Dominique Meeùs
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Atomic Heat Sources

From Science in the USSR (ISSN 0203-4638), USSR Academy of Science, n^o 4 (July-August 1988), p. 12-18.

L. Popyrin, Corresponding Member of the USSR Academy of Sciences, expert in heat and power engineering, department head at the Krzhizhanovsky Power Engineering Research Institute.

V. Sidorenko, Corresponding Member of the USSR Academy of Science, expert in nuclear reactors, Deputy Chairman of the USSR Safety Supervision Committee for Atomic Power Engineering.

Advances in nucleonics and nuclear power engineering during the last thirty years allow the energy balance in the Soviet Union to be restructured so that atomic energy is used not only for the production of electricity, but also of heat where it is economically justified. Such a change will necessitate a large-scale reconstruction of the network of trunk and distribution heat pipelines in cities, along with a modification of the role and operating mode of existing heat supply facilities. Great outlays are inevitable since heat generators using atomic fuel are much more expensive than their fossil-fuel counterparts. Highly topical therefore are all questions of cost-efficiency of building such generators and all major engineering decisions concerning their application.

Naturally, when discussing the future of nuclear power engineering we should always remember what happened at Chernobyl and treat matters of safety with the utmost care and thoroughness. Nevertheless, this does not mean that Chernobyl will halt the development of nuclear power engineering, for without it the national economy cannot be provided with the required amounts of energy.

USSR Fuel Balance

The Soviet Union is the only industrial power whose prospected reserves of fossil fuels are large enough to take care even of its long-term demands. However, the main fuel consumers are far removed from the areas where power resources are concentrated. Over three quarters of all power generated is consumed in the European Soviet Union (including the Urals), while fossil-fuel production here tends to decline. As a result, great amounts of oil, natural gas, coal, and electric power have to be transported from the east to the west of the country (table 1). At present, fossil-fuel supplies from the east make up nearly two-thirds of the energy balance of the European regions (including export). By the end of this century the absolute volume of all power resources (oil, coal, gas, etc.) transported from the east is expected to increase even further.

Transportation of such amounts of fuels over distances of some 3000-4000 kilometers involves great expenditures on the development of the transport network, hence increases the cost of the fuels.

The most promising way out of this situation is to develop the nuclear power industry. The atomic power plants in the European USSR are capable of producing enough electricity to meet the increasing demand. By the end of this century the share of electricity generated by atomic power plants may rise to 25 or 30 percent, substantially reducing the consumption of fuel oil and natural gas. It should be noted, however, that only a quarter of all primary energy resources are used to produce electric power (table 1). This means that atomic power plants alone cannot serve as a means of wholly eliminating shortcomings in the energy balance of the European USSR.

The situation can significantly be improved by using nuclear energy for heat generation. [*] The heat supply service distributes low-potential (up to 100 °C), medium-potential (up to 300 °C) and high-potential (above 300 °C) heat.More fuel is required to generate steam and hot water, i.e., heat of medium or low potential [*], than to generate electric power. The generation of high-potential heat (for high-temperature processes in ferrous metallurgy, machine building, construction materials industry, etc.) also requires a considerable amount of fuel.

Heat Consumption Trends in the USSR National Economy

The development of the power generation complex during the last 25 years has revealed certain important trends in heat consumption (table 2). [**] End power is the mechanical, thermal and other types of energy supplied to the national economy upon deduction of losses. (Ed. )One such trend is the stable share of low- and medium-potential heat in the total consumption of end power [**] ; a somewhat decreased supply of heat to the public and housing sectors and services ; and an increased consumption of heat by industry. Another trend is a larger share of steam and hot water in the generation of low- and medium-potential heat, and an increased proportion of low-potential heat in the total power consumption.

There is every reason to believe that these trends will prevail in the future as well. This fact opens up wide prospects for the use of the existing types of atomic reactors to generate heat of a relatively low temperature.

[***] One GJ per hour is equivalent to 0,28 MW ; thermal loads are usually evaluated in GJ per hour, power in MW. (Ed.)The changeover to “atomic” heat is facilitated by the high concentration of thermal loads typical of the Soviet Union : the share of heat consumption by cities and small towns has reached 80 percent ; 70 percent of this amount is consumed by cities with thermal loads above 2000 GJ per hour [***].

Centralized heat supply provided 50 percent of all heat in this country in 1985 and is expected to reach 60 to 65 percent in the future, with centralized heating in cities attaining 75 to 80 percent.

Thus, there are favorable conditions for building large heat generating facilities, including atomic ones.

Atomic Thermal Plants : What will They be Like ?

To date several versions of an atomic heat production facility based on pressurized water nuclear reactors have been developed.

One of the most promising ones is the atomic power-and-heating (plant generating both heat an electricity. The atomic power-and-heating plant (APHT) is designed as a traditional thermal power installation (fig. 1). After performing its function in the turbine (4) the steam is split into two streams. One is directed to the condenser (6) where it is condensed at a temperature of 20 to 30 °C, and the heat it carries is lost. The second stream is conducted to the water heater (7) and its heat is added to the heat supply system. The APHP is therefore more efficient than the conventional atomic power plant in which all steam passes into the condenser (for this reason they are sometimes referred to as condensing plants) and is lost. It is planned to build 2000-MW APHPs to supply heat to large cities.

Fig. 1. Thermal APHP circuit with a VVER-1000 reactor.
1—reactor ; 2—circulation pump ; 3—steam generator ; 4—steam turbine ; 5—electric generator ; 6—condenser ; 7—water heater for heating system ; 8—feed pump.

Another way to improve the efficiency of a heat generating nuclear reactor is to split heat into two streams (see fig. 2), one of which will be used for power generation and the other for heating. The first steam is used in the traditional condensation cycle and does not provide any power for heating. Although its heat utilization efficiency is lower than that of the previously described version, it has advantages that become evident when using, say, boiling water cooled reactors.

Fig. 2. Thermal circuit of a nuclear power supply with a VK-500 reactor.
1—reactor ; 2—steam turbine ; 3—electric generator ; 4—condenser ; 5—water heater for heating system ; 6—feed pump ; 7—circulation pump.

The point is that water in the reactor’s fuel core is in two phases : as a liquid at the inlet and as a steam-and-water mixture at the outlet. If the water at the reactor inlet is additionally cooled, as is done in this case, it will tap more energy produced during uranium fission, at the same time improving the depletion of the fuel. Calculations show that this scheme provides significant economic gains. To select the best scheme though, it will be necessary to examine all the design versions available.

Engineers are now working on a special purpose generator which will produce heat for some specific need, say, for domestic consumption. A generator of this type, termed atomic heat plant (AHP), is shown in fig. 3. The current USSR five-year plan envisages the completion of two large AHPs to meet the demand for heat of Gorky and Voronezh.

Fig. 3. Thermal circuit of a nuclear heating installation.
1—reactor ; 2—water-water heat exchanger ; 3—circulation pump ; 4—water heater for heating system.

In the design and construction of any reactor plant special attention is paid to operation safety. But before discussing safety matters involved in AHPs we will have to say a few more words about the APHPs. At present these plants use shell-type water cooled reactors of the same design as those in operation at condensing atomic power plants. A standardized design approach means that all nuclear and radiation safety regulations, as well as safety regulations concerning the siting of atomic plants, are virtually the same.

As far as the atomic heat plants are concerned, they should be located near large settled areas so that the losses of heat during transportation are minimal. The APHs have therefore to meet stricter operation safety standards which necessitates designs differing from those of the APHPs.

Any nuclear power installation is equipped with hardware providing for radiation safety of the staff and nearby population and preventing radioactive matter penetrating into the heating system or into the technological heat carrier. The safety measures and technical equipment may vary, depending on the type of reactor an its location, but each and every installation should rule out the very possibility of accidents like the one at Chernobyl. The task today is to design a new generation of reactor installations of improved safety, and the AHPs should be included in this category. Operation safety has been considerably improved at a reasonable cost by developing special low-temperature shell-type reactors for use at AHPs oriented to standard domestic heating system parameters (water temperature of up to 150 °C at the inlet).

The very fact that such an installation should operate under moderate conditions (at 200 °C which is about 100 ° lower than usual, and at pressure which is only an eighth of the standard) makes the installation much safer. At the same time a moderate energy intensity in the fuel core improves the reliability of the fuel element.

Operation safety is given priority in designing reactors or the AHPs. Account is taken even of the least probable situations such as damage to the shell, the fall of an aircraft, the impact of a blast shock wave, etc., and every precaution is taken during the removal of solid and liquid radioactive waste materials.

Heat from the reactor is conducted to the user by means of an intermediate heating circuit in which the pressure is lower than in the heating system, owing to which radioactive penetration into the heating system is impossible even in the event of a leakage. In this reactor installation natural circulation of the heat-transfer agent does the work of the pumps. Therefore, the reactor is insensitive to electric power supply failures. Also provided for is an additional safety shell which houses the main shell with a clearance. Thus even if the shell should be ruptured (this is one of the most serious failures) the core of the reactor will not be left without water and a situation fraught with the danger of a radioactive discharge into the atmosphere will be prevented.

Some of these design solutions may be used in the future for special-purpose reactors of improved safety intended for industrial heating supply systems using high-potential heat.

Optimization of Heating Systems

As a next stage in its development, centralized heating can be provided by several types of installations. These include heat-and-power plants working on fossil fuels, atomic heat-and-power plants, district heating plants using fossil fuels, heating plants using nuclear fuel, including the AHPs and industrial AHPs generating medium- and high-potential heat.

To set up an efficient centralized heating system it is important correctly to define areas of application of its component parts. One of the directions of the study discussed in the present article was related to identifying problems that may arise in the course of optimization of the heating system. These problems have been formulated and arbitrarily grouped at four hierarchic levels : the power producing system of the country as a whole ; the integrated electric power systems and atomic power systems that are its component parts ; the heating systems of a specific city or industrial center ; and finally, individual heat sources.

For the solution of the problems in each group specific tasks, methodologies and mathematical models have been worked out. The main criterion has been minimum cost for the national economy. Other considerations were the probability that the production of fossil and nuclear fuels will be limited, and that there will be restrictions connected with transportation and with environmental questions. Computations have been carried out by means of powerful computers using modern methods of optimization and decision-making.

At the first hierarchic level (the country as a whole and individual regions) optimization means working out an optimal policy of developing a centralized atomic heating system within the national power system, and deciding the scope of construction of the APHPs, AHPs and industrial AHPs, taking into account the power consumption patterns ex1sting in the USSR.

At the second hierarchic level two groups of problems have to be tackled. The first is related to questions of the power supply : integration of the power generated by APHPs into the power grid and determination of total capacities and operation modes depending on its demands. The second group of problems concerns the development of nuclear heating installations as component parts of a nuclear power generation system. Future requirements and prospects serve as the basis on which major specifications of nuclear reactors for future APHPs, AHPs and industrial AHPs are determined.

At the third hierarchic level dealing with urban heating problems the policy is towards optimal development of heating systems using both nuclear and fossil fuels. This involves optimization of unit power, of the composition of major APHP, AHP and industrial AHP equipment, and of the deadlines for putting it into operation.

Optimization of nuclear heating systems is treated at the fourth hierarchic level. The most efficient arrangement and production schemes, designs and parameters of APHPs, AHPs and industrial AHPs are selected.

A measure of inertia is typical of the development of the power systems, therefore the problems mentioned above are dealt with in advance. Computations precede actual realization by 5 to 7 years, and in some cases by 20 or even 30 years.

Economic Considerations

The studies carried out indicate that in the European regions of the Soviet Union an extensive use of nuclear fuel in the heat supply system can be efficient if the atomic heat plants are rationally integrated with plants using fossil fuels.

A comparison of the technical and economic characteristics of traditional and atomic heat-and-power plants suggests the latter to be advantageous under heat loads of over 6000-7000 GJ per hour. While having similar economic characteristics, the APHPs provide for greater savings in fossil fuels and thus reduce the need to increase their production in undeveloped regions of Siberia.

At present the area where AHPs can be efficient can be defined only tentatively owing to the uncertainty of some of their technical and economic parameters. What can be stated definitely is that they can compete with the APHPs under relatively low heat loads, that is, loads of 1800 to 3500 GJ per hour, since in this mode (with the VVER-1000 reactor) the latter’s technical and economic indicators are reduced significantly.

A comparison of AHPs with district boiler facilities using fossil fuels shows that in the European regions the former are more efficient under heat loads of 2000 GJ per hour.

As far as e APHPs are concerned, they are most economical when they supply only a regular amount of heat within a heat supply system. Peak loads should be carried by “peak” facilities using fossil fuels. The same is true of the systems using AHPs as the major heat sources.

* * *

How will the structure of the supply system be affected by the development of the atomic heat sources ? Respective studies taking into account considerations of economic efficiency and recommendations concerning applicability of each type of heating installation show that the share of atomic heat sources has been growing rapidly (fig. 4) ant that in future they will produce one-third of the total heat output, thereby reducing fossil fuel consumption by about 400 million tons of equivalent fuel per year.

Fig. 4. Structure of heat sources.
1—heat-and-power plants ; 2—atomic heat-and-power plants (APHP) ; 3—district boiler facilities ; 4—atomic heat plants (AHP) and industrial AHPs ; 5 —secondary sources ; 6—small boiler installations ; 7—new heat sources ; 8—other sources.

Gradually building in nuclear installations into centralized heating systems is not only expedient but imperative, since today this is the only feasible way to provide the country with the necessary amount of thermal energy.

Artist A. Kolomatsky.

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