Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 51−52, 2000

51

Notes
Estimation of Total Fission Events in the JCO Criticality Accident by Passive
γ-Ray Measurement of 140La in Radioequilibrium with Precursor 140Ba
Toshiaki Mitsugashira,*,a Takashi Nakanishi,b Riki Seki,c and Sadao KojimadaThe Oarai-branch, Institute for Materials Research, Tohoku University, Narita-machi, Oarai-machi, Ibaraki-ken
311-1313, Japanb
Department of Chemistry, Faculty of Science, Kanazawa University, Kadoma-machi, Kanazawa 920-1192,Japanc
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki-ken 305-8571, Japand
Radioisotope Research Center, Aichi Medical University, Nagakute-machi, Aichi-gun, Aichi-ken 480-1195,
Japan
Received: November 24, 1999; In Final Form: March 2, 2000

La ( 140Ba 12.75d) 1596keV
140

La (140Ba 12.75d) 926keV
140

La ( 140 Ba 12.75d) 816keV
La (140Ba 12.75d) 868keV
140131The detector was set on the top of the northwest corner of ashielding wall made of sandbag heaped up around the Uranium
Conversion Building (UCB) in which the PV was placed.
Intense γ-ray signals mainly due to 140La that was in radioequilibrium with its precursor 140Ba (t1/2=12.75d) was recorded by
the 4096ch PHA. The dead time ratio of the PHA system wasabout 20% and we noticed that the detector was at the nearest
position practically allowed to access. Within about 30 minutes, three spectra were recorded and one of the spectra isshown in Figure 1. The main γ-ray at 1596 keV is very clearlyobserved with its sharp Compton edge, i.e., that implies the1596 keV γ-ray comes from a single remote radiation source.
Although scattering components are quite large at below about
500 keV and intense bremstrahlung are recognized up to about
2000 keV, many characteristic γ -ray peaks of 1 4 0 La areobserved. Rather weak γ-ray peaks of 131I and 95Zr-95Nb are
also observed. The reference data of the 66th meeting of NSC1
suggested that the majority of fission products was retained inPV. Therefore, TFE can be estimated without the sampling of
the uranium solution in PV by passive γ-ray spectrometry of
appropriate fission product radioactivity.

140I (8.02d) 637keV

La (140Ba 12.75d) 487keVBa (12.75d) 537keV
140

1000

140

La (140Ba 12.75d) 328keV

10000

140

counts/ch(0.434keV)

1000001311000000

I (8.02d) 364keV

The distinctive feature of the JCO criticality accident is thecontinuous release of fission neutron without appreciable
release of fission products. Hence, the estimation of total fission events (TFE) is the most important task for evaluating
environmental impact. The sampling of uranium solution in
the precipitation vessel (PV) in which the criticality accident
took place, was performed by the members of JAERI to know
TFE and the analytical results were reported to the Science andTechnology Agency (STA) and the Nuclear Safety Commission(NSC).1
On Oct. 25, the Ministry of Education, Science, Sports andCulture decided to organize the Japanese university member tomake research for the environmental impact due to the JCO
accident and about 12 scientists and their coworkers started
their research under the management of the group leader Prof.
K. Komura, Kanazawa University. This research group has
already performed five times the sampling of soil and plants in
the JCO site. On the 2nd sampling on Oct. 26, the presentauthors carried a portable Ge detector and a 4096ch PHA tomake the in-situ γ-ray spectrometry.

10010103Ru(39.3d) 497keV
95Zr(64d)-95Nb(35d) 757keV, 724keV, 766keV

10200

400

600

800

1000

1200

1400

1600

1800E/keV

Figure 1. An in situ γ-ray spectrum obtained by a portable Ge detector set on the top of the northwest corner of a shielding wall made of sandbag
heaped up around the Uranium Conversion Building.
*Corresponding author. E-mail: mitsug@imr.tohoku.ac.jp. FAX:
+81-29-267-4947.

© 2000 The Japan Society of Nuclear and Radiochemical SciencesMitsugashira

52 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

1.0m

16.10m

2.70m

9.60m

Ge detector
15.10m0.96m

Sandbag shielding I

Precipitation vesselSandbag shielding II

Figure 2. A schematic view of the Uranium Conversion Building of JCO Co. Ltd., and the Ge detector arrangement.

To calculate TPE from the observed γ-ray spectra, we estimated the attenuation of γ-ray by the wall of UCB, the uranium
solution, SUS304 wall of PV, and the sandbag for shielding.
Figure 2 is a plane figure showing a schematic arrangement ofUCB, PV, the sandbag for shielding, and the Ge detector. The
height of the sandbag shielding I is about 170cm and the head
position of the Ge detector was about 175cm from the groundlevel. The bottom position of PV is estimated to be about
100cm from the ground level. From the geometric configuration, the γ -rays from PV penetrated the wall at an incident
angle of 15.6 degree to its surface and at about 5.5 degree ofelevation. The distance between the Ge detector and PV wasdetermined to be 7.21m. The wall material of UCB is Siporex(ALC) that is calcined foam products of the mixture of silica,aluminum, quicklime, and cement. The density of Siporex is0.65 g/cm3 and the wall thickness is 10 cm. From the angles,
we estimated the penetration depth of the Siporex wall to be
37.3 cm. The concentration of the uranium in the solution andthe penetration depth of the solution were evaluated to be 0.28
g/cm3 and 22.5 cm, respectively.1 The thickness of SUS304
wall of PV and the cooling jacket is 3 mm. Thus, we assumed
that the penetration depth of SUS304 was 6.7 mm. The heightof the sandbag shielding II is about 2 m. The shielding II washeaped up at random and only its north end edge crossed theline between the detector and PV. We estimated the penetration depth of the sandbag to be 15 cm. Thus, the total attenuation factors of 329 keV, 487 keV, 816 keV, and 1596 keV γrays were estimated to be 0.000036, 0.00051, 0.0039, and0.020, respectively.
The counting rate values of 329 keV, 487 keV, 816 keV,and 1596 keV γ-rays were 5.77±0.51, 11.48±0.77, 7.83±0.42,
and 52.9±2.2 cps, respectively. The very large attenuation for329 keV and 487 keV γ-rays compared with those for 816 keVand 1596 keV was mainly attributed to the uranium solution
and sandbag shielding. The group leader Prof. K. Komura
pointed out that the soil samples collected at the outside points
close to the UCB wall were weakly contaminated with 140La(Ref. 2), which would be explained by the activation of soil orthe trace release of the β −-decay chain with mass 140 throughXe (t1/2=13.6 sec).3 Thus, we omitted the 329 keV and 487
keV γ-rays from the TFE calculation since the traces of theexistence of 140La supported by 140Ba in the outside of UCB
greatly affected the results calculated under such large attenuation. Absolute counting efficiency values from a 7.21m point
source for 816 keV and 1596 keV γ-rays were determined tobe 2.74×10−7 and 1.41×10−7, respectively. The TFE values
were calculated to be 2.95×1018 and 1.96×1018 for 816 keV and1596 keV γ -rays, respectively, by adopting 0.0627 as thecumulative fission yield for 140Ba (Ref. 4). This rough estimation for TFE of 2−3×1018 was in fairly good agreement withReference 1.
Our estimate of TFE based on passive γ-ray spectrometry isprincipally independent of the amount of uranium and the volume of uranium solution. It should be mentioned that we did
not confirm the 1/r2 dependency of the observed γ-ray intensity. In spite of the above fairly good agreement, the authors
consider that the values are a rough approximation. Thus, ourpresent result must be ensured by much more sophisticatedmeasurement and such attempts are proposed to JCO, STA,and NSC by the present authors.
The authors are grateful to Prof. M. Sasaki, Kyoto University,and Dr. Y. Igarashi, Geochemical Research Department,
Meteorological Research Institute, for their assistance andtenacious negotiation with NSC and STA that made possible
in-site γ-ray spectrometry as well as the environmental sampling in the JCO site.
140References(1) The Materials No. 1-1, The 66th meeting of Nuclear Safety
Commission (NSC), Nov. 4, 1999.
(2) K. Komura (private communication).
(3) R. B. Firestones and V. S. Shirley, Table of Isotopes (8thEdition, John Wiley & Sons INC., 1996).
(4) E. A. C. Crouch, Atom. Data Nucl. Data Tables 19, 417(1977).