7.5.1 Availability

Improved performance of current plants has been discussed earlier in this book. This is being achieved by better ways of processing information on the plant condition, e. g. components, better surveillance and diagnostics. The causes of reduced level of performance can be determined by analysing the better data obtained and improved management techniques can be implemented. Clearly these types of practices equally apply to advanced as for current generation plants.

Potential improvement in performance of evolutionary plants can be established in the design phase as indicated in Table 7.1. It may also be possible to take advantage of specific improved technology, e. g. the use of high burn-up fuel to enable longer length of cycles, more advanced computer-based systems, and simpler hydrogen control systems, which require less testing during outages and thereby reduce outage time.

Other technological improvements, some of which have already been tested on current plants, concern the utilisation of better materials. For example, Inconel 690 has better corrosion resistance compared with Inconel 600 in a steam generator environment. This improved material can be used for SG replacement in current plant as well as being used for new advanced plants.

Another way, which will reduce operating costs, is to reduce the number of welds, using better forging techniques. This reduces the need for weld inspection in areas of high — radiation fluence.

Future designs should achieve improved energy availability; targets of 87% for average energy availability factor have been put forward (Juhn, 1999) for future plants. Values of high 70s% are being achieved on current plant. These figures for advanced plants can be achieved by incorporating, at the design stage, the experience gained from currently operating plant.

7.5.2 Man-Machine Interface

Over the past few decades there has been very considerable progress in instrumentation and control (I&C) including the man-machine interface (Wahlstrom et al., 1999) (Table 7.2). New digital instrumentation has been developed; bringing both benefits and some difficulties. This new technology has been rapidly assimilated into conventional industry but has been incorporated to a lesser extent into the nuclear industry. The partial reason for this has been a significant downturn in the building of new plants in the last two decades of the 20th century. Other reasons are the lack of drive to replace proven old systems by new systems and in a similar vein, the conservatism of the nuclear industry and its regulators.

Nevertheless new technologies have been implemented in modernisation projects and good experience has been obtained. For new reactors, the new technology will be incorporated at the design stage. It will cover instrumentation, cabling, signal conditioning, many aspects of control, process computers and all aspects of an efficient man-machine interface. Developments relate to hardware, software, the development of information networks, interfacing and back-fitting with older systems (in the case of existing plants) and management of these aspects.

As noted above, I&C systems bring both benefits and some disadvantages. Digital systems are more flexible than analogue systems, which are limited in both practical and financial constraints. Storage capacity is not limited by physical constraints, ease of duplication of signals, better functionality of the control room, better reliability, etc. Other beneficial features are that new functions can easily be included; computers can be embedded into different components. Nevertheless digital systems are more unpredictable than analogue systems, because the software may be complex. A disadvantage of digital systems is their lack of robustness to different environmental factors such as temperature, moisture and radiation. However, commercial off-the-shelf systems can be designed to apply to the nuclear as well as the non-nuclear sector. This ensures better validation for application in some of the more challenging environments existing in nuclear plant.

Modernisation projects have been in progress in various countries — Finland, Germany, Netherlands and Sweden. Different strategies for establishing a mix between new and

existing I&C systems have been developed. In Korea, for example, upgrades of the Korea Standard Nuclear Plant (KSNP) are proposed which will be implemented into the new Ulchin Units 5&6 under construction.

The I&C systems for new plant designs clearly build on the experience gained from modernisation projects on current plants. However, for new reactor designs, a more generic approach to I&C systems is being adopted. The approaches being put forward for evolutionary plant though, do not vary substantially from the more developed systems already in place on the newer present generation plants. In both cases, I&C systems are based on digital distributed systems. Control room layouts follow the approach of compactness with information displayed on visual display units (VDUs). The main future developments are likely to be simplifications in regard to redundancy and physical independence; these have been put forward in some of the more innovative designs of the future.

Differences across the reactor vendors are relatively small. The KNSR design (a typical design) implements the utility requirements of the EPRI URD, including three redundant consoles, a separated console, large display panels and additional monitoring consoles. This concept relies on the 2/4 redundancy principle. The man-made interface incorporates computerised operating procedures and the I&C design is a plant-wide digital system. The plant protection and safety control system are four-channel programmable logic controller-based systems. Non-safety controls are implemented in a two-channel system with diverse processors; similarly plant monitoring has two independent diverse systems (Wahlstrom et al., 1999).