Tuesday, May 1, 2018

Multi Point Initiation

Multi Point Initiation or MPI was in the news recently.

First a few basics on building an atomic bomb.

It is inadvisable to store your lump of fissionable material in a solid sphere.  It tends to make noise when you do so. More typically, the super-critical mass is stored as two hemispheric shells somewhat like grapefruit rinds.  The two, hollow hemispheres are then assembled to create a hollow sphere.

The fissionable material (U235 or Plutonium) will not go into a run-away chain reaction until a high enough a percentage of the released neutrons are capture by neighboring atoms, thereby going unstable and releasing even more neutrons.  Early US efforts shot a slug of fissionable material through the hole of a donut.  These bombs did not have a very high yield considering the amount of material used.

The technical challenge of building a fission bomb is to get the lump to go super-critical as quickly as possible and to keep it super-critical as long as possible.  In a way, fission bombs are self limiting.  The U235 or Plutonium turns to plasma and expands, thereby turning the reaction off.  The slug-and-donut design approached super-critical mass too slowly and turned itself off too quickly.

We don't normally think of solid metal as being compressible but it is.  By surrounding the grapefruit rind with a shell of very uniform, military grade explosive and detonating it at multiple points...like the intersection of seams on a soccer ball, the frugal weapons engineer can make a lump of U235 or Plutonium that is sub-critical under normal conditions to undergo the run-away chain reaction.  The metal is compressed when the rapidly collapsing hemispheres collide with the material collapsing from the other side of the ball.  Compression increases its mass density.  The nuclei are closer together than normal and that makes them more likely to "catch" a stray neutron.

If you know a smidgen about nuclear weapons design (hold my slide-rule, it is only math) then you realize some of the implications.

The first implication is that the weapons engineers are trying to milk as many weapons as possible out of a finite supply of fissionable material.  The sophisticated electronics and testing is simply not worth the effort for just one or two war-heads.

Pulling a number out of my nethermost regions, Iran is probably planning to make 25 or more warheads.  That should make western Europe and the Eastern US very nervous.  The first five would be enough to obliterate Israel.  The remaining 20-to-100 nuclear bombs could be transported in standard cargo containers and detonated anywhere in the world.

The second implication is that the weapon designers are shooting for high yield.  The hydrodynamic, explosive forging of the grapefruit rinds is one way to keep the fissionable mass super-critical for a longer period of time and to get higher megaton yields.

The third implication is that the nuclear devices that use MPI are petite.  It is not hard to explosively forge a metal grapefruit rind.  Bury it in 1000 pounds of TNT and contour the thickness of the rind so it is thicker where the detonation front first encounters the shell.  But burying the grapefruit rind in a 1000 pound canister of TNT makes it harder to transport and limits the range if used as a warhead on a missile.

To repeat myself:
  • Iran intends to mass produce nuclear bombs.
  • Iran intends to make enough nuclear bombs to threaten the US, Europe and parts of Asia in addition to vaporizing Israel.
  • Iran intends to make nuclear devices that are suitable for missile delivery.
MPI is a very big deal.


  1. You should read

    "The Making of the Atomic Bomb" by Richard Rhodes. (if you haven't already)

    Some of the xray photographs of the MPI compression are pretty cool. From a grapefruit to a ping pong ball.

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