Due to the typical parabolic rate of increase of sur­face corrosion and oxide film thickness with time (Mann 1976 [36]), it is logical to allow the surface oxide to establish before fuel is loaded. The period of most rapid corrosion and corrosion product release therefore occurs when no fuel is loaded and the sys­tem can be cleaned up by the CVCS. This can be achieved during hot functional testing when the water temperature reaches 300°C due to heat input from the pumps, by operation with lithium hydroxide and boric acid present. Corrosion and corrosion-product release is then reduced during subsequent reactor operation at power.

A more positive chemical pre-treatment under de­velopment has the objectives of controlling the rate — of-growth, structure and composition of indigenous oxide films. The process is required to be simple and to produce oxide films that are stable, resistant to uptake of Co-60 and repairable after shutdown.

The production of a stable oxide film, known as pre-filming, can be achieved by treatment with ethy — lenediamine tetra-acetic acid (EDTA) at 200°C. This produces a glassy film of СГ2О3 and FejCU, followed by conversion to black crystalline material containing Fe}04 and FeCr204 by raising the temperature to 250°C.

However, the process kinetics and the stability of the resultant films at 300°C are unknown and further development work is required, although the process has been used on Canadian BWR systems (CANDU).

A further pre-filming development is the controlled growth of nickel ferrite in an alkaline/reducing envi­ronment, with an excess of benign cations present (e. g., zinc or manganese) to fully substitute the lat­tice structure of nickel ferrite. This produces a stable nickel ferrite film which is unable to take up Co-60 cations and therefore cannot accumulate activity due to grown-in active species. However, as above, the long term stability of the film at 300°C is unknown.

All pre-operational chemical treatments are designed to produce an oxide layer of enhanced adhesion and chemical stability, which is resistant to uptake of active species such as Co-60.

Start-up

Early establishment of the co-ordinated primary cir­cuit specification is judged to be important, although any benefit will only emerge after several fuel cycles, when radiation levels may be shown to be lower than on a reactor where early establishment of co­ordinated chemistry was not adopted. The lithium hydroxide concentration should be established during pre-operational testing through to normal operating temperatures, and oxygen levels should be below the recommended value as soon as possible. The latter may require nitrogen purging during start-up and the use of hydrazine to ensure less than 0.1 ppm oxygen above 80°C.

Charging the RCS up to pressure may cause a transient in the dissolved hydrogen level, due to a changing CVCS volume control tank pressure. This will subside when charging is complete.

Shutdown

The reactor shutdown causes transients in the primary circuit chemistry due to the removal of hydrogen and the addition of shutdown levels of boric acid (2000 ppm), necessary to suppress residual core reactivity. As noted earlier, the end of fuel cycle chemistry will already be characterised by low boron and low or zero lithium. In addition, there are temperature, pres­sure and hydrodynamic changes, and ultimately the primary coolant water is oxygenated by exposure to air via the refuelling pond water.

It is not surprising that these transient conditions cause an order-of-magnitude increase in circulating activity levels due to the release of crud to the pri­mary coolant. Currane, Stagg and Shaw (1979) [37] have identified three phases in this increased activity release. Initially, an increased Co-58 solubility ac­companied the cooldown and boration to 2000 ppm, followed by a maintenance of the Co-58 level while reducing chemistry was retained. Finally, when oxy­genation of the primary coolant occurred, there was a large increase in soluble Co-58 levels.

In terms of PWR shutdown, it is possible to deli­berately generate oxidising conditions by the addition of hydrogen peroxide (H202), and remove the re­leased Co-58 activity by use of the CVCS ion ex­change units before the system is opened and allowed to oxygenate via the refuelling pond water. However, this technique is not universally used.

It should be noted that normal short duration non­refuelling shutdowns would not be subjected to this oxygenation, and efforts would be made to retain the normal operational chemistry.