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Appendixes
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Appendix A
Autoignition and the M55 Rocket
As a consequence of the applicability of the Army's
broad experience with energetics in conventional weap-
ons, there is no reason to expect autoignition of either
the fuze or burster components of chemical munitions.
However, the M55 rocket also contains 19.1 lb of M28
propellant, which is of a double-base composition, con-
taining nitroglycerine (NG), nitrocellulose (NC), and
2-nitrodiphenylamine (NDPA) as a stabilizer. The NO
and NC components degrade slowly under storage con-
ditions, generating heat and releasing nitrogen oxides
(NOX). These oxides react with trace moisture to form
nitrogen acids, which lead to corrosion problems and
heat generation. The NDPA stabilizer reacts with the
NOX to form, successively, three disubstituted and two
trisubstituted daughter products, each of which can fur-
ther react with NOX, although with lower effectiveness.
Each NDPA molecule (including subsequent daughter
reactions) can theoretically combine with up to six NOX
molecules. Because NDPA has only a limited capacity
for absorbing NOX, which diminishes for its daughter
species, the rate of propellant degradation increases as
aging proceeds. This process also generates thermal
energy. If the heat generation rate exceeds the rate of
heat dissipation through the rocket and its packing, the
propellant temperature will increase, thereby increas-
ing degradation rates and, ultimately, the likelihood of
autoignition. In addition to the stabilizer depletion that
takes place normally in propellant under storage condi-
tions, concerns have also been raised about GB leakage
55
into propellant grains and the impact that such leakage
has on propellant stability. Further, when leaking GB
M55 rockets are overpacked, the ability to dissipate
heat from the rocket to the surroundings is reduced.
Overpacking can thus contribute to heating of the pro-
pellant and consequently to higher reaction rates, which
might increase the possibility of autoignition.
The original stabilizer concentration in the M28 pro-
pellant was a nominal 1.8 percent, although, as noted
earlier, a small sample of M55 rockets evaluated in
1985 showed propellant NDPA levels ranging from
about 1.6 to 2.2 percent (NRC, 1994~. MITRE Corpo-
ration studies done prior to 1994 suggested that if the
rockets were stored at 95°F, the primary NDPA would
be depleted to 0.5 percent by 2007 (see, for example,
Perry et al., 1993~. At the 0.5 percent concentration
level, Army guidelines call for increasing surveillance
of the munitions, since autoignition has been observed
to occur when levels are 0.2 percent or lower. How-
ever, this does not consider the additional stabilization
capacity provided by the NDPA-daughter species. A
subsequent Army study included the effect of stabili-
zation by the daughter species and estimated that there
would be a 50 percent likelihood of an autoignition in
the M55 rocket stockpile in the time frame from 2041
to 2120 (U.S. Army, 1994~.
In September 1996, the Army assembled a panel of
outside experts to assess the likelihood of autoignition
of M28 propellant in the M55 stockpile. This group,
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56
EFFECTS OF DEGRADED AGENT AND MUNITIONS ANOMALIES ON CHEMICAL STOCKPILE DISPOSAL OPERATIONS
TABLE A-1 Calculated Best-Estimate Autoignition Probabilities for M55 Rocket Sites
Year Tooele Anniston Umatilla Pine Bluff Blue Grass
2010 7.8x 10-~7 9.0x 10-~6 1.4x 10-~5 1.2x 10-~5 1.7x 10-~5
2015 5.11 x 10-~4 5.2 x 10-~3 8.3 x 10-~3 6.6 X 10-~3 9.1 X 10-~3
2020 5.2 x 10-~2 5.2 x 10-~i 8.2 x 10-~i 4.9 x 10-~i 8.5 x 10-~i
2025 2.0x 10-~° 1.7x 10-9 2.7x 10-9 1.8x 10-9 2.7x 10-9
2030 3.8 x 10-8 3.1 x 10-8 4.8 x 10-8 3.0 x 10-8 4.7 x 10-8
Source: U.S. Army (2002b).
through expert elicitation techniques that considered
detailed mechanisms and uncertainties, developed fur-
ther insights on the effectiveness of NDPA and its
daughter products in capturing NOX generated during
M28 propellant degradation, estimated that NDPA lev-
els would have to be less than 0.2 percent to allow
autoignition, and suggested that autoignition might not
occur immediately after depletion of the stabilizer (U.S.
Army, 1997~. Using all the information developed, es-
timates of the likelihood of autoignition were prepared
for all sites having M55 rockets (Table A-1~.
The study estimated that in 2010, the frequencies of
autoignition at the sites were on the order of 10-~5 per
year or less; by 2020, the frequencies had risen to
around 10-~° per year. Science Applications Interna-
tional Corporation (SAIC) uses a frequency of 10-8 per
year as the criterion for inclusion of a risk contributor
in a quantitative risk analysis (QRA), and autoignition
risks would not be above this threshold until 2025. It is
common practice in QRAs to establish a quantitative
value to screen out scenarios that do not singularly or
collectively contribute significantly to the risk metrics
of interest. According to the available analyses,
autoignition will not significantly contribute to risk for
the anticipated operational life of the sites. Further, the
contribution of autoignition to overall storage risk is
negligible, since the frequencies of ignition by a light-
ning strike at the sites storing M55 rockets vary from
6 x 10-4 to 5 x 10-3 per year.
The expert group also advised the Army to focus
special attention on the effect that leaking GB may have
on the stability of M28 propellant. Experience has
shown that about 0.5 percent of GB M55 rockets have
leaked (in contrast to 0.02 percent of the VX M55 rock-
ets). As leakers are discovered, they are overpacked.
When 99 overpacked GB leakers at TOCDF were
sampled 11 to 32 years after overpacking, some of the
overpacks showed pressure buildup due to continuing
NOX generation. Further, NDPA levels were measured
and found to be lower than would be expected in the
absence of GB contamination. A second expert elicita-
tion conducted in November 1998 evaluated the effect
of agent contamination on the likelihood of
autoignition Since VX rocket leakers are rare, atten-
tion was focused on GB rockets. The expert panel con-
cluded that M28 propellant degradation was enhanced
by the presence of GB, but that additional work was
needed to clarify the characteristics of the interactions.
The Army selected Midwest Research Institute to
perform both experimental studies and analysis of
leaker sample tests to assess the effects of agent con-
tamination on M28 stability (U.S. Army, 2000, 2002a).
Experiments were conducted at elevated temperatures
(50°C and 65.5°C) to increase reaction rates. Only
modest increases in stabilizer depletion rates occurred
with small amounts of GB contamination, and NO and
NC also were depleted along with the stabilizer. How-
ever, at concentrations above about 6 percent GB in the
propellant, the stabilizer depletion rate increased rap-
idly. A detailed model was developed to predict degra-
dation (in terms of remaining effective stabilizer) and
thermal behavior for GB-contaminated propellant
grains. A significant feature of this model was that va-
por transport was a major pathway by which GB agent
could reach the propellant in addition to direct liquid
leakage. This pathway became increasingly important
at higher temperatures (due to higher vapor pressures)
and in sealed containers.
Thermal modeling included detailed heat transfer
paths through the rocket and in the two types of over-
pack systems. Heat generation from internal chemical
iNotes from conference Expert Elicitation on Autoignition of
Agent-Contaminated M28 Rocket Propellant, Science Applications
International Corporation, November 18-20, 1998.
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APPENDIX A
reactions was also modeled. The possibility of leakage
that was not externally detected, i.e., leakers that were
not overpacked, was also considered. Where the analy-
sis showed that the peak heat generation rates exceeded
the rate of heat transfer out of the system, autoignition
was assumed to occur.
The analysis confirmed that the degradation process
over time results in increasing NOX generation, which
in turn further increases degradation as the NOX pres-
sure increases. Overpacking may be accelerating deg-
radation. Controlled venting of overpacks may reduce
degradation rates and heat buildup.
A comprehensive Army report in March 2002 con-
cluded that the probability of an autoignition event from
propellant stabilizer depletion (including consequent
thermal effects) is as follows (U.S. Army, 2002b): for
nonoverpacked (undetected) leaking GB M55 rockets,
Pa = 1.8 x 10-7 to 1.4 x 10-6 per year, and for over-
packed leaking GB M55 rockets, Pai = 1 x 10-5 to
6 X 10-5 per year. Estimates were reported for the years
2000, 2005, 2010, and 2020; the above ranges include
the median values to the year 2020 (U.S. Army,
2002b).
The probabilities of propellant ignition by other
57
means range from 6 X 10-4 to 5 x 10-3 per year. By
comparison then, the degradation of GB-contaminated
propellant leading to autoignition appears to be a fairly
small part of the total storage risk. All of these values are
subject to uncertainties of about an order of magnitude.
REFERENCES
NRC (National Research Council). 1994. Evaluation of the Johnston Atoll
Chemical Agency Disposal System, Operational Verification Testing:
Part II. Washington, D.C.: National Academy Press.
Perry, J., G. Nelson, and D. Tripler. 1993. Assessment of the U.S. Chemical
Weapons Stockpile: Integrity and Risk Analysis, April. McLean, Va.:
MITRE.
U.S. Army. 1994. M55 Rocket Storage Life Evaluation, December. Aber-
deen Proving Ground, Md.: Program Manager for Chemical Demilitari-
zation.
U.S. Army. 1997. Report on the Expert Elicitation on Autoignition of
Nonleaking Rockets, May. Aberdeen Proving Ground, Md.: Program
Manager for Chemical Demilitarization.
U.S. Army. 2000. Evaluation of GB Effects on M28 Propellant. Aberdeen
Proving Ground, Md.: Program Manager for Chemical Demilitariza-
tion.
U.S. Army. 2002a. Preparation and Analysis of M55 Leaker Rocket Propel-
lant Samples Final Test Report. Aberdeen Proving Ground, Md.: Pro-
gram Manager for Chemical Demilitarization.
U.S. Army. 2002b. M55 Rocket Assessment Summary Report. Aberdeen
Proving Ground, Md.: Program Manager for Chemical Demilitarization.
Representative terms from entire chapter:
stabilizer depletion
