The application of supercritical pressure in both thermal and fast

pressurized water power reactors with thermodynamic efficiency up to 44% was considered in many designs in the 60’s. The main difficulties in the construction of such reactors were connected with the development of reactor materials required for work under high temperature conditions, for example, for the efficiency of 44% it was necessary to have a primary coolant temperature of about 550°C. The SCP LWR construction was postponed because of this problem and good perspectives for the development of LWRs with subcritical pressure.

The RSC Kurchatov Institute returned to the idea of the NPP

development with SCP at the beginning of the 80’s. After severe accidents at NPPs the accent was transferred from the increase of NPPs efficiency to enhancing the safety while keeping economic competitiveness. The RSC

Kurchatov Institute has carried out studies which show that in the case of pressure increase over the critical value in an integral LWR all safety requirements for the future reactors can be met with economic characteristics at least on the level of NPPs of traditional layout.

The following four main requirements formed the basis of the

development of the reactors at SCP carried out jointly by the RSC Kurchatov Institute and EDO Hydropress. They are:

— its safety level must meet safety requirements for the reactors of future generation,

— its economic characteristics must have advantage over other reactors,

— it must have potentially lower environmental impact,

— it should not depart significantly from existing LWR technology and take into

consideration the utilization experience of the water at SCP in heat power stations.

It must be noted, that nowadays water at SCPs is widely used in fossil plants. The operation of these power plants has shown a high reliability of the equipment under these conditions.

Fig. l shows water enthalpy and density at SCP 23.6 MPa (Pcr=22.1 MPa) versus temperature. The temperature at which the derivative of enthalpy versus temperature is maximum is denoted as Tm. For comparison physical properties of water at 15.7 MPa are given in Fig. l.

2800

tuO

Л

З 2300 J a

л 1800

r

1300

300 320 340 360 380 400

Temperature °С, T

Fig. 1. Water enthalpy and density vs. temperature.

There are some correlations, e. g. V. Protopopov, V. Silin [2,3] for the calculation of heat transfer of SCP water flow inside the tubes and correlations

for the conditions at which heat transfer deterioration was observed.

The large amount of heat transfer experimental data of SCP water flow

in large bundles was obtained in the RSC Kurchatov Institute.

The experimental ranges covered are:

Pressure: 23.5 and 29.4 MPa,

Mass velocity: 350 to 5000 kg/(m2/c),

Bulk water enthalpy: 1.0 to 3.0 MJ/kg,

Heat flux: 0.18 to 4.5 MW/m^.

Experimental heat transfer was satisfactorily described by correlations obtained at SCP water flow in tubes as a resulting. Note that in the experiments conducted in the RSC Kurchatov Institute there was no heat transfer deterioration in the experiments with multirod bundles within the same test parameter range at which heat transfer deterioration occurred in tubes. Available information on supercritical heat transfer and hydrodynamics allowed reliable estimation of thermohydraulics characteristics of the core, SG and primary loop.

The value of primary operation pressure (P0) was determined from the requirement to maintain the supercritical pressure during transients taking into account pressure support system features. The minimal margin relative to

critical pressure Pcr was adopted equal to 1.5 MPa which defined the PD value equal to 23.6 Mpa. A greater pressure margin would cause increase in vessel mass and consequently would worse the technical and economic characteristics. The coolant temperature was chosen on the basis of the following ideas:

— thermodynamic efficiency increases with the temperature but if it is above the Tm value it is more difficult to supply the NPP with necessary materials and the water properties of as a coolant become worse,

— to improve the fuel cycle features and safety level a decision was made to use

the sharp change of the coolant density in the vicinity of Тщ temperature to maintain the core criticality during fuel lifetime.

Taking into account these considerations the core inlet temperature was chosen below Tm and the core outlet temperature was chosen close to Tm that is approximately 380°C.

To maintain the core criticality between refuelings the coolant density increases smoothly during fuel lifetime. This is achieved by primary coolant temperature decrease in the SG which increases the feedwater flow at the given reactor thermal power. The growth of feedwater flow decreases the steam overheating in the SG and increases the heat transfer to the secondary side. As a result the primary coolant temperature decreases at SG outlet leading to the decrease of core inlet and core average coolant temperatures.

The chosen coolant parameters provide:

— growth of NPP efficiency up to 38%,

— several times decrease of SG heat transfer specific surface due to the increase

of temperature difference between the primary and secondary sides as compared to an NPP with subcritical pressure,

— decrease of coolant mass flow due to the high coolant heat capacity in the region of Tm,

— significant difference of coolant densities at core inlet and outlet,

— use of a fuel cycle with high conversion coefficient and core criticality maintenance by varying the neutron spectrum during fuel lifetime due to the sharp density increase in the region of Tm temperature,

— high heat transfer coefficients in the core and SG,

— high average coolant heat capacity.

Taking these factors into account it was decided to develop the design of reactor of maximum unit power with natural circulation with regard to the capabilities of the existing reactor technology. The parameters of steam fed to the turbine at the chosen coolant temperature were adopted on the basis of optimization calculations. In these calculations the NPP efficiency increase with pressure and steam overheating and simultaneous decrease of average specific

heat flux in the SG were taken into account The decrease of specific heat flux in the SG requires the increase of SG heat transfer surface at given thermal reactor power.