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Thorium / Uranium Chemical Waste
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Date: Fri, 28 Jul 95 17:34:55 PDT
From: ehs5park@ucsbuxa.ucsb.edu (Duncan H. Parker)
Message-Id: <9507290034.AA05600@ucsbuxa.ucsb.edu>
To: ehs5park@ucsbvm.ucsb.edu
Alan Enn asked:
>
> thorium nitrate ~1 lb
> uranol (spelling?) nitrate ~600 g
> uranol acetate ~700 g
>
> does anyone know which isotopes of uranium and thorium are in these
> chemicals and what their specific activity is?
>
------------------------------------------------------------------------
Thorium Nitrate
------------------------------------------------------------------------
Thorium nitrate, as distributed by many chemical suppliers, consists of
the quadra-hydrate (Th(NO3)4)*(4H2O) and possesses a molecular weight of
approximately 552 grams/mole. The contained thorium consists, by mass, of
overwhelmingly Thorium-232 (M.W. = 232.04; halflife = 1.41E+10 y), the
isotope which is commonly referred to as "natural thorium" and is subject
to general licensing within the United States. (Similar in Canada?)
Pure Thorium-232 has a specific activity of approximately:
3.58E+05 Ci*y/M
----------------------------- = 1.09E-07 Ci/gram (0.11 uCi/gram)
(232.04 g/M) * (1.41E+10 y) (4.04 KBq/gram)
Or, for your hydrated nitrate:
232.04
-------- * 4.04 KBq/g = 1.70 KBq/gram
552
However, it is reasonable to assume that your chemical storekeepers have
not been continuously repurifying this thorium nitrate since its initial
purchase. When originally mined, the thorium ore also contained significant
activities of Radium-228 (halflife ~ 6 y) and Thorium-228 (halflife ~ 2 y),
relatively long-lived decendents of Th-232, and eight other relatively short-
lived decendents (Ac-228, Ra-224, Rn-220, Po-216, Pb-212, Bi-212, Po-212,
Tl-208; all halflives < 4 d). After the initial careful(?) refining of
the ore, only Th-232 and Th-228 should have remained. Yet, within 30 days
all seven of the Th-228 progeny would have returned to within 99.8% of their
original activities. Over the next five to ten years, most of this original
Th-228 would have decayed away, along with its progeny. However, over this
same period, Ra-228 would have built-up to roughly 50 to 75% of the original
Th-232 activity, in the process, replenishing the supply of Th-228. The end
result is that for "natural thorium" of greater than ten years age, most
of the contained "activity" results from progeny, and can be up to ELEVEN
TIMES that of the original Th-232. (I am assuming that the original ore
contained relatively little Thorium-230 from Uranium-238 decay, and thus
there has been only an infinitesimal buildup of Radium-226.)
Thus, your one pound of "thorium nitrate" may contain up to 8.5 MBq (225 uCi)
of mixed alpha and beta activity, with significant accompanying high energy
gamma emissions. Since it is also a strong oxidizer (nitrate), which
is considered a hazardous chemical waste, the combination with significant
radioactivity makes it a "mixed waste."
----------------------------------------------------------------------------
Uranol Acetate / Nitrate
----------------------------------------------------------------------------
The original bulk uranium for such chemical products may have been obtained
from the three most common sources:
1) direct mining -> "natural uranium" (U-238 = 99.3%, U-235 = 0.7%)
2) reactor fuel -> "depleted uranium" (U-235 = 0.3 - 0.1%)
enrichment plant
3) weapons material -> "depleted uranium" (U-235 <= 0.1%)
enrichment plant
In my experience, most of the uranyl acetates and nitrates that are received
as waste (>95%) are depleted uranium, possibly because these enrichment
plants often have large stocks of this "waste material." The plants are
thus eager to sell the chemically equivalent depleted materials to chemical
companies, CHEAP! During the "depletion" process, most of U-234 is also
removed, but there is no way to infer its concentration from that of the
U-235, as depletion processes differ in effectiveness.
Without careful alpha and/or gamma spectrographic analysis of the uranium,
it is difficult to know exactly what the U-238 / U-235 / U-234 ratios are
in any particular sample. However, since in all these cases most of the
"activity" results from U-238 and its relatively short lived daughters
(Th-234, Pa-234), it is possible to give some activity ranges. Thus, each
gram of metallic uranium, assuming secular equilibrium (>240d for depleted),
contains approximately:
Isotope "Natural Uranium" "Depleted Uranium"
-------- ------------------ ------------------
U-238 0.34 uCi (12 KBq) 0.34 uCi (12 KBq)
Th-234 0.34 uCi 0.34 uCi
Pa-234 0.34 uCi 0.34 uCi
U-234 0.34 uCi ~ 0.17 - 0 uCi
U-235 15.4 nCi (570 Bq) ~ 7.0 - 0 nCi
Th-231 15.4 nCi ~ 7.0 - 0 nCi
-------------------- ------------------
Total 1.4 uCi (51 KBq) 1.2 - 1.0 uCi (44 - 37 KBq)
The actual specific activity for your chemical compounds would have to be
derived (as for thorium) from their molecular weights (including hydration),
and the mole ratio of U-238. (It is quite possible that your compounds
have significant hydration, and that this is the reason the "ol" suffix for
hydroxy groups was incorporated into their chemical names.)
Again, some of this uranium might be classified as "mixed waste," depending
upon Canadian and provincial regulations. It is almost assuredly classified
as radioactive waste, and thus prohibited from uncontrolled disposal.