Chapter 4#—Monitoring Accidental Radianon Releases * 63 PREDICTING FALLOUT PATTERNS Figure 4-3—Projected Faliout Dispersion Pattern Tonopah e 170 mR @ Pioche 500 mR Caliente ®@ Alamo ‘N N \ Glendale \ 0 e 50‘ \ Scale in miles venting occurred 10 to 15 secondsafter detonation.® Las as Vegas V ¢ \ ‘ The predicted fallout pattern from an underground test depends on manyvariablesrelated to the type of nuclear device, the device's material composition. type of venting, weather conditions, etc. With so many variables and so little experience with actual ventings, fallout predictions can only be considered approximations. The accuracy of this approximation, however,is critical to the decision of whether a test can be safely conducted. Fallout predictions are made by the Weather Service Nuclear Support Office using up-to-date detailed weather forecasts combined with a model for a “prompt massive venting.’” The model uses scaling technique based on the actual venting of an underground test that occurred on March 13, 1964. The test. named **Pike,”” was a low-yield (less than 20 kilotons) explosion detonated in a vertical shaft. A massive Ne » Key: H+ number= time of detonation plus elapsed hours: mR= milliREM Predicted fallout pattern tor the case of an accidental venting. SOURCE Modified from: “Public Safety for Nuciear Weapons Tests,” U.S. Environmental Protection Agency, January 1984. pattern to measure exposure and perform remedial actions should they be necessary. The preferred weather conditions for a test are a clear sky for tracking, southerly winds (winds from the south), no thunderstorms or precipitation that would inhibit evacuation, and stable weather patterns. During the test preparations, the Weather Service Nuelear Support Office provides the Test Controller with predicted weather conditions. This information is used by the WeatherService to derive the estimated fallout pattern should an accidental release occur. About one-third ofall nuclear tests are delayed for weather considerations; the maximum delay in recent years reached 16 days. The venting continued for 69 seconds. at which time the overburden rock collapsed forming a surface subsidence crater and blocking further venting. The vented radioactive debris, consisting of gaseous and particulate material, rose rapidly to about 3.000 feet abovethe surface. The Pike scaling model has been used to calculate estimates of fallout patterns for the past 20 years because: 1) the large amountof data collected from the Pike venting allowed the development of a scaling model, and 2) Pike is considered to be the worst venting in terms of potential exposure to the public.’ The Pike model, however, is based ona very small release of radioactive material compared to what would be expected from an aboveground test of the same size.’ The percentage of radioactive material released from the Baneberry venung (7 percent from table 3-1). for example, is many times greater than the percentage of material released from the Pike test.? It would therefore appear that Baneberty provides a more conservative model than Pike. This. however, is not the case because Baneberry was not 5Pike was conducted in alluvium in Area 3 of the test site. The release was attributed to a fracture that propagated to the surface. Other factors contributing to the release were an inadequate depth of burial and an unadequate closure of the line-of-sight pipe. 7*1985 Analyses and Evaluations of the Radiological and Meteorological Data from the Pike Event.’ National Oceanic and Aumospheric Administration, Weather Service Nuclear Support Office, Las Vegas, NV, December, 1986, NVO-308. §The exact amount of material released from the 1964 Pike test remannsclassified. 9See table 3-1 for a comparison ofvariousreicases.