52
C. H. Pyeon
different from the current design of ADS where a target is located at the center
of the core for effective utilization of the generated neutrons. Consequently, the
introduction of a neutron guide is requisite for effectively directing the high-energy
neutrons generated from the tritium target to the center of the core for experiments on
ADS with 14 MeV neutrons. The neutron guide, which is very similar to the neutron
shield and the beam duct [1, 2], is composed of several shielding materials, including
iron, boron, polyethylene, the beam duct, and a special fuel assembly with a void.
Five major core settings are presented here: a core without the neutron guide
(Fig. 3.1a; Reference core: Case 1); a core including only streaming void (SV) in
the fuel region (Fig. 3.1b; SV core: Case 2); and three cores with the neutron guide
including SV (Fig. 3.1c–f; neutron-guided core with beam window: Cases 3, 4, 5,
and 6, respectively). Numerals 12, 20, 22, 26, and 24 correspond to fuel plates in the
partial assembly/ies used to reach criticality (Fig. 3.1a–f, respectively).
Each fuel assembly was set in a 2.1
× 2.1
and 1.5 mm thick aluminum (Al)
sheath; the cross section of the elements within the assembly was 2
× 2
. The
fuel assemblies constituting the core are reproduced in Fig. 3.2. The standard fuel
assembly shown (F; Fig. 3.2a) was composed of 36 unit cells of 1/16
thick and a 93
wt% enriched uranium plate with Al clad and two (1/8
and 1/4
) thick polyethylene
plates. The active height of the core was about 16
, with additional about 23
and
21
upper and lower polyethylene reflectors, respectively. Case 1 (Fig. 3.1a) was
composed of 20 regular fuel assemblies and one partial fuel assembly of 12 fuel unit
cells. In Cases 2–6 (Fig. 3.1b–f, respectively), the fuel region consisted of 18 regular
fuel assemblies, SV assembly composed of one 5.08 × 5.08 × 5.08 cm center void
(Fig. 3.2b), 32 fuel unit cells, and two partial fuel assemblies. Details of the partial
fuel assembly of (14, M and 16, M) used for Case 5 are presented in Fig. 3.2e. The
active part was centered with the rest of the core using an Al cell identical to the fuel
cell but with Al replacing the fuel plates.
The purpose of the neutron guide was to reproduce the conditions of a highenergy neutron beam entering the fuel region from an isotropic source. Thus, the
role of the SV assemblies was to direct the highest possible number of the highenergy neutrons generated in the target to the center of the fuel region, in order to
improve neutron multiplication. Moreover, to reproduce a high-energy source, it was
necessary to reduce the thermal component of the external neutron source, i.e., the
neutrons were moderated before they reached the fuel region. This was achieved
by shielding unnecessary fast neutrons and by capturing parasite thermal neutrons.
For deflecting unnecessary fast neutrons, the close vicinity in front of the target
included an ion block around the guide void to shield the fast neutrons by inelastic
scattering. For capturing parasite thermal neutrons, polyethylene blocks containing
10 wt% boron around the guide void were included around the Fe shielding near
the target and in the two rows next to the assemblies. The rest of the neutron guide
consisted of polyethylene assemblies and one void space. The detailed composition
of the neutron guide was presented in Ref. [1].
14 MeV neutrons were produced with a yield of about 8 × 10
8 s
−1 from the tritium
target in pulsed mode. The duty ratio and the duration of irradiation were adjusted
C. H. Pyeon
different from the current design of ADS where a target is located at the center
of the core for effective utilization of the generated neutrons. Consequently, the
introduction of a neutron guide is requisite for effectively directing the high-energy
neutrons generated from the tritium target to the center of the core for experiments on
ADS with 14 MeV neutrons. The neutron guide, which is very similar to the neutron
shield and the beam duct [1, 2], is composed of several shielding materials, including
iron, boron, polyethylene, the beam duct, and a special fuel assembly with a void.
Five major core settings are presented here: a core without the neutron guide
(Fig. 3.1a; Reference core: Case 1); a core including only streaming void (SV) in
the fuel region (Fig. 3.1b; SV core: Case 2); and three cores with the neutron guide
including SV (Fig. 3.1c–f; neutron-guided core with beam window: Cases 3, 4, 5,
and 6, respectively). Numerals 12, 20, 22, 26, and 24 correspond to fuel plates in the
partial assembly/ies used to reach criticality (Fig. 3.1a–f, respectively).
Each fuel assembly was set in a 2.1
× 2.1
and 1.5 mm thick aluminum (Al)
sheath; the cross section of the elements within the assembly was 2
× 2
. The
fuel assemblies constituting the core are reproduced in Fig. 3.2. The standard fuel
assembly shown (F; Fig. 3.2a) was composed of 36 unit cells of 1/16
thick and a 93
wt% enriched uranium plate with Al clad and two (1/8
and 1/4
) thick polyethylene
plates. The active height of the core was about 16
, with additional about 23
and
21
upper and lower polyethylene reflectors, respectively. Case 1 (Fig. 3.1a) was
composed of 20 regular fuel assemblies and one partial fuel assembly of 12 fuel unit
cells. In Cases 2–6 (Fig. 3.1b–f, respectively), the fuel region consisted of 18 regular
fuel assemblies, SV assembly composed of one 5.08 × 5.08 × 5.08 cm center void
(Fig. 3.2b), 32 fuel unit cells, and two partial fuel assemblies. Details of the partial
fuel assembly of (14, M and 16, M) used for Case 5 are presented in Fig. 3.2e. The
active part was centered with the rest of the core using an Al cell identical to the fuel
cell but with Al replacing the fuel plates.
The purpose of the neutron guide was to reproduce the conditions of a highenergy neutron beam entering the fuel region from an isotropic source. Thus, the
role of the SV assemblies was to direct the highest possible number of the highenergy neutrons generated in the target to the center of the fuel region, in order to
improve neutron multiplication. Moreover, to reproduce a high-energy source, it was
necessary to reduce the thermal component of the external neutron source, i.e., the
neutrons were moderated before they reached the fuel region. This was achieved
by shielding unnecessary fast neutrons and by capturing parasite thermal neutrons.
For deflecting unnecessary fast neutrons, the close vicinity in front of the target
included an ion block around the guide void to shield the fast neutrons by inelastic
scattering. For capturing parasite thermal neutrons, polyethylene blocks containing
10 wt% boron around the guide void were included around the Fe shielding near
the target and in the two rows next to the assemblies. The rest of the neutron guide
consisted of polyethylene assemblies and one void space. The detailed composition
of the neutron guide was presented in Ref. [1].
14 MeV neutrons were produced with a yield of about 8 × 10
8 s
−1 from the tritium
target in pulsed mode. The duty ratio and the duration of irradiation were adjusted