106
M. Yamanaka
was 1.25 at most, the reproducibility was considered acceptable by comparison with
experiments involving variations of subcriticality shown in Table 4.13.
4.3.2 Kinetics Parameters
4.3.2.1 Subcriticality in Dollar Units
The measured ρ $ was compared with the calculated ρ
MCNP
$
by using MCNP6.1 and
deduced as follows:
ρ
MCNP
$
=
1
β
MCNP
eff
1 −
1
k
critical
eff
−
1 −
1
k
case
eff
=
1
β
MCNP
eff
1
k
case
eff
−
1
k
critical
eff
,
(4.44)
where k
case
eff is the effective neutron multiplication factor in each of the cases shown in
Table 4.13, and β
MCNP
eff
the effective delayed neutron fraction obtained by MCNP6.1
corresponding to each case. In the comparison between ρ $ and ρ
MCNP
$
by varying
the neutron source shown in Figs. 4.7 and 4.8, ρ
MCNP
$
values were comparable until
deep subcriticality.
Also, notable is that no spatial effect of detector location was observed except
for that of BF3#4 detector. The results by BF3#4 detector placed near the neutron
source were not reliable since ρ $ values were significantly overestimated in deep
subcriticality (over 6$) in both 14 MeV and spallation external neutron sources. The
large error in deep subcriticality of 9$ in BF3#1 and BF3#2 was caused by the low
count rate of delayed neutrons. As an examination of detector type dependency on
ρ $ , LiFCAF fiber detector indicated almost the same ρ $ value compared with that by
BF3#2, validating the measurement results and capability of the λ-mode calculation
for ρ $ .
4.3.2.2 Prompt Neutron Decay Constant
Measured α was compared with calculated one (α
MCNP ) by using MCNP6.1 and
deduced as follows:
α
MCNP
=
1
Λ MCNP
1
k
case
eff
−
1
k
critical
eff
− β
MCNP
eff
,
(4.45)
where
MCNP is generation time obtained by MCNP6.1. In addition to α
MCNP , the
prompt neutron decay constant by the ω-mode calculation with PARTISN (α
PARTISN )
was added to compare the difference between λ-mode and ω-mode calculations, as