While studying accident situations, the major attention was focused on accidents associated with the depressurization of primary circuit components and the disturbance of heat removal over the secondary circuit.

4.3.1.1.Rupture of the pipeline between MM and pressurizer

Consider the behaviour of the MM channel in the case of the disrupture of the pipeline connecting the micromodule with the pressurizer. The peculiarity of this situation is that it is impossible to feed water to the MM from the pressurizer. The calculations obtained for this accident did not predict severe consequences; however, this fact needed an experimental verification. The problems to be studied were as follows:

— The MM depressurization results in ceasing or reversing of the circulation in the MM or degrading the heat removal from fuel elements in the first phase of the accident;

— the amount of water remaining in the MM compared to the amount at the moment the pressure reduces up to the atmospheric value in it;

— the maximum permissible power of the fuel assembly filled with water under the flooding conditions (zero flowrate);

— the rate of coolant losses at the final accident phase at a pressure near to the atmospheric.

Fig.4 shows the results of one of the experiments investigating the break-down of the pipeline connecting the MM with the pressurizer. The rupture is located just in the vicinity (100 mm) of the MM vessel and is simulated using a fast-responce device. The initial power of the channel amounted to 1070 kW and then reduced according to the law of residual heat variation. The delay time was accepted to be 10 s. which is considerably heigher than the design-basis value (i. e below 1 s).

The experiments indicated that the pressure in the MM reduces to the atmospheric pressure for 1-2 min. The water flow rate through the fuel assembly slitly increases at the beginning of the process (to — 1.9 kg/s) and then for 2-2.5 min is practically equal to the nominal one (1.5 kg/s), providing the reliable cooling of fuel elements. No negative outcomes were found in the temperature behaviour of fuel elements over this time period.

The amount of water remained in the MM to the moment of reducing the pressure to the atmospheric value amounted to 54 kg., the water level being ~ 2.6 m above the upper edge of the fuel assembly.

To define the limiting fuel assembly power under sarbotage conditions occurring after reducing the pressure, some preliminary tests were carried out using a 7-rod bundle with the inlet cross-section blocked, (i. e. under more severe conditions as compared to the MM). They revealed, that for a 30-rod fuel assembly MM the limiting power should be 120-130 kW, thich is higher than the net power of the residual heat in fuel elements and the heat influx from graphite (for the present 100 kW design). In the full scale MM without blockage at the fuel assembly inlet (i. e. for real conditions) at the atmospheric pressure, a power of 150 kW was reached without any indications of an> critical phenomena. Thus, the condition of the fuel assembly filled with water is sufficient to remove the residual heat under no-crisis conditions.

After the MM pressure is reduced to the atmospheric value, the further development of the process is characterized by the ongoing loss of coolant. The presence of the heat exchanger in the MM results in that the overwhelming portion of steam generated in the fuel assembly condenses on the surface of this heat exchanger and returns through the downcomming line of the circuit to the fuel assembly inlet. As a result, the rate of steam losses through the rupture is considerably lower than the rate of steam generation in the fuel

assembly. The measurement of the rate of mass losses in this phase of the accident is taken at different values of the fuel assembly power, and different levels of water in the MM. The obtained data allowed the time interval to be defined, over which the fuel assembly has reliable cooling, and the fuel element cladding temperature is close to the saturation temperature of water at the atmospheric pressure This time interval ranges from 36 to 44 hrs, and only then the fuel element heat-up will occur, but no higher than 610 C m accordance with the calculations