The Quantity-Distance (QD) is determined by a formula using the Net Explosive Weight (NEW ) of the explosive material as seen below that is expressed for distance in 'K' or 'K-factor'. It is about how much risk do you want to accept. Wiki has got a good review of this:
"Quantity-Distance
Quantity-Distance (QD)[4] criteria represents physical limits which cannot be breached without incurring risks, and acceptance of these risks at the appropriate command level. QD applies the “Cardinal Principle” of Explosives Safety which is to expose the least amount of people to the least amount of explosives for the least amount of time. QD experts (Explosives Safety Specialist) determine the risk of exposure by examining blast, fragment, and thermal hazards of a given amount of explosive using the net explosives weight (NEW) and applying accepted protection principles to determine levels of hazard a person is exposed to at a given distance from the explosives. The quantity (amount of NEW) and a given distance from the explosive equals the QD. There are factors that can be used to provide some protection from the effects of the detonation of explosives; however the effects of a blast wave from an explosion cannot be mitigated. Only distance from the blast will provide protection from the blast wave of an explosion.
"Net Explosives Weight (NEW)
The net explosives weight (NEW) is based on explosives compounds that are equal to one pound of trinitrotoluene (TNT). A compound may weigh two pounds but have the blast effects of only one pound of TNT it is then said to have a NEW of one pound. If the compound weights one pound but has the blast effects of two pounds of TNT the NEW is considered as two pounds NEW. NEW is used to calculate QD by means of a formula of the type D (ft) = K•W^1/3, where “D” is the distance in feet, “K” is a factor (also called K-factor) that is dependent upon the risk assumed or permitted, and “W” is the NEW in pounds. When metric units are used, the symbol “Q” denotes Net Explosive Quantity (NEQ) in kilograms. In the formula D (m) = Km•Q^1/3, the distance “D” is expressed in meters. Thus, the units of the K-factor ("K" in the English system) are ft/lb^1/3 and ("Km" in the metric system) m/kg^1/3. The value of “K” in English units is approximately 2.52 times “Km.” For example, if D (m) = 6•Q^1/3, then D (ft) = 15.12•W^1/3. Distance requirements determined by the formula with English units are sometimes expressed by the value of “K,” using the terminology K9, K11, K18, to mean K = 9, K = 11, and K = 18."
And some history on how this came about...,
"Explosives safety originated as a formal program in the United States in the aftermath of World War I when several ammunition storage areas were destroyed in a series of mishaps. The most serious occurred at Lake Denmark Naval Ammunition Storage Depot, New Jersey, in July, 1926 when an electrical storm led to fires that caused explosions and widespread destruction.[citation needed] The severe property damage and 19 fatalities led Congress to empower a board of Army and Naval officers to investigate the Lake Denmark disaster and determine if similar conditions existed at other ammunition depots. The board reported in its findings that this mishap could recur, prompting Congress to establish a permanent board of colonels to develop explosives safety standards and ensure compliance beginning in 1928. This organization evolved into the Department of Defense Explosives Safety Board (DDESB) and is chartered in Title 10 of the US Code. Today, the DDESB authors DOD Manual 6055.9, Ammunition and Explosives Safety Standards. It also evaluates scientific data which may adjust those standards, reviews and approves all explosives site plans for new construction, and conducts worldwide visits to locations containing US title munitions. "
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And the military explosive safety research and development has been adopted for the civilian world as seen in Title 10 of US Code.
"Explosives Safety Specialist must often travel to different storage sites to verify that the military installation is meeting the service explosives safety regulations.
Explosives Safety Specialist often works with other safety professionals. They are required to know OSHA, EPA, NFPA and other consensus standards when looking at safety and if these regulations are stricter than their service regulation they must apply these standards and regulations. They must also know Alcohol, Tobacco, and Firearms (ATF) regulations dealing with A&E and apply those standards if it is required. They must be able to convince people the need for following prescribes explosives safety standards/regulations. They must also work with ammunition cleanup sites insuring that safety laws and regulations as well as industry standards are followed. They should be good at solving problems.
The military is not the only industry to use explosives safety specialist but are by far the largest employer. Mining and construction also use explosives safety specialist to evaluate hazard and risk from explosives and blasting operations. Ammunition and explosives manufactures also use these professionals. Outside the military explosives safety specialist must apply and be knowledgeable of ATF, OSHA, EPA, NFPA, as well as state and local regulations dealing with safety of A&E."
-end snip-
The rest here.
http://en.wikipedia.org/wiki/Explosives_safety#Quantity-Distance
One more comment about the excerpt:
"There are factors that can be used to provide some protection from the effects of the detonation of explosives;"
The cheapest means that the plant should have done is built an earth berm around where they stored their ammonia nitrate, which could have deflected the explosive blast upwards instead of going into the town.
I remember seeing those charts at a military ammo dump. Some of the circles for the safe distance went out 20-30km.
That was with berms but it wasn’t ammonium nitrate. :)
A berm really doesn’t work. The overpressure will still get those within the safety arcs if calculating the plot plan as an ammunition depot.
Same applies if you are driving by a tractor trailer hauling gasoline.