The main primary coolant circuit construction ma­terials, together with a typical hard facing material and fuel clad are identified in Table 1.20 in terms of і heir elemental composition. It should be noted that other elements will be present at trace levels, and that not all the materials in Table 1.20 are necessarily present in all PWR designs. For example, for steam generator tubing Incoloy 800 is widely used in Euro­pean PWRs whereas American designs currently use inconel 600.

These materials will always be covered with an initial oxide film, which develops and may undergo chemical modification due to prolonged exposure to the primary coolant water chemistry over the tempera­ture range of ambient to 300°C. Typically the oxide film on a stainless steel would increase from 1-2 fim to — ~ -1b Mm over 30 years. In general, such oxide films give protection to the metal surfaces against dgnilicant corrosion and loss of metal thickness, how — aer> ^1еУ can lead to contamination of the primary coolant by two mechanisms:

tow (up to 10 ppb), the nature of the species and their activation by passage through the core can lead to significant primary coolant activity levels.

• In addition, corrosion products can be released as discrete insoluble particulate material, containing insoluble and sparingly soluble elements. Such ma­terial is transported around the primary circuit and can deposit in the core, activate, re-release and re­deposit on out-of-core surfaces.

These two processes will be largely responsible for the circulating and deposited primary circuit activity levels, and hence plant radiation levels by a transport mechanism as summarised in Fig 1.54. Species re­leased initially from in-core surfaces will already be activated, whereas out-of-core material will become activated by residence in the core.

The extent to which metal species are transferred from circuit materials to the primary coolant will be a function of a number of inter-related factors, which determine the corrosion rate and corrosion product release rate. The corrosion rate will be largely deter­mined by the temperature, chemistry and any pretreat­ment of metal surfaces, whereas the corrosion product release rate will be a more complex function of these factors, including hydrodynamic and erosion effects. It should be noted that the chemical composition of released material will not necessarily reflect the com­position of the base material. For example, nickel release is often reduced and iron release enhanced with respect to the base metal composition. In addi­tion, there will be a contribution from wear processes in moving components such as valve internals and components of the control rod drive mechanism.

In order to relate the circulating and deposited levels of activity to the system corrosion and corrosion product release-rates, it is necessary initially to esti­mate the amount of corrosion products released by means of surface areas. In Table 1.21 system materials are listed against typical applications, corrosion and corrosion product release-rates and surface areas for a four loop PWR. The surface areas can be sub­divided in terms of in-flux, out-of-flux, and high temperature/low temperature wetted fractions, if there is a sufficient data base to permit refinement of the estimate of the amount of material released to the system. It should also be recognised that corrosion and release data can be an order of magnitude greater for the first few months of operation, and that sig­nificant increases can result from system transients (e. g., temperature, flowrate and pH).

The concentration and nature of circulating and deposited activated species has been the subject of much investigation, both to correlate with corrosion product release data and to optimise methods of re­moval. For a typical PWR at full pow’er the metal ion concentrations (soluble plus particulate) would be at sub-ppm levels and have been reported as iron

Table і.20 Composition of materials in contact with the primary coolant Cr Ni Fe Mn Sr C Mo W Co Nb Zr Sn 316 Stainless steel 16-18 11-14 Balance <£2.0 2-3 304 Stainless steel 18-20 8-11 Balance 0.07 Inconel 600 14-17 >72 6-Ю <£1.0 lncaloy 800 19-23 30-35 Balance <1.5 Inconel 690 27-31 >58 7-11 <0.5 1 Inconel 718 19 52 18 3 5 Zircaloy 4 0.1 <0.5 0.2 і Balance 1.5 Stellite 26.5 -30 0-3 0-3 0-0.5 0.7 — 1.5 0.9 — 1.4 0-1 3.5 -5.5 Balance

(10-20 ppb), nickel (2.3-3 ppb), cobalt (0.2 ppb), chromium (0.8-2.3 ppb) and manganese (0.3-0.9 ppb). A consideration of such data, together with release rates and surface areas, leads to the conclusion that 2-20 g of material is present in the coolant at any given time. It is apparent also that up to 25-30 kg of material can be released to the coolant per year, which can deposit on circuit surfaces or be removed by the CVCS clean-up system.

The insoluble component of circulating and de­posited material is generally referred to as ‘crud’, and post irradiation examination (PIE) is used to both remove and identify the nature of crud deposited on fuel clad surfaces (fuel crud).

In discussing the nature of the species present it is necessary to differentiate initially between soluble and insoluble. Conventionally, soluble material is de­fined as that which passes through a 0.45 /tm filter,
accepting that some particulates will therefore be classified as soluble. The chemical identity of metal cations present can be inferred from the composition of the circuit materials and corrosion products. Table 1.22 gives the principal nuclides together with their formation reactions and half-lives. This data could be expanded to include Mn-56, Mn-99, Te-99, Sb-122, Sb-124, Zn-65 and Nb-95, which derive from minor constituents, impurities and welding consumables.

As Ni-58 (67.9% natural abundance) is a signifi­cant proportion of the metal content of released cor­rosion products, Co-58 production is high. With a half-life of 72 days it will equilibriate in one reactor cycle and dominate the plant radiation levels during early life.

Co-60 is produced from Ni-60 (26.2%) and Co-59 (100%), where the latter represents a trace impurity in construction materials and a major constituent of

some hard facing alloys. Typical cobalt levels would be 0.1-0.2% in stainless steels and 0.01-0.02% in nickel alloys, but 60% in some hard facing alloys. The Co therefore has a lower activation rate than Ni, however, the 5.2 у half-life results in continued increase over many years. Although the rate of in­crease will depend upon the Co-59 trace impurity level, corrosion/wear of hard facing alloys, rate of release to the circuit and residence time in the core, ultimately it is possible for the Co-60 level to dominate the ra­diation level.