BN-350 reactor facility. Collected experience. Part 1.

Fast neutron reactor topic is one of the top priority topics of our edition. Today we offer our readers to return to the origins of the domestic fast neutrons nuclear program - fast neutron reactor BN-350. What was the part assigned to the power generating unit in Shevchenko and what experience could gain Russian organizations by its exploitation?

Rudolf Baklushin, IAEA expert with a significant length of service that had been working for a long time as a deputy chief engineer of BN-350 facility, will help us to look into it. Today we publish the first article from a series of those, prepared by Rudolf Petrovich.

BN-350 - the first nuclear fast neutron reactor

The reactor facility BN-350 is the first facility with the fast neutron reactor. Experience of its designing, building, installing and exploitation has helped to realize and solve many problems with NPPs of a fast sodium type.

At the time of BN-350 it was defined a goal to get as much secondary nuclear fuel - plutonium - as it was possible, in order to ensure a broad development of nuclear energy. As plutonium recovery was proportionally to the capacity of the reactor. Its large-scale and profitable producing could be organized only by a significant capacity of the reactor facility. Economic estimations showed that plutonium producing could be profitable only by the capacity of 1000 MWe (thermal) and higher. This conclusion was made to 1960 and predetermined the creation of BN-350.

In May 1960 IPPE has made a draft proposal of a fast neutron reactor facility designing with sodium coolant and 1000 Mwe capacity. Later this project was called BN-350. The same month government of the USSR has made a decision to begin the designing. The launch of the reactor was planned to 1966. But these terms had to be carried over several times because of the complexity and novelty of the arising problems, some of those were caused by a scale factor.

Main steps of BN-350 life:

Reactor facility in Shevchenko was designed by the leading Russian organizations. More than 10 organizations of different specializations were working on this project. But the most important role in BN-350 constructing played IPPE, OKBM, OKB "Gidropress", VNIPIET.

The same organizations were managing a launch and exploitation process of the facility, were solving arising scientific-technical problems and were supporting operatives.

I have to stress that at the very beginning of the work on BN-350 project knowledge and practical experience of handling liquid metals were concentrated in IPPE. At the same place nuclear research reactor BR-5 was constructed. International scientific exchange in the sphere of fast reactors didn't exist at that time. Domestic specialists had to make a rapid progress. Just compare, sodium volume in the first circuit of the research reactor BR-5 was 1,7m3, and in the first circuit of BN-350 it was 510 m3.

The designing of BOR-60 reactor, that is sometimes called a predecessor of BN-350, has been started 4 years later. In its design and even in the start-up program we can find the influence of BN-350 ground works. Launch of BOR-60 had taken place earlier than BN-350 was launched, and it happened so because of the less scale of BOR-60.

Principal elements of BN-350 design

Principal elements of BN-350 design were chosen in accordance with knowledge and experience that was in the early sixties. Triple-purpose nature of the new reactor - generating of thermal energy and electricity and recovery of the secondary nuclear fuel - has left its imprint on the project.

The following principal design decisions were taken for BN-350:

Peculiarities of fast neutron reactors demanded a principally new approach to the construction of the equipment and technological systems, to the control units and protection systems, to the excess absorption factor and to the providing of reliability and security of the facility. Numerous calculations and experiments were held to justify physical and technological decisions. We had to design the equipment for sodium systems of the industrial scale for the first time, and to create the unique methods of control over the technological parameters of sodium systems.

Tasks of BN-350

First of all, Russian specialists planned to prove or to correct scientific and technique conceptions of fast neutron reactors while constructing and exploiting BN-350. It was rather important to specify BN peculiarities and processes, flowing both by normal operation and by abnormalities and incidents.

It was required to prove or to disprove the principal decisions on equipment constructions, on technological systems designing and on security principals. It was planned to examine reasonability and necessity of making changes in structural and circuit designs of fast neutron reactors, and to optimize them for the future facilities.

Undoubtedly, one of the main tasks of BN-350 was to achieve a practical experience of bringing into service of fast rector with sodium coolant. Domestic specialists had to learn operating conditions managing, to realize the rightness of foreseen precautions (or to detect an excess of this precautions), and finally, to optimize the behavior of personnel in different situations.

It was also necessary to gather the statistics of concrete equipment failures, standard operating conditions failures and of personnel mistakes, and to establish their influence on the reliability and capability of the facility. An important moment was to gather the information about initial events, causing these or that abnormal situations, which were either not provided by the designers or were created by the operative staff because of their inexperience or carelessness.

And, certainly, one of the most important tasks was to train the personnel, which had already had an experience of thermal reactors exploitation, for operating a reactor of a new type with an exotic at that time coolant (sodium).

Collected experience - brief results

At first, I would like to stress, that main scientific concepts and principal design elements were corroborated during BN-350 exploitation. We can say about one exclusion, which was the problem of self-development of small water-leaks to sodium in steam generators; we shall speak about it in the next articles of the cycle.

BN-350 exploitation has proved reliability and security of power generating units with sodium coolant, their handleability. Its exploitation made possible to collect a significant amount of information, which provided a solid base for the development of the following reactor facilities.Certainly, by the exploitation, and especially during the initial phase of work and the first period of mastering the facility malfunctions were observed, failures of some elements and assemblies. Reason for some of them was the lack of experience of the operative staff and of adjusters. Reason for the others was an incorrect estimate of real operating conditions, influence of the scale factor and simply mistakes.

For example, wrong estimate of the temperature conditions of the equipment caused a sodium vapors deposition in narrow spacing. And this caused difficulties by the launch of the reactor coolant pump of the first circuit after a longtime suspension or by the rotation of the reactors rotary cocks. But the personnel managed to detect promptly the reasons of failures and to debug them.

One of the most unpleasant defects may be nearly one and a half excess of the real sodium flow rate in the loops of the second circuit over the designed value, which was detected by installation. That caused the formation of funnels and a gas capture form the free levels, and also hydroblows and reactor coolant pump overload. To debug these defects we had to cut the pipe-lines 500mm - part of those had been already filled with sodium - and to weld the throttles into them. Therefore we had to delay the launch of the facility for a month.

But knowledge about such failures and understanding of their reasons is not the main issue in the exploitation experience. Having analyzed experience, received by designing, launching and exploiting BN-350, Russian organizations could critically estimate the decisions, taken in the design of BN-600 power generating unit, and to make necessary changes there, that provided a successful launch of the reactor.

Collected experience and knowledge can give us an answer practically to any question concerning the designing or exploitation of the fast neutron reactors. And it opens us today a way to the serial fast reactors.

What natural physical processes were used in BN-350 project? What did the BN-350 physical research program contain? How did the sodium armature approved itself? You will find these and a lot of other topics in the next article of the cycle.

SOURCE: Rudolf Baklushin, AtomInfo.Ru

DATE: April 02, 2008

Topics: NPP, Russia, Fast breeders

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