Code Manual for CONTAIN 2.0 - Federation of American Scientists

code project. In addition, the committee found that CONTAIN could be applied to both selected
design basis accidents and severe accidents for both current reactors and advanced light water ~
reactors. In the peer review committee’s final report, a number of recommendations were made for
improvements to the code and its documentation. ~oy95] The relatively small number of high
priority recommendations have since been addressed either in the CONTAIN 2.0 code or this code
manual, or resolved through separate communications with the USNRC. 1
1.1 The Need for Nuclear Reactor Containment Analvsi~
Since the inception of nuclear power as a commercial energy source, safety has been recognized as
a prime consideration in the design, construction, operation, maintenance, and decommissioning of
nuclear power plants. Nuclear reactor systems are sufficiently complex that dismissing the
possibility of an accident followed by the release of radioactivity to the environment would be
imprudent. Such a release would require the failure of multiple safety systems and the breach of
three physical barriers to the release of radioactivity: fuel cladding, reactor cooling system, and the
containment. Probabilistic risk assessments have shown that the required multiple system failures
are improbable but not negligible. The accident at Three Mile Island Unit 2 (TMI-2) demonstrated
that significant core melting could occur in a commercial power reactor and that at least the frost of
the three physical barriers to radioactivity release can be breached. The reactivity-driven explosion
at Chernobyl showed that all three barriers could be breached and the public exposed to radioactivity.
Although US power reactors are not vulnerable to the type of energetic event that occurred at
Chernobyl, that accident reinforced the perception that highly off-nomml system and/or operator
behavior, though extremely improbable, could in fact occur. Consequently, the USNRC’S reactor
safety research program has historically been focused on understanding system behavior and <
underlying phenomena associated with the low-probability, high-consequence class of accidents
referred to as severe accidents. The CONTAIN code was developed as a tool to assist the analyst
in understanding the complex phenomena and interactions that occur in the containment building
of a nuclear reactor during such accidents.
The need for such a tool was born out of the recognition that the extreme nature of severe accident
sequences presents unique dh%culties for the reactor safety analyst. First, unlike many other safety
engineering disciplines, very few relevant historical records of vessel failure accidents exist. Second,
the reactor and its containment are extremely complex systems, and their designs vary substantially
from one plant to another. Third, the fuel debris and the materials it contacts are expected to be
subject to such severe conditions that the behavior of even small sub-elements of these complex
systems is not easy to predict. Fourth, even though scaled experiments are vital to improving our
understanding, it is often difficult to extrapolate the experimental data to the physical scale and
system complexity of actual nuclear reactors. All these considerations lead to two conclusions
concerning reactor safety research methodology. First, detailed, system-level, phenomenological
computer models are essential tools for understanding how the reactor and its containment might
1It should be notedthat the revisedsevereaccidentcodestrategyde-emphasizesthe role of CONTAINfor sourceterm
analysis. This revisedstrategywastakeninto accountin the peerreviewmodelrankingsandrecommendations.Nevertheless,by
directcomparisonof the CONTAINaerosoland fissionproductmodelswiththoseof othercodes,the user willfind themin
manycasesto be state-of-the-artand suitablefor manytypesof sourcetermanalyses.
Rev O 1-2 6/30/97