Document text
Machine-read from the scan, so expect some errors. Lines that couldn’t be read are left out.
Page 1
NAVAL OCEAN SYSTEMS CENTER SAN DIEGO, CA 92152 To: Captain Andrew G. Nelson, Commonwealth Bldg., Room 772 1300 Wilson Boulevard, Arlington, VA 22209 Subj: Gamma Dose Estimates for PATAPSCO Crew Encl: (1) Estimation of Dosage Received by Personnel of USS PATAPSCO (AOG-1) following the BRAVO Shotof Operation CASTLE. 1. Enclosure (1) contains the estimates you requested (ref. a). The copy you requested for SAI is also enclosed in its addressed envelope. Your review and approval of the classification of appendix A extracted from the San Bruno Archives would be appreciated before mailing in this manner. We believe appen- $+x A is now definitely unclassified if it indeed ever required security classt cation above official use only. Another letter will follow shortly covering the many items which we have pending. Please call if you have any questions.
Page 2
-Estimation of Dosage Reveived by Personnel of USS PATAPSCO (AOG-1) following the BRAVO shot of Operation CASTLE. response to the Navy Nuclear Test Personnel Review office request following analysis of personnel exposures was undertaken. During the USS PATAPSCO from Eniwetok Atoll to Pearl Harbor Hawaii, the the PATAPSCO encountered an unknown amount of radioactive .fallout that resulted from the Castle-Bravo shot (ref. 1). The captain of the ship had been alerted to the possibility of radioactive material contaminating the ship and so made attempts to measure the radioactivity but because of inadequate equipment only uncertain readings were obtained (ref. 2). When PATAPSCO arrived at Pearl Harbor the ship and personnel were monitored and the ship was found mates of the range of possible external gamma radiation doses received by the crew from this fallout. Approach: Two primary sources of radiation data for dose estimation are used in this analysis: (1) published unit reference (H+l) dose rate contours (ref. 3) and, (2) radiological survey data obtained on arrival at Pearl Harbor. Extrapolations of these data are then utilized to estimate the range of dose variations caused by environmental conditions, exposures situations and times. Valuable information contained in the PATAPSCO captains recollections (ref. 2), the ship's logs (ref. 4) and notes describing'the survey and decontamination 'procedures employed at Pearl Harbor (ref. 4) was used in the preparation of this analysis. The time interval for the dose estimates undertaken,here will ? form fallout arrival at the ship to the time of crew evacuation as reported
Page 3
( in the PATAPSCO deck log In order to estfmate it is first necessary to the cumulative gamma dose (D) over various.time intervals evaluate the dose rate (Rt) as a function of time. This is usually accomplished by defining a set of "unit reference dose rate" fallout deposition contours so that subsequent decay corrected dose rates can according to the equation ‘Actually since the fallout is seldom if ever all down at H+l hour at any location, and since the Rl values must be based on the total cumulative deposition, R, is a fictitious quantity defined and used to simplify computations. In practice Rl is calculated from dose rate measurements made at various locations and times from the relationship. This is approximately valid, assuming 100% retention of bomb debris deposited on an infinite plane surface. All these gamma dose rates are defined for a point three feet above the surface and are proportionally reduced if some .fraction of the radioactivity is removed by wind, rain or other decontamination,
Page 4
or if the surface deviates from an infinite plane. In the following, the process of dose estimation will be broken down in five sections which cover ship movement, fallout dose deposition, radiological survey, dose rate variations and cumulative dose estimates. This'will then be followed by an overall sunrnary. B. Ship and Fallout flovement Ship position data was obtained from the PATAPSCO deck log (ref. 4) and plotted on an HO chart Iy81007with an open water extention to include the track of interest. Figure 1 illustrates the time-based track of the ship together with the track of fallout "hot line." The position and speed of advance of the fallout "hot line" up to BIKAR Atoll were obtained from reference 3 and superimposed on the ships track. Past BIKAR, available BRAVO reports do not agree on the c."hot line" location (ref.5)and data to extend this Tine are not available. For this reason the extremely conservative assumption that the "hot line" coincides with the PATAPSCO track after 011200, was made. Figure 2 shows the advance of the leading edge of the fallout cloud along the "hot line" as extracted from reference 3 along with the calculated and extrapolated 78 knot downwind speed. Table 1 combines the time based locational data for the ship and fallout cloud and shows the intercept distance of 586 nmi down range at ti+ 32.5 hours. These values were obtained from the large scale plot as illustrated in figure 1.
Page 6
TABLE 1 Time Based Locational Data For Ship and Leading Edge of Fallout Ship Position incremental A S d Distance Distance Speed of .
Page 7
The calculation of the shipboard (H+l) r/hr dose rate contour distances along the hot line was accomplished after the location of the fallout intercept had been determined. Table 2 shows the H+l hr (unit reference) dose rate values obtained from ref. 3 together with the calculated values of the H+l dose rates extrapolated down range to the ship. Various 'functionalforms were investigated for this extrapolation. A power law curve of the form paxb provided the closest fit over the reference 3 distance and was used to obtain the extrapolated unit reference value of 12.2 r/hr for the ship at its intercept location. Making the conservative assumption that fallout with an 18 knot downwind'speed could persist for roughly 10 hours at a specific geographic location (ref. 6), deposition on the deck of PATAPSCO (12 knots) moving relative to fallout cloud at only 6 knots might persist for 30 hours or 360 additional nautical miles 4I wnwind. For the purposes of the maximum unit-reference dose will be used with the all-down the conservative dose calculations presented here rate of 12.2 r/brat the initial intercept point time of 021520 or H + 32.5 hours. Table 2 Unit Reference (H+l) Dose Rate Contour Curve Fitting and Extrapolation Dose Rate Reference 3
Page 8
D. Pearl Harbor Radiological Survey After arrival at Pearl Harbor Naval Shipyard at 071550 (H+l75), the Atomic Defense Officer discovered radiation levels as high as 40 mr/hr (ref. 7) and the crew was "evacuated to the receiving station" (100200) except for special details. Subsequent radiological survey (appendix A) revealed the foilowing general gamma backgrounds adopted here to a reference time of 120900 March, (K I:'hileappendix A lists specific items scattered throughout the ship which ex- ( ,ieeded these levels, the average dose rates were probably below these levels. Converting these dose rates to H+l unit reference rates (Rl) gives; E. Variation of Dose Rate With lime Two primary factors can change the dose rate at a given location. First, radioactive decay which will be estimated by use of equation (2) for each of the times of interest (see Table 5 below) and; second accumulation or depletion of activity by physical processes. Of specific interest here is the effect of the weather in removing some of the fallout which is assumed to have been deposited on PATAPSCO at H+32.5 hours. Table 3 extracted from the PATAPSCO c. deck log summarizes appropriate wind and rain periods during the return trip to Pearl Harbor.
Page 9
Table 3 PATAPSCO Deck Log Weather Summary (Numbers in parenthesis are reference time zones) Fallout Deposition Initiation 031530 (-12) 2 Mar (-12) No rain; winds 11-22 knots. 3 Mar (-12) Slight imtennittant rain (2100-2300); wind lo-12 knots. 3 Mar (+12) No rain; winds less than 20 knots. 4 Mar (+ll)Moderate thunderstorms (2000-2300); winds 12-24 knots. 5 Mar (+ll)Moderate thundershowers (0800~.lOOO);winds 22-35 knots. 6 Mar (+ll)No rain; winds 17-28 knots. Arrive Pearl Harbor Naval Shipyard 071550 (+lO). During these intervals of rain and relatively high winds some portion of the fallout was undoubtly removed, the question is how much? At Operation CASTLE it was observed that moderate rain reduced ship (YAG) deck contamination \, approximately 50% (ref. 8). Applying this 50% reduction to the PATAPSCO Rl is in remarkable agreement with the Rl (deck) value of 4,5 r/hr estimated from the radiological survey when the ship reached Pearl Harbor. While this decrease continues as long as the rain persists, the first rain removes the most actively and an effective time of decrease in the estimated weather deck dose rate from the 12.2 to 6.1 r/hr at 042100 (+ll) or H + 109 will be assumed here. Another important factor is the departure of the ships decks from idealized deposition surfaces. For a person moving about,these departures inject variable shielding which can considerably reduce the effective dose rate. It is also possible for a person to consistently occupy positions which increase his effective dose rate. For these reasons, combined with the difficulty and uncertainty in computation, no attempt will be made here to compensate for complex
Page 10
Within the interior of the ship, dose rates can also vary apprqiably. This gananafield can result from deck deposItIon altered by complex shielding and variable distances to multtple surfaces,or more directly Prom activity tracked inside the ship. While the deck deposttion component i-sreduced by weather, the interior contribution Is constantly jncreasing as activity is tracked in and spread throughout the ship. Shielding factors from exterior contamination are reported to vary between 0.0001 and 0.2 for various ships and locations within the ship (ref. 9). A 10% transmittance giving an RI = 1.2 r/hr (interior) is a high (ma% dose) estimate which will be assumed for the interior gamma field between fallout deposition (H + 32.5) and partial fallout reduction at storm arrival (H + 109). After this time the RI = 0.9 rjhr value determined from the Peari Harbor (survey) measurement is thought the best characterization of the ii..driorgamma field for the remainder of the time interval. Figure 3 show the estimated variation of dose rate with time as it relates to input data for four hypothetical cases for which total accumulated doses will be'presented in the next section. These include an estimated "maximum" case which assumes no reductions in the gamma field after fallout deposition; two more realistic cases where minimal reductions have been applied to both the deck (A) and ship interior (B) gamma fields; and a "best estimate" for a hypothetical individual who spends eight hours per day outside on the weather decks (C). This later case (C) is a simple combination of cases (A) and (8). The unit references dose rates (RI) applicable to these cases are summarized in Table 4. Table 4 Unit Reference Dose Rates r/hr for Dose Calculations Weather Dose Ship Interior Hypothetical Crewman
Page 11
Estimated from Extrapolated Fallout Deposition ‘ Fallout Storm Pearl Crew Radiological Arriy+l Arrival Harbor Evacuation Survey Hours After Shot at Critical Events Figure,3 Conceptual Dose Rate (mr/hrj vs Time for Cumulative Dose Calcblations (not drawn to scale).
Page 12
F. Gamma Dose Estimates The gamma dose for each interval of the cases specified above is calculated by multiplication of the approximate unit reference dose rate (R,)fram Table 4 by the time integral (I) from Table 5. Table 5 Time Parameters of Interest for Dose Calculation The total cumulative dose for each case is then computed by summing of appropriate intervals covering the desired time period, For the time interval of interest here, from fallout depostion on PATAPSCO on 2 March (H+32.5) to crew evacuatjon at Pearl Harbor,on 10 March (H+233), the total cumulative doses estimated for each case are shown below.
Page 13
Estimated from extrapolation fallout deposition for a crewman on deck 24 hours a day with 100% retention of all radioactivity on deck. Deposition to Storm Storm to Pearl Harbor Arr. Pearl Harbor Arr. to Crew 0 Estimated from extrapolated fallout deposition for a crewman on deck 24 hours a day with 50% removal of activity by weather enDeposition to Storm Storm to Pearl Harbor Arr. Pearl Harbor Arr. to Crew 0 Estimated for external fallout plus internal ship contamination for a crewman spending 24 hours per day inside the ship structure. Deposition to Storm Storm to Pearl Harbor Arr. Pearl Harbor Arr. to Crew 0 Estimated for a crewman who spends 8 hours per day on deck and 16 hours per day inside using conservative storm decontamination Oeposition to Storm Storm to Pearl Harbor Arr. Pearl Harbor Arr. to Crew
Page 14
The range of dose estimates presetitedhere for a "typical PATAPSCO crewman" vary between 0.9 to 10r depending on the exposure conditions. Our conservative "best estimate:'that we believe is on the high side because of the nature of the worst case analysis used here, is 3.3r. This dose was accumulated between the estimated start of fallout deposition aboard ship at 011520 March and crew evacuation at 100200 March at Pearl Harbor. So many assumptions were made in this analysis that confidence limits cannot be assigned to the values presented. All that can be done is to list those assumptions most crucial to the above estimates. 1. The fallout "hot line" coincides with the shids track during the period of {..ides an adequate description of fallout deposition. 3. The dose rate is adequately described by the t-lo2 law. 4. The choice of an instantaneous storm decontamination time ly change the primary dose contribution. In order for trc fallout deposition to grossly exceed the estimated value something like a "catastrophic" high altitude rainout would have to have occurred. However "catastrophies" of this type are not predictable. Action Susqested. It will be noticed that the subject of internal personnel doses was not discussed. It will also be noticed in the Appendix, that reference is made to a report covering the events leading to the ships contamination and possible dose to the crew. This report was to be prepared by then LCDR Royce K. -Skew. Captain Skow (USN Ret*) was contacted and asked if the report was ever
Page 15
( tten. He did not recqll its preparation,,but thought that since there must have been pressure to prepare it, Tt was probably done. A document search of the DDC reports authored by Skow was made, but no such title was found. This report, if published, would probably be worth the effort to find it, since a71 the details of the dose rate measurements and the contamination status of the crew would have been available to Skow. More importantly, this document might also describe any internal dose measurements that were made on the crew and thus provide the only source of information for determining internal radiation doses.
Page 16
encl to ref 1 above. Figure 149. Operation CASTLE - Shot 1 - BRAVO. Off-site dose rate contours in r/hr at H+f hour (NRDL), encl to ref 1 above. See also Fallout SympoPages extracted form the Deck Log of USS PATAPSCO dtd 27 Feb 54 to 23 April, encl to ref 1 above. R. R. Rapp, USAF Proj. Rand Research Memorandum RM1855 16 Jan 1957, SECRET RD. The Effects of Nuclear Weapons, 1977. Memorandum report covering TAD at …