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The depleted uranium available to us contains about 99.7%
U
and less than 0.3%
U which may be compared to the natural
abundances of 99.3%and 0.7%respectively. In addition it is alloyed
with 1.75%Nb. Depleted uranium differs strongly from the natural one
since it was separated chemically and physically from all other
elements, especially from the daughters of the various
decay chains. 
U alone decays with the mean life time
of 6.5
10
yr yielding a decay rate of 12.210
Bq/g.
U decays under the emission of 
particles. These
are hazardous only if uranium is inhaled or ingested.
(Limits for these are given as well as for surface contaminations in
working areas in table 
[20.5.2]). In the DU
calorimeter they are totally shielded by the uranium itself and
by surface coatings and walls.
The daughters 
Th and 
Pa decay in 34.8 d and 1.7 min,
respectively, by 
decay releasing particles up to 2.3 MeV and
rays between 0.03 to 1.83 MeV. The decay chain is then blocked
by the long lifetime of 3.610
yr of the next daughter
U.
With the isotope 
U and some residual 
U
(7.810
Bq/g and 2.310
Bq/g respectively)
the specific activity of DU increases to
1.610 Bq/g.
The total
amount of 499 t alloyed DU yields a decay rate for uranium of
8 TBq (i.e. 220 Ci). The daughters of 
U,
Th and 
Pa are the only ones which contribute to the
observed radioactivity in the calorimeter modules. They decay with the
same rate as 
U since they are in radioactive equilibrium. The
radioactivity of the DU metal may be enhanced for short periods if
material is activated by intense particle radiation.
A great fraction of the and rays are absorbed in the
DU plates themselves. The dose rate at the surface of an uncoated plate
of DU is 2 mSv/h [20.5.5,6]. At the lateral surfaces of the
genuine encapsulated ZEUS modules a dose rate of
45 
S/h has been measured. Its residual radiation is dominated by
rays with an energy of about 1 - 2 MeV.
The decay of the dose rate perpendicular to a 4.6 m high FCAL
module is plotted in figure . An iron plate of 5 cm thickness
attenuates the dose rate by a factor of 10. In figure the
dose rates at ZEUS during the installation period is sketched.
The content of 
in depleted uranium is too low to
start and maintain a nuclear chain reaction. In studies performed
for ZEUS and for the UA1 detector at CERN, the criticality k
was calculated for large calorimeters [20.5.7,8]. The value found,
k 
0.6, is far below 1, the value
needed for a nuclear reactor. Even for natural uranium with a
content of 0.7%of 
U, k is below 1.
Next: Studies on Fire
Up: Hazardous properties of
Previous: Reaction of Uranium