also an additive used in many plants to control the shutdown radiation dose rate. Nickel and chromium are corrosion products in BWR plants fi-om stainless-steel piping. The best selectivity and sensitivity for achieving low to submicrogram/Liter detection limits for transition metals can be obtained by separating transition metal complexes using pyridine-2, 6-dicarboxylic acid (PDCA) or oxalic acid as chelators in the eluent, followed by postcolumn derivatization with 4-(2-pyridylazo)resorcinol (PAR) and absorbance detection at 520 nm (see Section 8.2.1.2). This approach was successfully used to determine trace concentrations of iron, copper, nickel, and zinc in BWR and PWR matrices [197]. Figure 10.113 compares the chromatograms from the... 

Polythionic acid solutions can readily form at ambient temperatures in process equipment during shutdowns. Only the interaction of moist air and iron sulfide corrosion products is required. 

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