CEA,

France

III — 1. DESCRIPTION OF THE SCOR DESIGN

The Simple Compact Reactor (SCOR) is a 2000 MW(th) integral design pressurized light water reactor (PWR). The design for the reactor was developed at the Nuclear Energy Division of the Commissariat a l’Energie Atomique in Cadarache, France. A detailed description of SCOR design and features is provided in [IV-1].

The SCOR is mainly being developed for electricity generation, providing competitive costs, when compared to large sized reactors, through system simplification and compactness in plant layout. However, the SCOR could be used in cogeneration schemes, such as seawater desalination using low temperature processes, as well as thermo-compression or multi-effect distillation.

The SCOR is an integral design reactor having new features with respect to the designs of typical integral type reactors, which usually contain several modular steam generators inside the vessel. Such architecture has led to the design of a large vessel, limiting the output of the reactor to a maximum of 1000 MW(th). In the SCOR concept, the steam generator is located above the vessel and acts as the vessel head. This layout component provides space inside the vessel to increase core size and therefore, has the same safety advantages (elimination of a large break loss of coolant accident); the SCOR unit power is twice as high as the maximum power of a typical integral design reactor [IV-1, IV-2].

Passive safety features allow the SCOR to respond safely to all initiating events within the design basis, with few operator actions required. Except for loss of coolant accidents (LOCA), where low electric power is needed in the mid term (a low pressure safety injection with a power of about a few tens of kW is required for less than one day), no alternative current (AC) power is needed for accident management. Most of the design extension[43] conditions are eliminated or passively managed as accidents within the design basis. This simplifies the scope of operator training, equipment qualification and surveillance to meet safety requirements.

The main characteristics of a nuclear power plant (NPP) with a SCOR reactor are given in Table IV-1. A schematic view of the SCOR plant is shown in Fig. IV-1.

The plant control scheme will be specifically designed for operation with a single steam generator and will be based on a ‘reactor follows the plant load’ strategy.

The SCOR is an integral type PWR with a compact primary circuit. The reactor pressure vessel houses the main primary system components including the core, the pressurizer, the reactor coolant pumps, the control rod drive mechanism (CRDM), and the heat exchangers of the decay heat removal system. Such design configuration eliminates large penetrations through the reactor vessel, excluding the possibility of large break loss of coolant accidents. A single steam generator acts as the reactor vessel head; see Fig. IV-2 (this figure also illustrates the flow path of the coolant).

From the lower plenum, water flows upward through the core and the riser and through the centre of the pressurizer. At the top of the vessel, fluid flows upward and downward through the U shaped tubes of the steam generator. Then, the fluid is collected in an annular plenum and passes to the inlet of the reactor coolant pumps. From the pump outlet, the coolant flows through a venturi and then across the tubes of the decay heat exchangers to the lower plenum.

A design with integrated pumps eliminates large diameter loops typical of a standard PWR and substantially eliminates large break LOCA events. The number of smaller diameter pipes is also reduced, limiting the probability of occurrence of small breaks and small break loss of coolant events.

Characteristic	Value
Installed capacity
Power plant output, net	630 MW(e)
Reactor thermal output	2000 MW(th)
Reactor core
Active core height	3.66 m
Equivalent core diameter	3.04 m
Average linear heat rate	12.9 kW/m
Average fuel power density	24 kW/kg UO2
Average core power density (volumetric)	75.3 kW/l
Thermal heat flux	430 kW/m2
Reactor pressure vessel (RPV)
Cylindrical shell inner diameter	4983 mm
Wall thickness of cylindrical shell	141 mm
Total height	14813 mm
RPV head	No (steam generator)
Base material: cylindrical shell	Carbon steel
Liner	Stainless steel
Design pressure/temperature	9.78/309 MPa/°C
Transport weight (lower part)	280 t

The SCOR concept is based on well-proven nuclear reactor technologies; its major innovations are related to safety design and the design of auxiliary systems. The innovative features of SCOR are as follows:

• Elimination of large diameter penetrations through the reactor pressure vessel;

• Integrated passive emergency core cooling systems based only on natural convection and using external air as the ultimate heat sink;

• A soluble boron free core with control rod drive mechanisms located inside the reactor pressure vessel;

• Relatively low core power density, enabling a large margin (i. e., departure from the nucleate boiling ratio (DNBR)) within the whole range of operating parameters;

• Reduction of reactor building maximum pressurization;

• Reduction of human factors affecting safety systems;

• Easy testing and maintenance of all safety systems.

Reactivity control is achieved through the use of control rods with in-vessel drives; no soluble boron system is foreseen. To reduce reactivity at the beginning of the cycle, the loaded portion of fuel contains burnable poison. As in standard pressurized water reactors (PWRs), the clusters of control rods are moved in guide thimbles but, as the steam generator acts as a vessel head, there is no possibility of using an external mechanism to move the control rod clusters. The control rod drive mechanism (CRDM) appears as an integrated hydraulic system. There is around one control rod cluster per two fuel assemblies; such selection is sufficient to control reactivity from a full power to a cold shut down state. In accident conditions, redundancy is achieved by another device, called the MP98 system [IV-3]; this system enables the movement of a liquid neutron absorber in dedicated tubes in the guide thimbles of the assemblies without control rod clusters. Main characteristics of the reactivity control system are summarized in Table IV-2.

System type/characterization	Availability/value
Burnable absorbers	Yes
Number of control rods	78
Absorber rods per control assembly	24
Drive mechanism	Hydraulic
Soluble neutron absorber	No
2nd system for accidental conditions	Yes

The SCOR design philosophy is based on finding an optimum between economic and safety approach issues:

• SCOR is a larger size integral design PWR, compatible with the option of industrial manufacturing in series and also offering a compact plant layout;

• The safety approach is based on architecture with which as many as possible accident initiators are eliminated or reduced, or the possible consequences of accidents are limited, by relying upon both inherent safety features and active and passive systems.

The design options of SCOR were selected to facilitate safety demonstration:

• The integral design eliminates large primary penetrations of the reactor vessel; therefore, large break loss of coolant accidents (LOCAs) are practically eliminated;

• The integrated control rod drive mechanisms eliminate the risk of rapid reactivity insertion through control rod ejection;

• The residual heat removal system on the primary circuit (RRP) with heat exchangers located in the vessel, very close to the core, eliminates an additional loop with the primary water typical of a standard residual heat removal system.

The design philosophy of SCOR results from reactor studies conducted in the 1990s, based on such PWR designs as the AP600, SIR, PIUS, low pressure PWRs, and the EPR, and incorporates the results of CEA (France) studies of safety systems and several PWR core types [IV-1, IV-2].

The SCOR design concept provides for a simplification of the main systems. Such selection contributes to simplified plant operability and reduced plant costs and also improves safety and reduces machine-human interactions.

Low primary operating pressure enables a reduction of the wall thicknesses of pressure bearing components and reduces the required pressurizer volume.

The elimination of alternate current (AC) powered safety systems2 contributes to a reduced complexity of the active systems, which otherwise would need sensors, actuators, etc. that must be qualified for reliable operation over the full range of conditions which might be encountered (e. g., fire, seismic events, etc.).

Another important implication of the design simplification targeted for SCOR may be related to improved human reliability [IV-4], as discussed in more detail below.

Most human reliability assessment (HRA) models acknowledge the fact that human performance in operating a system (especially in performing cognitive, demanding tasks) is largely influenced by complexity characteristics of the system. Although this notion of complexity may appear somewhat subjective at a certain level (the perceived complexity of a system is highly dependent on the knowledge and skills that the operators have developed), it still exhibits an objective component directly correlated to the intrinsic complexity of the features of a system. For example, minimizing the intrinsic complexity of a system, particularly in the early

Except for the safety injection system, which operates at low pressure and with a low flow rate.

FIG. IV-3. Characterization of the complexity features (illustration) [IV-1].

phases of its design, appears to be an attractive way of improving the system operation taking into account human factors.

The abovementioned considerations form a basis for the approach proposed by the CEA (France) to assess the relevance of human factors in advanced nuclear reactor concepts, particularly during the very early phases of the design, that is, when it is still possible to propose alternative solutions at a limited cost. Such an approach was followed in the SCOR design.

The method consists of characterizing design features, especially within safety system architecture, that are likely to pose problems in operation, notably during degraded situations in which plant safety strongly depends on human reliability. The characterization of the intrinsic physical behaviour of plant processes (safety functions), of the operating constraints of the safety systems, and, finally, of the interrelations between these entities[44] (most of the complexity theories consider these interrelations to be the main contributors to the complexity of a system), lead to the definition of an operational complexity index and to the identification of sources of operational constraints bearing on operation crews. Figure IV-3 illustrates such complexity features, as defined by the relationships between safety functions and safety systems.

Figure IV-4 illustrates the principles applied for quantification of complexity (operational complexity index (OC)), on the basis of functional architecture shown in Fig. IV-3.

Each parameter used in the expression of Fig. IV-4 is evaluated on the basis of a discrete scale, considering the potential human factor impact of a certain feature. For example, in the case of the reversibility (REVJ) of an engineered safety system, a 3-level scale has been defined:

REV = 1 — for a system in which the effects are totally reversible (easily achieved by making a reverse action);

REV = 2 — for a system in which the reverse action requires more effort than a normal action;

REV = 3 — for a system in which the consequences of an action are irreversible (the worst case).

FIG. IV-4. Quantification of complexity — Operational complexity index (OC).

The basic idea behind this quantification is the notion that it is possible to undo the effects of a (potentially erroneous) action, which is a definitive factor in human decision making. If such a possibility is not understood, operators may be reluctant to take an action, even though it might be vital for plant safety. This characteristic has a strong link to what is called the ‘forgiving features’ of a design. On its basis, comparative studies among various designs are possible, outlining a new approach to design optimization which considers human factors at a very early phase in the conceptual design, whereas customary approaches only consider these aspects during instrumentation and control (I&C) and man-machine interface (MMI) design phases.

Even though the SCOR design is still at an early conceptual phase, the present knowledge of its safety design options is sufficient for a preliminary assessment of the operational complexity. Figure IV-5 presents the first results of such an assessment, performed in comparison with a standard loop-type PWR.

The presented results point to a potential decrease in the operational complexity of the SCOR as compared to a standard loop-type PWR. The reasons behind this expected simplification are twofold [IV-1]: [45]

FIG. TV-5. Operational complexity safety functions for the SCOR and a standard PWR [1V-1 ].

elimination of soluble boron in the SCOR, and for the coolant inventory control (INV) systems — simplification of the configuration of a low pressure safety injection in the SCOR.

Even though the assessment of human factors for the SCOR concept is preliminary (it focuses on degraded operation, but similar analysis is required for normal operation, maintenance and testing), results confirm that the design options for SCOR may lead to a considerable simplification of operation and to a possible improvement of human reliability in operation. This conclusion appears particularly valuable as probabilistic safety assessments (PSA) indicate that human failures make a major contribution to the global risk in existing nuclear power plants.