In the modified STACY, the core configuration consists of fuel rods loaded in the
core tank (up to 900 rods) and light water fed as moderator. Because the maximum
thermal power is only 200 W, fuel burn-up is negligibly small and cooling water is
unnecessary. The reactivity of the core is controlled not with control rods but by
water level, and with safety plates (cadmium) in the case of emergency shutdown,
similar to the present STACY. The fuel rods contain 5 wt.%-enriched UO 2 pellets
and have zircaloy cladding. A soluble neutron poison (boron) can be added to the
light water moderator. Major core specifications and a schematic diagram of the
modified STACY are shown in Table 22.1 and Fig. 22.1, respectively.
Table 22.1 Major core specifications of the modified Static Experiment Critical Facility
(STACY)
Item
Present STACY
Modified STACY
Core tank
Closed tank
Open tank
Replaceable (cylinder, slab, heterogeneous, interaction)
Cylinder (1.8 m in diameter,
1.9 m in height)
Core size
Same as each core tank
Maximum 60 cm  90 cm
Critical height 40–140 cm
Critical height 40–140 cm
Maximum thermal
power
200 W
200 W
Maximum integrated
power
100 W · h/operation, 300 W · h/week,
3 kW · h/year
100 W · h/operation,
300 W · h/week, 3 kW · h/year
Fuel Fuel solution
6-, 10 wt.%-enriched uranyl nitrate
solution
Not used
Maximum
concentration
500 gU/l
Fuel rods
5 wt.%-enriched UO 2 pellets
5 wt.%-enriched UO 2 pellets
(<10 wt.% available)
Cladding
Zircaloy cladding (9.5 mm in diameter, 150 cm in length)
Zircaloy cladding (9.5 mm in
diameter, 150 cm in length)
Maximum
loading
400 rods
900 rods
Volume ratio of moderator to fuel (lattice
pitch of fuel rods)
1.9–15 (13.0–29.0 mm)
0.9–11 (10.9–25.5 mm)
Moderator
Solution fuel
Light water
Temperature
Room temperature ~40
C
Room temperature ~70
C
Reactivity control
Solution level
Water level
Maximum excess
reactivity
0.2 dollar in normal operation
0.3 dollar in normal operation
0.8 dollar in abnormal transient
0.8 dollar in abnormal transient
Maximum reactivity
addition rate
3 cent/s
3 cent/s
Emergency shutdown Insertion of safety plates
Insertion of safety plates
Drain of fuel solution
Drain of light water moderator
Shutdown margin
<0.985 in k eff
<0.985 in k eff
One-rod stuck margin <0.995 in k eff
<0.995 in k eff
22 Modification of the STACY Critical Facility for Experimental Study on Fuel. . .
263
core tank (up to 900 rods) and light water fed as moderator. Because the maximum
thermal power is only 200 W, fuel burn-up is negligibly small and cooling water is
unnecessary. The reactivity of the core is controlled not with control rods but by
water level, and with safety plates (cadmium) in the case of emergency shutdown,
similar to the present STACY. The fuel rods contain 5 wt.%-enriched UO 2 pellets
and have zircaloy cladding. A soluble neutron poison (boron) can be added to the
light water moderator. Major core specifications and a schematic diagram of the
modified STACY are shown in Table 22.1 and Fig. 22.1, respectively.
Table 22.1 Major core specifications of the modified Static Experiment Critical Facility
(STACY)
Item
Present STACY
Modified STACY
Core tank
Closed tank
Open tank
Replaceable (cylinder, slab, heterogeneous, interaction)
Cylinder (1.8 m in diameter,
1.9 m in height)
Core size
Same as each core tank
Maximum 60 cm  90 cm
Critical height 40–140 cm
Critical height 40–140 cm
Maximum thermal
power
200 W
200 W
Maximum integrated
power
100 W · h/operation, 300 W · h/week,
3 kW · h/year
100 W · h/operation,
300 W · h/week, 3 kW · h/year
Fuel Fuel solution
6-, 10 wt.%-enriched uranyl nitrate
solution
Not used
Maximum
concentration
500 gU/l
Fuel rods
5 wt.%-enriched UO 2 pellets
5 wt.%-enriched UO 2 pellets
(<10 wt.% available)
Cladding
Zircaloy cladding (9.5 mm in diameter, 150 cm in length)
Zircaloy cladding (9.5 mm in
diameter, 150 cm in length)
Maximum
loading
400 rods
900 rods
Volume ratio of moderator to fuel (lattice
pitch of fuel rods)
1.9–15 (13.0–29.0 mm)
0.9–11 (10.9–25.5 mm)
Moderator
Solution fuel
Light water
Temperature
Room temperature ~40
C
Room temperature ~70
C
Reactivity control
Solution level
Water level
Maximum excess
reactivity
0.2 dollar in normal operation
0.3 dollar in normal operation
0.8 dollar in abnormal transient
0.8 dollar in abnormal transient
Maximum reactivity
addition rate
3 cent/s
3 cent/s
Emergency shutdown Insertion of safety plates
Insertion of safety plates
Drain of fuel solution
Drain of light water moderator
Shutdown margin
<0.985 in k eff
<0.985 in k eff
One-rod stuck margin <0.995 in k eff
<0.995 in k eff
22 Modification of the STACY Critical Facility for Experimental Study on Fuel. . .
263