216
M. Yamanaka
8.1 Experimental Settings
8.1.1 Core Configuration
The experiments of reactivity measurement [1, 2] were carried out in the A cores
(EE1 and E3 cores in Figs. 8.1a, b, respectively) that have polyethylene moderator and
reflector rods, and different fuel assemblies: “F” and “f” (Figs. 8.2a, b, respectively).
In EE1 core (Fig. 8.1a), fuel assembly “F” (1/8
P60EUEU) in Fig. 8.2a is composed
of 60 unit cells, and upper and lower polyethylene blocks about 25
and 20
long,
respectively, in an aluminum (Al) sheath (2.1
× 2.1
× 60
). For the fuel assembly,
a unit cell in the fuel region is composed of two highly enriched uranium (HEU) fuel
plates 1/8
(2 × 1/16
) thick and a polyethylene moderator plate 1/8
thick. In E3
core (Fig. 8.1b), another fuel assembly “f” (3/8
P36EU) in Fig. 8.2b is composed of
36 unit cells with an HEU fuel plate 1/16
thick, polyethylene plates 3/8
thick, and
upper and lower polyethylene blocks about 23
and 21
long, respectively, in the Al
sheath as in fuel assembly “F”. The neutron spectrum in EE1 core is compared with
that in E3 core as shown in Fig. 8.3, demonstrating representatively hard (EE1 core)
and soft (E3 core) neutron spectra in the KUCA A-core.
8.1.2 Reactivity Measurements
For measuring the excess reactivity, the critical state was adjusted by maintaining a
certain position of C3 rod in EE1 core and C1 rod in E3 core, and by withdrawing
fully the other control (C1 and C2 in EE1; C2 and C3 in E3) and safety (S4, S5,
and S6) rods from the core, respectively. Furthermore, excess reactivities in EE1
and E3 cores were measured by the positive period method, when C3 and C1 rods,
respectively, were, under the critical state, fully withdrawn from the core.
For measuring the control rod worth, in cases of C1 and C2 rods in EE1 core,
criticality was maintained by C3 rod, and the control rod worth of C1 or C2 rod was
acquired by the rod drop method, after full insertion of C1 or C2 rod into the core.
For C3 rod in EE1 core, criticality was adjusted by C1 rod, and the control rod worth
of C3 rod was obtained by the rod drop method, after the full insertion of C3 rod into
the core. Almost the same procedures were followed for the E3 core, with the use of
the rod drop method, and the control rod worth of C1, C2, and C3 rods was obtained
experimentally.
To estimate the experimental uncertainty of excess reactivity and control rod
worth, the dimensions of the HEU plate comprising of core components are considered manufacturing tolerances among the significant factors taken into account, as
shown in Table 8.1. In addition to the dimensions of the HEU plate, as previously
demonstrated at the Fast Critical Assembly in the Japan Atomic Energy Agency [3],
other important uncertainty factors include mechanical reproducibility of control rod
position, measurement errors of doubling time by the positive period method, core
M. Yamanaka
8.1 Experimental Settings
8.1.1 Core Configuration
The experiments of reactivity measurement [1, 2] were carried out in the A cores
(EE1 and E3 cores in Figs. 8.1a, b, respectively) that have polyethylene moderator and
reflector rods, and different fuel assemblies: “F” and “f” (Figs. 8.2a, b, respectively).
In EE1 core (Fig. 8.1a), fuel assembly “F” (1/8
P60EUEU) in Fig. 8.2a is composed
of 60 unit cells, and upper and lower polyethylene blocks about 25
and 20
long,
respectively, in an aluminum (Al) sheath (2.1
× 2.1
× 60
). For the fuel assembly,
a unit cell in the fuel region is composed of two highly enriched uranium (HEU) fuel
plates 1/8
(2 × 1/16
) thick and a polyethylene moderator plate 1/8
thick. In E3
core (Fig. 8.1b), another fuel assembly “f” (3/8
P36EU) in Fig. 8.2b is composed of
36 unit cells with an HEU fuel plate 1/16
thick, polyethylene plates 3/8
thick, and
upper and lower polyethylene blocks about 23
and 21
long, respectively, in the Al
sheath as in fuel assembly “F”. The neutron spectrum in EE1 core is compared with
that in E3 core as shown in Fig. 8.3, demonstrating representatively hard (EE1 core)
and soft (E3 core) neutron spectra in the KUCA A-core.
8.1.2 Reactivity Measurements
For measuring the excess reactivity, the critical state was adjusted by maintaining a
certain position of C3 rod in EE1 core and C1 rod in E3 core, and by withdrawing
fully the other control (C1 and C2 in EE1; C2 and C3 in E3) and safety (S4, S5,
and S6) rods from the core, respectively. Furthermore, excess reactivities in EE1
and E3 cores were measured by the positive period method, when C3 and C1 rods,
respectively, were, under the critical state, fully withdrawn from the core.
For measuring the control rod worth, in cases of C1 and C2 rods in EE1 core,
criticality was maintained by C3 rod, and the control rod worth of C1 or C2 rod was
acquired by the rod drop method, after full insertion of C1 or C2 rod into the core.
For C3 rod in EE1 core, criticality was adjusted by C1 rod, and the control rod worth
of C3 rod was obtained by the rod drop method, after the full insertion of C3 rod into
the core. Almost the same procedures were followed for the E3 core, with the use of
the rod drop method, and the control rod worth of C1, C2, and C3 rods was obtained
experimentally.
To estimate the experimental uncertainty of excess reactivity and control rod
worth, the dimensions of the HEU plate comprising of core components are considered manufacturing tolerances among the significant factors taken into account, as
shown in Table 8.1. In addition to the dimensions of the HEU plate, as previously
demonstrated at the Fast Critical Assembly in the Japan Atomic Energy Agency [3],
other important uncertainty factors include mechanical reproducibility of control rod
position, measurement errors of doubling time by the positive period method, core