BWR coolant chemistry

Normally, BWR reactor water is not treated by the addition of chemicals. The relevant specifications require high purity of the reactor water and the feedwater as is shown in Table 2.3. (Riess, 1991). This high purity guarantees a minimum corrosion of the materials of the circuits. 

During operation, impurities are inevitably introduced into the reactor water, though all necessary measures are taken to minimize the ingress of foreign substances. Impurities that are introduced by the feedwater into the reactor water are 

Cation conductivity (25 °C) Total iron Total copper Oxygen 

In order to minimize contamination buildup in the plant primary system, addition of zinc to the reactor water has been proposed. Laboratory investigations have 

In RBMK reactors, oxygen (up to 0.2mg/kg) is injected into the feedwater circuit downstream from the condensate polishing system in order to prevent corrosive attack on the perlitic steels, and then removed from the water in the deaerator (Dragunov et al., 1992). Attempts have also been made to inject hydrogen as a remedy for intergranular corrosion attack on stainless steels. 

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Since its first introduction in the early 1990s, HWC [58,59] has undergone steady development, particularly with regard to enhancing the effectiveness of the technique by noble metal chemical additions (NMCA), after the limitations of HWC had been made clear by electrochemical analysis. By the mid-1990s, it was evident that many of the answers that were sought with regard to the appKcation of HWC would be best provided by sophisticated BWR coolant chemistry models [6, 9,13,14,19,... 

Experimental studies of the effect of flow rate on the ECP of Types 304SS and Alloy 182 in high-temperature water have been reported by Macdonald et al. [51], on Type 316SS under simulated BWR chemistry conditions by Kim et al. [52, 53], and on Type 304SS by Prein and Molander [54]. In the work of Macdonald et al. [51] tubular flow at low flow rates was employed which, while not simulating the exact conditions in BWR coolant circuits did provide a sensitive test of the MPM. A comparison between experiment and theory is shown in Fig. 21. The experimental... 

At this point, it is worth enquiring if these calculations have any relationship to reality. While it is very difficult to obtain information on crack extension in reactor coolant circuits for a variety of reasons, Tang etal. [63] published the data shown in Fig. 35. The data refer to the extension of a crack adjacent to the H-3 weld on the inner surface of the core shroud of a GE BWR in Taiwan. The authors had monitored the growth of the crack as a function of time after the eleventh outage for refueling. The reactor model was the same as that employed in our previous modeling and the coolant chemistry conditions could be estimated with sufficient accuracy to make a comparison between the observed and calculated crack extensions... 

D. D. Macdonald, The Electrochemistry of IGSCC Mitigation in BWR Coolant Circuits, Chimie 2002, Water Chemistry in Nuclear Reactor Systems, Avignon, France (April 22-26, 2002). 

The important parameters of the BWR primary coolant chemistry are conductivity, pH level, dissolved oxygen, sulfate and chloride. The BWR coolant is a high purity electrolyte. Therefore, conductivity is very low. 

G.M. Gordon, K.S. Brown, Dependence of creviced BWR component IGSCC behavior on coolant chemistry, in D. Cubicciotti, E. Simonen (Eds.), Proc. 4th Int. Symp. On Environmental Degradation of Materials in Nuclear Power Systems — Water Reactors, JekyU Island, Georgia, National Association of Corrosion Engineers, August 6—10,1989, pp. 14.46-14.62. 

A variety of nuclear reactor designs is possible using different combinations of components and process features for different purposes (see Nuclear REACTORS, reactor types). Two versions of the lightwater reactors were favored the pressurized water reactor (PWR) and the boiling water reactor (BWR). Each requites enrichment of uranium in U. To assure safety, careful control of coolant conditions is requited (see Nuclearreactors, water CHEMISTRY OF LIGHTWATER REACTORS NuCLEAR REACTORS, SAFETY IN NUCLEAR FACILITIES). 

All over the world, 432 nuclear power reactors are under operation and more than 36 GW of electricity could be produced as of December 31, 2001. There are several types of reactors such as boiling water reactor (BWR), pressurized water reactor (PWR), Canada deuterium uranium (CANDU), and others. In these reactors, light water is normally used not only as a coolant, but also as a moderator. On the contrary, in CANDU reactors, heavy water is taken. It is widely known that the quality control of coolant water, the so-called water chemistry, is inevitably important for keeping the integrity of the plant. 

In consideration of the features of NHR-5 low temperature, lotv power density, and refueling interval bemg longer than PWR, and by reference to the operation experience of nuclear powered ship " Otto Hahn" a water chemistry sjstem different from the PWR and BWR is adopted in the operation of NHR-5 This water chemistry system is m neutrol water, not to contain boron and not to add hydrogen in premaiy coolant, and oxygen removed by chemical additive (CjH )... 

The low coolant saturation temperature, high heat transfer coefficients, and neutral water chemistry of the BWR are significant, advantageous factors in minimizing Zircaloy temperature and associated temperature-dependent corrosion and hydride buildup. This results in improved cladding performance at long... 

Rooth, T., Ullberg, M., Karlsson, E., Persson, I. Hydrogen peroxide in BWRs An experimental determination of the actual level. Proc. 5. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, 1989, Vol. 2, p. 55-60 Ruiz, C. R, Lin, C. C., Robinson, R., Burns, W. G., Curtis, A. R. Model calculations of water radiolysis in BWR primary coolant. Proc. 5. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth 1989, Vol. 1, p. 131-140 Wunderlich, F., Eberle, R., Gartner, M., Gross, H. Brennstabe von Leichtwasserreaktoren. Auslegung und Betriebsverhalten. KTG Seminar Band 5, Verlag TUV Rheinland, Koln, 1990... 

On the basis of the C activity concentrations measured in the off-gas, power-related C emissions were calculated to amount to 300 to 450 GBq/GWe a for PWR plants and to about 500 GBq/GWe a for the only investigated BWR plant. These measured values are higher than the calculated ones mentioned above since these measurements were conducted over short time periods only, the influence of statistical fluctuations cannot be ruled out. Whereas in the BWR plant (normal water chemistry conditions) more than 95% of the C released was in the chemical form of CO2, in PWR plants about 80% was in the form of CO and/or alkanes (which could not be distinguished from each other by the analytical techniques applied). These differences in the chemical speciation of between PWR and BWR plants are assumed to result from the stronger reducing conditions in the PWR coolant, which are caused by the presence of excess hydrogen. 

The pronounced differences in the chemistry conditions of the BWR reactor water and the PWR primary coolant have a profound influence on the chemical state of the corrosion products and of the radionuclides produced from them. The most important parameters in this context are the pH of the water, which in a BWR is virtually neutral, and the O2 concentration. The latter parameter shows significant differences from one plant to the next and also may vary over time. As a result, the question of the chemical nature of the radionuclides cannot be answered as definitively as for the PWR primary coolant. 

Honda, T., Ohashi, K., Furutani, Y, Sato, Y, Mochizuki, H. Suppressing effect of surface treatment on radiation buildup in a BWR. Proc. 4. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, 1986, Vol. 1, p. 139-143 Ishigure, K. A review of models describing the behaviour of corrosion products in primary heat transfer circuits of BWRs. Report IAEA-TECDOC-429 Reactor Water Chemistry Relevant to Coolant-Cladding Interaction. Annex III, p. 165—214 (1987)... 

Boiling water acts as coolant and moderator, while separated steam is used to drive the turbine. Maintenance of the water chemistry is performed by the systems that are conventional for reactors of BWR type. The coolant circuit equipment is made of stainless steel, or has a welding deposition made of stainless steel. The fuel assemblies are also made of stainless steel. The outer coatings of micro fuel elements are made of silicon carbide (SiC). 

The EPRI guidelines for BWR primary coolant system water chemistry [4.3] is listed in Table 4-2a for Normal Water Chemistry (NWC) and Table 4-2b for HWC (Hydrogen Water Chemistry) or HWC+NMCA (Noble Metal Chemical Addition). The NWC guideline is also followed in Finland. 

Figure 85. Predicted ECP around the primary coolant circuit of a BWR under (a) normal water chemistry (NWC) conditions (no hydrogen added to the feedwater) and (h) hydrogen water chemistry (1.2 ppm of H2 added to the reactor feedwater. Reprinted from Ref. 9, Copyright (1995) with permission from Elsevier.

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