Activated corrosion products

Oxygen is a prime factor in the corrosion of system materials and the release, activation, and redeposition of activated corrosion products. 

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. 

Other activities are also found in the steam Na, activated corrosion products like Co, and fission products like Sr, " Tc, and Cs. These activities are many powers of ten lower than for the gases. 

Corrosion and fission products appear in dissolved ionic form and "precipitated" in the crud, d nding on the chemistry and water conditions. Most of the corrosion products giving rise to induced activity enter with the feed water. The dominating activated corrosion products are Cr, Mn, Fe, Co, Co, Zn, and Sb, and the dominating fission products are H, Cs, and Cs. Other fission products and actinides are released... 

The main difference between the BWR and the PWR is that the former possesses only one cooling circuit. Therefore, the water of the primary cooling circuit is directly driving the turbines. This has the disadvantage that the turbines have to be shielded in order to protect the personnel from the radioactive water. The radioactivity of the steam from the primary circuit is not due to the presence of fission products, because these remain inside the fuel elements, or to (n,y)-activated corrosion products, because they do not enter into the vapor phase, but to the nuclide N, an emitter of hard y rays of 6.1 MeV with a half-life of 7.13 s. It is produced inside the reactor core from the oxygen of the cooling water by the following reaction (n,p) N. Despite of its short half-life, reaches the turbines because of the... 

The activation products of the coolant, with the sole exception of N, are not of substantial importance in plant operation in some cases, however, they have to be taken into consideration environmentally following release of off-gas or waste water. The fission products and the fuel activation products represent by far the greatest proportion of the radionuclide inventory in the reactor, from the viewpoint of radioactivity as well as from that of radiotoxicity. However, with the exception of severe accidents (which will be treated in Part C), during plant operation they are reliably confined within the fuel rods, so that only the very small amounts released from failed rods to the primary coolant are of interest in this context. Finally, the activated corrosion products are the origin of the buildup of radiation dose rates at the surfaces of the circuits, which potentially complicate the performance of operational procedures, in particular of inspection and repair work. 

Under normal operating conditions, the water-steam circuit (or secondary circuit) of pressurized water nuclear power reactors is completely free of radionuclides. Activated corrosion products and tritium, which have been reported in very low activity concentrations from the water-steam circuits of some high-temperature reactors and which are caused by the neutron Held reaching into the steam generator or by diffusion through intact steam generator heating tubes, do not appear at the PWR secondary side. 

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... 

The radionuclides incorporated into the oxide layers, which lead to a radiation field in the surrounding area, are mainly the activated corrosion product nuclides, above all Co and Co. Out of the fission products present in the primary coolant during plant operation with failed fuel rods in the reactor core, iodine and cesium isotopes are not deposited into the surface oxide layers this reactor experience is consistent with the general chemical properties of these elements which do not allow the formation of insoluble compounds under the prevailing conditions (with the sole exception of Agl, see Section 4.3.3.1.2.). On the other hand, fission product elements that are able to form insoluble compounds (such as oxides, hydroxides or ferrites) in the primary coolant are incorporated almost quantitatively into the contamination layers (see Section 4.3.3.1.4.). However, because of the usually low concentrations of polyvalent fission products in the primary coolant, only in very rare cases will these radionculides make a measurable contribution to the total contamination level for this reason, they will not be treated in this context. 

Thornton, E. W. Activity transport mechanisms in water cooled reactors. Report IAEA TECDOC-667 Coolant Technology of Water Cooled Reactors, Vol. 3 Activity Transport Mechanisms in Water Cooled Reactors., Vienna, 1992, p. 9—51 Walker, S. M., Thornton, E. W. Reanalysis of oxide solubility data. Proc. 5. BNES Conf Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, 1989, Vol. 1, p. 89—95 Walton, G. N., Hesford, E. The migration of activated corrosion products in high-pressure water loops. Proc. Conf Corrosion of Reactor Materials, Salzburg 1962, Vol. 2, p. 547-556... 

Some NPR wastes will, or may, contain fission products or activated corrosion products contamination of a degree to high for river disposal, but ordinarily too low to merit the special treatment afforded decontamination wastes. Such waste streams Include ... 

Besides the activation products and the activated corrosion products (e.g. Co 60, Fe 55, Ni 63), there exist dust-bound fission products (Sr 90, Cs 137, Cs 134 etc.) and partly nuclear fuel fines caused by abrasion. 

The activation of oxygen-17 in the primary coolant results in the formation of C-14. Also present in activated corrosion products (see below). Emits beta radiation and presents no hazard during power operatioa Present in all coolant liquor released during maintenance activities and is an inqxrrtant isotope in radioactive waste management. 

In the design of the coolant circuit and auxiliary circuits, traps where fluid can stagnate and where activated corrosion products can collect should be... 

The main contributor to dose rates during maintenance and repair is activated corrosion products, such as Co, Co, Mn, Fe and Cr. These are present as deposits on all the components and pipes of the primary coolant circuit and the circuits that are connected to it. Fission products such as 1, Cs and Cs make a low contribution to dose rates around these circuits because both the source term and the deposition rate are low. However, this contribution to dose rates may increase significantly in situations where components such as heat exchangers and valves are opened or entered for maintenance and repair. 

In cases where there is a separate oxygen containing fluid moderator system (such as in a pressure tube reactor), the isotope that is the major source of radiation during reactor operation will be N. After shutdown, the radiation levels around the primary coolant system will be due mainly to activated corrosion products. The tritium present in the water coolant or moderator contributes to the radiation hazard only if it is released from the system and becomes airborne. This hazard has to be taken into account in the design of LWRs also since operation with a limited leakage of primary coolant is tolerated. 

Adequate facilities shall be provided for removal of radioactive substances from the reactor coolant, including activated corrosion products and fission products leaking finm the fuel. The capability of the necessary systems shall be based on the specified fuel design limit on permissible leakage with a conservative margin to ensure that the plant can be operated with a level of circuit activity which is as low as reasonably practicable, and that radioactive releases meet the ALARA principle and are within the prescribed limits. 

Dissolved air in the water coolant of a research reactor and pneumatic conveyors will contain radioactive gases, such as from the activation of oxygen, from the activation of carbon in CO2, Ar from the activation of Ar present in air, tritium from the activation of vapour in the air in heavy water reactors, and other gases or vapours that may be released from the core cover gas systems or through leakages from experimental devices. Air may also contain dust particles that have become radioactive, as well as activated corrosion product particles that were released into the air during maintenance operations. Particulate contamination in the air may also arise from irradiations and from various experiments. 

Carbon steel, monel-400, zircalloy-2 and stainless steel form the major materials of construction in the PHWRs. Out of the total surface area of these construction materials exposed to the PHT system about 94% can be attributed to carbon steel, monel-400 and zircalloy-2, and only about 6% to stainless steel. The major chemical constituents of the oxide film are magnetite and nickel ferrite. Activated corrosion products Co-60, Mn-54, etc., and fission products Cs-137, Ce-144, Ru-103, Ru-106 are the contaminants in the oxide film. Stainless steel is the chief construction material for Boiling Water Reactor systm and the major contaminant film consists of chromium rich ferrites. 

The carbon steel and monel-400 coupons exposed to PHT system of PHWR contained both activated corrosion products and fission products. Bench scale and dynamic loop decontamination runs indicated that decontamination factor varying from to 5 to SO could be achieved using the above dilute chemical formulations. 

The adsorption by ion exchange resins of metallic ions removed by dissolution of magnetite layer of the coupon specimens in the decontamination formulations was studied in the glass loop experiments. Inactive metal ions of iron, nickel and activated corrosion products Co-60, Co-58, Mn-54 and fission products Cs-137, Ce-141, Ce-144, Pr-144 were also effectively removed by the cation exchange resins. Ru-103, Sb-125 were taken up by anion exchange resins. Ru-106, Zr-95 and Nb-95 were taken up by bodi the ion exchange resins. 

Nuclear pressurized water reactors (PWRs) use hydrogen peroxide during the plant shutdown to force the oxidation and dissolution of activated corrosion products deposited on the fuel. The corrosion products are then removed with the cleanup systems before the reactor is disassembled. 

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