64
C. H. Pyeon
and S6) rods by the rod drop method and its calibration curve by the positive period
method. Furthermore, in Cases II-4, II-5, and II-6, the subcriticality was numerically
obtained by the MCNP6.1 [11] code with the JENDL-4.0 [12] library, because the
reactivities of control and safety rods were varied by the substitution of fuel assembly
rods for polyethylene ones.
The Pb–Bi target was located inside the core at the location (15, L) shown in
Fig. 2A.11 on the basis of the characteristics of the location of the target outside the
core, as discussed in the previous study [13]. Note that the location of the original
target is not easily moved to the center of the core, because control and safety rods are
fixed in the core to function as the control driving system at KUCA. The Pb–Bi target
was 50 mm in diameter and 18 mm thick. The main characteristics of proton beams
were 100 MeV energy, 0.7 nA intensity, 40 mm beam spot, 20 Hz beam repetition,
100 ns beam width, and 1.0 × 10
7 s
−1 neutron yield.
3.2.1.2 Measurements
During the injection of 100 MeV protons onto the Pb–Bi target located at (15, L)
shown in Fig. A2.11, the time evolution of prompt and delayed neutron behavior was
examined by the optical fiber detectors [14] set at three locations: Fiber #1 at (12-11,
T-R) in Fig. A2.16 between the polyethylene moderator rods, Fiber #2 at (14-13,
P-O) in Fig. A2.16 outside, and Fiber #3 at (15-14, O-M) inside the
235 U-fueled and
Pb–Bi-zoned core. The optical fiber was shaped with a mixture of lithium-6-enriched
LiF and ZnS (Ag) scintillator pasted at its 1 mm diameter tip.
From the results of neutron signals shown in Fig. 3.9, prompt neutron decay
constant α was deduced from the exponential function fitting of the PNS measurements in the region of prompt neutron behavior as follows:
N = C PNS · exp(−αt) + B PNS ,
(3.1)
where N indicates the counting rate of the neutron signal, and C PNS and B PNS the
constant values obtained by the least-squares fitting. Additionally, subcriticality ρ $
in dollar units was deduced by the PNS method, on the basis of the following theoretical background: in the area ratio method [15], subcriticality ρ $ in dollar units
was determined by the ratio of two prompt and delayed components in the decay of
neutron density as follows:
ρ $ =
ρ
β eff
= −
A p
A d
,
(3.2)
where ρ indicates the subcriticality in pcm units, β eff the effective delayed neutron
fraction, A p the area of the decay curve by prompt neutrons, and A d the area of
delayed neutrons. For reducing the spatial higher mode components of neutron
flux, the extrapolated area ratio method [5] was introduced into the measurement
of subcriticality as follows:
C. H. Pyeon
and S6) rods by the rod drop method and its calibration curve by the positive period
method. Furthermore, in Cases II-4, II-5, and II-6, the subcriticality was numerically
obtained by the MCNP6.1 [11] code with the JENDL-4.0 [12] library, because the
reactivities of control and safety rods were varied by the substitution of fuel assembly
rods for polyethylene ones.
The Pb–Bi target was located inside the core at the location (15, L) shown in
Fig. 2A.11 on the basis of the characteristics of the location of the target outside the
core, as discussed in the previous study [13]. Note that the location of the original
target is not easily moved to the center of the core, because control and safety rods are
fixed in the core to function as the control driving system at KUCA. The Pb–Bi target
was 50 mm in diameter and 18 mm thick. The main characteristics of proton beams
were 100 MeV energy, 0.7 nA intensity, 40 mm beam spot, 20 Hz beam repetition,
100 ns beam width, and 1.0 × 10
7 s
−1 neutron yield.
3.2.1.2 Measurements
During the injection of 100 MeV protons onto the Pb–Bi target located at (15, L)
shown in Fig. A2.11, the time evolution of prompt and delayed neutron behavior was
examined by the optical fiber detectors [14] set at three locations: Fiber #1 at (12-11,
T-R) in Fig. A2.16 between the polyethylene moderator rods, Fiber #2 at (14-13,
P-O) in Fig. A2.16 outside, and Fiber #3 at (15-14, O-M) inside the
235 U-fueled and
Pb–Bi-zoned core. The optical fiber was shaped with a mixture of lithium-6-enriched
LiF and ZnS (Ag) scintillator pasted at its 1 mm diameter tip.
From the results of neutron signals shown in Fig. 3.9, prompt neutron decay
constant α was deduced from the exponential function fitting of the PNS measurements in the region of prompt neutron behavior as follows:
N = C PNS · exp(−αt) + B PNS ,
(3.1)
where N indicates the counting rate of the neutron signal, and C PNS and B PNS the
constant values obtained by the least-squares fitting. Additionally, subcriticality ρ $
in dollar units was deduced by the PNS method, on the basis of the following theoretical background: in the area ratio method [15], subcriticality ρ $ in dollar units
was determined by the ratio of two prompt and delayed components in the decay of
neutron density as follows:
ρ $ =
ρ
β eff
= −
A p
A d
,
(3.2)
where ρ indicates the subcriticality in pcm units, β eff the effective delayed neutron
fraction, A p the area of the decay curve by prompt neutrons, and A d the area of
delayed neutrons. For reducing the spatial higher mode components of neutron
flux, the extrapolated area ratio method [5] was introduced into the measurement
of subcriticality as follows: