one of the conditions of the reactor reliability and safety is the favourable water-chemical conditions for the construction materials. The traditional measures of water purification and the introduction of correcting agents in the MM reactor being the "blind" part of the loop are practically impossible.

The study of the water-chemical conditions was performed using MM models under in-pile conditions. All in all 5 micromodules were tested in the reactor of the First Nuclear Power Station. In total, the time of operation during testing amounted to about 150 000 days and nights. The duration of the operation of four micromodules amounted to about 10 years for each of them. The main results of experiments are as follows:

— while filling the gas space of the pressurizer with nitrogen, the ammonia synthesis takes place in the water of the primary circuit and the pH value is set up at a level of 9-11;

— the content of corrosion products in stainless steel is on the average — 0.15 mg/kg for iron, for nickel and chrome it was 20 mg/kg;

— the amount of chlorides does not exceed 0 05 mg/kg. The amount of gasses amounts to 30-150 H. cm5 /kg;

After 5 years of testing, one of the micromodules was withdrawn out of the reactor to assess the corrosion condition. The visual inspection showed that the fuel elements and the MM vessel are in a satisfactory state. No mechanical damages of fuel elements and spacing grids were detected and no visible depositions were present on the fuel element surfaces. After examination, this MM was placed again into the reactor, where it operated for about 5 years more.

In addition to the above mentioned studies, the work was performed on a number of directions of supporting the RKM-150 reactor design. They included comparative experiments on enhancing the absorbing materials control rods system. The best results were obtained with usinig Dl/2 Ті 05 which was adopted in the technical design of the reactor

Also, the problems of the fuel element cladding tightness control system were studied. It was found that such a control, which does not require sampling the coolant can be implemented using gamma-ray detectors being moved between the micromodules in the above-reactor space, when the reactor is shut down.

Conclusion

1. The concept is proposed to enhance the reactor safety at the cost of subdividing the primary circuit into small parts. This is achieved by means of micromodules incorporating a fuel assembly, a heat exchanger and a natural circulation of coolant.

2. Wide experimental and computational investigations confirmed the design characteristics of such a reactor (RKM-150) regarding:

The thermohydraulics involving the temperature regimes; flow rate and natural circulation stability; thermal core safety under the natural circulation; the water-chemical condition;

the manoeuvrability in var.-Ing the power during load variation; and the correspondence to the nuclear and radiative safety requirements.