RRFM 2009 Transactions - European Nuclear Society

Picture 2:
Original sketch out from [6] to
illustrate the tailor-made model of
NBK for thermohydraulic calculations
between the envolute shaped plates. It
is shown a cross section of one
cooling channel between two plates at
the area of uranium density stepping
(3.0 and 1.5 g/cc, s. arrow). The
cooling channels are divided horizontally
each into about 60 threads. Any
thread sees a different heating from
left and right side. What is not shown
here are the finite elements in the
cladding and the ‘meat’ layer which
have the same edges as the threads and can transfer heat in both directions of the drawing.
As long as the channel width is kept constant (as done here) there may not be regarded any mass exchange or
friction between the channels. The threads are adjusted to the same pressure value at any height.
For the actual work the code was now adapted for:
f) Running different states of operation in series. The time steps to be used were now the same as those
given by the former neutronic calculations, that simulated now 14 time steps from begin to end of the
fully 60 FPDs operation of FRM II.
The temperatures at the surfaces and in the coolant can now be written by the NBK code (together with the
calculated channel dimensions) to a specific file and in series for all calculated time steps.
4 Results
Next a program for displaying the data has been adjusted to read the values written by the NBK code and to
show the single temperature profiles for any of the time steps as well as an adapted integral, or better averaged
value profile. This is shown next.
There where taken rather conservative values for the operation during the calculated cycle. The water inlet
temperature was fixed to 38°C. The velocity of the coolant between the plates was taken to be 17.5 m/s in
average and the heat transfer coefficient was taken by the formula from Colburn with a wall temperature
correction. The formula showed best results in comparable cases [7].
The main result of this work is a more moderate maximum value for the time averaged wall temperature:
• The temperature maxima are fairly smoothened in contrast to the power density with sharp peaks,
that are input data here for the thermohydraulic NBK code.
• There exist two regions on the plates of practically the same maximum time averaged temperature.
Both maximum show up to be rather broad and are located at a height between -5cm and –
20 cm relative to the core center plane. The first maximum is close to the inner side of the channel
and the second one more to the channel center slightly inside the edge where the uranium
density stepping occurs (about fluid element 45 in Picture 2).
• The time averaged maximum temperatures for FRM II are calculated to be nominally 85°C when
taking conservative water inlet temperatures of 38°C.
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