Having described the reasons for the major require­ments of RCS primary coolant water chemistry control, and identified a typical primary coolant water speci­fication, it if. necessary to consider how such a chem­istry is best maintained in operation. The plant to enable such control is described later.

Early PWR operation tended to place emphasis on maintenance of the chemistry specification without any additional need to control the Li/B ratio and, by implication, the pH over a restricted range. A typical operational sequence would start with a high boron concentration at the beginning of a fuel cycle, with a lithium 7 concentration of the order of 1.0 ppm in the middle of the range. As power was increased the Li-7 concentration increased due to the B-10 (n, a) Li-7 reaction, together with a gradual reduction in boron on reactivity grounds, therefore the pH in­creased. Later in the fuel cycle Li-7 would be removed from the coolant by ion exchange operations in the CVCS in order to meet the specification, and towards the end of the fuel cycle Li-7 production decreased further as the boron concentration was reduced to a low level. Eventually, in anticipation of the reactor shutdown the Li-7 concentration would be allowed to fall to a very low value. The pH therefore varied significantly throughout the fuel cycle.

During the early years of operation there was also considerable awareness of the increasing primary cir­cuit activity levels, which were leading to increased doses to operational and maintenance personnel.

The most extensively monitored location was the steam generator channel head after shutdown, as the radiation field contributes significantly to the dose to personnel engaged in routine steam generator tube inspection. Such data is also the most reliable, as the activity can be monitored directly without attenuation by shielding and the dimensions and geometry of the location with respect to the detector is reproducible in most plants. Other areas routinely monitored in­clude various primary coolant pipework locations.

The conclusion drawn from the analysis of a con­siderable amount of plant data, including studies of in-reactor crud samples taken from fuel and pipework, is that the increasing activity levels are primarily due to corrosion products from out-of-core components.

It is concluded therefore that there is a mechanistic link between an unco-ordinated control of the RCS chemistry, which allowed wide variation of pH through a fuel cycle, and the release and deposition of acti­vated corrosion products. The corrosion products are known to have complex composition and morpho­logy, and the process of release/transport/deposition shown in Fig 1.56 is a function of system chemistry, hydrodynamics, heat flux, surface condition and sur­face temperature.

Investigation of the effect of chemistry on corrosion product solubility and release indicates that there is
a relationship between corrosion product solubility, temperature and the pH resultant from a range of Li/B ratios. Early studies of magnetite solubility in dilute aqueous acid and alkaline solutions saturated with hydrogen (Sweeton and Baes 1970 [32]) yielded graphs of iron solubility against temperature at var­ious pH as shown in Fig 1.57. If the isotherms are plotted for Fe solubility and OH- concentration (or pH) for relevant PWR temperatures (Fig 1.58), it is apparent that the temperature coefficient of the iron solubility could change from negative to positive as the pH of the primary coolant changed due to varying Li/B ratios.

In order to emphasis the point, data from Fig 1.58 which is relevant to PWR operation is presented in Fig 1.59 (Solomon 1977 [29]), where 300°C approxi­mates to the fuel clad temperature and 250°C is representative of the in-core coolant temperature.

For a coolant condition in which magnetite has a negative temperature coefficient of solubility it will precipitate on hot surfaces, whereas for a positive temperature coefficient magnetite will tend to dissolve from hot surfaces. The hypothetical situation for no crud transport by dissolution would require a zero coefficient of solubility and this is calculated to be pH

6.6 at 300°C for magnetite.

As noted earlier, Sandler (1979) [30] has shown that PWR fuel crud was based on the non-stoichiometric composition and structure of nickel ferrite. Subse­quently Sandler and Kunig (1981) [33] studied the so­lubility of nickel ferrite in boric acid solution in the

Fig. 1.58 Iron solubility as a function of hydroxide
molality

presence of hydrogen, using an experimental flow system used earlier for high temperature alkaline solutions by Sandler and Kunig (1977). This indicated that a zero temperature coefficient of solubility for nickel ferrite occurred at pH 7,2 at 300°C.

In applying a requirement to minimise the tem­perature coefficient of solubility of corrosion products

to a PWR operational fuel cycle, it is necessary to display Fe solubility as a function of temperature for relevant combinations of В and Li concentration. This is shown in Fig 1.60, 1.61 and 1.62 for 1100, 500 and 100 ppm В respectively at various lithium concentra­tions. The condition of minimum iron solubility (mini­mum change of iron solubility with temperature) over the range 285-330°C is achieved at progressively lower Li concentrations as В concentration decreases.

If this process of dissolution and precipitation of corrosion product derived species is analysed in terms of chemical equilibrium, it is possible to mathemati­cally predict the lithium concentration for a given boron concentration that corresponds to the minimum iron concentration and zero temperature dependence of solubility. The results of such a calculation are shown graphically in Fig 1.63 which represents the со-

Fjc. 1.60 Iron solubility plotted against temperature
for solutions containing 1100 ppm boron and varying
lithium concentrations

Fig. 1.62 Iron solubility plotted against temperature
for solutions containing 100 ppm boron and varying
lithium concentrations

ordinated control of Li and В throughout a PWR fuel cycle, for optimum control of circulating corrosion products and activity levels. RCS chemistry conditions outside the operational window would be expected to lead to dissolution or precipitation of corrosion pro­ducts and result in enhanced activity transport around the circuit.

Figure 1.63 indicates that the co-ordinated chemistry control is continued down to low lithium/low boron levels at the end of the fuel cycle. Operationally, there may be advantage in maintaining a minimum lithium concentration (e. g., 0.7 ppm) for boron concentrations below about 360 ppm. This would be to maintain a pH high enough to minimise corrosion and is illu­strated by the horizontal section on Fig 1.63.

Fig. 1.63 PWR operational chemistry —
recommended lithium concentration range as a function
of boron concentration