Thursday, December 29, 2016

What nukes does Russia want to test?

As you may have heard, President Putin and President Elect Donald Trump have indicated a desire to build stronger nuclear arsenals [1].

I think most of us have some guesses as to what that means at the US end - but what does it really mean at the Russian end?

As you might expect, it is very difficult to find actual information about Russian nuclear weapons design. The usual (American and British) sources are reluctant to talk about what they know about Russian designs and issues as this information is often secretly sourced. Only very skimpy descriptions of Soviet nuclear designs are known in the public domain. Wikipedia maintains a pretty decent archive of these things [2] but as you can see there is very little meat on the bone there.

We know that the basic soviet fission design centered around ideas gleaned from American sources via a formidable spy network. However as the spy network was disrupted and the American knowledge of higher yield fusion based devices was shaky at best when the network was still active - the Soviets had to come up with their own ideas for how to use fusion reactions in nuclear explosives. For some detail see LANL's wonderful history collection [3]

As indicated in the Wiki article - there is a tiny bit of insight into the workings of Soviet physics packages. We know that in 1948 Dr. Andrei Sakharov's team expanded existing notions about fusion boosted fission devices (capable of ~ 10kT) and developed a "layered cake/sloika" design. The bulk of the yield in this device came from fission reactions and the fusion yield was just to enable boosting the fission yield of the device (capable of ~100kT). We also know that by 1955, Dr. Sakharov's team had worked out how to build two stage devices and use the X-ray emission from a fission primary to compress fusion fuel (capable of ~ 1MT). The Sakharov team also pushed aggressively to see how far they could go with this technique and built a three stage device with a fission primary and two fusion secondaries. The X-rays from the first fission primary were used to compress the first fusion secondary and the explosion of the first fusion stage heated and compressed a second fusion stage. The result was a 50 MT design. A plan to add a boosted fission element to the design was abandoned after it was felt the yield might exceed 100 MT. In this fashion the Soviet physicists were able to create a design ladder that straddled the 1kT - 100 MT range.

These experiments in nuclear explosive design were sufficient to demonstrate Soviet mastery of the core ideas of nuclear weapons design (i.e. knowledge of the precision needed to control stacked sets of fission and fusion reactions) - but it too completely different tests to prove that it was possible for the Soviets to make practical nuclear weapons.

A practical nuclear weapon is one which can be used to deter an adversary from launching a nuclear attack.

A practical nuclear weapon is reliable - as in when you ask it to explode - it always explodes.

A practical nuclear weapon is safe - as in it doesn't go off when you don't want it to.

A practical nuclear weapon is economical - as in when you have it sitting in storage - you don't have to keep pouring ungodly amounts of money into refurbishing it.

The Soviets spent a great deal of time and effort in pursuing practical aspects of nuclear weapons design. And the Russian inherited all this when the USSR fell.

Things were pretty messed up for a while in the FSUs but over the last two decades Russia has consolidated its hold on *all* nuclear weapons mission critical resources. Perhaps the most revealing comment in  [3] is in the conclusion slides -

"The Russian nuclear weapons complex has downsized while modernizing within a smaller and more efficient footprint." 

This speaks to the Russian sensibility of maintaining their nuclear deterrent at economically acceptable costs. This is very reassuring to me as a human being - because I really do not believe the caricature of the mad vodka slugging Russian general who pushing the button while totally wasted. The Russians are fundamentally speaking a cultured and rational people - but their sense of culture and rationality is very different from what we might easily recognize.

I think all this together tells us what Russia wants to do on the nuclear front.

I feel that if the Russians want to test a new physics package - it will lighter, more survivable and more easily serviceable.

Leaving aside fantasies about a Gen-4 direct fusion package, one can visualize a small two stage or a FBF design as being more suitable for modification to such narrowly defined aims.

The first part of that is probably the least difficult given how much the Russians have inherited in package design from the Soviets. The Russians are also very good in the chemistry side of things, so it would not be too hard for them explore different compact explosive lens designs. The Russians have also made a lot of progress on the electronics side, so you can imagine that they are able to produce lighter weight rad-hard electronic controls quite reliably.

The second and third parts of that goal are somewhat more difficult as the design window narrows quite rapidly and trade-offs come into play.

If you want a physics package that can handle the shocks that accompany a sudden maneuver at Mach 27 (as you might want in a MARV as opposed to plain old MIRV)  you need to bulk up the warhead to handle things like the thermal shock and vibration. If you put a nice thick shell on the package and rigidly anchor everything inside it - you get a package that is very difficult to service and not very light.

There have been some advances in material science recently that permit you to push ahead with certain compromises but the utility of these in the demanding environment of a mass produced physics package is unknown at this time.

So that is where things may lead us in the near future.

If the Russians want to test new warheads - the best place to go would be to lower yield but better guidance. You can always use some Laser, RF signals or GPS to guide your weapons platform, but this is not secure and we all know that "Real Men only use INS". Most INS require a bullet-proof idea of what the errors in your gradiometer and gyroscope are. So any improvement on this side requires more work in gradiometry and gyroscopy. This in turn requires better gravitational mapping. Some of that work is going on [4,5] , one will need to see how such knowledge is transferred to the Russian weapons design side.

For external observers of the Russian special weapons program, the challenge will be to maintain a state of highly informed opinion. This is challenging at the best of times - and this is why an Arms Race as postulated by several people in the Non-Proliferation community might develop.

The Arms Race of the 50s developed because of an imperfect state of information on the size of the Soviet and American nuclear arsenals. When Edwin Land and others were able to bring the CIA's scientific intelligence operations to a level of maturity - the IMINT was able to reduce ambiguity and the Arms Race gave way to strategic arms limitation talks.

In our age - independent OSINT has taken the place of pride once occupied solely by the all seeing eye in space. This is where the bulk of the work will need to be done in the coming years.

Perhaps the best way to cope with a potentially dangerous situation would be for the Russians to educate the world at large about their plans. 


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