BN-350 reactor facility. Collected experience. Part 2.

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

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 second article from a series of those, prepared by Rudolf Petrovich.

The use of natural physical processes in BN-350

By the reactor facility BN-350 designing in the early sixties, the principle of maximum usage of natural physical processes was stated and sequentially realized. It had significantly simplified the facility, increased a reliability of exploitation and security, reduced the necessity of technological parameters automatic regulation and excluded the necessity of interfering of the personnel by the transitional processes and by various incidents.

It should be noted, that Russian fast sodium reactors appeared to be almost the first reactor facilities, which had been widely using natural physical processes. On the reactors of the other types the idea of using this principle was implemented only after serious incidents on NPPs (TMI-2 and Chernobyl).

Let us have a look at two examples of using natural processes in BN-350 project.

Both sodium circuits the first and the second one don't demand coolant make-up or regulation of its levels during the work of the facility at normal capacity, its launches and halts. By the increasing of the capacity occurs the sodium volume expansion at the expense of its levels changing within the limits inside the vessels of the main equipment.

In order to limit the volume changing value and peak-to-peak value of the levels, BN-350 designers adopted an idea of a so-called "reference temperature" - minimum acceptable sodium temperature. Its value (260°C) was chosen as the closest one to the operating temperature of the "cold" pipelines, and it was provided at the expense of maintaining of a constant steam pressure in steam generators by the special regulators.

It is interesting to compare this decision of domestic specialists with the approach adopted by designing of an American NPP with the fast sodium reactor "Enrico Fermi", which was in progress approximately at the same time. The sodium level in the first circuit was maintained at the expense of sodium pouring from the circuit into the special tank by the growth of the temperature and, correspondingly, by the growth of its volume, and sodium pumping into the circuit with the small pumps by the temperature decrease.

The second example of the natural processes using in BN-350 concerns the method of excluding of significant gas pressure changes in the gas blanket of the reactor by the changing of an operational mode of the facility from the anchor load to the work at full capacity. In this situation the growth of a gas volume was 28m3. It could course a significant growth of a gas pressure in the gas cavities of the circuit; its volume in operating conditions was 15m3.

How have the BN-350 designers solved this problem? Gas cavity of the reactor was connected to the two dump tanks of the first circuit with the pipeline of 200-mm diameter. At the expense of their volume an excessive pressure within the circuit by the launch increased from 0.04 just to 0.09 MPa.

Let us address ourselves to the experience of "Enrico Fermi" again, American specialists had to use automatics to maintain the constant pressure, which switched on compressors for gas pumping out by the growth of the pressure and provided the gas giving from the receivers by the pressure decrease.

Mentioned decisions of using the natural processes were successfully implemented in the other projects of Russian reactor facilities with the sodium coolant (BOR, BN-600, BN-800).

Design of the first circuit of BN-350 reactor facility

There are two different possible lay-out drawings of the first circuit - loop and tank ones (integral).

The loop lay-out drawing was chosen for the BN-350 project as more traditional. Principal equipment of the circuit was situated in the separate vessels, connected with each other with pipelines. I have to admit, that it caused many difficulties by the designing process. We had to think over the providing of pipelines temperature compensation, how to achieve a reliable hermetization of the first circuit boxes, that were planned to be filled with nitrogen, and to find a decision for some other technical problems.

That is why in 1964 in USSR the tank lay-out drawing of a power reactor had been worked out in detail. Practically at once it became clear, that reactor tank has acceptable dimensions and a sodium volume and could be implemented by the technological sophistication of that time. Principal decisions of the equipment lay-out in the general tank were contemplated, that allowed to optimize size of the tank. Later, this lay-out was used for the next Russian reactor BN-600.

Nowadays the majority of fast neutron power reactors in the world are implemented by the integral scheme.

Physical researches on BN-350

A large program for physical characteristics research was elaborated and realized by the commissioning of BN-350 reactor facility. Among others the following parameters were examined:

The work of physicists on BN-350 was not limited only by the commissioning program. Many of researches mentioned above were repeated in the future by the transition to the middle-stationary state of the active zone, by the change of fuel assembly construction and in some other cases.

Special methodology and technologies had to be created for BN-350 measuring. Particularly, a unique methodology , based on the activation of the needle sensors, located directly within the operating fuel assemblies, was implemented to measure the neutron fields.

The first criticality of the reactor and the following researches proved the main characteristics of an active zone and a reactor protection control system. Measuring showed, that main characteristics of the active zone could be prognosticated with the sufficient peculiarity. Some underestimate in the calculations was found for the effectiveness of the temperature compensator and in the values of the thermal and power effects of reactivity.

Twice - during the BN-350 first criticality and after it - flow rate through every fuel assembly was measured by the special device. It turned out, that flow rates through the fuel assemblies of an active zone and of an internal screen were different from the estimate values not more than on 5%, and in the external screen they were a little underestimated.

For physicists the first BN-350 criticality gave a lot of useful information, which helped them to check the accuracy of calculation methods in the real conditions of a power reactor. Collected experience allowed improving of computer codes, that helped in designing and exploitation of BN-600.

Sodium equipment and BN-350 fittings

First of all, I would like to say, that the experience of commissioning and exploitation of special equipment and fittings, designed for sodium systems, proved their design objectives. All types of the equipment were working well, excluding only steam generators and back-flow prevention valves of the first circuit. Some changes, that we had made, were not of a great importance.

Reactor junctions and mechanisms (including control system and overloads)

All BN-350 reactor mechanisms were checked at several stages:

Such a thorough checking methodology has proved its value. Generally speaking, a stable and reliable exploitation of the reactor was provided. By the launch and during the first time of the exploitation, failures of some elements were observed. Though they hadn't been principal and couldn't cause the functional failure of the systems and were removed immediately.

Principle of the recharging operations division, adopted by the designers, was proved. On the BN-350 reactor facility such operations were distributed between different mechanisms, each of them was making only one or two operations and that is why it was of a simple construction. Possibilities of the recharging system appeared to be so, that by some departure from the designed procedure and limits, personnel managed to unload fuel assemblies from the reactor with the sag value up to 15 mm.

I have to add, that by the results of researches during the exploitation process, lifetime of the control system and transfer mechanisms was enlarged against the designed one.

Centrifugal vertical reactor coolant pumps of the first and the second circuits

The reactor coolant pump of the first and the second circuit had a cantilevered construction and a floating-type device for the level maintaining in the tanks of the pump. That is why they were situated in the top of the circuits. Designers knew about all the drawbacks of the cantilevered constriction - however about advantages of it - but to the moment of BN-350 creating there was no experience of using bearings in sodium in our country.

During prestarting checkout works pumps have passed a necessary cycle of tests. Particularly, the joint work of the pumps, their overshoot, behavior by the short (1.2 and 3 seconds) power supply breaks and other dynamic characteristics.

On the first stage of exploitation appeared some difficulties in the work of the pumps of the first circuit - complicated launch after a long halt and a cavitation wear of the pump runners by the launch of the reactor with the incomplete number of loops. Reasons of these effects were defined rather quickly. Measures excluding their repetition were elaborated.

Pumps had been exploited successfully during the whole lifetime of the facility. Running gear and shaft seal have shown an insignificant wear by its examinations. They had made the prolonging of an overhaul life of the reactor coolant pump possible - by the end of its lifetime it was already 80000 hours (10 years!).

Sodium fittings of a small diameter (to 150 mm)

Elaboration of the sodium fittings of a small diameter had been implemented on the BR-5 research facility in the IPPE. It is not surprising, that there were no problems with its exploitation on BN-350.

Sodium valves with the frozen shaft seal (roughly speaking, with the gasket of the frozen sodium) of 500-mm and 600-mm diameter had shown themselves to advantage. No leaks through the sealings were being observed, non-leakage value of the shut valves was better than it had been designed. During the exploitation optimal modes of the freezing belts cooling were defined.

Another history concerns 500-mm back-flow prevention valves, mounted on the power conduit of every loop of the first circuit. Their characteristics haven't been satisfying the designed requirements for the first time.

By the shut-down of a one pump the valve shut when the backflow rate of the coolant was more considerable, that it supposed to be. That brought to the strong mechanical strokes of the valve disks against the valve seat, hydraulic impacts and visually observed swaying of the pipe-lines of the shut-down loop.

To ensure the safe BN-350 reactor facility exploitation, the control safety systems activity algorithm by the halt of a one pump was temporary changed. According to the design, reactor shouldn't stop in this case, but should be switched to a low-power operational mode by the automatics. According to the temporary algorithm, activation of the emergency protection system with the switching of all the pumps to the reduced rotary speed was provided.

All valves have been modernized later. Backflow rate that caused their shut-down was significantly reduced. That made it possible to return to the designed algorithm of the safety system activity. Let us stress, that revalving have not caused any difficulties, as a valve construction was maintainable.

General conclusions on the BN-350 sodium equipment and fittings

To be able to judge how successful the experience of BN-350 facility and its equipment operation had been, let us give two following facts:

How was the sodium industrial producing of the reactor purity provided? How was the exploitation of large-volume sodium circuits mastered and how was the collected experience transmitted? What was happening with the "sodium-water" steam generators? About these and a lot of other topics you can read in the next article of the cycle.

SOURCE: AtomInfo.Ru

DATE: April 06, 2008

Topics: NPP, Russia, Fast breeders

Rambler's Top100