Atomic Energy, Vol. 77, No. 6, 1994

ARTICLES

SMALL NUCLEAR POWER PLANTS BASED ON SHIP-PROPULSION

NUCLEAR REACTORS

V. K. Ulasevich UDC 621.039

The utilization of the ship-propulsion nuclear power plant technology - for which there is a great deal of operating experience - in the national economy is now being actively discussed. In a climate of substantially increasing costs of fossil fuel and the cost of transporting it, especially to the remote eastern and northern regions of the country, the wave of

the post-Chernobyl disenchantment with nuclear power industry is gradually being replaced by the realization that energy intensive fuel, such as nuclear fuel, has certain advantages. The ecological advantages of nuclear power plants for the vulnerable and virtually unrestorable environment of the northern parts of the country is also important.

The present paper is a review of developments at NIKIt~T in this field of nuclear power in the period 1991-1994. The review is based on the results of previous and current investigations, which have shown that the industrial and social structure which has been erected in the main part of the territory of the regions of Russia mentioned above and the primitiveness of the roads and power linkages in these regions create a great demand for small nuclear power plants (mainly up to 30 MW(th)) which generate, as a rule, both electricity and heat. If nuclear power miniplants are considered as such sources, then the specific nature of the construction and operation of such plants determines the obvious basic properties which both the plants as a whole and their nuclear reactors must have:

maximum factory assembly of all units of the nuclear power plant and minimum construction and assembly work

at the plant site; ease of transport of the units to remote regions, both in delivering the units to the plant site and removing units

after the plant ceases operation; high maneuverability of the characteristics of the nuclear reactor and complete automation of operation with a

minimum number of skilled service personnel; possibility of operation of a nuclear power plant in regions with little or no water, including under conditions of

large seasonal differences of the surrounding air temperature; operation of the plant over a long period of time without reloading the nuclear fuel; and, the nuclear fuel must be economically more advantageous than the traditional fuel. Of course, in addition to these properties, the power plants must meet current and the constantly improving safety

standards. The experience, gained over many years of NIKIET in the field of nuclear power for ship propulsion and the

principles, systematically developed by specialists at the institute in the course of these works, for increasing plant autono- my and plant safety, and especially the degree of plant integration, which has been implemented in recent years in designs of nuclear power plants with one-unit steam-generating plants, make it possible to propose, together with the long-term partner AO "Kaluga turbine plant", some of these designs as a basis for low-power nuclear power plants with capacities ranging from 0.5 to 12 MW(el).

As one can see from Table 1, the nuclear power plants being proposed can be built on the basis of the nuclear power plants for which prototypes with a long history of operation are available or which have been designed using proven solutions to technological problems. This makes it possible to reduce to a minimum the testing and construction work and, as a result, to reduce the cost and construction period of the advance plant.

The plants indicated in Table [ are similar in appearance, and they have similar schemes and configurations as well as similar safety approaches. It is helpful to examine these plants for the example of the "Uniterm" nuclear power plant with an electric capacity ranging from 1.5 to 6.5 MW. If the plant must be used for producing electricity as well as heat, the heat extraction could amount to 20-30% of the nominal thermal power.

NIKII~T. Translated from Atomnaya I~nergiya, Vol. 77, No. 6, pp. 407-414, December, 1994. Original article submitted September 12, 1994.

I063-4258/94/7706-0889512.50 �9 Plenum Publishing Corporation 889

TABLE 1. Main Opierbtional Qualities of Nuclear Power Plants Based on Ship-Propulsion Nuclear

Reactors

Characteristic

Run time of a prototype of the nucle- ar power plant, yr

Electric power, MW

Heat production, Gcal/h

Number of reactors

Operating time without core reload- ing, yr

Number of protective barriers in the path of radioactive substances

Form of cooling of the condensers in the turbogenerator plant and safety systems

Earthquake resistance

Servicing of the reactor during oper- ation

Servicing of the turbogenerator unit during operation

Number of transported umts and their mass, taking into account the transport platforms

"Krot"

15

upto 1

upto3

2

8

Conventional name of the nuclear plant 1

"Kedr" "Uniterm" [ "Shel'f-3"

10 Main solutions to techmcal problems checked in the practice of building and operating low-power propulsion

upto2

upto 5

1

10

plants

up to 6.5

up to 7.5

l

20

up to 12

upto 15

1

20

5 5

Air. Local water sources not required

8 on the MSK-64 scale. Containment of radioactive substances pro- vided up to 9 on the MSK--64 scale.

Not required. Scheduled servicing is performed by a mobile crew once a year during a period of two weeks.

Not required. Scheduled Required. Personnel works outside servicing is performed by a the irradiation zone (category B). mobile crew once a year The three-loop design of the nuclear during a period of two power plant prevents radionuclides weeks, from entering the turbogenerator

plant.

10, 80 tonnes 4, 50-150 15, 100-180 [ 3,250-600 i

tonnes tonnes [ tonnes

The heat source of the nuclear power plant "Uniterm" is a one-unit nuclear steam-generating plant (Fig. 1), in

which the basic elements of the first-loop system - the active zone, the steam generator, the surge tank, and the control

and containment elements - are housed in a single vessel. Moreover, they are structurally combined with the plant and the

heat exchangers, which transfer heat from the first loop into the intermediate loop. This makes it possible to eliminate

virtually all pipes in these loops and to achieve the maximum possible compactness of the arrangement of the sources of

ionizing radiation and the potentially hazardous working medium - the coolant in the first loop. The structural layout of

the steam-generating plant makes possible cooling of the core and transfer of heat into the steam generator by natural

convection of the coolant in the first loop. This makes it possible to eliminate forced-circulation equipment. This approach

to the design of the main element of the nuclear power plant - the nuclear reactor - is used in order to achieve maximum

reliability and simplicity of the construction by eliminating active elements with constantly moving mechanical parts. The

only moving elements in the steam-generating plant are groups of the working units of the safety and control system

together with their drives, which provide protection during an accident and compensate for a change of reactivity. These

elements are moved once over the period of continuous operation of the nuclear power plant during the startup of the

steam-generating plant during normal operation. In the case of an accident the units of the safety and control system, which

provide the emergency shielding, can be actuated. The thermohydraulic scheme of the "Uniterm" nuclear power plant (Fig. 2) allows for the use of three coupled

technological loops, the last of which contains all heat users (the turbogenerator plant or heat users).

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Fig. 1. O n e - u n i t s t e a m - g e n e r a t i n g p lan t : 1 - c o r e ; 2 - s t e a m g e n e r a t o r ; 3 - vesse l ; 4 - i n t e r m e d i a t e - l o o p h e a t

e x c h a n g e r ; 5 - e m e r g e n c y c o o l i n g hea t e x c h a n g e r ; 6 - a c t u a t i n g m e c h a n i s m o f t he s a f e t y and c o n t r o l s y s t e m ;

7 - s u r g e t a n k .

The heat energy in the first loop is transferred to the intermediate loop and the emergency cooling loop by means of phase transitions of the coolants. This decreases the required flow rate of the coolants and increases the driving head for natural circulation.

The technological parameters of the first-loop coolant were chosen from the reliably determined range of working pressures and temperatures that is characteristic for the first loop of a nuclear plant with water-cooled reactors as well as on the basis of experience in operating propulsion nuclear power plants under operating conditions with natural circulation of the coolant. Moreover, the limits of the range of the coolant parameters during the period of operation of the core without compensation of reactivity by displacement of the control rods were taken into account. The choice of the working temperature and pressure of the coolants in the intermediate loop and the loop of the steam turbine plant was strongly affected by the need to achieve acceptable plant efficiencies and by the desire to make use of the experience in the con- struction and operation of operating prototype steam turbine plants. Analysis of the heat-engineering characteristics of steam turbine plants of this type made it possible to choose, on the basis of the considerations mentioned above, the parameters of the coolant in the technological heat-user loop and the intermediate-loop coolant parameters were deter-

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~ o . . , o . . , , , r

. . . . ~ ; . w "

, .

17

Fig. 2. Schema t i c d iagram of the "Un i t e rm" nuclear p o w e r p lant : 1 - core ; 2 - one -un i t s t e a m - g e n e r a t i n g p lant ; 3 - s team genera to r ; 4 - i n te rmed ia te - l oop hea t e x c h a n g e r ;

5 - c o n t a i n m e n t enve lope ; 9 - wa te r -a i r - coo led heat exchange r o f the e m e r g e n c y coo l - ing s y s t e m ; 10 - reac to r p lant ; 11 - t u r b o g e n e r a t o r p lant ; 12 - t r a n s f o r m e r s ; 13 -

e lec t r ic gene ra to r ; 14 - s team turb ine; 1 5 - a i r -coo led condense r o f the t u rb i ne .

mined so as to distribute efficiently the temperature difference of the coolants in the first loop and the heat-user loop. The parameters of the coolants in the technological loops of the "Uniterm" nuclear power plant were determined on the basis of these considerations and computational studies:

first-loop coolant (highly pure water with the quality of the first-loop coolant): pressure, MPa temperature at the core entrance/exit, ~

intermediate-loop coolant (water with the quality of the second-loop coolant):

pressure, MPa temperature, ~

coolant of the heat-user loop (water with the quality of the second-loop coolant): pressure. MPa steam/feedwater temperature, ~

16-16.5 245-225/325-305

2.4 220

1-1.2 207-210/45-60

It should be noted that the comparatively high energy potential of the user loop coolant of the "Uniterm" power

plant makes it possible to use the coolant not only for domestic but also, if necessary, for technical purposes. The choice of characteristics and structure of the core and control units was based on the following principles: maximum possible decrease of the operating reactivity factor and, in particular, the fraction of the total efficiency

of the working units of the safety and control system for one group of units moved by individual drives; power, coolant temperature, and fuel temperature feedback coefficients optimized for safety; specific energy intensity guaranteeing achievement of both the chosen operating period of the core without opening

the fuel casings and the degree of residual heat release required for reliable cooling of the fuel elements and other elements of the core in the case of serious accidents; and,

greater reliability of the reactor shutdown system achieved by using in the control system additional passive emergency shielding channels whose principle of operation is different from that of the main functional units.

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The overall reactivity factor of the reactor was decreased by means of technical solutions which almost eliminate

the need for extracting the control rods from the core during the entire period of cominuous operation of the nuclear power plant in the power-generating mode. This is achieved because the core operates in a regime of self-regulation of the power

and reactivity, which in turn is achieved by changing the average temperature of the coolant and bumup of the neutron

absorbers. Safety of the "Uniterm" nuclear power plant is achieved by a complex of technical solutions, of which the follow-

ing should be noted. The nuclear power plant employs a water-moderated water-cooled reactor. This plant possesses the properties of

internal self-shielding, which reflect its capability to remain safe on the basis of internal feedbacks, natural physical

processes, the use of passive systems for removing the residual heat released, and automatic shielding devices that quench the efficient chain reaction without intervention by the operator. The reactor plant also has the property of self-regulation of the power and matching of the power with the load by means of optimal negative temperature, power, and void reactivity factors. The physical characteristics of the core were chosen so as to achieve the optimal reactivity factors in the

entire temperature range throughout the operating period of the core under both normal and accident operating conditions. This eliminates spontaneous runaway of the reactor during startup and heating and stabilizes operation in stationary and

transient regimes during the change of operating regimes of the heat-user loop; this does not require the displacement of the control rods. After startup and after the steam-generating plant reaches a prescribed power level, extraction of all

control rods is mechanically blocked; this eliminates the possibility of an unauthorized increase of the reactivity. The structural implementation of the steam-generating plant is such that the potentially possible leaks are located at

the top of the plant, and the maximum equivalent diameter of such leaks is small and does not exceed 20 ram. The use of an integrated arrangement of the steam-generating plant together with an effective iron-water radiation shield between the

core and the plant vessel prevents brittle fracture of the vessel as a result of neutron irradiation of the metal. All this makes

it possible to ignore accidents determined by large and medium-size leaks and to avoid a dangerous development of accidents associated with disruptions of the cooling of the core. Such disruptions are eliminated by the use of an additional

vessel designed to contain coolant leaks in the first loop within the internal volume of this loop. The use of a three-section

system for feeding a liquid absorber guarantees that the additional vessel is filled during the accidents considered by a liquid up to a level above the possible rupture locations in the first loop; this eliminates the possibility of loss of coolant in the core during any of the initial events taken into account in the design and accident scenarios. In summary, an accident with the appearance of a leak in the first loop of a conventional equivalent size not exceeding 20 mm can be regarded as

the maximum design accident for the "Uniterm" nuclear power plant. Calculations have shown that such an accident develops according to a scenario that is characteristic for small

leaks, and the consequences of such an accident can be localized within the additional vessel without unsealing and damage to the elements of the core. This is achieved by efficient backup of the emergency cooling system and the passive nature of

its operation without the use of forced circulation. The inclusion of an additional passive containment safety barrier in the "Uniterm" power plant - an additional

vessel - made it possible to prevent, even during off-design accidents, emissions of radioactive substances into the environment and the danger of loss of coolant in the core. An off-design accident, resulting from unsealing of the addition-

al vessel with a postulated destruction of 10% of the fuel elements in the core and emission of first-loop coolant and radionuclides into the containment envelope, does not present a significant radiation hazard to the population: The individu-

al dose load in this case will not exceed 0.11 rem/yr. The three-loop thermohydraulic scheme of the "Uniterm" power plant, in which the heat users can be reliably

protected, even with two successive interloop leaks, by means of efficiently backed up cutoff fittings, which prevents emissions of radionuclides into the heat-user loop, eliminates irradiation of personnel by ionizing radiation. As a result, the

plant personnel are not subject to a radiation hazard. Under normal operating conditions, the dose rate for ionizing

radiation on the surface of the shielding structures of the nuclear power plant is lower than the natural background dose

rate, and with the maximum design accident it exceeds background by only 10% at a distance of 100 m from the steam-

generating unit. In the design of the "Uniterm" power plant special attention was devoted to the emergency reactor cooling system,

which plays an important role in ensuring plant safety. To a large degree, this is due to the fact that the traditional

technical solutions were found to be ineffective. For this reason, new solutions were developed which take into account the specific conditions of operation of a nuclear power plant and the climatic characteristics of the proposed site locations.

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The emergency cooling system, intended for removing the residual heat from the core, consists of an autonomous,

technological loop, coupled with the intermediate loop. During accidents the heat removed from the core through the steam generators, which are located in the vessel of the steam-generating unit, flows into the intermediate loop, from which it is removed by heat exchangers into an autonomous loop of the emergency cooling system toward the heat-transfer surfaces, which are cooled by atmospheric air. The low winter temperatures in the regions where the nuclear power plants will be

used determines the choice of the low-boiling coolant for the autonomous loop. Ammonia can be used for this purpose. A characteristic feature of the emergency cooling system is that it has no cutoff fittings, i.e., the system is in constant operation. To reduce the loss of heat, removed by this system into the atmosphere, the air flow through the heat exchangers is seasonally regulated. In addition to its main function, the emergency cooling system allows for the possibility of maintaining the nuclear power plant in a "hot" reserved state, i.e., at the minimum possible power level of the core.

Another technical solution for increasing the reliability and safety of the "Uniterm" nuclear power plant is the passivity of the system for controlling and shielding the core. Since the reactor power is self-regulated during reloading in the process of plant operation and the change of reactivity during the period of continuous operation is compensated by burnup of the absorber and by the temperature effect, the reactivity is corrected only once per year by remote change of the position of the absorbing rods. Reactor shutdown during an accident and maintenance of the reactor in a subcritical ;tate are achieved by inserting the control rods by free fall and by compressed springs, i.e., independently of the presence

of power sources at the moment of an accident. To obtain a quantitative assessment of the safety of operation of a "Uniterm" nuclear power plant, possible

scenarios of the most serious accidents (rupture of the first loop, complete stoppage of electricity flow for plant needs) ~ere examined. The results of the calculations showed that the probability of damage to the core as a result of rupture of ,he first loop is of the order of 10-11 1/yr and the probability with complete .stoppage of electricity flow for internal needs does not exceed 10 -7 1/yr, i.e., in both cases the probabilities are much lower than the probability recommended by the

standards 10 -5 1/yr. The design of the nuclear power plant makes it possible to build, assembly, and adjust the plant at machine-

building plants and to deliver large units to the operating site, where only the minimal assembly, startup, and adjustment

work is performed. Floating means (barges, pontoons, and so on) can be used to transport the units. After the units are off loaded, they can be transported by means of large-load platforms and tractors. After operating for the established period of time, the units of the nuclear power plant are disassembled and transported to special enterprises for dismantling and

utilization. The spent fuel can be either reprocessed or stored. For the "Uniterm" nuclear power plant the assembly work is best performed on a prepared a site. After the units

are installed and fastened to the foundations, the connective pipes and cable runs are mounted, including the heat and power grids, the technological loops are filled with the working media, and the startup and adjustment tests of the systems and equipment are performed. The assembly time for the "Uniterm" is estimated to be 4-5 months from the day the plant

units are delivered. With regard to the other plants indicated in Table 1, the critical analyses, which form the basis for the suggestion

regarding the "Shet'f-3" nuclear power plant, of the autonomous underwater power-generating unit, which were performed in NIKIt~T while searching for a solution to the problems of power supply, exploration, production, and transportation of

oil and gas on the Russian Arctic shelf under the difficult conditions of an ice environment, should be noted. The "Shel'f- 3" plant as well as the "Kedr" and "Uniterm" plants, as floating stations with a small draft or as stationary power sources for operation of ground-surface oil and gas pipelines, may turn out to be reasonable and economically useful for certain

sites. As mentioned above, the need for such low-capacity nuclear power plants, which can be transported as separate

units, for the northern and eastern regions of Russia appears to be substantial in the future. A large number of sites, where the use of such plants is economic, especially since preliminary estimates show that the replacement of fossil fuel by nuclear fuel compensates the cost of small nuclear power plants for the period of operation, have already been identified.

In turn, this outlook suggests the possibility of creating regional structures for servicing a park of such nuclear power plants that provides for transportation of the plants to their sites, assembly, startup and adjustment operations, preventative maintenance during operation and servicing, and removal after the plants have operated for their allotted period of time.

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The possibilities for converting the experience and knowledge gained in building ship-propulsion nuclear power plants and utilizing the scientific- and technical resources for the purposes of the national economy are extensive and they have been proven. They make it possible to count the adaptation of atomic nuclear power plants of different capacities for solving future problems of power generation.

LITERATURE CITED

1~

2.

3. 4.

5.

Reports at the Scientific Seminar of the Nuclear Society of the USSR on Autonomous Nuclear Power Sources of Low Capacity for Decentralized Heat and Power Supply [in Russian], Moscow (199 l). V. K. Kovalenko, V. K. Tarasov, and V. K. Ulasevich, "Seventy MW(el) floating atomic electric power station (New construction concept. Ecological questions)," Morskoi flot (1991), No. 11. Annual Report of NIKII~T [in Russian], (1993). F. M. Mitenkov, "Prospects for the use of ship-propulsion nuclear reactor plants," At. l~nerg., 76, No. 4, 318-326

(1994). L. A. Adamovich, G. I. Grechko, V. V. Rumyantsev, et al., "Autonomous nuclear power plant with a one-unit nuclear reactor for production of electricity and heat in remote regions," Report at IAEA Technical Committee [in

Russian], Obninsk (1994).

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