RRFM 2009 Transactions - European Nuclear Society

q
q
P
SK
nom i
max = ⋅Ri
⋅fi
⋅Kth
(1)
P
S
nom i
maxH2O
= K ⋅ Kth
⋅qmax
= ⋅ Ri
⋅fi
(2)
Where: P i [W] is the power generated in the fuel cell ‘i’; S [cm 2 ] is the total heated surface of
the fuel element; K=1.055 is coefficient taking into account the fraction of the total power that
is generated in the coolant channels; K th =1 to 0.945 is correction diffusion coefficient taking
into account the non-uniform heat distribution over the plate thickness; R i =1.52 to 1.38 is
axial peaking factor; f i =1.48 to 1.3 is azimuth peaking factor.
2.2 Determination of fabrication tolerances and thermal-hydraulic
uncertainties
The errors affecting the maximum value of the heat flux include: (i) the errors of the neutronic
measurements in determination of the fuel power per channel and the errors in axial and
azimuth power peaking factors; (ii) the manufacturing tolerances in the fuel density and in the
quantity of burnable absorber in the fuel elements. The following uncertainties have been
determined: error in determination of the specific power of a fresh fuel element: ε 1 = 18.7% ;
error in determination of the axial peaking factor, ε = 7 2
% ; error in determination of the
azimuth peaking factor, ε = 7 3
% ; uncertainties of the surface of the fuel meat, ε = 4 4
% ;
error in the correction coefficient K, ε =1 5
% ; uncertainties in determination of the fuel
concentration in the fuel plates, ε 6 = 20 % .
To establish an operational limit, in addition to the errors and tolerances, a correction
factor is considered for the effective power deposited in the coolant flow and for the thermal
diffusion of heat through the fuel cladding. Assuming a statistical distribution of all errors,
influencing the calculated heat flux, the following ‘security factors’ have been determined: (i)
'security factor' for the maximum probable heat flux at the hot spot: this flux has to be
compared to the heat flux giving an allowed wall temperature of fuel cladding, which
corresponds to the nominal hydraulic conditions:
hot spot
max
5
2
i=
1
sq = 1+
∑ εi
= 1.22
(3)
(ii) 'security factor' for the maximum probable mean heat flux along the hot streamline: from
the point of view of flow instability, taking into account the total void fraction in the water gap,
this flux has to be compared with the heat flux giving a possible flow instability at the nominal
hydraulic conditions, where all factors and tolerances are taken with the most unfavourable
value:
hot line
max
6
2
i=
1
sq = 1+
∑ εi
= 1.30
(4)
2.3 Determination of maximum admissible heat flux
The Onset of Nucleate Boiling (ONB) (Bergles & Rohsenow) with sub-cooling occurs when
the fuel temperature exceeds the saturation temperature of the coolant by ΔT onb . The model
of Hsu, calculated by Bergles and Rohsenow for maximum heat flux 600 W/cm 2 at nominal
0
hydraulics conditions, gave ΔTonb = 9.5 C using the following expression:
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