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By Viktor SIDORENKO, RAS Corresponding Member, adviser to director, Russian Research Center "Kurchatov Institute";

Alexander ZHURBENKO, deputy department head, Institute of Nuclear Reactors, Russian Research Center "Kurchatov Institute"

At the end of 2006 Bulgaria's National Electric Company announced the results of the competition for the "Belene" nuclear project: the Russian R&D Association "Atomstroiexport" (that puts up nuclear facilities abroad) won the right to build an atomic power station in Bulgaria. As many as 200 experts of eight countries had spent 17 months studying the blueprints with the aid of the latest methods of all-out evaluation, and using a specialized program for assessing the possible risk factors according to the criteria elaborated by the International Atomic Energy Agency (IAEA).

The "Belene" nuclear station slated for construction in Bulgaria will have two 1,000 MW power-generating units; the design and specifications were a concerted effort of the Moscow-based "Atomenergoproyekt" Institute, of the "Gidropress" Design Office at Podolsk near Moscow, and of our "Kurchatov Institute", too. The "Atomstroiexport" Association enlists the services of its regular partners, the leaders of

стр. 49

Diagram of the "Belene" nuclear project.

Russia's nuclear power engineering, such as the Company "Izhorskye zavody" ("Plants of Izhory" at St. Petersburg), and the machine-building plant of Podolsk ("ZiO-Podolsk). Bulgaria will handle about 30 percent of the job.

Our experts have attached particular attention to safety. This is a multifactorial indicator depending on the available level of know-how, technology and equipment, and expertise.

Any "revolutionary ideas" in such matters should be well-considered and foolproof. Rigorous safety standards are naturally a must-the system of such standards draws upon the experience amassed worldwide in nuclear engineering and condensed in documents of the International Atomic Energy Agency (IAEA). In addition, in 2003 participants in the International Project (INPRO) on innovational nuclear reactors and fuel cycles formulated basic criteria for this field. Compliance with these standards is meant above all to preclude failures and emergencies in the performance of nuclear generators and, should such mishaps occur, to minimize their harmful effects on man and the environment.

The present-day concept of nuclear safety provides for making the key standards of nuclear engineering legally binding. Such standards apply to "Belene", too, in practical terms identical to those in force here in Russia, and based on the strategy of defenses organized in depth at all operational levels. One level concerns an adequate location of atomic power stations; another-early fault detection and trouble shooting. The third level of defense is designed to prevent and minimize the aftereffects of failures allowed for at the gestation stage; the forth line of defense provides for a worst-case and most unlikely scenario; and the fifth stands for precautionary measures.

Controlling the nuclear chain reaction in the reactor's active section (fuel core) is all-important to nuclear and radiation safety. Likewise important is to ensure heat

стр. 50

Reactor protective shielding.

removal (i.e. removing the heat produced in the course of nuclear fission), and proper protection of the personnel and population. Such safety techniques incorporated in the "Belene" project were adopted at our home nuclear facilities of previous generations, and their reliability was confirmed by tests at experimental setups and full-scale stands.

"Internal safety" precautions will be an established part of the Bulgarian nuclear project. This is above all self-sustainability of the station's components in conformity with natural physical processes. For one, fuel charges of the reactor should satisfy definite physical parameters so that the chain reaction should stop in off-standard situations with the afterheat withdrawn by means of the natural (not forced) circulation of the coolant.

Most stringent safety standards are an essential part and asset of "Belene". It combines both active and passive systems of safeguards. The active system of conventional generators is equipped with pumps, gate valves and safety catches supplied with moving and rotating power-driven parts. This means they may go out of commission because of power failures or mechanical defects; all that downgrades equipment reliability. Passive safeguards performing the selfsame functions work, as a rule, on compressed gas, and are fitted with relief (safety) or check (non-return) valves. Say, if the pressure in the reactor starts falling and the temperature rising, borated water will be dumped from compressed gas pressurized receptacles to cool the fuel core (active section) and arrest the chain reaction.

Adequate safety is also ensured by constituent elements operating by different principles, a factor that rules out failure due to a common cause. Yet another, no less important safeguard: safety systems will keep on even in case of power supply failure: special diesel generators will be switched on automatically just within seconds. May we stress once again that the work of the

стр. 51

equipment will proceed in line with state-of-the-art physical processes, and prototypes of this equipment are at work at operating nuclear facilities. Russian designers took account of human factors by means of an automatic ban on interference to block faulty actions on the part of the personnel (error protection).

Certain precautions deal with preventing or minimizing a discharge of radioactive substances and emissions (in hypothetical emergencies) beyond the protective boundary - in this particular case, beyond the reactor's superstructure (shielding). It is composed of two structural parts - the inner (made of prestressed reinforced concrete lined with steel) for checking dangerous discharges, and the outer one shielding the generators and premises against disastrous natural and technogenic effects. Both shields ensure biological protection of the environment against ionizing radiation.

Special equipment sustains the permissible pressure and temperature under the reactor's shielding and protects it against damages. If micropores come up in its steel lining or concrete in an emergency situation, a passive system of filtration will be activated to prevent radioactivity from escaping into the outer medium. This system will continue in operation for a long time even if all power supply sources are off: this will be done through pressure differentials generated by heat exchangers of hot air circulating about the power plant's ventilation tube.

Excluding the possibility of explosive mixtures that could be formed during bad emergencies is also a key element of nuclear safety. To do this it is necessary to keep the concentration of hydrogen in the air below a definite level. This job is tackled by a system monitoring the concentration and emergency removal of hydrogen from the reactor. Passive catalytic recombiners burn up part of this gas. Automatic gas analyzers monitor the concentration of hydrogen in outer premises and detect any rise in its level instantaneously.

Once again, the "Belene" nuclear project allows only for exclusively hypothetical emergencies. The probability of one of the worst failures, fuel core melting and discharge of core fragments beyond the reactor's shielding, is absolutely minimal and estimated at less than one event per year for 10 million similar generators. But the concept of multiple defenses developed in depth envisages technical countermeasures even for such extraordinary cases.

In such emergencies it is important to confine and cool the melt of liquid and solid components of the core (active section), reactor vessel and fixings. The "trap", a localization device within a concrete pit under the reactor, will capture (entrap) the fragments and thus secure nuclear safety by prohibiting a nuclear chain reaction. It will minimize the release of radioactive substances and hydrogen, and forbid stress maxima in structures beneath the reactor. This will occur with minimal interference of the human personnel. The "trap" that can be operational for an indefinite period of time is activated only in a bad emergency described above (heat-up and core melting). This safety "trap" - dry - is always at the ready at normal operational conditions or standard deviations.

The endorsed "Belene" version is remarkable for high technical and economic standards making it possible to cut the down time and up the service life to 60 years. The output efficiency will be up to 90 percent, and the heat produced by the unit can be used by the utilities. The nuclear fuel will enable operators to regulate "Belene's" capacity 100 - 75% even at high burnup. The project is a happy combination of the latest design solutions with up-to-date nuclear engineering technologies developed in Russia and elsewhere in Europe. A large portion of "Belene" equipment will be supplied by the international AREVA NP Company and SIEMENS of Germany (SIEMENS will bring in a modern digital automated system for controlling technological processes). Up-graded systems developed in Russia will take care of the control and diagnostics of the reactor's work.

By winning the competition in Bulgaria, "Atomstroiexport" has consolidated its leading positions on the world market of nuclear engineering.



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VIKTOR SIDORENKO, ALEXANDER ZHURBENKO, RUSSIAN SCIENTISTS' VICTORY // Минск: Белорусская электронная библиотека (BIBLIOTEKA.BY). Дата обновления: 01.10.2018. URL: (дата обращения: 17.10.2018).


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