RRFM 2009 Transactions - European Nuclear Society

Figure 4 : Electromagnetic pump
6. Thermal balance and heat exchanger
The NaK temperature in the sample zone is the result of the thermal balance between the
injected power and the heat losses. The main power deposits into the NaK come from
nuclear heating into the samples, the test device structures and the NaK itself, but also from
the electrical heater and the electromagnetic pump. The heat losses of the circulating NaK
toward the water of the reactor primary circuit are controlled by three parameters:
- the NaK flowrate,
- the thickness of the Helium gas gap between the two tubes of the containment rig,
- and the length of exchange.
The temperature homogeneity in the sample zone depends on the heat exchange through
the separator shell between the two reversed part of the NaK flow.
Sensibility studies were performed to determine the optimum geometry and operating
conditions of the test device. The main parameters were tested taking into account thermalmechanical
dimensioning consideration. Therefore, to cope with the target of less than 7.5°C
difference over the 600 mm of the fissile zone, and to cover a large range of nuclear heating
(11 W/g - 23 W/g in steel) and NaK temperature (225°C - 450°C), the studies showed the
necessity to have a variable length of exchange: from 100 mm to 800 mm.
A component called heat-exchanger located at the lower part of the sample-holder, below the
active zone, is designed to be removed and replaced in hot cells (4). It allows having the NaK
flow return at a variable altitude (Figure 5).
Figure 5 : Principle of the heat exchanger
255 of 455