by Marina KHALIZEVA, journalist

The accident at the Fukusima Nuclear Power Station that took place in Japan in 2011 as a result of a destructive earthquake and the tsunami that followed urged on development of safety systems for nuclear stations. In this context, the experience and know-how offered by Russian scientists stir up indubitable interest. Several years after the disaster at the Chernobyl NPS (Ukrainian SSR, 1986), our design engineers offered an original invention-- the so-called trap for melt or a device for localization and cooling of fuel masses in cases of severe accidents leading to melting of a core. This device was installed at some national and foreign nuclear plants and was acknowledged worldwide as a reliable "security blanket". In critical situations it can retain nuclear fuel for an indefinite period of time, preventing products of radioactive decomposition from exceeding the bounds of the nuclear reactor, thus protecting the population and the environment from radiation. This essential element of any modern nuclear plant was developed by scientific institutions of the state corporation for nuclear power Rosatom in cooperation with its partners from academic institutes.

Before the Chernobyl disaster, nuclear power industry was considered one of the main sources of power supply. The industry achieved its heyday in 1980-1985: the total capacity of nuclear plants operating at that time was 117 MW, and development programs promised further increase of this indicator. But a technogenic tragedy in a small Ukrainian satellite town Pripyat broke the plans--after 1986 active construction of power generating facilities was stopped, and in the 1990s the number of shut-down reactors exceeded the number of commissioned ones. After national referendums held in Sweden, Italy and Austria, these countries adopted a decision to stop operation of nuclear plants; Germany gradually took out of operation power generating units constructed under Soviet designs. At the same time, Russia made attempts to find solution of complex safety problems: national specialists actively developed new advanced designs of nuclear power units based on state-of-the-art equipment and newly designed additional "passive" automatic security systems that in combination with conventional security systems could increase greatly safety of nuclear stations. The trap for melt is a unique development of Russian scientists and the last "defense" line in case of grave nuclear accidents.

The trap for melt looks like a big steel "pressure cooker" weighing 800 t filled with the so-called sacrificial material cartridges (aluminum and iron oxide grains), installed at the bottom of the reactor shaft. In case of emergency at a nuclear plant, a mix of melting nuclear fuel and structural materials of the active zone (corium) will flow down into this device and react physically and chemically with the sacrificial material, which will not allow a radioactive material to spill out.

This project was initiated in 1994, when specialists of the Russian Research Center "Kurchatov Institute" headed by Vladimir Asmolov, Dr. Sc. (Tech.), in cooperation with the RAS Institute of Safe Development of Nuclear Power Engineering (Moscow), research and production enterprises Luch (Podolsk, Moscow Region), Elektroterm, TermlKS (Moscow) organized large-scale high temperature experiments aimed to observe and measure interaction of the corium with the bottom of a water-cooled power reactor (WCPR). The main

Basic corium localization scheme:

1--reactor vessel;

2--concrete trunk;

3--bottom plate;

4--heat exchanger;

5--basket;

6--filler.

task was to develop a mathematical model and codes describing behavior of the melted corium in case of serious accidents and work out recommendations for their localization within the limits of a nuclear plant. Thus, a complex of small-, middle-, and large-size experimental plants Tigel, Korpus, Rasplav-Salt and Rasplav-AW200 with the operating temperature up to 2,700°C was designed at the institute, where scientists carried out experiments using original prototype materials of the active zone--uranium and zirconium oxides with metal inclusions, and metal fluorides, making it possible to simulate thermohydraulic processes in the conditions close to real in the reactor. The result was a proven maintenance of integrity of a water-cooled power reactor body with a capacity less than 1,000 MW due to external cooling of the reactor with water, as for more powerful reactors (WCPR-1000, 1500)--scientists substantiated the necessity of special traps--devices filled with light metal oxides (Al₂0₃--Fe₂0₃) for localization of a liquid melt.

Tianwan Nuclear Power Station (China) was the first power station to get such equipment; to be constructed with the help of Russian specialists from Atomstroiexport, a leading engineering company of the state corporation Rosatom, under the contract signed in 1997. In the course of design works engineers of St. Petersburg Institute Atomenergoproekt had to fulfill some very stringent safety requirements of Chinese customers: in case of failure of the cooling system, the melt should stay within the limits of a power generating unit. This made scientists remember recommendations offered by Kurchatov Institute--to use the trap.

Let's point out that similar devices were first put into operation on EPR reactors by European specialists. The French company Framatome, a close partner of the German Siemens, offered a concept of an "external" localization of fuel masses. After melting of the reactor body, the corium first got to the so-called "pre-trap", accumulated there and, after changing its condition, forced out a "plug" and spread in a room of about 170 m², where it cooled down and crystallized. This process had a number of defects, in particular, a lot of extra space required inside the protective cover, while the design of widely used water-cooled power reactors did not allow to do that. In addition, our specialists did not want to increase the size of the most expensive module of a nuclear plant. And then Atomenergoproekt offered the Research Technological Institute named after A. Alexandrov (Sosnovy Bor, Leningrad Region) to develop an original design of a protective device in cooperation with the Design Branch of Rosatom, RAS Institute of Chemistry of Silicates named after I. Grebenshchikov (St. Petersburg), All-Russia Research Institute of Chemical Technology, RAS Institute of Safe Development of Nuclear Power Engineering and Kurchatov Institute (Moscow).

It was not by chance that the Institute in Sosnovy Bor was chosen as the main partner of the project. From the late 1980s it was a place where the Scientific and Industrial Center of Nuclear Power Engineering operated and developed a trap capable of keeping the melt inside the average capacity WCPR-640 (it was planned to install the similar reactor at the Sosnovy Bor Institute). But due to a lack of financing, the project was suspended, but useful designs remained. They were later used as a basis for the original Russian invention.

Vladimir Khabensky, Dr. Sc. (Tech.), recollected in an interview to the Mayak newspaper (Sosnovy Bor): "We faced a complex task: we had only 3 years to substantiate the concept of the trap. The project budget was about 1,200,000 USD, while French and German specialists had already spent more than 800 mln Euro. In other words, our budget was less 800 times. Nevertheless, we achieved the goal".

The national project was based on a crucible structure--an "iron tank"--6 m high and 6 m in diameter with walls of up to 6-9 cm thick, as Khabensky called it, installed directly under the bottom plate of the reactor. In case of emergency the fragments of the reactor body would remain on this plate, and the melt, through the "funnel" at the top of the tank would get into the trap with the external water cooling and water supplied to the surface of the melt bath. The sacrificial material inside such structure should perform a number of functions: decrease energy of the corium and its temperature, making up 2.4 thous. °C, prevent emission of hydrogen in the process of zirconium oxidation and ensure subcriticality of the melt (impossibility of the chain reaction). At the same time, it should quickly dissolve in the corium to increase the mass contact with the walls of the cooling down reactor body and decrease the thermal current on the structure to a safe level.

The composition of such "filling" was developed by specialists of the RAS Institute of Chemistry of Silicates named after I. Grebenshchikov under the guidance of Acad. Vladimir Shevchenko, its director. Experts of the laboratory headed by the RAS corresponding member Viktor Gusarov managed to develop a material with a safety factor designed for 60 years. Samples of the material were manufactured at St. Petersburg plant Magneton, and the industrial technology was tested at Borovichi plant of fire-proof materials (Borovichi, Novgorod Region). Today, its production lines can manufacture up to 200 t of the sacrificial material (in the form of dark grey bricks). Cartriges filled with the sacrificial material are put into the trap in the course of its installation and do not require replacement in the whole 60-year operation period of the nuclear station.

In 1998, the Russian system of "passive" safety was examined and approved by the International Atomic Energy Agency (Vienna, Austria), and in 2001 it was recommended for delivery to China. A year later it was installed at Tianwan Nuclear Power Station. The system consisted of 12 modular heat exchangers resembling high boots. According to Russian specialists, modular structure of the unit was the only way to meet the requirements of the construction schedule agreed upon with the Chinese contractors. At the present

moment, these efficient safety systems are already installed at the Kudankulam NPS (India) and newly built power units of Leningrad and Novi Voronezh nuclear power stations.

Let's point out that melt localization devices developed for national NPS have a number of innovations. Unlike the Chinese modular trap, the national one is made in the form of an integral body, resembling a reactor that improves strength properties of the unit. There are also some differences in the system of superheating control too. Today such equipment has a double body: thickness of the first wall is 60 mm and of the second wall--30 mm correspondingly. The space between the walls is filled with aluminum and iron oxide grains. In case of local meltings of the inner wall, they interact with the fuel mass and create additional protective barrier preventing destruction of the outer shell.

It must be emphasized that delivery of the trap from the production plant to the destination point is another complex technological process. The integral body weighing 150 t is more convenient to transport by water using a barge. A special unloading procedure has been developed: the barge is first filled with the ballast, then it runs aground; and, the front side is cut off from it to let a powerful truck inside to take away the trap. Sometimes it is necessary to improve the infrastructure to take the trap to the place of installation: to construct new bridges and roads. It was the case in 2009, when the trap was delivered to Leningrad NPS, where a new bridge span and 3 km of a new road were constructed.

It is generally accepted that national projects are too complex in terms of safety. They say, it costs too much. But the accident at the Fukusima NPS showed that they are the most competitive. Russian traps have already been ordered by Byelorussia, Bangladesh, Vietnam, India and Turkey. By the way, designers are constantly improving their invention. For example, reduction of the volume of the sacrificial material made it possible to reduce the price of the unit. Today it costs 250-300 mln RUR--it is less than 0.5 percent of the total value of the modern block. Its production has already mastered 7 national machine-building plants, which will allow to guarantee a special protection system for almost all national NPS construction projects, and offer such systems to our foreign partners.

Illustrations from the web-site of OAO Rosatomenergoproekt" and Scientific Production Company "TermIKS"