While breakage of the MM vessel, and the link between the MM and the pressurizer remains intakt, the MM is provided with water from the pressurizer. However, with a large break of the MM vessel, the flow rate of the loss of coolant exceeds the flow rate of the supply; therefore the pressure and the amount of the coolant in the MM decreases until the amounts of losses and supplied liquid are equal

To study such the situation, the MM vessel was provided with pipe connections of which the breaks at different elevations were simulated (i. e 5190 mm and 1140 mm above the upper edge of the fuel-assembly) All experiments were conducted at an initial power of 1070 kW reducing in accordance with the residual heat expected.

It is experimentally established that if the disrupture of the MM vessel occurs at these elevations m the initial phase of the accident, the heat removal from the fuel assembly is effected in crisis-free conditions with the fuel assembly filled with water Therefore the subsequent cooling of the fuel assembly is beyond question even if only the feed is present in these cases. The contrary is the case when the MM vessel ruptures below the fuel assembly where the MM is incapable to retain the supplied water and the flow rate entering it from the pressurizer is insufficient to provide a continuous flow over the entire fuel assembly cross-section. It is possible under these conditions that water will not flow around a portion of the fuel elements, which will lead to their overheating.

The experiments were conducted as follows: A preset flow rate of water ot room temperature (15- 20OC) was fed to the upper part of the micromodule, then the power of a predetermined value was supplied to the fuel assembly and the micromodule vessel; the fuel element temperature being recorded at four points over the fuel assembly cross-section and at ten points of the entire elevation. Considerable attention was paid to the investigation ot the influence of the feed water conditions (e. g. below or above the "flow-over" wiridoiqs of the heat exchanger; from one or two sides of the MM vessel) Tables 3 and 4 present some results of these experiments It is obvious from Table 3 that there is a significant scatter in tuel assembly temperature (65-400°C) This shows a non-uniform distribution of inlet water over the tuel assembly cross-section. Of great interest is the fact associated with reduction of the fuel element surface temperature as the power increases from 10 to 15 kW in the present case. To our mind, this is due to the boiling-up of water on those simulators where water is available, and its more uniform distribution over the fuel assembly cross-section At a certain stage, further increase in power logically results in a rise ot the rod temperature

With increasing the flow rate of water being ted to the MM low tuel assembly temperatures are observed at sufficiently high powers (Table 4) In the RKM-150 reactor, the parameters of water flowing around the secondary circuit and the net MM power (together, with the heat influx from graphite, amounting to — 40 kW) meet the conditions ot the test the results, ot which are shown in the second column ot temperatures, Table 4. However in the RKM-150 reactor the flow rate of water flowing to the MM from the pressurizer is significantly higher (no less than 4000 kg/hr) than in the above mentioned case (700 kg/hr). Thus, the reliable cooling of the fuel elements without their overheating will be provided in each phase of accidents accompanied by the MM vessel rupture at any elevation.

In addition to the above consideration, experiments were conducted, simulating accidents with MM vessel rupture at the elevated pressure and a power level of ~ 600 kW. These experiments confirmed the absence of the fuel element overheating under conditions ot this accident even at flow rates of feeding water considerably ‘ower than in the RKM-150 MM •

Table 3

THE THERMOCOUPLE INDICATIONS AT WATER CIRCULATION ROUND THE MM AT A FLOW RATE OF 100 KG/HR AND ONE-SIDED FEED.

(The MM vessel is not heated and the water flow rate over the secondary circuit is equal to zero)

4.3.2. Secondary Circuit Zero Flowrate,

Experimental study has been carried out of an accident with ceasing the second circuit water flow rate through the heat exchanger of one MM, which can be caused by blocking the flow area by an outside subject. The. initial parameters of the micromodule were set up to correspond — to its operation at a maximum design-based power of 1070 kW. After terminating the flow rate of the secondary circuit water, the MM power reduced according to the law of residual heat variation with a delay of 10 s.

Experiments showed that in such an accident, the pressure over the primary circuit slightly exceeds the nominal value (by 0.2 MPa) during a short period of time 15s) The circulation over the primary circuit provides a crisis-free cooling for the fuel assembly As for the whole of the reactor, the pressure in the emergency MM reduces after decreasing the power The duration. of the experiment was 10 mm By that time about 32 kg of water retained in the MM whereas to fully cover the fuel assembly, 18 kg is sufficient (Note, that according

Table 4.

FUEL ELEMENT TEMPERATURE AT WATER CIRCULATION AROUND THE MM WITH A FLOW RATE OF 720 KG/HR (ONE-SIDED FEED.

(The water flow rate in the secondary circuit is 11 i/iir and inlet temperature is — 65 C)

to calculations, the time required to decrease the water amount in the MM to 18 kg is 13 min). Thus the experiment reasonably well justified the predicted time of starting the fuel assembly uncover, which is assumed in the fuel element temperature behaviour evaluations.