NATURAL CIRCULATION SYSTEMS IN NEW DESIGNS

1.1. THE ROLE OF NATURAL CIRCULATION SYSTEMS

The role of the natural circulation systems should be considered in the general context of implementation of passive safety systems into new nuclear power plant designs. It may be noted that many systems and equipment proposed in future reactor concepts include natural circulation phenomena as the main mechanism that determines the passivity of the fulfilment of a designated function. Naturally driven systems have been used in the existing reactors both to remove the core heat under normal operation conditions including full power operation (like the Russian boiling water reactor VK-50 or the Dodewaard reactor in the Netherlands) and to fulfil some safety functions (like the passive part of the emergency core cooling system [2] in PWRs). So, there is a good technical basis and some operational experience to use passive systems in new reactor concepts.

With respect to plant safety, application of passive systems/components is intended to simplify the safety systems and to improve their reliability, to mitigate the effect of human errors and equipment failures, and to provide increased time to enable the operators to prevent or mitigate severe accidents. Natural circulation systems typically do not require or accommodate repair or maintenance work during power operation. Therefore, a reduced number of safety system trains may be needed to perform the designated safety function with the required reliability. An IAEA conference [1] included discussions on the safety of future plants, and noted that the use of passive safety systems is a desirable method of achieving simplification and increasing the reliability of the performance of essential safety functions, and should be used wherever appropriate.

However, natural circulation systems have their own advantages and drawbacks in comparison with forced flow systems, both in the areas of plant safety and plant economics. Passivity itself does not mean that a natural circulation system should be automatically considered as more reliable with regard to fulfilment of the designated safety function. Therefore a reasonable balance of traditional systems and new passive means is adopted in many future reactor concepts as a possible way to improve safety and public acceptability of nuclear power, and at the same time to keep nuclear power competitive with conventional power technologies. Many considerations govern this balance and define the final design decision, such as:

(a) Application of passive systems should reduce the number of components, and yield design simplification, so that the number and complexities of safety actions can be reduced;

(b) Passive means should be taken, to the extent possible, from similar ones having operational experience at power plants or elsewhere, so that the efforts needed to demonstrate the reliability and licensability are not too large;

(c) Passive systems actuation should be more reliable than for an active system providing the same function; otherwise the increase of the system reliability projected by implementation of the passive system may be lost;

(d) Passive systems should eliminate the need for short-term operator actions during accidents

(e) Passive systems should minimise dependence upon off-site power, moving parts, and control system actions for normal operation as well as during design basis and beyond design basis accidents;

(f) Passive systems should reduce the construction, operation and maintenance costs.

The main drawbacks of natural circulation systems include lower driving forces and less possibility to alter the course of an accident if something undesirable happens (i. e. less operational flexibility). In particular, in certain conditions where forceful or rapid actions are required, active systems may be more suitable to carry out certain safety functions. Also, load follow operation may be limited in the reactors based on natural circulation of the primary coolant. Therefore, in some new reactor designs originally designed for natural circulation, forced circulation flow (by pumps) has been introduced to allow for a better load follow capability and to increase the reactor rated power. Scaling for natural circulation systems is more difficult than for active systems. Therefore usage of experimental or operational data obtained for a system with a size that differs from that of the system being designed may not be appropriate. Due to low driving forces, the operation of a natural circulation system may be adversely affected by small variations in thermal-hydraulic conditions. The lower driving forces might also lead to quite large equipment, and this factor may reduce the cost savings projected from elimination or downsizing of active components. Besides, larger components may cause additional difficulties in seismic qualification on some plant sites.

The design decisions with regard to balancing active/passive features may also depend upon the functions assigned to the given system. In particular, the system having an important role in the mitigation of severe accident consequences that is located in a potentially contaminated area (e. g. the part of the containment cooling system which is located inside the containment) could be designed to be as passive as reasonably achievable. This is because of the difficulty or even impossibility of access to such areas and because passive components may not require maintenance even during long-term operation.

All the above aspects are being taken into account by nuclear power plant designers, and as a result, both novel and more or less proven passive systems and features are proposed in many new designs [2]. Some designs have only added a few passive components to the traditional systems. Some other designs make use of the passive systems/components and natural circulation phenomena for power production in normal operation, or to fulfil a number of safety functions, intended to prevent severe accidents and mitigate their consequences.