BN-350 reactor facility. Collected experience. Part 1.
BN-350 reactor facility. Collected experience. Part 2.
The topic of fast neutron reactors is of the top priority for our edition. We offer our readers to come back to the backgrounds of our nuclear fast neutron reactors program - the fast neutron reactor BN-350. 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 third article from a series of those, prepared by Rudolf Petrovich.
"Sodium-water" steam generator
The weakest element of BN-350 reactor facility appeared to be "sodium-water" steam generator (SG). Otherwise, time, spent by the designers and operative staff to remove problems with SG, gave us literally, a priceless experience, used by the designing of other BN reactors.
Two reasons that caused difficulties at BN-350 facility can be singled out:
By the designing of the control units and SG protection system a serious consideration was given to the detecting and localization of large leaks. Incidentally, designed systems coped with this task perfectly. But authoring control systems were very inertial to detect leaks timely.
In 1973 and 1974 BN-350 had eight leaks, including two large ones. Practically all of them appeared on the lower edge of the weld, which connected stamped bottom of the Fild pipe with the pipe itself. The result of the incidents was a withdrawal of the SG with the loop for repair, that demanded limitation of the facility capacity.
Repair, which is carried out by the defective pipes opacification, was not successful. By next SG launches leaks in the other pipes were appearing.
We managed to cope with this problem after the complete overhaul of the 5 SGs with the total replacement of the evaporator matrixes in 1974-1975. Fild pipes 32×2 mm with the stamped bottoms were replaced by the 33×3 mm pipes with the chiseled bottoms. Though we haven't managed to exclude leaks, but we have reduced their amount significantly.
To the moment of the decommissioning of the facility operating time of the evaporators was 160000-180000 hours. Because of the leaks maximum capacity of SG was reduced and correspondingly capacity of the reactor facility was also reduced. Maximum achieved capacity of the BN-350 reactor facility was 75% of the rated capacity.
I have to stress, that one of the first SGs had been working without leaks for more than 56000 hours, and then was replaced by SG of a new construction. That fact indicates the importance of a qualitative steam generators producing.
General conclusion that we have come to was the confirmation of the reasonability of the single-shell SG design. Besides, it was proved, that SGs, created for the BN-350 facility, provided the secure exploitation, though being not so perfect.
A big number of leaks, including those, when about 1000 liters of water poured into 70 m3 of sodium, and protective system failures gave us a unique experience. Among others, we managed to formulate the demands to the SG construction, to its control and protection systems, methods of defective pipes detecting by the shutdown after a small leak and a following defect closure, different methods of repair and leaktightness control after them, and finally, the sodium circuit cleaning technology after very significant coolant contaminations.
Results, achieved at BN-350, were taken into account by BN-600 reactor facility designing. Section-module SG construction was chosen for the last one. Modern protection and leaktightness control systems and elements were used for the BN-600 steam generators, principally new decisions were taken on SG binding and actions in case of the leaks.
Other equipment
Cold traps
Cold traps with sodium-potassium cooling proved to be good at BN-350 reactor facility. They appeared to be very comfortable by work.
By cleaning of the second circuit loops after large leaks and significant sodium contaminations by the products of water-sodium reaction, some traps drawbacks were detected. This drawbacks were connected with the scale factor - transition from the research reactor of a small capacity to the power generating facility.
Particularly outlet pipes of the trap at the point of the flow meter installation didn't have heat insulation. At that place oxides, that were in sodium at the trap outlet as a suspension, fell out. After some time, sedimentations were totally blocking the section.
Detected problem was removed at the expense of the trap binding scheme modification. A set of recommendations for the designers of new facilities with sodium coolant had been worked out.
It is notable, that transition to the air-cooled cold traps at BN-600 facility was not connected with BN-350 trap drawbacks, but with the simplification of the technological part of the facility.
Electromagnetic pumps
Three types of electromagnetic pumps with the productivity from 10 to 150 m3/h were used at BN-350 in the auxiliary sodium systems. They are comfortable in exploitation, allow easy flow rate regulation. No principal difficulties occurred, but there were some damages of the pumps by irregular defrosting.
Sodium technology
Creating and mastering sodium technology in industrial scales demanded to solve the following problems:
All the decisions, elaborated in sixties for BN-350, were used in the projects of all the domestic fast neutron reactors. Some of the decisions were not ready to the moment of beginning of works on BN-350 and were directly used in BN-600 and the following projects.
Providing of industrial producing of reactor-pure sodium, possibility of a quality control, industrial sodium deliveries, preparation to its pouring into the NPP circuit
In this direction, methods of sodium chemical analysis, temporary technological conditions for the reactor-pure sodium were elaborated; sodium supplier, capable of following these conditions, was chosen; and problems of its delivery in special tanks were solved; demands and conditions of preparing to sodium pouring into the circuit of the reactor facility were elaborated.
Not all the decisions on the delivery were considered to be successful at that time. Sodium was delivered in 1m3 tanks, that demanded receiving and processing of 1200 of such tanks. Besides, in tanks was a paraffin admixture, used to protect sodium from oxidation. This paraffin had to be removed by the preparation of sodium. But these problems were connected with the possibilities of the manufacturing plant and couldn't be solved at that time.
Analysis of the collected at BN-350 facility experience allowed to elaborate a new approach by BN-600 constructing. All the problems were solved and coordinated with the industry. Particularly, new supplier adjusted the technology - Bereznikovsky chemical enterprise, necessary amount of sodium was provided, and it was delivered in 32m3 railway cisterns.
I would like to mention one more important and new for that time decision, connected with sodium preparation for BN-350, - justification of refusal from supplied sodium distillation and of its purification on the cold traps.
Elaboration of methods of preparing circuits to filling them with sodium, excluding pipes and equipment blocking by different contaminations
This issue has nearly become the "apple of discord" and has become the subject of furious scientific discussions. IPPE in 1963 - 1965 prepared a justification of refusal from water or some other rinses for the mounted liquid-metal circuits and BN-350 systems. Such approach provoked criticism of many organizations involved in the project.
Nevertheless, Obninsk's decision was implemented, and how it turned out in the future, was correct. In the justification of refusal from rinses IPPE has elaborated demands to the condition of equipment and pipe-blocks supplied for the installation, to the quality and purity of sodium systems mounting. Further these demands were included into the mounting technology, and a tough control over these demands was provided by the equipment installation.
Mastering sodium circuits exploitation with sodium volumes of hundreds of cubic meters, explore the systems operating and influence of the scale factor
Work on this issue began at the time of elaborating BN-350 precommissioning program in 1965. Program enveloped all the sequence of work over the sodium systems, beginning with acceptance, purification and collecting inert gas and sodium and ending with the work at generating capacity. Problems, detected during the work over the program, demanded revision of some design solutions. Later launch programs of BOR-60 and BN-600 reactor facilities were implemented based on this program.
Standard operations, testing and verifications sequence by the commissioning of sodium systems was elaborated in this program. For the most complex systems - the first and the second circuits - a separate sequence of actions was provided.
Complexity of the program for the first circuit consisted in the fact, that it was a ramified hydraulic system with parallel working pumps. Efficiency of each pump, stability of the system by different amount and composition of operating reactor coolant pumps and by their different speed was to be experimentally tested and checked out. At the same time it was necessary to work out the algorithm of the reactor coolant pump launching, as operating only of one or two pumps at full capacity was impossible.
For the loops of the second circuit the most complex was the safety problem by the first combination of water and sodium in the steam generator. The matter is, that at that moment there was no experience of such works in the country. That is why we had to provide the alternate check of every separate circuit, control and protection systems adjustment, and only after it - combination of coolants in the SG. Such a task had been solved, and collected experience allowed to simplify the designed sequence of operations.
The program provided a big set of sodium systems and equipment operation researches at different stages of work, necessary for the verification of calculations, justifying exploitation and emergency modes, security measures and so on.
Operating of cold traps without any problems provided maintaining the wadding temperature in the first and the second circuits within the fixed range - 130-150°C. In 1990 trap regeneration was implemented, that allowed to restore their effective capacity.
Creation of reliable sodium-water steam generator constructions, control and protection systems for them
We have spoken already about "water-sodium" SG at BN-350 reactor facility in the beginning of the article. Here I would like to add, that at the early stages of the project in IPPE a necessity of a single-shell division of sodium and water in SG was justified. After this were started calculations and experimental researches Obninsk to explore the conditions of large leaks, detection systems, protection from the high pressure in the circuit and so on.
All that allowed us to solve a problem of creation of compensation techniques for the high chemical activity of a sodium coolant. BN-350 SG leaks haven't caused inadmissible damages, such as pipe or vessel ruptures, though they have limited the capacity of the facility. Changes, made on the base of this experience in BN-600 design, provided a successful operation of the facility at designed capacity.
Mastering the methods of work of sodium systems and equipment, optimizing the sodium protection methods against oxidation in the circuit being opened, and protection of personnel against possible contact with melted sodium
Repair work at BN-350 didn't cause any troubles. Reactor coolant pump, heat exchanger matrix, 500 mm fittings were designed as repairable elements. There was no need to cut the pipes of a large diameter or to enter the room of the first circuit to extract them.
To protect the inner cavity of the circuit from oxidation by extracting large-size equipment from it we were using soft and hard clean booths, with the biological shielding for the active equipment. Special bathtubs with the admission of nitrogen, vapor, water and deactivating solutions were provided for cleaning the equipment from sodium.
Besides, during such repairs sodium freezing in communications was widely used. For example, by SG repair sodium was frozen in 500mm pipes. Warming up of sodium after repair was realized from the free surface and by the corresponding precautions didn't cause equipment or fitting damages.
There is one important conclusion: repaired area should be separated from the operating systems not less than by two barriers on the way of the sodium flow, it means either by two valves or by a closed valve and a frozen section of a pipe.
Translation of the experience of sodium technologies to the designing and engineering organizations, operative staff of NPP
During BN-350, BOR, BN-600 designing and mastering employees of designing and engineering organizations gained necessary knowledge and experience and are solving a big part of problems without any assistance now.
Special attention is paid to the training of operatives and giving them necessary experience of work with sodium. This task had been solved both by reading lections and by sending experienced specialists on a mission to the NPPs.
What demands met BN-350 reactor facility security? What nuclides defined the environment of the rooms of the first circuit? Should we be afraid of a sodium coolant? About these and a lot of other issues read in the final part of the cycle.
SOURCE: AtomInfo.Ru
DATE: May 04, 2008
Topics: NPP, Russia, Fast breeders