The UNITHERM NPP thermal-hydraulic cycle (Fig. 1) includes three interrelated process loops, the last of which accommodates all heat consumers (turbine-generator unit and heat system or process steam boilers).

Selection of coolant parameters for the above-mentioned process loops was based on the proven range of operating pressures and temperatures typical for NPP water-cooled reactor primary circuits, and on the experience with mobile NPP operating with primary coolant natural circulation Besides, variation limits of the primary coolant parameters without reactivity compensation of nuclear fuel bumup by control rods were taken into account On the other hand, selection of coolant operating temperatures and pressures for the intermediate and steam-turbine •loops was greatly influenced by the conditions which could provide acceptable efficiency and orientation on the development and operating experience with steam turbine units that might be considered as existing prototypes. The analysis of thermal characteristics of steam turbine units of such type enabled selection, taking into account the above reasons, of coolant parameters for the process loop of heat consumers

Transport of heat from the primary circuit to heat consumers during the phase change in the intermediate loops reduces the required coolant flows and increases pressure m the natural circulation system, while the desire of optimal distribution of the available temperature difference between the coolants of the primary and third circuits determines the intermediate loop coolant parameters Considering the above reasons, the parameters of the UNITHERM NPP process loop coolants are as in Table 1 below.

The proposed UNITHERM NPP has been designed to employ integrated water-cooled NSSS as a heat source (Fig. 2). NSSS combines in one vessel the main primary circuit components — core, steam generator (SG), pressurizer, control and protection elements. This allows to avoid primary circuit pipework, reach extremely compact location of ionizing radiation sources and potentially dangerous working fluid — primary coolant. The NSSS design ensures core cooling and heat supply to steam generator by convection of primary coolant and thereby allows to eliminate forced circulation. Such approach to the design of the NPP main component — nuclear reactor — is necessary to reach maximum possible reliability and simplicity due to absence of active elements with continuously moving mechanical parts. The group of absorbers with

associated drives, which perform the function of emergency protection and compensation for reactivity variation, is a single movable element. The absorbers are displaced once during NPP continuous operation when starting NSSS unit in normal operation. In emergency situations it is possible to drop the absorbers which perform the function of emergency protection.

The integrated NSSS vessel made of 15X2MFA-A steel with corrosion-resistant cladding consists of the shell with welded elliptical bottom and flange.

The central part of the vessel accommodates the removable shield with the core, lattices of absorber rods and devices of additional emergency protection. The thermal shield which also functions as core reflector and radiation and thermal shield, is arranged around the removable shield in the bottom section of the vessel.

Inside the removable shield above the core there is a cavity with pressurizer equipped with a set of flat screens serving as upper radiation shield.

In the annular space between vessel and removable shield the steam generator tube system banks are located. Intermediate circuit heat exchanger is heat exchanger with coil-type heat transfer surface, where primary coolant moves in the tubes while the intermediate circuit fluid moves in the intertube space. The heat transfer surface is made up by 24 banks of the same type, the shells of which are designed to withstand the primary circuit pressure. The banks are pairwise combined into 12 assemblies which are connected to 12 sections of the intermediate circuit heat exchanger.

Steam collecting headers and water distribution chambers of intermediate circuit heat exchanger sections are located in the vessel flange area. The chambers are also connection points of SG sections to the NSSS vessel. The shell of each SG section is a cylindrical vessel with welded spherical bottom with a coil-type tube system inside cooled by the coolant from the heat consumer circuit. In each section of the SG above the tube system the coil-type system of independent cooldown circuit is arranged. From the top the shell is closed with a spherical head.

In the choice of characteristics and structure of the core and its control elements the following priority concepts have been adopted:

• maximum possible reduction of operative reactivity margin, in particular, the fraction of total efficiency of shim elements, per group of absorber rods with individual drive;

• optimal power, coolant temperature and fuel feedback factors;

• specific power density (about 15 kW/І) which guarantees specified long-term operation without fuel element clad leaking, and minimum specific levels of residual heat release for reliable heat removal in severe accidents;

• increased reliability of emergency core chain reaction suppression system by using in the control system of additional passive emergency protection channels with operation mechanism differing from that of main functional components.

To reduce core overall reactivity the adopted design philosophy excludes withdrawal of control elements from the core in power generation mode during continuous NPP operation. In this period, the core operates in self-control mode due to variation of coolant average temperature and absorber bumup.

The additional emergency protection actuator is a structure with leaktight vessel, which by its lower part enters the core instead of one fuel assembly, while the upper part is flange connected to the NSSS vessel head. Inside the actuator vessel, actuating element consisting of the accumulator with absorber and interconnected receiving chamber made of two elastic membranes. The space between the vessel and the actuating element is filled with nitrogen, control fluid. Gas (He-З or boron trifluoride) is used as absorber.

When there is no emergency signal, the membranes of the receiving chamber are under control fluid pressure and the absorber is displaced into the accumulator. The emergency signal activates the electromagnetic switch and control fluid is discharged to the NPP containment volume. By its pressure in the accumulator the absorber is displaced to the receiving chamber spacing the membranes apart. To return additional emergency protection to initial position, the control fluid from the tank outside NSSS vessel is fed to the actuator which compresses the membranes and displaces the absorber to the accumulator.