Nuclear Safety Research

2-5 Acquisition of Data Required for Severe-Accident Evaluation in a Reprocessing Plant
ᴪ Understanding the Release Characteristics of Ruthenium from Highly Active Liquid Waste during a Drying Step ᴪGas flows only one side of condenser
by operation of three-way valve
Gas absorber(5 ℃ cooling)

Pump

Condenser
Coolingwater(5 ℃)

s-HALW sample
initial: 200 mℓ

0.1ℓ/min

MFC
Mass flow
controller

ElectricfurnanceNOx gasanalyzer
Collection ofcondensate

Two-way valve
Three-way valve

Only use at measurementof NOx gas release rateDrying Drying
step step(1)(2)

400Temp. in measurementof Ru released
Temp. in measurement
NOx gas release rate

20010002030Time (h)4050

Fig.2-13 Condition of temperature increase of
the s-HALW
The rate and time of temperature increase of thes-HALW were experimentally simulated based on
the computer-code analysis of a HALW-storagetank in the postulated accident.

Time (h)
40.6

42.4
5

Drying step (2)2.0×10−44

1.5×10−43

1.0×10−4
0.5×10−401033.6

Boiling
step Drying step (1)Mole of released Ru (mol)Temp. of s-HALW (℃)

Boiling step

300

0

2.5×10−4500100

Mole ofreleased Ru
Volume ofcondensate
Nox gasrelease rate

200300Temp. of s-HALW (℃)

210400

7.5×10−4
6.0×10−4
4.5×10−4
3.0×10−4
1.5×10−4

NOx gas release rate (mol/min)

120 ℃
heatingFig.2-12 Diagram of the experimental apparatus
The temperature of the simulated highly active liquid
waste (s-HALW) was increased, as shown in Fig.2-13.
Released gases from the s-HALW containing steam andRu were collected as the condensate in a condenser.
The number of mols of Ru in the condensate samples
was analyzed by inductively coupled plasma mass
spectrometry (ICP-MS).

Exhaustto draft

Air filter

Volume of condensate (mℓ)

Thermocuples
Air: measurement of150 ℃ heating
Ru released
N2 gas: measurement ofNOx gas release rate
Gas suction
Slicagel

0Fig.2-14 Measurement results for mole of released Ru, volumeof condensate, and NOx-gas release rate in terms of the s-HALW
temperature
Under the s-HALW-temperature condition shown in Fig.2-13, it was found that
release behavior of Ru is dependent upon three divided zones of the s-HALW
temperature during the period from the boiling step to the drying step.

When the cooling function for HALW in the fuel-reprocessing
plant is lost, the HALW is heated by the decay of dissolved
radioactive materials, which may cause boiling and dryout.
Since the radioactive materials may be released from theHALW to the air, it is necessary to quantitatively understandtheir release and transport behavior to evaluate the safety at aplant during such an accident. In particular, since Ruthenium(Ru) can transform into volatile species during the boiling anddrying steps, the release ratio of Ru from the HALW would
be larger than those of other non-volatile elements. Therefore,
Ru is considered an important element from the viewpoint ofestimating the public dose from the accident.
Therefore, we performed experiments to acquire practical
data on Ru-release behavior using s-HALW, which wasprepared with stable isotopes including lanthanide elements
based on the composition of HALW. The test conditions
included a very slow rate and time of temperature increase
based on the computer code analysis for an HALW-storagetank in a postulated accident.
The s-HALW was heated in the electrical furnace shownin Fig.2-12 under the experimental condition of s-HALWtemperature rise, as shown in Fig.2-13. Measurement results forthe mole of released Ru, the volume of condensate, and the NOxgas-release rate in terms of the s-HALW temperature are shownin Fig.2-14. It was found that the release behavior of Ru wasdependent upon three divided zones of the s-HALW temperatureduring the period from the boiling step to the drying step.
The release mechanism of Ru in each region was considered
as follows. In the boiling step, the concentration of s-HALW
increased owing to steam release. Ru was released as a volatile
species transformed by the oxidizability of highly concentrated
nitric acid. When the release of steam went to the end, theamount of the Ru release was temporarily decreased. In thedrying step (1), in which the condensate was not collected, Ru
release resulted from progress of thermal decomposition of Ru
nitrate with NOx release in the dried s-HALW. In the dryingstep (2), a small amount of steam was released, probably owing
to desorption of hydrates of lanthanide nitrates. The NOx
release was owing to thermal decomposition of lanthanide
nitrates. In contrast, the amount of Ru released monotonicallydecreased because of the regress of thermal decomposition ofRu nitrate in the drying step (1). We are the ﬁrst group to obtain
detailed data on the amount of Ru released depending on theHALW temperature during the drying-step period. These data
will be useful for investigating effective accident-management
measures to mitigate the amount of the Ru release, considering
the HALW temperature if the accident is in the drying step
owing to loss of the cooling function.

Reference
Tashiro, S. et al., Release Characteristics of Ruthenium from Highly Active Liquid Waste in Drying Step, Nippon Genshiryoku Gakkai Wabun Ronbunshi(Transactions of the Atomic Energy Society of Japan), vol.14, no.4, 2015, p.227-234 (in Japanese).
JAEA R&D Review 2016-1735