F. M. MITENKOV, A. V. KURACHEN KOV,

V. A. MALAMUD, Yu. K. PANOV,

B. I. RUNOV, L. N. FLEROV OKB Mechanical Engineering,

Nizhny Novgorod,

Russian Federation

Abstract

Design bases of hew generation nuclear power units (nuclear power plants — NPP, nuclear co-generation plants — NCP), nuclear district heating plants — NDHP), using integral type PWRs, developed in OKBM, Nizhny Novgorod and trends of design decisions optimization are considered in this report.

The problems of diagnostics, servicing and repair of the integral reactor components in course of operation are discussed. The results of safety analysis, including the problems of severe accident localization with postulated core melting and keeping coriurn in the reactor vessel and guard vessel are presented. Information on experimental substantiation of the suggested plant design decisions is presented.

INTRODUCTION

The integral lay-out realized in boiling water reactors and BN-type reactors is a result of the search for optimum technical and economically substantiated decisions.

An analogous search process is also characteristic of the reactors of PWR type.

Investigations and developments allow the conclusion to be made that certain conditions integral reactors have considerable advantages as for mass and size in comparison with loop-type and unit-type plants.

Besides the integral lay-out, the reactor has advantages as for safety, quality of fabrication, mounting, building time and removal from operation. But the integral lay­out objectively complicates the reactor design and the problems of operational service, it causes the necessity to use highly reliable in-reactor equipment.

In the development of integral reactors especially important are specific characteristics of the heat exchanger (steam generator) built into the reactor, because the reactor vessel dimensions depend largely on heat exchange surface dimensions.

The lifetime reliability of the reactor components should be confirmed by operational experience as a part of operating reactor plants and their prototypes or by broadened complex representative tests at testing facilities in the conditions corresponding to operation conditions in the plant.

For some decades, OKBM specialists developed ship nuclear power plants and experience has been accumulated on the development of some equipment and the NPP as a whole, fabrication and experimental development of some equipment, designer supervision of the fabrication at the factories and in course of operation.

The afore-mentioned allowed the development of new generation nuclear power plants with integral reactors.

First of all is the reactor plant AST-500 for NDHP, which may be located in the vicinity of large cities.

The AST-500 reactor plant is the first in the group of the plants with integral PWRs. Its characteristics are widely known. Its main peculiarities are following: natural coolant circulation in the reactor, high safety level provided bv passive means.

The high safety level of the RP AST-500 was recognized by national technical and ecological expert examination, supervision bodies and a special commission PRE-OSART IAEA.

The main fundamental decisions of the NPP, such as integral reactor design, use of guard vessel, use of passive safety systems of various principles of operation with deep redundancy and self-actuation became the basis of the whole group of the developed plants of ATETS-200, VPBER-600 type and the others.

The main advantages of the integral design in comparison with traditional loop — type designs:

— localization of radioactive coolant in one vessel (excluding purification system);

— absence of large diameter pipelines and nozzles in the primary circuit;

— keeping the core under water level at any loss-of-tightness due to the proper choice of guard vessel volume;

— decrease of neutron fluence to the reactor vessel to the level, excluding any noticeable change of the vessel material properties, radiation embrittlement (fluence <1017n/cm2);

— higher completeness of the reactor plant important equipment, of the guard vessel at the delivery to the site and as a result increase in the quality of mounting the power units as a whole;

— reduction of NPP building time to the reducing of the installation work and simplification of construction work;

— considerable simplification of the technology and operations at NPP decommissioning and RP change for repeated use of NPP structures.

Possible negative consequences of the integral reactor design are the following:

— delivery of off-gauge heavy cargo from the factories;

— the necessity to increase considerably the rated load of mounting cranes at the

site.

Corresponding analysis and the experience of delivery of AST-500 reactors and guard vessel to the sites of Nizhny Novgorod and Voronezh confirm the feasibility of such delivery by the existing engineering means.