Atomic Insights

Atomic energy technology, politics, and perceptions from a nuclear energy insider who served as a US nuclear submarine engineer officer

In the news: December 1995

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Watts Bar Receives Low Power Licence

(November 11, 1995) TVA’s Watts Bar nuclear power plant has been under construction for more than 23 years. The Nuclear Regulatory Commission has just approved a permit that will allow the Tennessee Valley Authority to begin loading it with fuel for low-power testing.

The fuel has been stored in a vault on site since 1979. Though the plant has been plagued with difficulty, officials state that the problems have been fully resolved.

Minnesota Questions Fees

(November 10, 1995) The Minnesota Public Service Department is investigating options regarding the nuclear waste storage fee that has been collected from utility customers since 1982. Northern States Power, the state’s only nuclear utility has collected more than $246 million from ratepayers, and sent it to the federal government, yet there seems to be no progress toward a permanent fuel storage system.

MPS Commissioner Kris Sanda states that the DOE program responsible for waste has been mismanaged and constitutes a consumer “rip-off.” Sanda said that the investigation will determine if it is possible to put the collected fees into an escrow account and whether or not Minnesota has any non-federal options for safe disposal of spent nuclear fuel.

Rapid Energy Growth in China

(November 14, 1995) China reported that its energy demand growth outpaced its energy supply growth. This has led to a growing level of energy imports and a lifting of the ban on foreign equity investments in the Chinese energy sector.

To help combat the gap between supply and demand, and to head off the possibility of future shortages slowing economic growth, China is aggressively pursuing an expanded role for nuclear power. A Chinese spokesman states that China “has no choice but to embrace nuclear energy, despite the growing anti-nuclear sentiment in the developed world.”

USEC For Sale

(November 14, 1995) The United States Enrichment Corporation (USEC) could be ready for sale either to the public or to another company as early as spring 1996. The company currently has an 86% share of domestic sales of enriched uranium and a 37% share of the global market. Some analysts expect that the sale will produce between $1.7 and $2.0 billion in revenue for the Treasury.

Protection for Fuel Elements: Ensuring Saftey

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Once Rickover’s team selected water as their primary reactor coolant, other material criteria became readily apparent. Though pure water at room temperature is a relatively benign environment, water at high temperatures is quite corrosive. Because radioactive fission products can be dangerous to human health, prudent reactor designers must devise methods to ensure that the fission products are kept in a carefully controlled location.

Uranium vs Uranium Dioxide

Uranium, in pure metal form, is a chemically reactive metal, especially in a hot water environment. Put simply, uranium rusts rapidly. Allowing fuel material to rust is obviously not a smart engineering decision, particularly since any rust that flaked off would be radioactive and difficult to remove from the piping systems.

Two means of ensuring fuel stability and resistance to corrosion were available. As usual, Rickover and his team covered their technical bets and chose both of them.

Instead of using uranium metal in its pure form, the team decided to use uranium dioxide fuel material. In essence, they chose to use uranium that was already completely “rusted” so that its behavior would be more predictable. Uranium dioxide is not reactive, and it will not dissolve readily even in very hot water.

However, the team was not content with the protection that uranium dioxide offered. To more completely separate the fuel from the coolant, the team decided to sheath the fuel plates in a corrosion resistant covering called cladding.

Stainless or Zircalloy

Of course, deciding to cover the fuel material is one thing. Choosing an appropriate material for the covering is quite another. The material had to be compatible with uranium dioxide, it had to have good resistance to corrosion by hot water, it had to be strong enough to resist the stresses of reactor operation and, most importantly, it had to have a low affinity for neutrons.

There were essentially two groups of materials that seemed appropriate. Various grades of stainless steel had been used in test reactors and demonstrated adequate performance. Stainless steel was a familiar material to most engineers and its methods of manufacture were well developed. Its main disadvantage was that it had a relatively high affinity for neutrons.

The second group of materials, alloys of zirconium, had a much lower affinity for neutrons, but it had other, rather significant disadvantages. Zirconium, though naturally abundant, was not readily available in a useful form.

New Industry

Pure zirconium metal was an exotic material manufactured only in laboratory quantities. In a natural state, it was contaminated with a small portion of hafnium, a metal with very similar chemical properties.
Unlike zirconium, however, hafnium has a strong affinity for absorbing neutrons. The similar chemistry of zirconium and hafnium makes it somewhat complicated to achieve efficient separation. A manufacturing scale process had to be developed to fully separate the zirconium from the hafnium.

Many of the engineers associated with the nuclear submarine development program recommended stainless steel as the Nautilus fuel cladding, apparently because it was a known quantity that was perceived as low risk.

Rickover, looking long term and understanding that the use of stainless steel would mean that far more uranium would be needed to fuel his planned nuclear fleet, directed that zirconium alloys be chosen. Almost by shear will power, he forced the creation of a completely new metals industry that rapidly developed technology allowing production to move from several pounds per year to thousands of tons per year. At the same time, the cost of zirconium dropped from about $300.00 per pound to about $5.00 per pound!

Like many of Rickover’s choices, zirconium and uranium dioxide became nuclear industry standards. A large portion of the world’s reactors use that combination as the basic materials for their fuel elements. The combination has proven itself suitable for use in water cooled reactors, allowing high fuel integrity and low coolant radioactivity levels.

Controlling Power: Temperature and Rods

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Pressurized water reactors turned out to be extremely stable power producers. Because of the fact that water is used to moderate the energy level of neutrons, making them more effective in causing fission, the concentration of water in the core is an important part of determining the reactivity of the core. Temperature control An increase […]

Technical Hurdles: One Step at a Time

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Though water was chosen partially because it was a familiar fluid for power engineers, the choice almost doomed the nuclear submarine program. Laboratory testing of the pumps, bearings, valves, and piping demonstrated to Rickover’s team that the simple, familiar fluid was not so simple at high temperatures and pressures, particularly when exposed to neutron radiation. […]

Letter from the Editor: A History of Success

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The United States Navy submarine reactor program has a record of achievement and respect dating back to the early 1950s. In a field full of failed projects begun with high expectations, the Naval Reactors program is worthy of admiration and study. In this issue of AEI, we will focus on several early technical decisions that […]

Pressurized Water: Best Choice for the 1950s Subs

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When Rickover first began studying nuclear technology, he found a program in severe disarray. The Army’s Manhattan Project had accomplished its mission of completing a workable bomb before the end of the war. Many of the key scientists and engineers had left the program, eager to leave the security restrictions and poor working conditions behind. […]