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2.4 The gravitational pull calculation<br />

In order to complete the present model definition, we have to mention how to calculate the<br />

gravitational pull ∆P g0<br />

and ∆P gk<br />

to be used in equation (18).<br />

In steady state we use equation (12). For the further time steps, we have to directly apply<br />

equation (10). Practically, we have to follow (every time step) the distribution of the coolant density<br />

and velocity in the core, in the heat exchanger and in both the a and b loop legs. To do that, we have<br />

also to apply a heat exchanger model 1 to evaluate the primary coolant variations of the heat exchanger<br />

outlet temperature.<br />

3. Results<br />

The natural convection model presented above has been already coupled with the Tieste-Minosse<br />

“point dynamics” code elaborated at ENEA Casaccia. Some Tieste-Minosse results, that can be useful<br />

to preliminarily evaluate the natural convection impact on the EADF core dynamics, are presented in<br />

this section.<br />

3.1 The proton beam interruption and the short duration beam trips<br />

Figure 1 shows the trends of the outlet coolant temperature induced by short duration beam trips<br />

in the EADF. The neutron source transients have been performed either considering or neglecting the<br />

inlet coolant velocity variations due to lead-bismuth natural convection.<br />

It can be easily seen that the impact of the primary coolant natural convection on the outlet<br />

coolant temperature trends is not significant for the EADF core dynamics. The main reason of this<br />

result can be related to the presence of an active system of coolant flow enhancement in the EADF<br />

design. In particular, the pull due to the system of cover gas injection into the bottom part of the riser<br />

is about five times the gravitational pull due to the coolant natural convection. Practically, while this<br />

active system of flow enhancement is working, the most of the pull that moves the coolant remains<br />

constant and the gravitational pull variations due to the natural convection transient do not induce<br />

significant effects.<br />

1 The definition of a specific Heat Exchanger Model will be the object of a further paper and therefore is not<br />

defined in the present one.<br />

515

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