26.08.2015   Study on Neutron Spectrum of Pulsed Neutron Reactor

Experimental Study on Criticality Control for Fuel Debris

22.2.1 Modification of STACY

To implement the new criticality control measures for fuel debris, the Japan Atomic Energy Agency (JAEA) has been carrying a project to modify the Static Experiment Critical Facility (STACY) to pursue critical experiments on fuel debris [6]. STACY, a facility using solution fuel (low-enriched uranyl nitrate), is to be converted into a thermal critical assembly using fuel rods and a light water moderator.

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 UO2 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 x 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 UO2 pellets

5 wt.%-enriched UO2 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 keff

<0.985 in keff

One-rod stuck margin

<0.995 in keff

<0.995 in keff

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