Whole report - STUK

STUK-YTO-TR 175
Table VIII. Parameters of the 244 Cm neutron yield vs. initial enrichment fits with different burnup values.
Burnup
(MWd/kg)
s s s
r s r
R 2
7 7
15
3.3663
· 10
0.2643
· 10
0.9774
0.0394
0.9945
20
8
1.0148
· 10
8
0.0632
· 10
0.8694
0.0304
0.9955
25
8
2.2148
· 10
8
0.0998
· 10
0.7769
0.0212
0.9971
30
8
3.9399
· 10
8
0.1231
· 10
0.6961
0.0143
0.9983
35
8
6.1714
· 10
8
0.1273
· 10
0.6250
0.0092
0.9991
8 8
40
8.7467
· 10
0.0927
· 10
0.5590
0.0046
0.9997
tinides and thus higher neutron emission. [12] In
other words, for the same burnup, lower enrichment
should give a higher 244 Cm neutron source.
The initial operator declared enrichment of the
measured assemblies is given in Table I.
The type of the assemblies #16732 and #13857
is ABB SVEA-64 and the type of the assemblies
#0425 and #0104 is Siemens 9×9-1. All other
assemblies are of type ABB 8×8-1.
The enrichment dependence of the 244 Cm neutron
source at discharge was investigated with
help of ORIGEN-S calculations. The enrichment
varied from 1.6% to 4.4% and the burnup from 15
MWd/kg to 40 MWd/kg. The fuel assembly model
was a common ABB 8×8-1 BWR fuel without rods
containing burnable absorber. The average power
of the bundle was set at 4.5 MW and the desired
burnup was achieved by adjusting the time that
the bundle spent in the reactor.
The calculations were performed using void
fraction values of the coolant 0, 0.2, 0.4 and 0.5
inside an assembly and 0 in the channel. The
ORIGEN-S calculation results for 244 Cm neutron
source with void fraction 0 and burnup 20, 30 and
40 MWd/kg are found in ref. [11]. The results of
the calculation performed with the void fraction
0.5 corresponding to the moderator density 0.392
g/cm 3 were chosen for use in the enrichment
correction, because this void fraction value corresponds
best to the real average situation in a
BWR reactor. The calculated 244 Cm neutron sources
at discharge are shown in Table A2.I in Annex 2.
To obtain a recipe for the enrichment correction
the 244 Cm neutron source was correlated to
the initial enrichment while the burnup and void
fraction were kept constant. The 244 Cm neutron
source was correlated to the initial enrichment
according to the equation
N s e - *
r IE
O
= * (12)
where N O
denotes the 244 Cm neutron source at
discharge
IE denotes the initial enrichment and
s and r denote burnup and void fraction
dependent parameters received from the
fit.
The parameters received from the fits corresponding
to different burnup values and the void fraction
0.5 are shown in Table VIII. Correlation curves
with six different burnup values are shown in
a semi-logarithmic scale in Figure 16.
Utilizing Table VIII, the parameter r was correlated
to the burnup according to the equation
r = t1 + t2
* B
(13)
where t 1
and t 2
denote void fraction dependent
paremeters received from the fit and
B denotes the discharge burnup.
When the void fraction is 0.5, the values of parameters
received from the fit are t 1
= (1.2073 ±
0.0224)% –1 , t 2
= (–0.0166 ± 0.0008) (%*MWd/kg) –1
and R 2 = 0.9913 (Figure 17). In fact, the linearity
improves as the void fraction decreases. When
considering the measurement data, fitting to the
first degree correlation curve is sufficient.
The enrichment correction equation corresponding
to the void fraction value of 0.5 can be
calculated using equations (12) and (13):
N N e
R
= *
( 1.2073- 0.0166* B) *( IE-IER
(14)
)
where N R
denotes the 244 Cm neutron countrate
of an assembly at discharge as corrected
to the reference enrichment,
N denotes the measured 244 Cm neutron
count rate of an assembly as corrected
to the discharge date,
IE R
denotes the reference enrichment,
which was chosen to be 2.95%,
20