As explained in the introduction, a hot channel model has been attached to the core average channel "c", in order to assess commonly used safety quantities such as max fuel temperature, fuel heat content, max clad temperature, dnb (departure from nucleate boiling ratios,...), used in safety evaluations.
Hot channel representation is activated by the keyword hot_channel which is, normally, called after step_end when the feedback's are stabilized, because the hot channel does not retroact on average core conditions, which are evaluated on the average channel only.
In variables's names, hot channel is pinpointed with a "
7" appended to average channel "
c"; for example,
f2ci for average becomes
f2c7i [nuclear power F; at eos 2; in the Core; hot channel 7; Interval].
The hot channel is processed exactly like the average channel by (do, core) except that power
f27ci in mesh
ci is not derived from the local neutron flux but simply scaled from
f2ci by means of a radial power form factor
fxykn correlated, versus
vm, ig, u, h, ehec.
The effect of boron on power peaking is neglected.
f27ci: power deposited in pellet
flu: linear (W/m) power deposited in hot fuel rod.
qlu: linear power heating the water.
qsc7i: heat flux (W/mē) [thermal power Q; per unit surface S; in c7i] in clad-water film.
u2rc7i: [pellet temperature U; Radial distribution; in hot channel cell] on
2,3,4,5,for the 4 successive sectors (),
6for pellet edge,..
t17ck: water temperature in node
c7kof hot rod.
fluc7: max flu
fqc= fluc7 / fluc, where
flucis the average of
iu27ci: [mesh Index;] where max
hu2c7: fuel enthalpy (0 J/kg at 0 K) at that location. This property is used in rod-ejection accidents evaluation.
A simplified evaluation of DNB flux
qsdnb7ci, [thermal power Q; per unit Surface; at onset of Departure from Nucleate Boiling; in each
c7ci] is computed by inserting "dnb" after end_step, hot_channel.
It is the heat flux at onset of boiling crisis from which clad temperature suddenly increases by degradation of film heat-transfer.
This value, calculated by empirical correlations, depends on the local thermal properties and their upstream distribution.
dnbrc7: (Departure from; Nucleate Boiling; Ratio: in c7] is the minimum value of the ratio
qs7ci/qsdnb7cialong the fuel channel.
DNB correlations were established at steady state test conditions and their extension to transient conditions has not, to author's knowledge, been demonstrated.
Although dnb correlations are being developed for each type of fuel, depending on flow mixing grids geometry, the classical Tong correlation has been implemented here (recast in SI units) in order to get a first feeling of the dnbr behavior.
cp4are pressure only dependent factors .
cx2are function of local
G7is the flow density;
cd1depends on hydraulic diameter
This correlation (2) applies for uniform axial heat flux.
qs7ci: local heat flux
z_ci: elevation of end of ci
z12: boiling length: elevation from nucleate boiing onset position
qs7ci: local heat flux
Nucleate boiling is evaluated, together with film transfer coefficient, by means of the classical Jens-Lotte correlation.
The proposed single, hot channel, representation provides a bounding evaluation of safety margins because:
fxy's at state point conditions (at min dnb margin time) and repeat the problem.
The application covers only the ini condition (ini, core, hot_channel, dnb);
idnbrc7=21. Note that nucleate boiling starts already at
ci=9, the Tong non-uniform corrector is 1.0999, water is still sub-cooled at min dnbr point.