Friday, September 25, 2009

The ABC of N-subs

A LARGE number of "experts" have written numerous articles after India's strategic nuclear submarine, Arihant, was launched on July 26, 2009 by Mrs Gursharan Kaur, wife of Prime Minister Manmohan Singh. Also, nuclear scientists proudly unveiled the "half boat", or the shore-based miniaturised nuclear reactor, at Kalpakkam on July 29, 2009.

Earlier, in February 2008, the Defence Research and Development Organisation made an unprecedented announcement in the press about the testing of an SLBM (submarine launched ballastic missile).
In my opinion, the unnecessary publicity given to these three events should have been avoided.
The nation -- in keeping with worldwide practice -- should have announced only the commissioning, post-successful sea and weapon trials of the Arihant, which is the first in its class.

Unfortunately, in a hurry to get individual achievements lauded, a lot of premature and needless publicity was given. Hopefully, this project will fructify as planned, without the recent "fizzle" controversy surrounding our 1998 thermonuclear test.

This article tries to explain, in simple terms, the unclassified, but misunderstood mysteries of a nuclear submarine.

Let us begin with the miniaturised reactor (known as PWR or pressurised water reactor, similar to the light water reactors, being imported for civilian power plants after the Indo-US nuclear deal of 2008).
Common sense dictates that a submarine crew living, eating and working within 20 to 50 metres of the submarine reactor needs to be "safe" from radioactive materials, that is, alpha and beta particles and gamma radiation. Hence, the submarine reactor must be safe, simple, rugged and capable of operating in all situations that a nuclear submarine encounters on surface and underwater.

Secondly, it must be silent. This stealth is achieved by ensuring that the number of pumps, specially the PHT pumps (primary heat transfer pumps), that re-circulate the "closed" first loop radioactive water between the reactor core and the steam boilers, are reduced to a minimum or, in some cases, not used at low submarine speeds by having a "convection system" which does the work of the PHT pumps.
Till the '70s, nuclear submarines used two reactors, each with steam boilers and PHT pumps for redundancy though these added to the radiated noise.

But once reactor reliability was established, most advanced nations now use a single reactor for more silent operations. Theoretically, a single reliable PWR reactor, with a single steam boiler and a single PHT pump, would produce the least noise.

The second issue mentioned by "experts" is the reactor life and how the Americans have been operating their ninth generation submarine reactors with 25-year life spans, as compared to the rest of the world. Our "experts" should remember that the world's first nuclear submarine, the American USS Nautilius, had a reactor core life of only two years. There is no doubt that the Americans are 50 years ahead of India in this regard, but we must understand that a safe submarine reactor requires a combination of metallurgy and enriched Uranium-235 (U-235) to achieve longer reactor life. And this data is a closely-guarded secret for obvious reasons.

Some articles in the Indian media, about using 80 to 93 per cent enriched U-235 in a submarine reactor core, are incorrect.

This use of weapon grade U-235, may, under some emergent situations, "convert" a reactor into a fission bomb by making the transition from "critical" to "super critical" stage during operations.
We should be happy that our scientists have made a submarine reactor (irrespective of whether it is a few generations behind the Americans and Russians). The important thing here is to see how the reactor operates in harbour and sea trials. Based on this initial experience, improvements can be made to the life and stealth qualities of this reactor.

The third issue mentioned by the "experts" is the reactor power and why we need an SSBN to have higher reactor power to transit at a speed of 30 knots.

Reactor power, as indicated, is thermal and not electrical (eg 100 MW is 100 megawatts of thermal power). Most SSBNs need a transit speed of below 20 knots since their task is to avoid contact with enemy warships and submarines and launch their SLBMs when ordered in a second strike.

Given our geographical location vis-à-vis our two nuclear-armed adversaries, an SSBN with a speed of 20 to 24 knots, but with SLBMs of about 5,000-km range, should suffice. It should be noted that SSNs (or tactical nuclear attack submarines), which are required to search and sink enemy warships and submarines, would need higher speeds (over 30 knots, which, depending on the SSN size, would require a single reactor of 160 to 200 MW or two reactors, each of about 80 to 100 MW).

There are a few other factors which decide submarine stealth -- improved shock mounts, "rafting" (where the reactor and machinery are not in direct contact with the pressure hull), hydrodynamic hull shape, skewed propellers or the new pump jet propulsion system, "static" electrical machinery, anechoic tiling, silent weapon and garbage discharge systems among others). Similarly, greater diving depths are a combination of metallurgy, pressure hull thickness and frame spacing (steel frames are the inner skeletons which, along with the keel, provide support to the pressure hull.

For example, an expensive titanium-hulled submarine can dive to twice the depth of a modern steel-hulled submarine.

Also, in the case of two "similar" steel-hulled submarines, the one with a thicker pressure hull and "closer, thicker frame spacing" would dive deeper, but would pay a penalty in loss of some speed.
Diving depths of submarines are a closely guarded secret, as are noise figures and weapon-firing depths. A nuclear submarine also needs a system to generate oxygen and absorb carbon dioxide and other gases to enable human beings to live and operate in demanding conditions underwater. Production of drinkable sea water and the ability to get rid of garbage (food and human waste) are equally important and technologically demanding.

Our scientists and the Navy personnel involved in work on the Arihant have indeed achieved a major milestone. However, much more needs to be done, and I hope it is done without any unwise and unnecessary publicity.

In addition, India now also needs to begin work on a faster, deeper-diving SSN (tactical nuclear attack submarine) to provide its Navy with a major sea denial capability in the vast expanses of the Indian Ocean.
This SSN will require a single 160 to 200 MW reactor and improved stealth and complex metallurgy. We are beginners in the field of nuclear submarines and have a lot of catching up to do before we start celebrating. VICE-ADMIRAL ARUN KUMAR SINGH retired as Flag Officer Commanding-in-Chief of the Eastern Naval Command, Visakhapatnam

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