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WSRC-TR-98-O0463
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Effect of Americium-241 Content on Plutonium Radiation Source
Terms
by
R. Rainisch
Westinghouse Savannah River Company
Savannah River Site
Aiken, South Carolina 29808
A document prepared for AMERICAN NUCLEAR SOCIETY 1999 ANNUAL MEETING AND EMBEDDED
TOPICAL at Boston, MA, USA from 6/6/99 - 6/10/99.
DOE Contract No. DE-AC09-96SR1 8500
This paper was prepared in connectionwith work done under the above contractnumberwith the U. S.
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Effect of Amencium-241 Content on Plutonium Radiation Source Terms
Reuben Rainisch (Westinghouse SRC)
WSRC-TR-98-O0463
Effect of Americium-241 Content on Plutonium Radiation Source Terms,
Reuben Rainisch
Westinghouse Savannah River Company
BACKGROUND
The management of excess plutonium by the US Department of Energy includes a
number of storage and disposition alternatives. Savannah River Site (SRS) is supporting
DOE with plutonium disposition efforts, including the immobilization of certain
plutonium materials in a borosilicate glass matrix.
Surplus plutonium inventories slated for vitrification include materials with elevated
levels of Americium-241. The Am-241 content of plutonium materials generally reflects
in-growth of the isotope due to decay of plutonium and is age-dependent. However,
select plutonium inventories have Am-24 1 levels considerably above the age-based
levels. Elevated levels of americium significantly impact radiation source terms of
plutonium materials and will make handling of the materials more difficult. Plutonium
materials are normally handled in shielded glove boxes, and the work entails both
extremity and whole body exposures.
This paper reports results of art SRS analysis of plutonium materials source terms vs. the
Americium-241 content of the materials. Data with respect to dependence and magnitude
of source terms ordvs. Am-241 levels are presented and discussed. The investigation
encompasses both vitrified and un-vitrified plutonium oxide (Pu02) batches
OUTLINE OF PLUTONIUM BATCHES
The analysis pertains to weapons grade (WG) plutonium. Isotopic compositions of WG
plutonium are given in DOE Standard DOE-STD-3013-96 (Criteria for Preparing and
Packaging Plutonium Metals and Oxides for Long-Term Storage, 9/1996). The
compositions include 93.6 weight percent (wt%) Pu-239 and 0.4 wt% by Pu-241. In each
class batches with low and high Americium-241 concentrations were analyzed.
The un-vitrified batches have an overall weight of 3.5 kg plutonium oxide, and the Am241 level ranges from comparatively low levels corresponding to in-growth following 15
years decay of WG plutonium and estimated at 0.18 wt% Am-241 to the maximum
expeeted Am-241 level of 5.6 wt%. The vitrified batches have overall weights of 25 kg,
plutonium content of 2,490 gm, and Am-241 concentrations range from 0.02 to 0.77 wt%.
Page 1 of 3
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Effect of Americium-241 Content on Plutonium Radiation Source Terms
Reuben Rainisch (Westinghouse SRC)
WSRC-TR-98-00463
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ANALYSIS
Parameters investigated include: (a) Neutron and gamma source”terms; (b) Contact dose
rates, needed to assess to extremity exposures; and (c) Shielded dose rates, applies to
whole body exposure of operators at glove box shielded windows.
The RASTA computer code was used to calculate the neutron and gamma source terms of
the plutonium masses. RASTA is an SRS in-house computer codes used for engineering
calculations. Included in the source terms are neutrons from plutonium and Am-241
spontaneous fission, neutrons from (et, n) reactions in plutonium compounds, and
subcritical multiplication neutrons. Gammas from decay of plutonium and plutonium
daughter nuclides, prompt and delayed gammas from plutonium and Am-241
spontaneous fission, and gammas from (q n) reactions in plutonium compounds.
Batches were modeled as spheres and the multi-group neutron/gamma shielding
calculations were performed with the ANISN Code.
RESULTS
The more important findings are:
Vitrification Effects – Vitrification of plutonium oxide with a 0.18 weight percent Am241 yields neutron source terms that are 20 times higher than the un-vitrified source
terms. Similarly, vitrification of PU02 with 5.6 wt% Am-241 results in neutron source
terms that are 40 times higher than the un-vitrified plutonium. The increase is attributed
to additional (a, n) reactions with Boron-11 target isotopes present in glass. Gamma
source terms are not impacted by vitrification and remain unchanged.
Neutron Source Terms vs. Americium Content - Increasing the Am-241 content of
plutonium batches results in significant increases in neutron source terms. Increasing the
Am-241 content of un-vitrified plutonium from 0.18% to 5.6% by weight (a 30 fold
increase) results in neutron source terms that are 2.2 times higher ( 120~0increase). . The
increases are attributed to more alpha particles (et), emitted by Am-24 1, and additional
(et, n) reactions with 0-18. In higher concentrations Am-241 surpasses Pu-239 as the
dominant alpha emitter. For vitrified plutonium an increase in Am-241 (from 0.02 to
0.77 wt%) results in 4.4 times increase in neutron source terms. For vitrified plutonium
most of the (a, n) reactions are with Boron-11 target isotopes present in glass.
Gamma Source Terms vs. Americium Content - Increasing the Am-241 content of
plutonium oxide and vitrified plutonium from in-growth levels to maximum expected
levels results in significant magnifications in gamma source terms. The analysis shows
magnifications of x 18 and x23 in gamma source terms for oxide and vitrified materials
respectively. Americium-24 1 decay yields a 0.06 Mev gamma. The majority of gamma
photons are in the low energy range of 0.01 to 0.06 Mev.
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Effect of Americium-241 Content on Plutonium Radiation Source Terms
Reuben Rainisch (Westinghouse SRC)
WSRC-TR-98-O0463
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Contact Dose Rates vs. Americium Content – As expected additional Am-241 result in
higher extremity dose rates. For un-vitrified materials gamma photons together with
neutrons control extremity exposures. Increasing the Am-241 content of un-vitrified
plutonium from O.18% to 5.6% by weight increases total contact dose rate from 2.3 to 6.1
mSv/hour. For vitrified plutonium neutrons alone control contact dose rates, and the
gamma contribution is limited to about 5%, or less. Increasing the Am-241 content of
vitrified plutonium from 0.02~0 to 0.77!Z0by weight causes contact dose rate to increase
from 8.3 to 36.5 mSv/hour.
Shielded Dose Rates vs. Americium Content – For both un-vitrified and vitrified
plutonium materials dose rates were calculated at the glovebox shielded windows,
Independent of the Am-241 content shielded dose rates are controlled by neutrons. This
is because of the soft gamma spectrum and attenuation of the photons by the materials
that make up the shielded window (i.e. lead glass). Increasing the Am-241 content of unvitrified plutonium from O.18% to 5.6% by weight increases total dose rate at the shielded
window from 1.4E-3 to 4.OE-3 mSv/hour. Increasing the Am-241 content of vitrified
plutonium from 0.02% to 0.77% by weight increases total shielded dose rate from 2.2E-2
to 9.7E-2 mSv/hour.
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