RRFM 2009 Transactions - European Nuclear Society

the primary circuit in the central CR region, but most of it outside the primary cooling channel in the heavy
water (HW) tank and its installations.
control rod (CR) position [cm]
40
30
20
10
first criticality with warm-cold correction
'cycle 6', no stop
burn up calculation at total power: 20MW
'burn up'- calculation operation 'cycle 6'
Picture 1:
CR driveway calculated and
measured. The red crosses
mark the positions given by
the first ‘no-stop’-operation
‘cycle 6’ in 2006 of FRM II.
The blue dots mark the time
steps of the burn-up
calculation, that gives in
parallel the power distribution
data as input for the
thermohydraulic temperature
evaluation with the code NBK.
0
0 10 20 30 40 50 60
-10
FPD [d]
3 Detailed temperature evaluation for fuel plates with NBK code
TUM also developed a tailor-made thermohydraulic code NBK [5][6] for the description of all relevant parameters
during steady state cooling of the fuel element. It has not been used for safety evaluations where
standardized routines are mandatory. However the code NBK includes details that could not be modeled in
any of the very few qualified codes that were available at that time. Some of the general features of NBK are:
a) Full evaluation of the thermohydraulic parameters over the whole length of the cooling channels.
That means local coolant speed in the channels for the water; local pressure values, pressure drops;
local heat transfer values and resulting temperatures for the water and the wall surfaces.
b) Evaluation of the thermohydraulics also with differentiation over the width of the cooling channels
(the channel edges are also respected). Any 2d-power profile can be regarded.
The very particular features are:
c) The 2d-power profiles generated at the former neutronic burn-up calculations can be used and are
adjusted directly to the 2d-profile of the plates ‘meat’ layer.
d) Heat transfer over the width of the plates is evaluated. This is important for the plate sides where the
‘meat’ layer inside ends abruptly and sees more heat transferring aluminum around; again this is of
clear effect for the ‘meat’ area of FRM II fuel plates, where the uranium density changes abruptly
from 3 g/cc to the half value. There are several areas of heat transfer, the two claddings, the ‘meat’
and the remaining aluminum at the plates sides as well as the edges of the cooling channels which
are formed by the inner and outer core tubes. All of them are described by finite elements for the
heat transfer.
e) The involutes shape of the plates and channels is respected. That is, the heat transfer finite element
areas in the plates follow the involute shape and are thus not exactly rectangular. Because of this the
plate ‘meat layer’ sees also different channel areas left and right.
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