Primary coolant chemistry specification

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. 

The influence of coolant chemistry on radiation buildup can be expected to differ from plant to plant, due to plant specific parameters such as chemical composition and morphology of the surface oxide layers in the system, operating parameters (e. g. temperature), and the structural materials employed. In order to take into account these parameters in the selection of an optimum primary coolant chemistry, the empirical code Optimi was developed which uses a so-called Chemistry Influence Factor which is calculated from the differences in the concentrations of dissolved iron in the coolant at the primary system hot-leg and cold-leg side (Marchl and Greger, 1989). By comparing the results of the calculated chemis-... 

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 ... 

To fulfil the chemistry requirements, the primary coolant has to be treated by the addition of chemicals. The concentrations of the additives needed have been given in specifications elaborated by specialists in detailed discussions and which are continuously re-evaluated based on the current state of knowledge. Table 1.4. shows an overview of the primary coolant specifications established by various vendors and institutions for plant normal operation periods (Mathur and Narasim-han, 1992). From these data, it can be seen that there is a worldwide consensus on the use of LiOH or KOH as a pH control reagent. Different vendors or organizations may recommend minor changes in some of these specifications as, for example, KWU not specifying fluoride, but fundamentally the plant specifications are virtually identical. During startup and shutdown phases, deviations from these specified data are allowed for a limited period of time. 

To ensure that the chemistry specifications of the primary coolant are maintained the addition of chemicals is necessary. This is usually performed via the chemical control system. This system is part of the nuclear auxiliary systems just as are the primary coolant purification systems described in Section 1.1.3. In order to maintain the specified H2 concentration in the primary coolant, either an adequate H2 partial pressure is established in the volume control system or H2 is directly injected into the high-pressure system (newer Siemens/KWU plants). 

Special attention should also be paid to the selection of materials and to the coolant chemistry, which also make an important contribution to the reliability of the steam supply system for the nuclear plant. The compatibility of materials and coolant, which is of the utmost importance to minimizing the amount of maintenance, repair and statutory inspection necessary for primary circuit components, should be given careful consideration. Only those materials should be used that have been shown to be compatible with the coolant under the conditions (of temperature of coolant and material and coolant composition) that will prevail in the reactor. A specific concern is the possible occurrence of intergranular stress corrosion cracking. 

If the on-line chemistry monitoring system detects that the primary coolant is out of specifications, the source of the ingress of the impurities should be identified and corrective actions taken to meet the chemical specifications. If halogens are detected out of specifications, a cleaning or flushing operation will be required during the next outage. 

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