Corrosion neutron activation

Stainless steel contains iron and nickel—important materials in nuclear power reactors and possible constituents of the materials used to construct nuclear test devices or their supporting structures.8 9 During nuclear weapons tests, stable Fe and Ni isotopes are neutron activated, giving rise to radioactive Fe and Ni along with fission products. In nuclear power plants, moreover, stable Fe and Ni isotopes are released from stainless steel through corrosion, become activated, and are transported to different parts of the reactor system. 

In selecting the materials ( W), major emphasis is given to neutron activation and radiation damage effects in materials close to the plasma and further considerations are given to other factors such as corrosion, long-term creep strength, fabrication technology, and cost. 

Various isotope applications are used to monitor the quality of materials and structures. Isotopic tracer techniques measure wear, corrosion, moisture, leakage, and many other factors. Neutron radiography creates images of materials that are not as dense as those captured in X-ray photos. This method is used chiefly to check uranium fuel in nuclear reactors for flaws, to find cracks in the inner plastic or aluminium parts of airplanes, or to detect tiny fissures in gas turbine blades. Californium-252 is used for neutron radiography and neutron activation analyses. 

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... 

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.). 

The investigations cited above (like others performed in this field) yielded results on the amounts of corrosion products formed under PWR operating conditions, but not on the amounts of the associated radionuclides. As was emphasized above, Co source must not be confused with the term of essential interest Co source . Moreover, it has to be pointed out that the conclusions drawn from such evaluations of corrosion rates are only fully valid if the deposition of corrosion products on the fuel rod surfaces and subsequent neutron activation there, i. e. mechanism 1 in Fig. 4.26., were the most important contributor to the production of Co carried in the coolant. If this assumption does not apply, as will be discussed below, then these arguments would be of less significance. In order to correlate the two figures Co supply to the coolant and Co supply to the coolant , the activation period and the neutron flux density to which the different potential sources are exposed also have to be taken into account. Such calculations are comparatively... 

As was pointed out at the beginning of this section, the second possible mechanism of radionuclide production and of contamination buildup is neutron activation of corrosion products which are temporarily deposited inside the neutron field, in particular on the claddings of the fuel rods. These corrosion products mainly stem from the turbine cycle of the plant and are introduced into the reactor pressure vessel with the feedwater. In most of the BWR plants, where all the arising condensates are purified in the condensate polishing system (mainly using precoat... 

The concentrations of the corrosion product radionuclides in the reactor water depend on the same parameters as those that control the behavior of the total corrosion products and, in addition, on the intensity of neutron activation. As a consequence, the concentrations of radioisotopes may vary considerably from plant to plant and also within a plant. According to the observations reported by Anstine et al. (1984), the concentrations of Co and Co, the two most prevalent radioisotopes, do vary, but not as greatly as the iron concentrations. In the BWR plants examined in this context, the majority of Co and Co in the reactor water appeared in the dissolved state. On the average, the concentrations of dissolved Co increased for the first 3 to 5 fuel cycles and then leveled off to values on the order of 7 kBq/1, whereas the dissolved Co already reached its steady-state level in the same range of concentrations during the first fuel cycle. These differences in the time behavior of both cobalt isotopes probably are to be attributed to their different halflives. The concentrations of particulate Co and Co, on the other... 

Radiochemical methods, such as tracer methods [1-3], Mossbauer spectroscopy [4], neutron activation [5], thin layer activation (TLA) [5], ultrathin layer activation (UTLA) [5], and positron lifetime spectroscopy [6], are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows adsorption and electrosorption occurring on the surface of electrodes the role of electrosorption in electrocatalysis deposition and dissolution of metals corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes... 

A large reduction in potential exposure can be achieved at the design stage of reactors and experiments. In particular, for experiments and facilities, care should be taken in the choice of materials that are likely to be activated, with account taken of neutron activation cross-sections, half-lives and corrosion resistance. Impurities in standard materials should be carefully investigated as part of minimizing doses to operating personnel. 

The stainless steel samples for the corrosion experiments were neutron activated in a research reactor. 

Letters, numbers and coloured rings were painted, stenciled or stamped on the field-grey surface paint. Because of corrosion, such markings have disappeared in the course of elapsed decades. For this reason non-destructive techniques like X-ray photography and neutron activation analysis are used when possible for munition identification. 

Radioactivation Techniques Neutron and thin layer (TLA) activation are non-intrusive techniques ofi ering the prospect of continuous, direct component monitoring, in addition to coupon or probe, monitoring. In principle, localised corrosion can be monitored using a double-layer technique. Process plant applications of the technique have been limited to date. ... 

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. 

All other materials used in nuclear reactors for construction or as tubes should exhibit low neutron absorption, low activation, no change in the properties under the influence of the high neutron and y-ray fluxes and high corrosion resistance. These requirements are best met by zirconium which has found wide application in nuclear reactors. Al, Be and Mg have hmited applicabihty. Steel and other heavy metals are only applicable if their relatively high neutron absorption is acceptable. 

The main part of the HLLW is aqueous raffinate from the Purex cycle. It contains 99.9% of the nonvolatile FPs, <0.5% of the uranium, <0.2% of the plutonium, and some corrosion products. For each ton of uranium reprocessed about 5 m of HLLW is produced. This is usually concentrated to 0.5-1 m for interim tank storage specific activity is in the range 10 GBq m. The amounts of various elements in the waste and their concentration in 0.5 m solution is shown in Table 21.9. The HNO3 concentration may vary within a factor of 2 depending on the concentration procedure. The metal salt concentration is 0.5 M it is not possible to keep the salt in solution except at high acidity. The amounts of corrosion products, phosphate, and gadolinium (or other neutron poison added) also may vary considerably. Wastes from the HTGR and FBR cycles are expected to be rather similar. 

SiC/SiC composites are being considered as candidates for structural materials for fusion reactors because of their excellent high-temperature properties and stability, corrosion resistance, thermal conductivity, as well as their low-induced radioactivity by 14 MeV neutron irradiation, quick decay of activity, low aflerheat, low atomic number and good fracture resistance. 

Residual radioactivity. As it was pointed out in [7.4], the amount of the long lived radioactivity generated in sodium by neutrons is negligible. Activation of sodium reaches equilibrium state in about ten years of the first cycle of its use and will never exceed this level. The long-lived radionuclides furnished by fission products, sodium impurities and corrosion activation products are chemical elements alien to sodium, that makes possible its external contamination at the reactor plant decommissioning stage. 

The CVI SiC/SiC composites are also promising for nuclear applications because of the radiation resistance of the p phase of SiC, their excellent high-temperature fracture, creep, corrosion and thermal shock resistances. Studies on the P phase properties suggest that CVI SiC/SiC composites have the potential for excellent radiation stability [5]. Furthermore, because of excellent thermal fatigue resistance, start-up and shut-down cycles and coolant loss scenarii should not induce significant stmctural damage [5]. The CVI SiC/SiC are also considered for applications as stmctural materials in fusion power reactors, because of their low neutron-induced activation characteristics coupled with excellent mechanical properties at high temperature [6-8]. 

---