14
K. Hashimoto
2.1 Feynman-α and Rossi-α Analyses
2.1.1 Experimental Settings
2.1.1.1 Core Configurations
The Feynman-α and the Rossi-α neutron correlation analyses considering a nonPoisson character of the spallation source [1] are carried out in the A-core that has
two kinds of fuel assemblies: 1/8”P60EUEU and 1/8”P4EUEU. Figure 2.1 shows
a side view of the fuel assembly referred to as 1/8”P60EUEU. Fuel and moderator
plates of the assembly were set in a 1.5 mm-thick aluminum sheath and the cross
section of these plates within the assembly was the square of 2” (50.8 mm). The fuel
assembly consisted of 60 unit cells. The unit cell was composed of two (1/16” thick)
uranium plates with Al cladding and one (1/8” thick) polyethylene plate. Each of
the fuel assemblies had 120 sheets of the uranium plates. The active height of the
core, namely 60 unit cells, was 15” (about 38 cm). Adjacent to both axial sides of the
active region of each fuel assembly, about 22” (57 cm) upper and 24” (52 cm) lower
polyethylene reflectors were attached, respectively. Another fuel assembly referred
to as 1/8” P4EUEU consisted of 4 unit cells with upper and lower polyethylene
reflectors. The core configuration employed in this study is shown in Fig. 2.2. The
twenty-five 1/8”P60EUEU assemblies and one 1/8”P4EUEU assembly were loaded
on a grid plate to constitute a critical reactor core. The core was surrounded with
many polyethylene-reflector assemblies.
The subcriticality for the present experiment was adjusted by changing the axial
position of the central fuel loading [3], as well as the positions of safety and control
rods. These subcritical and critical patterns employed in the present experiments
are shown in Table 2.1, where these respective subcriticalities are calculated by the
continuous-energy Monte Carlo code MVP version 3 (MVP3) [3, 4] with the nuclear
data library JENDL-4.0 [5] are also included. The error of the subcriticality indicates
statistical uncertainty ±1σ of the MVP calculation.
Poly. moderator (p)
(10”)
+
Poly. reflector (PE)
(1/2” 17 + 1/4”)
(8.75”)
Al plate
(20.00 mm)
Gr (2”)
Gr (2”) + Void (1.7”)
(1/16”HEU 2 + 1/8”p) 60
(15”)
Reflector (21.54”)
(Lower)
Fuel (15”)
(Unit cell 60 times)
Reflector (23.45”)
(Upper)
Poly. moderator (p)
(10”)
+
Poly. reflector (PE)
(1/2” 19 + 1/4”)
(9.75”)
Fig. 2.1 Description of 1/8”P60EUEU fuel assembly (Ref. [2])
K. Hashimoto
2.1 Feynman-α and Rossi-α Analyses
2.1.1 Experimental Settings
2.1.1.1 Core Configurations
The Feynman-α and the Rossi-α neutron correlation analyses considering a nonPoisson character of the spallation source [1] are carried out in the A-core that has
two kinds of fuel assemblies: 1/8”P60EUEU and 1/8”P4EUEU. Figure 2.1 shows
a side view of the fuel assembly referred to as 1/8”P60EUEU. Fuel and moderator
plates of the assembly were set in a 1.5 mm-thick aluminum sheath and the cross
section of these plates within the assembly was the square of 2” (50.8 mm). The fuel
assembly consisted of 60 unit cells. The unit cell was composed of two (1/16” thick)
uranium plates with Al cladding and one (1/8” thick) polyethylene plate. Each of
the fuel assemblies had 120 sheets of the uranium plates. The active height of the
core, namely 60 unit cells, was 15” (about 38 cm). Adjacent to both axial sides of the
active region of each fuel assembly, about 22” (57 cm) upper and 24” (52 cm) lower
polyethylene reflectors were attached, respectively. Another fuel assembly referred
to as 1/8” P4EUEU consisted of 4 unit cells with upper and lower polyethylene
reflectors. The core configuration employed in this study is shown in Fig. 2.2. The
twenty-five 1/8”P60EUEU assemblies and one 1/8”P4EUEU assembly were loaded
on a grid plate to constitute a critical reactor core. The core was surrounded with
many polyethylene-reflector assemblies.
The subcriticality for the present experiment was adjusted by changing the axial
position of the central fuel loading [3], as well as the positions of safety and control
rods. These subcritical and critical patterns employed in the present experiments
are shown in Table 2.1, where these respective subcriticalities are calculated by the
continuous-energy Monte Carlo code MVP version 3 (MVP3) [3, 4] with the nuclear
data library JENDL-4.0 [5] are also included. The error of the subcriticality indicates
statistical uncertainty ±1σ of the MVP calculation.
Poly. moderator (p)
(10”)
+
Poly. reflector (PE)
(1/2” 17 + 1/4”)
(8.75”)
Al plate
(20.00 mm)
Gr (2”)
Gr (2”) + Void (1.7”)
(1/16”HEU 2 + 1/8”p) 60
(15”)
Reflector (21.54”)
(Lower)
Fuel (15”)
(Unit cell 60 times)
Reflector (23.45”)
(Upper)
Poly. moderator (p)
(10”)
+
Poly. reflector (PE)
(1/2” 19 + 1/4”)
(9.75”)
Fig. 2.1 Description of 1/8”P60EUEU fuel assembly (Ref. [2])