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OCR for page 87
G
Use of Atmospheric Models in Response to the
Chernobyl Disaster
Summary of a presentation by James Ellis, LLNLINARACi
The Chernobyl nuclear accident occurred on April 26, 1986. The size of this release was
unprecedented, releasing millions of curies of radioactive material including iodine-131, cesium-
137, and strontium-90 all of which are potentially harmful to human health.
The initial release occurred on a Friday night. By Sunday afternoon, contamination
readings were picked up on workers at a nuclear power plant in Sweden. Within a few hours it
had been determined that the contamination source was from a nuclear power plant to the south.
By Monday, the Russians admitted that a major accident had occurred, and on the same day,
LLNL/NARAC was notified by the Department of Energy to begin predicting the consequences.
NARAC worked round-the-clock for two weeks, providing assistance in modeling the transport
and deposition of the radioactive cloud.
NARAC utilized three different model codes for these analyses 2BPUFF, PATRIC, and
MATHEW/ADPIC. The 2BPUFF model is a two-dimensional long-range transport and diffusion
model used mostly for estimating the Chernobyl accident release amounts of radioactivity. The
PATRIC model is a three-dimensional puff and diffusion model that had been specifically
designed to treat continental and hemispheric scales. The MATHEW/ADPIC is a combined mass-
consistent wind flow model and a particle-in-cell dispersion model that was to calculate
consequences over 200 km or less. For the previous 12-year period, real-time radiological dose
assessments had been done at scales up to 200 km. For the Chernobyl event, MATHEW/ADPIC
had to be rapidly modified to expand its capability to approximately a 2000-km domain. The Air
Force Global Weather Center provided meteorological data for this work.
Based on samples collected around Europe, the time and strength of the release were
estimated, and approximately 40 percent of the total radioactive material was estimated to have
been released in the initial blast (the rest came from the ensuing fire over the next five to six days).
There was a considerable amount of rain in the area, which led to "hotshots" of wet deposition
across Europe. Scientists were not able to model these washout processes, because they did not
have the needed meteorological precipitation data or fine-scale forecast model precipitation pro-
~ Ellis emphasized that it is difficult to reconstruct the exact history of LLNL involvement in this event, since
those who participated directly in this response effort are no longer at LLNL.
87
OCR for page 88
88
APPENDIX G
ducts. With today's high-resolution mesoscale forecasts, they would have been able to do a much
better job of modeling the deposition patterns.
NARAC models were used to estimate air concentration and ground deposition of key
radioactive elements and the corresponding exposures and potential health effects. It was esti-
mated that iodine deposition in the United States was insignificant but that dangerous levels of
ground deposition of key radioactive elements had occurred throughout Europe. The greatest risk
was not from direct exposure but from exposure through the food chain. In particular, radioactive
material was deposited on farmlands and on grass eaten by cattle, forcing many countries to
destroy exposed milk and crops.
Aircraft measurements of radioactive material at 17,000-30,000-ft altitude above Europe,
the Japan Sea, and the West Coast of the United States indicated that radioactivity from the reactor
accident had gotten higher in the atmosphere than initially thought to have been possible. Based
on NARAC's knowledge of the thermal energy of the blast, it did not understand how the material
could have risen so high; after examining the prevailing weather patterns, NARAC surmised that
convective activity in the area had driven the radioactive material up to these high altitudes in the
atmosphere.
The upper-level flow reached the United States (from across the Pacific) by Day 10. LLNL
modeling of this event matched well with the readings from aircraft measurements. Even with the
limitations of the meteorological data and model prediction capability available in the 1986 time
frame, overall agreement between ground-based and aircraft measurements and model estimates
was within a factor of 2 or 3.
Since that event, there has been a lot of activity in Europe to improve the models used in response
to nuclear accidents, culminating in development of the Realtime Online Decision Support System
(RODOS) for nuclear emergency management (www.rodos.fzk.de/RodosHomePage). The dispersion
models being used in this system and those being used by other national organizations within Europe
have been improved from those originally used in the Chernobyl response. One of the strengths of
RODOS has been to link these dispersion models to better atmospheric prediction models and to various
dose pathway models, including sophisticated watershed models, with the objective of providing tools to
the decision-maker for making well-informed decisions.
Representative terms from entire chapter:
ground deposition
