Nuclear Attack Fallout Pattern Gif courtesy of Greg Walker*

Nuclear Fallout Downwind Models
by Robert Monaghan

Related Sites:
Nuclear Survival Site
Free FEMA Radiological Monitor Course(PDF)
Free FEMA Radiological Emergency Management Course (PDF)
U.K. Radiological Documents Online
Unconventional Warfare pages
Fallout Protection Booklet (USGovt)
Accidental Nuclear War (New Engl. Jrnl Medicine Article)

Potassium Iodide FAQ (blocks major radionucleide uptake with KI pill -low cost sources)...

Excel Spreadsheet Fallout Model (win 95/ver.7)
Excel Spreadsheet Fallout Model (ver.4)

[*Animated Nuclear War GIF by Gregory Walker [email protected] from Trinity Atomic Web Site and used pursuant to copyright and authorship notice at that site..]

This nuclear fallout downwind model calculates projected fallout dose rates, accumulated doses, fallout arrival time, and effects of various shelters on mortality and morbidity.

No spreadsheet can claim to model the exact effect of a given nuclear weapon explosion in the light of the many variables involved, including wind shear, rain, and weapons composition (fission/fusion ratio..) et. cet. But this model at least provides a generalized view of the downwind effects and the benefits of seeking early and effective shelter.

The summary section provides some useful notes and formulas used in these calculations.

If you remember nothing else, remember the Rule of 7 and 10:
For every 7x increase in time, the radiation dose rate will decline ten-fold (10x), viz:

TimeDose Rate
1 hr1000 rads/hr
7 hr100 rads/hr
49 hr (2 days)10 rads/hr
14 days (2 wk)1 rad/hr
90 days (14 wk)0.1 rad/hr

This 7/10 rule highlights the critical importance of seeking early shelter, as will inspection of the fallout dose rate table. The dose rate in rads/hr drops off greatly in just a few hours and days. You need to be in an effective shelter during that time to avoid a fatal radiation dose!

The model also documents the effectiveness of various materials and structures as a shelter. Effective shelter from fallout is literally dirt cheap! A basement can cut your exposure tenfold, while two feet of earth shielding can reduce your dosage to one percent of an unprotected person. We usually consider a shelter with 3 feet of earth shielding to be ideal. If you are farther from ground zero, you will have more time to locate and prepare an effective shelter. In any case, reviewing our shelter protection factor table will highlight the better shelter choices for you.

What follows below is a sample printout from running the above Excel model and entering the required 3 input parameters shown in Steps #1 thru #3 below:
[comments added in blue]


		Nuclear Fallout Downwind Model								

Enter required 3 parameters in steps#1 to #3:

Step #1:		1000	"weapon size (in kilotons 10kt to 10,000kt)"							
Step #2:		15	"cloud wind speed (miles/hr, at 25,000ft, 15mph nominal, 8 to 45 mph)"							
Step #3:		100	"shelter site distance downwind from target (miles, 15 to 300+ miles)"							



At shelter site (100 miles) R1=309 rads/hr and fallout 
arrival starts circa 6.333 hours after the explosion

		Shelter Site								
R1 (rads @1hr)	309	3000	1000	300	100	30	10	3	1
Miles		100	21.3	40.3	100.7	199.2	358.2	537.3	671.6	895.5
Arrival time (hrs)6.333	1.1	2.4	6.4	12.9	23.5	35.5	44.4	59.4

cloud radius=	5 miles		wind speed factor = 1	(15 mph=1.0 nominal)			



										
For Shelter site, rate drops to 15.6 rads/hr by 12 
hours, to 6.8 rads/hr after 24 hours (1 day)

Shelter Site		Dose Rate Table (Rads/hr)								
309	R1 (Rads@1hr)	3000	1000	300	100	30	10	3	1
100	Time	Miles	21.3	40.3	100.7	199.2	358.2	537.3	671.6	895.5
	(day.)	Time (hrs)								
0.0		1	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
0.0		2	1305.8	0.0	0.0	0.0	0.0	0.0	0.0	0.0
0.0		3	802.7	267.6	0.0	0.0	0.0	0.0	0.0	0.0
0.0		4	568.4	189.5	0.0	0.0	0.0	0.0	0.0	0.0
0.0		5	434.9	145.0	0.0	0.0	0.0	0.0	0.0	0.0
0.0		6	349.4	116.5	0.0	0.0	0.0	0.0	0.0	0.0
15.6	.5day	12	152.1	50.7	15.2	0.0	0.0	0.0	0.0	0.0
9.6		18	93.5	31.2	9.3	3.1	0.0	0.0	0.0	0.0
6.8	1 day	24	66.201	22.067	6.620	2.207	0.662	0.000	0.000	0.000
4.2		36	40.697	13.566	4.070	1.357	0.407	0.136	0.000	0.000
3.0	2 day	48	28.816	9.605	2.882	0.961	0.288	0.096	0.029	0.000
1.3	4 day	96	12.543	4.181	1.254	0.418	0.125	0.042	0.013	0.004
0.7	7 day	168	6.408	2.136	0.641	0.214	0.064	0.021	0.006	0.002
0.3	2 wk	336	2.789	0.930	0.279	0.093	0.028	0.009	0.003	0.001
0.1	1 mo	720	1.118	0.373	0.112	0.037	0.011	0.004	0.001	0.000
0.1	2 mo	1440	0.487	0.162	0.049	0.016	0.005	0.002	0.000	0.000
0.0	6 mo	4320	0.130	0.043	0.013	0.004	0.001	0.000	0.000	0.000
0.0	1 yr	8760	0.056	0.019	0.006	0.002	0.001	0.000	0.000	0.000
0.0	2 yr	17520	0.024	0.008	0.002	0.001	0.000	0.000	0.000	0.000
0.0	50 yr	438000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000
										


At shelter site, from initial fallout arrival (6.333 hrs) to 12 hours (.5 day) 
we accumulated 128 rads, by 24 hours we got 250 rads, and a 90% likely 
fatal dose of 585 rads in 2 weeks without shelter.


Shelter Site		Accumulated Dose Table (Rads)								
309	Rads/ 1 hr	3000	1000	300	100	30	10	3	1
100	Time	Miles	21	40	101	199	358	537	672	895.5
	(day.)	Time (hrs)								
0		1	0	0	0	0	0	0	0	0.0
0		2	1700	0	0	0	0	0	0	0.0
0		3	2717	200	0	0	0	0	0	0.0
0		4	3391	424	0	0	0	0	0	0.0
0		5	3887	590	0	0	0	0	0	0.0
0		6	4276	719	0	0	0	0	0	0.0
128	.5day	12	5633	1172	123	0	0	0	0	0.0
201		18	6344	1409	194	19	0	0	0	0.0
250	1 day	24	6814	1565	241	35	0	0	0	0.0
313		36	7433	1772	303	55	6	0	0	0.0
355	2 day	48	7843	1908	344	69	11	1	0	0.0
447	4 day	96	8738	2207	433	99	20	4	1	0.2
513	7 day	168	9375	2419	497	120	26	7	2	0.4
585	2 wk	336	10072	2651	567	143	33	9	2	0.6
653	1 mo	720	10735	2872	633	165	40	11	3	0.9
706	2 mo	1440	11256	3046	685	183	45	13	4	1.0
777	6 mo	4320	11947	3276	754	206	52	15	4	1.3
816	1 yr	8760	12317	3400	791	218	55	16	5	1.4
848	2 yr	17520	12633	3505	823	229	58	17	5	1.5
952	50 yr	438000	13642	3841	924	262	69	21	6	1.8
										


Fallout Deposition Pattern Table										

The 3000 rad/hr (R1@1 hr) zone extends 21.3 miles downwind, 2.9 mi. width, 
contaminating circa 52 sq. miles at this 3000 rad contour level (or above).
 
R1 (rads@1hr)			3000	1000	300	100	30	10	3	1
length (mi.)			21.3	40.3	100.7	199.2	358.2	537.3	671.6	895.5
Width (mi.)			2.9	6.9	12.4	24.0	36.4	54.5	69.6	90.9
Grnd zero width (miles)		1.4	3.1	5.5	7.1	9.0	11.5	15.1	25.5
contam. area est. (sq.mi)	52	238	1055	3958	10686	23915	38221	66965
										

										
		Lookup Table:				Calculation Tables				
		Cloud Radius				Selected Distance Unit Rate R1				
		kilotons	miles			4.467	259.26	309	Rads	
		10		1			0.95	3000	2352.9		
		100		2			1.8	1000	259.3		
		1000		5			4.5	300	45.5		
		2000		7			8.9	100	9.9		
		5000		15			16	30	2.5		
		10000		21			24	10	1.2
		100000		30			30	3	0.2
							40	1	


Radiation Effects Table	     Vomiting	Therapy		Deaths	Hospitalization		
		<100 rems	none	unneeded	none	"subclinical, mainly blood changes"		
		100-200 rems	infreq	effective	none	hospitalization generally not required		
		200 rems	common	effective	low	hospitalization recommended		
		300 rems	100%	effective	low	hospitalization recommended		
		400 rems	100%	guarded		moder.	hospitalization required		
		500 rems	100%	guarded		high	hospitalization required		
		600 rems	100%	palliative	90%	nearly hopeless		
		1000 rems	100%	palliative	100%	hopeless		


This table highlights the impact of an accumulated dose, and indirectly why it is  
important to seek effective shelter to keep accumulated dosage below 100-200 rems
For our purposes, accumulated (gamma) rads have similar effect as if rems dosage.
 


								
Dose Effects Table								
Dose in rems	Effect						
0		okay				
200		homestay				
300		sick				
500		very sick				
600		most die				
1000		all die				



Shielding Effects Table
	protection factor=	0.1	0.01	0.001	Thickness in:
		steel		3.3	6.6	9.9	inches
		concrete	11	22	33	inches
		earth		16	32	48	inches
		water		24	48	72	inches
		wood		38	76	114	inches


This table highlights how a yard of concrete or 4 feet of loose earth are
enough to reduce radiation exposure to only one-thousandth of unprotected dose. 
 


Shielding Factor Table						
			protection factor			Comments:
Shelter type:			high range	low range			
3 feet underground		0.002		0.004		Excellent	
frame house			0.8		1		bad	
basement			0.1		0.6		poor	
apartment bldg (multistory)							
		upper floors	0.8		0.9		bad	
		lower floors	0.3		0.6		poor	
concrete blockhouse shelter							
		9 inch walls	0.1		0.2		fair	
		12 inch walls	0.05		0.1		good	
		24 inch walls	0.007		0.02		very good	
shelter (partly above grade)							
 - with 2 ft earth cover	0.03		0.07		good	
 - with 3 ft earth cover	0.007		0.02		very good	


An underground shelter or thick concrete walls provide the best shelter listed.

 
							
Shelter Effectiveness table	shelter	distance=	100	miles
				2 wk accum. dose=	585	rads	
				      hi range              low range
Shelter type:		 		rads=  effect:   	rads=  effect:
3 feet underground			1	okay		2	okay
frame house				468	very sick	585	very sick
basement				58	okay		351	sick
apartment bldg (multistory)								
		upper floors		468	very sick	526	very sick
		lower floors		175	sick		351	sick
concrete blockhouse shelter								
		9 inch walls		58	okay		117	okay
		12 inch walls		29	okay		58	okay
		24 inch walls		4	okay		12	okay
shelter (partly above grade)								
 - with 2 ft earth cover		18	okay		41	okay
 - with 3 ft earth cover		4	okay		12	okay


At the 100 mile shelter site, an unprotected person in open gets 585 rads 
in 2 weeks, a 90% likely fatal dose. A basement, concrete wall shelter, or 
underground shelter provides enough protection to reduce risks greatly.


								
Notes:		
1		"contact surface burst assumed, t^ -1.2 decay characteristic (-0.9 to -2.0 range)"
2		"ignores surface geometry, wind shear effects, local rain, hot spots/cold spots"
3		"beyond 200 days, local weathering effects should reduce actual dose rate"
4		model assumes nearly instantaneous deposition at fallout arrival time
		"Actual deposition may take hours, largest error at closest distance to device"
5		ground zero width shows how upwind sites get fallout (mushroom cloud width etc.)
6		"rule of 7/10s - for every 7 fold increase in time, you get a 10 fold drop in radiation rate"
		"example: 100 rads/hr at 7 hr, 10 rads/hr at 49 hrs (2 days), 1 rad/hr at 14 days"
7		Shielding table extended to show effect of given shelter on reducing accumulated dose
		Accumulated dose at 2 weeks for selected shelter site used (distance shown in miles)
8		"Most important - nuclear weapons effects are highly variable, depending on wind etc."
		Above table can't reflect exact shelter location effects due to many variables involved
9		"Beware: Error correction and catching of wrong, negative, out-of-range values is minimal"
10		"protection factor of shielding, 0.1 factor means only 1/10th doserate, 0.01 is 1/100th"
11		local weapons effects ignored - model is aimed at downwind sites outside direct effects
12		"cloud altitude wind speed ranges 8- 45mph, avg 15mph - this is wind speed at cloud ht."
		"Surface wind speeds are not relevant here, clouds moved by winds at 25-45,000+ feet"
13		linear interpolation of radiation rate between isocontours is used rather than log interp.
14		Lifetime recommended worker radiation accumulated dose is 5 rems
15		"Rads are units of external (mostly gamma) radiation here, Rems are biological equiv."
		We are ignoring injested or inhaled radiation particles and dosages in above analysis.
16		Contaminated area est. as 80% of rectangular area based on length and width formula
		Area in square miles contaminated at or above the isocontour level (3000 rads.)



Summary:		
		This model is based on determining a one hour equiv. radiation rate R1@ 1 hr
		Rt radiation rate at time t is approx = R1 x t^ -1.2  (9.147.1 in No.50-3 reference).
		"Using this equation, knowing estimated R1, we can derive Rt dose rate at later time t."

		The accumulated dose is approx 5 x R1 x ((ta ^ -0.2) - (tb ^ -0.2)) (9.150.1 in No. 50-3)
		"Again, using R1 and later time tb, from initial time ta we can get accumulated doses."

		"Ta here is calculated as time of initial fallout arrival, based on radius of mushroom cloud"
		"and wind speed, e.g., 80 miles away, given 5 mile cloud radius, and 15 mph wind (avg),"
		we calculate  Ta= (80miles - 5 mile cloud radius)/15 mph = 75 mi/15 mph = 5 hours.

		R1 unit time @1 hr reference dose rate is calculated based on weapons yield and
		"formulas provided in table 9.93 of No.50-3 for selected iso-contours - 3000 rads, etc."

		Downwind distance is a constant times the weapons size (in kilotons) raised to 0.45;
		example: 3000 rad  = 0.95 x (Wkt ^0.45) = 0.95 x (1000^0.45)=21.3 miles
		example: 1000 rad  = 1.8 x (Wkt ^0.45) = 1.8 x (1000 ^ 0.45) =40.3 miles etc.

		A similar set of constants and formulae are used to derive width and ground zero width.
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		A lookup table is used to select cloud radius based on weapons size from table above.

		Calculation of R1@1hr for shelter at specified distance is based on linear table
		"interpolation between standard isocontours (3000 rads, 1000rads..) in calculation area."
		"Actual distribution likely higher nearer ground zero, below predicted farther away."

		No.50-3 is the Effects of Nuclear Weapons March 1977 version from Dept. of Army

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Page Accesses since May 22, 1998