Reactor water cleanup system

Fig. 6. Impurity flow paths of BWR radioactive contamination (24). RWCU = reactor water cleanup system.

The primary coolant circuit of a water-cooled reactor (including BWRs and PWRs) has several loops, including the main coolant loop, a core heat removal system, and a reactor water cleanup system. However, it is convenient, for computational purposes, to differentiate between the main loop, which has a high flow fraction, and the secondary loops, for which the flow fractions are small. The species concentrations and electrochemical potential (ECP) are solved for in the main loop and the values at the entrance to the secondary loop are used as the initial conditions for solving the system of equations for the secondary loops of interest. Mass balance is applied at each point where more than one section comes together. 

The primary coolant circuit of a PWR is shown in schematic form in Fig. 36. In this particular circuit, there are four loops between the reactor and the steam generators. The pressurizer is also shown, which maintains the pressure in the primary loop at a sufficiently high value (typically 150 bar) such that sustained boiling does not occur and maintains the desired concentration of hydrogen in the coolant. The reactor heat removal system (RHRS) and the reactor water cleanup system are not shown. The general operating conditions in a PWR primary loop are summarized in Table 2. 

The reactor building encloses the reactor primary containment and forms a secondary containment. The building also houses all primary process and service systems for the reactor, such as handling equipment for fuel and main components, fuel pools, reactor water cleanup system and engineered safety systems. 

C. CHEMICAL AND VOLUME CONTROL SYSTEM (CVCS) [REACTOR WATER CLEANUP SYSTEM (RWCU) IN BWRsI... 

The second block includes the reactor auxiliary and waste management building, housing the reactor water cleanup system and the liquid and solid radwaste systems, the radioactive maintenance shops, housing the active workshop, and storage rooms for potentially radioactive waste. 

In order to keep the impurity levels in the reactor water low, a continuous flow of the water is passed to the reactor water cleanup system, which in most of the plants is equipped with precoat filters with powdered ion exchange resins. 

The mixed-bed ion exchange resins of the PWR primary coolant purification system show a high retention effectiveness for P, with the purification factors being in the range 100 to 3000. In the BWR reactor water cleanup system the purification factors are lower, on the order of 20 to 300. is retained in the purification systems of both PWR and BWR plants to a lower degree, with the purification factors measured being in the range between 6 and just below 100. 

The isotopic composition of fission product iodine present in the BWR reactor water in the case of failed fuel rods in the reactor core is quite similar to that in the PWR primary coolant. Since the iodine purification factor of the reactor water cleanup system is on the order of 100, i. e. virtually identical to that of the PWR primary coolant purification system, this similarity in isotopic composition demonstrates that the release mechanisms of iodine isotopes from the failed fuel rods to the water phase are virtually identical under both PWR and BWR operating conditions. On the other hand, the resulting chemical state of fission product iodine in the BWR reactor water is quite different from that in the PWR primary coolant. The BWR reactor water usually does not contain chemical additives (with the possible exception of a hydrogen addition, see below) as a result of water radioly-... 

In the case when defective fuel rods are present in the reactor core, the BWR reactor water contains the other fission products and the activation products released from the fuel in concentrations well below those of fission product iodine. This applies as well for fission product cesium, which is retained on the ion exchangers of the reactor water cleanup system with a decontamination factor of about 100. As far as it is known, cesium in the reactor water is present as the Cs ion, whereas large proportions of most of the polyvalent fission products and of the actinides are attached to the corrosion product particles suspended in the water as yet, there is no detailed knowledge on the chemical state of these elements (i. e., adsorbed to the surfaces or incorporated into the Fe203 lattice). It was reported that the strontium isotopes as well as Np appear in the reactor water in the dissolved cationic state, while Tc was found in the reactor water as a dissolved anionic species, most likely Tc04 (Lin and Holloway, 1972). According to James (1988), discrete fuel particles were not detected in the BWR reactor water. 

In both PWRs and BWRs, corrosion of the primary circuit materials is an essential factor in the buildup of contamination layers on the surfaces of the pipes and the components. The materials used in BWRs which are in contact with the reactor water and, therefore, are potential sources of radionuclides are mainly stainless steels wear-resistant hardfacing alloys such as Stellite are also present in most of the plants. Zircaloy as the material of fuel rod claddings, spacers and fuel assembly casks need not be considered in this context, because of the extremely small release of activated constituents from this material. Due to differences in temperature and environment, the mechanisms of the corrosion process and the resulting metal release rates, which contribute to the input of corrosion products into the region of the reactor core, may show differences in different regions of the plant. Thus, corrosion of materials in the water-steam cycle exhibiting H2O phase transformations and considerable temperature differences will proceed differently than in the recirculation lines and the reactor water cleanup system, which are in contact with liquid water exclusively and show comparatively small variations in operating temperature. 

The concentrations of the corrosion products in the reactor water are controlled by a number of parameters, including feedwater input, particle deposition and resuspension, precipitation and dissolution, and quality of performance of the reactor water cleanup system. As a consequence, these concentrations vary considerably from plant to plant and also within a plant, frequently without showing a discernible trend. The concentrations of total iron may vary by more than 3 orders of magnitude, primarily as a result of variations in the concentration of insoluble iron species, while that of dissolved iron is relatively constant and typically below 10 ppb. The concentration range for total cobalt in different plants is considerably smaller, and is typically in the range 10 to 200 ppt. In general, even when extreme fluctuations are ignored, there is no consistent trend in the concentrations of the corrosion products in the reactor water. 

In the BWR plants, the out-of-RPV surfaces which are subject to contamination during steady-state operation are mainly those wetted by high-temperature reactor water. These are, in the main, the pipes leading to the reactor water cleanup system and the recirculation lines (as far as the plant is equipped with an external recirculation system). In addition to these surfaces, the main steam lines and the turbine, as well as part of the feedwater system, may be contaminated by radionuclides carried with the steam. In the course of a shutdown of the plant, certain regions of the main steam lines and of the feedwater lines are also in contact with low-temperature reactor water containing radionuclides. 

Besides reduction of the cobalt inventory of the primary systems and minimization of the generation of corrosion products by appropriate coolant chemistry control, prevention or at least reduction of the transport of non-radioactive as well as of radioactive corrosion products is another effective countermeasure, independent of the predominant source of radionuclide production. The reactor water cleanup system in its usual design is not efficient enough for a significant reduction in the concentrations of non-dissolved and dissolved corrosion products in the... 

Over the entire shutdown period, the reactor water cleanup system has to be operated at its maximum throughput in order to remove as much of the radionuclides as possible from the primary system and to minimize radionuclide redeposition on the out-of-core surfaces. 

As an example of the decontamination of subsystems, the treatment of a BWR recirculation loop and, in addition, parts of the residual heat removal system and the reactor water cleanup system, by using the Cord process in parallel to the... 

Table 4.15. BWR reactor water cleanup system decontaminations using the Cord process (1990-1993)...

Flow proceeds from the lower plenum, through the core. The steam and water are separated the steam is then dried and passed to the turbine. Other flow (see above) returns to the recirculation system. Feedwater is introduced to the annulus between the core shroud and reactor vessel (Fig. 4). The recirculation system piping is a primary pressure boundary for the high-pressure, high-temperature reactor coolant. Type 304 stainless steel was selected for recirculation system piping and numerous other auxiliary systems (such as the reactor water cleanup system, residual heat removal system, core spray, and other emergency core cooling systems) for its corrosion resistance and adequate mechanical properties. Failures of weld heat affected zones... 

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