C O N C E P T A N D D E S I G N S O F N E W - G E N E R A T I O N F A S T R E A C T O R S

F . M . M i t e n k o v UDC 621.039.5

The nuclear power plant disasters at Three-Mile Island and Chernobyl made it necessary to revise the views on the

potentialities and limitations of nuclear power, its place in the power system, and the conditions that the energy sources built

must meet. The safety of the nuclear plants, ways and means of ensuring it, and the substantiation were the determining factors

in the revision.

An exchange of information and an open discussion of the problems by experts from various countries make it possible

now to discuss the conceptual theses that determine the conditions for ensuring the safety of nuclear energy Sources and the

prospects for nuclear power development, which are supported by the international community.

Further development of nuclear power will depend to a considerable degree on a high-quality solution of the following

key problems:

- substantiating the possibility of building safe reactors;

- improving the economic characteristics with allowance for the life cycle of the nuclear plant; and

- overcoming the negative attitude of the public.

Since the potential danger from nuclear plants with reactors of any type is an irremovable feature, assurance of safety

should be reduced to a system of technical measures that would protect the environment from unacceptable effects in any

technically possible accidents. The directions and methods for ensuring the safety of nuclear plants have been determined and in

the reworked projects are distinguished not by principles but by the perfection of the technical measures that should prevent

accidents and ensure the minimum necessary monitoring and control of accident processes, as well as limiting the consequences

of the accidents.

Studies to date suggest that nuclear plant safety is attained if the following requirements are satisfied:

- internal self-protection of the reactor is achieved through negative temperature (power) feedbacks in the reactor core

and the thermal lag of the reactor;

- the use of passive means for protection and emergency cooling, not requiring external energy and intervention by an

operator to be switched on and function;

- allowance for operator error as a factor increasing the development of an accident; and

- organization of a sufficient system of protective barriers in the path of possible propagation of radioactive products so

as to prevent their entering the environment in any technically possible accidents; and

- the introduction of diagnostic means to ensure high-grade monitoring of the equipment and systems (primarily

responsible for safety) and the possibility of determining the residual service life.

The formulation of the requirements must take into account the prospect of using nuclear energy sources for more than

just producing electricity. Undoubtedly, with time nuclear power will also find its place in industries requiring much thermal

energy, in particular high-temperature heat. Communal heating systems for large cities are also based most rationally on

custom-built nuclear power plants (atomic power plants, atomic heat and power plants). Nuclear power plants have already

proved themselves not only on naval ships but also on merchant ships (atomic icebreakers, the container ship Sevmorput', and

the ore carrier Otto Hahn). Without the comprehensive introduction of atomic energy into energy-intensive branches of the economy it is not

possible to solve the ecological problem, which at the present level of knowledge only atomic energy can do this. Most experts

have no doubts about the statement that the types of reactors that will be built and used on commercial scales in the next few

decades have already been determined: VVt~R (water-moderated water-cooled power reactors), fast reactors, and high-tempera-

ture reactors. The field where one type of reactor or other is used will be determined by the independent directions of develop

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Engineering Testing-Design Bureau. Translated from Atomnaya I~nergiya, Vol. 74, No. 4, pp. 290-294, April, 1993.

1063-4258/93/7404-0272512.50 ©1993 Plenum Publishing Corporation

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Fig. 1. Loop (a), block (b), and integrated (c) layout schemes

for a nuclear steam-generating plant: 1) reactor; 2) electric

pump of primary loop; 3) steam generator; 4) short

"tube-in-tube" power connector.

ment of technical designs with its optimum criteria (with observance of the general requirements and conditions for ensuring

safety). Thus, the low parameters of the primary-loop coolant, which are characteristic of nuclear communal heating plants,

substantially affect the optimality of the circuit and structural designs of the reactors and plants as a whole. In low-capacity

nuclear plants intended for autonomous regions isolated from electricity grids, the low capacity also substantially affects the

choice of structural designs. The circuit, structural, and layout designs of reactor plants are determined to an even greater extent

by the specific conditions in which atomic energy is used on ships.

Finally, today, and even more so in the future, studies should be conducted on the creation of other reactors free of the

disadvantages or "bottlenecks" characte'ristic of those mentioned above. Thus far, however, no promising directions for the search

for reactors of a new type have appeared. If we consider that no less than 15 years is undoubtedly required for the construction

of a new type of reactor, with allowance for the unavoidable large volume of research and testing and design work, it seems

reasonable to say that the development of nuclear power for the next few decades will be based on reactors of the types already

known, but improved by taking into account the accumulated operational and practical experience and the more stringent safety

requirements.

The prospects for improving the economic characteristics of nuclear energy (with increasing outlays for improving safety)

will be determined by studies in the following directions:

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Fig. 2. Integrated layout of a reactor in a

containment vessel.

- optimization of the structure on the basis of a closed fuel cycle, i.e., with plutonium built up in various reactors being

used as nuclear fuel (the specific characteristics of plutonium in this case substantially determine the relation in the structure of

reactors of different types);

- reduction of the capital outlays on nuclear power plant construction by cutting the construction time, standardizing

the designs, technology, and organization of work, and increasing the service life of the equipment and the nuclear plant as a

whole;

further optimization of the composition of the reactor core and improvement of the technology for the production of

nuclear fuel and fuel elements, and improvement of the recharging conditions to increase burnup;

- improvement of the structural-layout schemes of reactor plants; and

- minimization of radioactive wastes.

The importance of the layouts requires explanation. The layout scheme of reactor plants determines much in the

methods and means of ensuring safety, the structural designs, operating conditions, and in the final account, the economic

indicators. Three types of layout schemes for plants are known: loop, block, and integrated (Fig. 1). These schemes are applica-

ble to all types of reactors. The optimum choice in each specific case is determined by various factors and, therefore, one cannot

speak of universal recommendations. The advantages and disadvantages of each of these schemes, however, can be discussed.

Loop reactor plants are spread out considerably and their primary loop is large, and large-diameter pipes are used to connect the

main equipment: steam generators, pumps, heat exchangers, volume compensators, etc. A serious problem of this scheme is that

of ensuring safety if large-diameter primary-loop pipes burst accidentally. A large portion of the operating nuclear power plants

use plants with a loop scheme. Practice shows that this scheme has a considerable advantage, namely, the equipment is accessible

for prophylactic inspection and maintenance work, which is particularly important when the equipment is not highly reliable.

The integrated scheme was first used in sodium-cooled fast reactors. In particular, such a BN-600 reactor has been

operated accident-free over the past 12 years at the Beloyarsk Nuclear Power Plant. An obvious advantage of this scheme is that

the coolant of the primary loop is localized in one volume (in a vessel), there are no short connections and large-diameter pipes,

which of course sharply reduces the probability of coolant leaks. With an integrated scheme the problem of embrittlement of the

reactor vessel by neutron irradiation is obviated. Indeed, the neutron fluence for the vessels of the AST-500 and VPBI~R-600

reactors, built with an integrated scheme, is less than 1017 cm -z. Such a fluence does not cause any appreciable change in the

mechanical properties of the vessel steel. The integrated layout of the reactor makes it possible to build a containment vessel

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(Fig. 2). In this case it is possible to eliminate the danger of the reactor core drying out and thus cooling of the reactor in

emergency situations can be simplified substantially. With an integrated layout the organization and execution of work for reuse

of site or its restoration to a "green town" are simplified fundamentally. In an integrated layout, however, access is more difficult

to the equipment inside the reactor, thus limiting or complicating maintenance work. The integrated layout, therefore, requires

the use of highly reliable equipment built according to designs that have been proved in operation and have passed representa-

tive service-life tests under laboratory conditions. The integrated layout considerably increases the mass and size characteristics

of the reactor. New solutions thus are needed for the organization of work on reactor fabrication and assembly. In the case of

the BN-600 and Superphenix reactors the welding of the reactor vessels and the assembly work were done on the building site.

Water-moderated water-cooled reactors with an integrated layout have a substantially smaller diameter than do fast reactors (the

diameter of the BN-600 vessel is 12 m while that of the VPBt~R-600 is 6 m) and thus can be delivered from the fabricating

works to the building site by transport means, as has been confirmed by experience from the construction of the AST-500.

The block layout in essence is intermediate between loop and integrated schemes. Instead of long primary-loop pipes

there are short large-diameter connectors between the main equipment of the plant (reactor, steam generator, pumps). The

diameter of these connectors is larger than in the loop layout because the block layout has the most rational organization of

coolant inflow and outflow by the "tube-in-a-tube" principle. With the block layout the power scheme of the plant changes

radically in comparison with the loop layout and requires more profound studies for implementation. The connectors should be

fully compatible with the reactor vessel in regard to fabrication technology, quality control, and requirements. Like the reactor

material, the material of the connectors and their load should satisfy the "leak before rupture" criterion. The block layout has

found application in plants on ships, e.g, nuclear icebreakers. It reduces the size of the plant and ensures accessibility for

maintenance.

The above comparative analysis suggests that the integrated layout brings additional qualitatively new- possibilities for

increasing nuclear plant safety, which the other schemes do not have, but its application is justified only if the in-reactor

equipment is highly reliable and highly developed. If we bear in mind that a considerable number of diverse passive safety

systems contrived for the different plant layouts, the above discussion suggests that technical designs already worked out can.

ensure reactor safety. Designers are in essence faced with the task of optimizing these designs with respect to the parameters that

determine the safety and economy of nuclear power plants.

Reactor core damage requires further discussion. The ensemble of solutions dictated by the safety conditions eliminates

large-scale damage and all the more, total meltdown of the reactor core. Moreover, it is very enticing to realize project designs

that would eliminate unacceptable radioactive discharges into the environment even in the event of a postulated meltdown. One

can certainly predict, however, that these solutions will not be simple and inexpensive and will most likely be redundant. What

is the way out?

It is desirable to study the ensemble of physicochemical processes associated with reactor meltdown, formulate a

representative mathematical model of an emergency situation and consider possible designs that would avert unacceptable

environmental consequences.

As already mentioned, the future of nuclear power is determined not only by technical considerations but also by the

attitude of the public at large. The public reaction to a nuclear plant accident is a natural protective reaction. The attitude of

experts to the public sentiments, therefore, should be the same as to any other objective factor that determines the acceptance

of technical or organizational solutions. The complexity of the problem lies in the fact that the public is unreceptive to technical

logic. A search must be made to find other ways and means of conveying to the public at large objective information about the

true state of nuclear power and its promise for our civilization.

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