Passive measure for preventing recriticality of fuel debris dust for defueling at Fukushima Daiichi nuclear power station

Figures & data

Table 1. The initial condition of resolidified fuel debris.

Item	Specification
Shape	Sphere
Radius	100.0 cm
Material composition	UO2
Enrichment of ^235U	4.9 wt%
Porosity	0%
Mass density of UO2	10.95 g/cm^3
Effective multiplication factor keff (νt)	0.791 (σ = 0.012%)
Effective multiplication factor keff (νp)	0.782 (σ = 0.012%)

Figure 1. Pattern A of debris dust distribution.

Figure 2. Pattern B of debris dust distribution.

Figure 3. Infinite multiplication factor for debris dust.

Figure 4. Increase in neutron multiplication for pattern A of debris dust distribution.

Figure 5. Increase in neutron multiplication for pattern B of debris dust distribution corresponding to a drill diameter of 10 cm.

Figure 6. Increase in neutron multiplication for pattern B of debris dust distribution corresponding to a drill diameter of 20 cm.

Figure 7. Debris dust guide in the form of an inverted frustum of a circular cone.

Figure 8. Limited increase in neutron multiplication by the debris dust guide.

Table 2. Comparison of the effectiveness of the debris dust guide.

	UO2, keff,initial = 0.782
	keff	Δkeff	Increase in keff (%Δkp/kp,ini)	Suppression rate (%)
Without debris dust guide	1.335	None	70.7	None
With debris dust guide	1.096	−0.239	40.1	56.8
	UO2+H2O ($\mathrm{V}{\mathrm{F}}_{{\mathrm{H}}_2\mathrm{O}}$ = 16%), keff,initial = 0.982
	keff	Δkeff	Increase in keff (%Δkp/kp,ini)	Suppression rate (%)
Without debris dust guide	1.314	None	33.8	None
With debris dust guide	1.112	−0.202	13.3	39.3