Reactor coolant chemistry

The requirement, then, for reactor coolant chemistry is to measure up to 300 C in light and heavy water the primary yields for low and high LET radiation, and the rate constants of the spm reactions. It is also of intrinsic interest to test the spur-diffusion model for water radiolysis over a wide range of temperature. 

Controlled water chemistry is maintained within the RCS. Control of the reactor coolant chemistry is the function of the CVCS which is described in Section 9.3.4. Water chemistry limits applicable to the RCS are given in Section 9.3.4. 

Returning to the Westinghouse AP reactor design, the primary coolant system water chemistry is selected to minimize corrosion. Routinely scheduled analyses of the coolant chemical composition are performed to verify that the reactor coolant chemistry meets the specifications. Other additions, such as those to reduce activity transport and deposition, may be added to the system. The CVCS provides a means for adding chemicals to the RCS. The chemicals perform the following functions ... 

Godfrey W. and Phennah P.J.. Bradwell Reactor Coolant Chemistry J. Brit. Nucl. Enersy Society, 7.151-157 and 217-232.1968... 

Maintain reactor coolant chemistry during plant start ups, provide normal dilution to compensate for fuel depletion, and provide shutdown boration also provide the means for controlling the reactor coolant system pH by maintaining the proper level of lithium... 

The primary water specifications for a PWR are given in Table 1 (4). Rigid controls are appHed to the primary water makeup to minimise contaminant ingress into the system. In addition, a bypass stream of reactor coolant is processed continuously through a purification system to maintain primary coolant chemistry specifications. This system provides for removal of impurities plus fission and activated products from the primary coolant by a combination of filtration (qv) and ion exchange (qv). The bypass stream also is used both to reduce the primary coolant boron as fuel consumption progresses, and to control the Li concentrations. 

Radioactivity transport in reactor coolant circuits involves both surface corrosion and deposition. Several XPS studies(8,9) of reactor boiler alloys have been reported which show the very strong effect of coolant chemistry on the films deposited. The chemistry of corrosion products precipitated on ZrO and Al O surfaces has been studied using XPS.ly More recently, chemical decontamination of radioactive boiler circuits has been assisted by XPS analysis of the surface-active decontaminating agent.(1 ) Surface oxidation in gas-cooled reactor circuits has also been investigated. AES has been used to follow the CC>2 oxidation of a chromium steel(H) and some pure metals. (12)... 

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... 

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). 

The organic-cooled CANDU concept was proposed by McNelly of CGE in 1958 (71), This began an extensive investigation of coolant properties, decomposition, control of deposition, and many other aspects of coolant chemistry. An organic-cooled, heavy-water-moderated research reactor, WR-1, began operation at WNRE in 1965. It has demonstrated reliable operation with coolant outlet temperatures of up to 675 K. Low corrosion and a low potential for activity transport result in very low radiation fields around the piping. 

Before the measurements in Figure 3 were made, it had been generally assumed that spur reactions which are difiusion controlled at room temperature remained so at elevated temperatures and early modelling of the radiation chemistry of reactor coolant water was based on this premise. However, it is now evident that there is no sure substitute for experimental measurements in this field. 

Research work has been undertaken to determine the dependence of irradiation embrittlement on material chemistry, heat treatment and service factors (irradiation conditions, temperature, coolant chemistry, etc.) in Russian research reactors and WWER-440 reactors. The irradiations were carried out mainly on surveillance specimens irradiated in nuclear power plants in locations where the inlet temperature was 270 °C (for WWER-440). The intent was to provide carefully controlled irradiation conditions in terms of temperature and neutron spectrum. The resulting major conclusion was that a substantial body of data established an irradiation-induced increase of the brittle fracture temperature, (similar to Tnj), of the general form of (CF) (FF) ... 

The charging pumps and letdown control valves in the chemical and volume control system (CVCS) are used to maintain a programmed pressurizer water level. A continuous but variable letdown purification flow is maintained to keep the RCS chemistry within prescribed limits. A charging nozzle and a letdown nozzle are provided on the reactor coolant piping for this operation. The charging flow is also used to alter the boron concentration or correct the chemical content of the reactor coolant. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

Sodium occurs widely as NaCl in seawater and as deposits of halite in dried-up lakes etc. (2.6% of the Earth s crust). The element is obtained commercially via the Downs process by electrolysis of NaCl melts in which the melting point is reduced by the addition of calcium chloride sodium is produced at the steel cathode. The metal is extremely reactive, vigorously so with the halogens and also with water, in the latter case to give hydrogen and sodium hydroxide. It is used as a coolant in fast-breeder nuclear reactors. The chemistry of sodium is very similar to that of the other members of group 1. 

Bergmann, C. A., Roesmer, J. Coolant chemistry effects on radioactivity at two pressurized water reactor plants. Report EPRI NP-3463 (1984)... 

Gott, K., Andersson, P.-O., Svenson, J. The build-up of radioactive corrosion products in Ringhals 4 and its relationship to primary system coolant chemistry. Proc. 4. BNES Conf Water Chemistry of Nuclear Reactor Systems, Bournemouth, UK, 1986, Vol. 1, p. 15-20... 

Another important parameter in controlling the buildup of radiation fields is employment of an optimum primary coolant chemistry throughout plant operation, even when all the facts mentioned above indicate that deposition and activation of corrosion products in the reactor core do not represent the most important contamination mechanism. As was discussed in detail in Section 1.3., the primary aim of coolant chemistry is to keep the corrosion of the materials and, consequently, the release of metal atoms to the coolant as low as possible. With the main source of radioactive atoms being the in-core materials, this requirement particularly concerns the conditions prevailing inside the reactor pressure vessel. A second requirement which is of equal importance is to keep the concentration of... 

Some authors have claimed that KOH-NH3 primary coolant chemistry as applied in the WER reactors leads to lower radiation dose rates on the primary system. In the course of their work at the DIDO Water Loop, Large and Wood-wark (1989) investigated this type of primary coolant chemistry under conditions which were comparable to those applied in the experiments with LiOH chemistry. Their results showed comparatively low corrosion product concentrations in the coolant, suspended solids as well as dissolved species the radioactivity buildup on the loop surfaces, however, was on the same order as that experienced with coordinated Li/B chemistry. From these findings the authors concluded that the comparatively low radiation dose rates which are reported from VVER plants are not due to the type of coolant chemistry employed, but to the absence of Stellite and Inconel in the primary circuit, i. e. to low cobalt and nickel inventories of the materials in contact with the primary coolant. 

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