Criticality accident at TOKAI (fwd)

[Bernard L Cohen <blc+@pitt.edu>]

	The following message with a little new info was received from a
Japanese friend.

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


---------- Forwarded message ----------
Date: Fri, 8 Oct 1999 18:22:43 +0900
From: Keizo TAKAHASHI <tkeizo@postman.riken.go.jp>
To: COHEN Bernard <blc+@pitt.edu>
Subject: Criticality accident at TOKAI

 There was a nuclear criticality accident at nuclear fuel conversion plant
of
JCO(former Japan Nuclear Fuel Conversion Organization?), 100% daughter
company of Sumitomo Metal & Mining on 10:35 am Sept 30, 1999.
But my house is situated in Tokai-mura (2km from JCO site), and I returned
every week end at Tokai, from Tokyo to see my family, and naturally I have
still interest in nuclear technology as usual.
The information, which is shown here,  is totally my personal opinion and
not official one.
1.  Time of Accident; 10:35 Thursday Sept 30, 1999
2.  Place of accident; Experimental Conversion Facility, Tokai Works, JCO.
3.  Persons who had received high doses:
worker A; 1700 rem, worker B; 1000 rem, worker C; 300 rem
4.  Situation of accident;
Three workers was preparing the Uranyl nitrate solution(U.E. 18.8%, 360
g/li) for final stage of conversion of U3O8 to UNH for experimental fast
reactor "Joyo" of JNC.  Worker;A poured the NHH solution in bucket (volume
10 li, ) to precipitation tank (diameter 40cm hight 70cm, volume 137
litres), and then criticality incident occurred. Worker;A was severely
irradiated and lost consciousness, making spasm and vomiting. Worker; B who
was near worker;A was hit also and lost consciousness. Two said later they
saw the blue flash light.
Worker; C, who was in next room helped worker; A and B with other helper.
Three workers went to Mito hospital initially and then lifted by helicopter
to Chiba's National Radiological Medical Institute around 15:25.  A and B is
now at hospital of Medical school of Univ. of Tokyo for transplantation of
blood cells (very highly medical methods are now applied for both first time
in Japan medical history). A is still in critical condition and B's
condition is better than A, but could be classified as critical.

5.  Termination of criticality incident
The workers of JCO did emergency intervention at the room to discharge
water from cooling jacket( cover of precipitation tank) at water cooling
circuit. The criticality chain action was terminated around 4:30 am, Oct 1,
1999, after 18 hours of criticality condition . Some intervention workers
got radiation dose of 10 rem, which is legal limit of emergency radiation
work.

6.  Effect to local population;
Voluntary Evacuation of families within 350 metres of the perimeter has been
implemented around 3:00 pm by mayor of Tokai-mura (around 160 persons).  A
decision to shelter the population (on voluntary basis),  out to a radius of
about 10 kilometers was in effect around 10:30 pm by Ibaraki-ken governor,
and this was lifted 4:30 pm next day Oct 1, 1999. Voluntary Evacuation was
lifted also in the evening of Oct 2, 1999.
The number of affected population for sheltering in their houses was
estimated as 310, 000 persons.

7.  Cause of accident.
(1)The mass criticality was attained with introduction of about 16 kg-U
(E.U; 18.8%) of UNH solution, totaling seven batch of bucket (2.3
kgU/bucket, 10 litre/bucket). Although maximum allowable limit for
precipitation tank is only 2.4 kg (mass control).
(2) The bucket is used for dissolution of Uran powder in these years (some
said 10 years) , although regulatory license specified that it could be done
dissolver. Operator of JCO think that using of bucket is more efficinet and
convenient than using dissovers which is not working smoothly. They used
this bucket, knowing that it is illegal to use without licensing
modification.
(3) Workers didn't know the criticality risk of medium level enriched
uranium.

(4) The procedure for homogenization of final UNH product solution was
specified to use storage tanks, which is geometrically safe design (diameter
17.5cm, height 3.5m, volume 87 litre). The reason is that use of
precipitation tank needed 30 minutes, but use of storage tank will required
three hours, and workers choose short circuited, wrong method.


Unfortunately, through this JCO accident, many people worried about
radiation effect and in some sense this is panic, although there is nothing
to afraid for effect of JCO incident environmentally. JCO accident surely
affected the workers, who received fatal dose, and population very near to
the JCO site, but environmental point of view criticality accident in
solution is not major danger as you surely .

n

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