The steam generator is an once-through vertical cassette surface-type heat exchanger consisting of straight-tube steam generating elements, where steam with the required parameters is generated due to the heat from primary circuit coolant. The flow of media in the steam generator is counter-current: primary coolant flows downwards between tubes inside block-section shrouds, secondary medium (water-steam) flows upwards inside the steam generating elements.

The design of the steam generator is based on know-how decisions which include:

1) chemical composition of tube system materials;

2) design of steam generating elements;

3) method of spacing of the steam generating elements;

4) design decisions for assuring SG hydrodynamic stability;

5) the method of assembling the steam generating elements into the block-section;

6) the device for fixing and sealing the SG block-section into the reactor vessel internals;

7) the technique for obtaining a strong, leak-tight joint of titanium alloy and steel.

The SG consists of 12 identical block-sections 1 (see Fig. l), located uniformly

in the annular space between the reactor vessel and core barrel.

Each block-section is individually isolated by valves for feed water and steam.

The block-section consists of steam generating modules (cassettes) 2, header 3, feedwater tubes 4, steam tubes 5, shroud 6, nozzle with the seal 7, strong and leak — tight joints 8 and 9.

Steam generating elements are assembled into steam generating modules 2. Module groups are united into independent subsections, supplied with feed water individually.

The block-section header 3 is intended to organize feed water supply, heat removal and block-section fixing in the reactor vessel. 18 holes for welding of feedwater tubes are located over the header centre and 18 holes for the attachment of steam tubes are located over the periphery.

Feed tubes 4 are in the compartment 10, and intended for the supply of feed water from the header 3 to subsections.

Steam tubes 5 serve for steam removal from the subsection to header 3.

The shroud 6 embraces steam generating elements of the block-section and serves for:

1) the organization of primary coolant flow;

2) forming of specified geometry dimensions of the block-section;

3) tightening of the steam generating elements to exclude vibration.

The shroud is fixed to the block-section header.

The nozzle with sealing 7 is located in the lower part of block-section shroud and intended for prevention of coolant leakage bypassing the steam generating elements and

1. Block-section

2. Module

3. Header

4. Feedwater tube

5. Steam tube

6. Shroud

7. Nozzle with the seal

8,9. Strong and leak-tight joints

10.

Compartment

for restraint of the lower block-section part from latteral displacements. The seal design assures longitudinal temperature displacement of the block-section.

Strong and leak-tight joints 8 and 9 ensure the connection of steam generator items, made of titanium alloys to the stainless steel items and are located on each feedwater tube 4 and steam tube 5.

The block-section is installed into the reactor vessel and headers 3 are welded to the vessel from outside.

The steam generator design permits inspection by non-destructive methods during fabrication, including 100% radiographic inspection of all welded joints subjected to primary and secondary pressures.

Unification of items and assembly components by block-section, modules and steam generating elements, application in steam generator design of a minimum quantity of items of standard sizes and assembly of components of different types and dimensions allows organization of parallel process flows for automated manufacturing. This assures high quality of a steam generator and reduces the fabrication cycle.

The steam generator operates as follows. Primary coolant from the pressure chamber enters the shrouds of block-section 1, flows downward between steam generating elements transferring heat to the secondary medium. In the lower part of the shroud the coolant leaves the block-section through the nozzle with a seal 7 and goes further to the core inlet.

Feedwater enters feed tubes 4, then it enters modules 2 and is distributed between steam generating elements. Then water goes upward through the steam generating elements and is converted to superheated steam. Steam from the modules passes through steam tubes 5 and holes in the block-section headers and is supplied to steam chambers and removed from the steam generator.

The steam generator design gives the possibility of diagnostic control in operation and during sheduled shut downs of the reactor by the methods adopted in Russia. It allows estimation of the real state of the steam generator and to ensure its operational safety.

The steam generator design is maintainable and allows to leakage wear in and to isolate any non-leaktight module in case of intercircuit leakage. If one module is isolated, the heat exchange surface is reduced by 1/216 part.

The modular structure of the steam generator allows performance of complex representative testing of steam generator including the confirmation of lifetime characteristics in the rigs of relatively low power.

In the steam generator project design decisions are realized which have resulted in limitations of cross-sections, through which primary coolant leaks in case of some structural failures, including the rupture of a feed tube sheet or of the nozzle in the vessel.

Examination of various SG element failures has shown that the maximum equivalent diameter of the leak is between 5-40 mm.