Research and Development Related to the Backend of the Nuclear Fuel Cycle and the Reprocessing of Spent Nuclear Fuel

8-3 Toward Radioactive-Waste Conditioning Suitable for Near-Surface Disposalᴪ Study on Standards Concerning a Solidification Method by using Mortar ᴪ(a) Composition(b) The voidage ratio after solidification

Waste which iseasily filledwith mortar

Kind ofwaste
Metal plate

Weightratio
20%

Metal piece

20%

Cable

Waste which isdifficult to fillwith mortar

Meter

Kind of shapeP-funneltest timeTopvoids

Innervoids(solidified)

Innervoids(drum)

4%20 s19.6%

3%

23%

Pipe

45%

30 s

19.3%

4%

24%

Valve

10%

40 s

19.8%

4%

24%

1%

50 s

19.8%

2%

22%

Top voids

Filling with mortar

Solidified

Fig.8-8 Composition of Simulated waste, (a) as filled with mortar and the voidage
ratio after solidification (b)The simulated wastes were placed in a 200-ℓ drum, which was then filled with mortar.
The voidage ratio in this drum was then evaluated after solidification.

We are planning for the disposal of radioactive wastes
generated from research, industrial, and medical facilities.
Before the radioactive waste is disposed of, it should beconditioned so as to meet technical standards. One technicalstandard is that radioactive waste should be solidiﬁed and stored
in a container in which no harmful voids remain after filling.
This is intended to prevent scattering of radioactive waste andthe collapse of disposal facilities after backﬁlling with soil.
Solidiﬁcation methods that ensure that wastes are immobilized
in 200-ℓ drums within cementitious matrices and no harmful
voids after ﬁlling are studied. Our aim is to conﬁrm the adequacyof our solidification methods for conservative “simulatedwastes” containing more valves and pipes (which are difﬁcult toﬁll and likely to be left with voids) than actual waste.
The fluidity of mortar is important. Mortar’s viscosity
decreases significantly in proportion to its water content,
increasing the likelihood that the voids between wastes in the
200-ℓ drum will be ﬁlled. However, the effect of such inviscid
mortar may cause faults after curing. In contrast, a strength gain
readily occurs when the amount of water contained is small but
the voids between wastes become difﬁcult to ﬁll with mortar.
The indicator of the fluidity is the test result of the P-type
funnel method, expressed as the time at which a given mixture
ﬂows down under the funnel. Four types of ﬂuid cement–water

Fig.8-9 A vertical cross-sectionalview of the simulated waste
The vertical cross-section of the waste
sample was investigated by visualinspection.

mixtures with ﬂuidities of 20, 30, 40, and 50 s are prepared. The
mix loses ﬂuidity with increasing time.
Simulated waste packages were produced by placing thewaste materials in 200-ℓ drums, which were then filled withmortar of the mix proportions at a general injection speed of
30 ℓ/min, followed by curing. The void ratios within thesewaste packages were less than the limit of 30vol% (Fig.8-8),
equivalent to the assumed average void ratios in the installation
ground of our disposal facilities. For this reason, it is considered
not to be harmful.
The vertical cross-sectional view of the waste packages
demonstrated that the wastes and mortar were in close contact
with the inner walls of the 200-ℓ drums, as shown in Fig.8-9.
For this reason, wastes are considered to be immobilized withina cementitious matrix and to exist in an un-scattered state.
From these results, an appropriate solidification method
corresponding to the technical standards of waste-disposal
conﬁrmation were demonstrated under the condition that wastematerials were placed in a 200-ℓdrum and ﬁlled with mortar ina controlled manner regarding the injection speed and ﬂuidity
described above.
On the basis of this result, this solidiﬁcation method serves asa reference when manufacturing waste packages from research,
industrial, and medical facilities.

Reference
Nakata, H. et al., Waste Acceptance Criteria for Waste Packages Destined for Near Surface Disposal Containing Radioactive Waste from Research, Industrialand Medical Facilities, JAEA-Technology 2016-001, 2016, 112p. (in Japanese).

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JAEA R&D Review 2016-17