Nuclear Safety Research

2-6 Evaluation of the Criticality Characteristics of Fuel Debrisᴪ Study of the Critical Experiment using the Modified STACY ᴪ
20.0
15.0

UO2 fuel rods with zircalloy cladding(diameter: approximately 0.9 cm,length: approximately 150 cm)

Relative Reactivity (cent)Light water(fed from the tank bottom)

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ht
fo
18
etertely
Diam proxima: ap

Height of the core tank: approximately 180 cm

Lattice plates(aluminum)

10.0
5.00
−5.0

1 sample
5 samples
9 samples
13 samples−10.0−15.0020406080100Concrete-volume fraction (%)

Fig.2-15 Overview of the modified STACY
Fuel rods will be supported by lattice palates. The moderator-tofuel-volume ratio will be adjusted by changing the interval of thefuel rods. The modified STACY will be able to simulate various
moderator conditions for the analysis of fuel debris.

In the accident at the TEPCOʼs Fukushima Daiichi NPS,
it is conceivable that fuel debris may have formed frommelted nuclear fuels and structural materials such as ironand concrete. It is important to evaluate the criticalitycharacteristics of fuel debris using computational models forthe decommissioning project including defueling. Furthermore,
we should validate the uncertainties of the computations using
critical experiments with pseudo fuel debris to clarify theircriticality characteristics. For this purpose, the modiﬁcation ofSTACY, whose core is being converted from a uranyl-nitratesolution-fuel type to a UO2-fuel-rod-and-water-moderator type,
is now underway at JAEA.
As shown in Fig.2-15, the fuel rods will be arrayed at someinterval and supported by lattice plates in the modiﬁed STACY
core tank. The moderator-to-fuel-volume ratios (Vm/Vf) of thetest core will be varied by changing the array interval. The
criticality characteristics of fuel debris are largely inﬂuenced
by Vm/Vf as well as the total amount of ﬁssile material and thegeometries of fuel debris. Furthermore, the pseudo fuel debris
including concrete will have specific critical characteristics
because concrete includes water, contributing to neutron
moderation. In the critical experiment, we will measure
variation of the critical water heights; one of them is a basecore configuration comprising only a normal fuel-rod array;the other is a core configuration with pseudo fuel debris.
The variation will be interpreted into a critical characteristic

Fig.2-16 Estimation of reactivity changes using a
number of samples and the concrete volume fractionsof pseudo fuel debris
The reactivity changes of pseudo fuel debris under certain
Vm/Vf conditions are dependent on the number of samples
and the concrete-volume fractions. We have studied theexperimental core configurations to measure their reactivity
changes with high accuracy.

called “reactivity” (unit: cent).
We studied the experimental core configurations and thenecessary numbers of samples for high-accuracy reactivity
measurements of pseudo fuel debris using the radiationtransport-calculation-code system MCNP 5.1 and theevaluated nuclear-data library JENDL4 evaluated by JAEA.
For example, when Vm/Vf is 1.2, the calculation results ofreactivity effects by the changing concrete-volume fraction
are shown in Fig.2-16. This figure shows that the concretevolume fraction has a big impact upon the reactivity, andthere is a tendency for reactivity to become more positivewith increase in the concrete-volume fraction. It is thoughtthat the number of samples should be at least five to realize
a condition with reactivity sufficiently large to be measured
easily. In contrast, when the concrete-volume fraction is30%, the reactivity change is too small to be measured.
In this case, we should consider changing the experimentalcore configuration. Following these studies, we are going toestablish experimental conditions and procedures using themodified STACY. Furthermore, the measurement data fromcritical experiments will be useful for developing the methodsfor critical control of fuel debris.
This study includes the results of contract work funded by
the Secretariat of the Nuclear Regulation Authority (NRA) ofJapan.

ReferenceGunji, S. et al., Study of Experimental Core Conﬁguration of the Modiﬁed STACY for Reactivity Worth Measurement of MCCI Products, Proceedings ofPHYSOR 2016, Sun Valley, Idaho, USA, 2016, p.3927-3936, in USB Flash Drive.

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JAEA R&D Review 2016-17