BN-350 reactor facility. Collected experience. Part 1.
BN-350 reactor facility. Collected experience. Part 2.
BN-350 reactor facility. Collected experience. Part 3.
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 forth final article from a series of those, prepared by Rudolf Petrovich.
Safety and safety systems
First of all I would like to remind you, that reactor facility BN-350 was designed and launched before the first domestic and international safety regulations appeared.
A lot of designing decisions applied at BN-350 satisfy the safety, quality management and other principals that were formulated much later. But the project didn't satisfy these principals totally. As a result, as the demands of regulating authorities were being toughened we had to provide measures, compensating (partially) departures from the norms.
Let us analyze how providing of three main security functions corresponded to the safety regulations:
Capacity (reactivity) management
Control and safety systems of the reactor were designed with a certain degree of conservatism and with necessary reservation. It made possible a longtime corresponding of the reactor to the safety regulations. Electronic units, logic units and other equipment were replaced because of obsolescence during the operation.
Main changes made in BN-350 safety control system during the operation were caused by the reduction of the number of safety system activation parameters and by improvement of this principals for some signals, taking into account dynamic experiments, carried out during the commissioning tests.
Activity retention within the limits
Radiation safety completely totally corresponded safety regulations.
Radiation environment at BN-350 reactor facility was examined as higher capacity levels were mastered, and was controlled further during the operation. Research program included definition of radiation flows at the reactor junctions, coolant and shielding gas activity measurings. Matching of measurements and calculations was satisfactory.
In the rooms of the first circuit radiation environment at operating reactor was defined by 24Na (about 10 Cu/l). After reactor was stopped main role were playing 22Na isotopes, nuclides of corrosive origin, 137Cs and 134Cs, getting out through the fuel elements leakages. Radiation load over the personnel was insignificant and didn't exceed control levels.
Speaking about radiation environment near the facility location, it was safe during all the lifetime of the reactor and was defined by the radionuclides of the natural origin. Radioactivity emissions were only some percent from acceptable.
Providing of heat extraction from fuel
Heat extraction systems had significant departures from new regulation.
First of all design provided the use of the same heat engineering and electrical equipment in emergency situations as by usual operation.
For these purposes the design provided the following measures:
Designers considered, that at least one of the loops will provide an emergency shut-down cooling and that will be enough.
They also believed that a big sodium leak from the first circuit was practically unreal. As arguments that justified their position they noticed low pressure and mild temperatures. Besides, more than 80% of the circuit surface, including reactor vessel and 500 and 600 mm pipelines, had a protection casing and space within the casing was toughly limited. Cut-off valves were used to disconnect the loops sectors without casing by small leaks.
But in 1982 main regulating document on NPP safety was issued. Its statements demanded providing the possibility of the total cross-section pipeline rupture in the design. BN-350 project was not designed for these demands and it was impossible to solve this problem without fundamental reconstruction.
One more important departure was impossibility of using natural circulation for heat extraction from the active zone. Experiments carried out during power startup showed, that natural circulation consumption is stable in the first circuit of the facility and is 3% of the nominal one, but in the second circuit circulation turns over, that causes big temperature drops in the intermediate heat exchanger.
Reasons for the circulation turn over appeared to be the designed steam generator and intermediate heat exchanger mutual location, and a character of the transition process in evaporators by the transition from the forced to the natural circulation. At analysis a decision to cool the reactor at the expense of the low speed work of the main coolant pump with power supply from the autonomous unit was taken. One more important problem that we faced at BN-350 was the detection of the possibility of "stop-water" mode. Analysis showed, that such a mode could appear by some failures on general reasons in the schemes of water feeding (general collector ruptures) and in the power supply schemes.
It is remarkable that at the time of BN-350 designing failures on general reasons considered to be not so important as nowadays.
We had to carry out a cycle of work that allowed creating of reliable schemes of water and power supplying to remove this problem. Finally the scheme of water supplying had a triple redundancy.
Final design of a power supply system was the following:
All these measures increased reliability and safety of BN-350 operation significantly.
Generality
BN-350 project didn't provide all the types of safety systems that are demanded nowadays.
Particularly, there are no safety devices that protect the first circuit from the pressure increasing. Designers believed, that there were no situations that could cause such a pressure. Operation experience showed that this statement had been false.
In order to exclude serious consequences BN-350 operation regulation was changed, particularly the facility systems condition. In the following projects, beginning from BN-600, safety systems were implemented.
Sodium and its influence over NPP safety
Finally I would like to speak about sodium and its influence over the NPP safety.People that don't have practical experience of work with sodium usually name the following drawbacks of this coolant:
I have to say here, that no explosions occur by leaks to steam generators, as there is an inert gas above sodium level within the circuit, and inert gas flows withdraw products of reaction (including hydrogen) outside the building. Safety system preserves second circuit from the exceeding pressure and damages of the equipment. It was totally proved during BN-350 operation.
During operation sodium leaks from the circuits were repeatedly observed. To say nothing of small leaks through the detachable sealings in the system of sodium intaking, big part of leaks is connected with the regulations violations by repair works or by heating of a defrosted sector after repair. Maximum scale of the leak was tens of liters.
Leaks usually were accompanied by the sodium burning. Though it didn't result in any serious circumstances. None of the leaks caused reactor failure of downtimes longer than some days.
Fine experience of leaks detection, suppression and fast localization was received at BN-350. As a result of this experience fire detecting and extinguishing systems were modernized at the following units. One of the most important results was a conclusion about reasonability of the total exclusion of sectional junctions in sodium communications.
Conclusion
Experience of BN-350 designing, launching and operation has given a priceless and multilateral experience to all Russian organizations. This experience initiated development of researches in the spheres where it was not so successful and gave confidence in the decisions where it was successful.
It has given a possibility to estimate the decisions taken at BN-600 designing critically, to change some of them, that provided a successful launch of the facility.
Experience is being collected and analyzed during BN-600 operation, designing of the following facilities. Collected knowledge and experience give us an opportunity to answer practically any question, arisen by designing and operation.
By BN-350 designing main task was to prove principal decisions, their optimization, and now main efforts are directed to the improvement of performance characteristics of new power units, creating of competitive NPP of BN-type. BN-350 experience contributes to the solution of this task.
But direct implementation of knowledge and experience is not always correct. Optimal or even reasonable decisions, constructions, modes depend of the concrete conditions and goals. That is why by creating new facilities concrete experience and knowledge should be interpreted.
SOURCE: AtomInfo.Ru
DATE: May 19, 2008
Topics: NPP, Russia, Fast breeders