Thursday, December 6, 2007

The Second Fifty Years of Nuclear Fission

The Second Fifty Years of Nuclear Fission
Alvin Weinberg
Early 1944 was when the Chicago Metallurgical Project re-established contact with our colleagues from Canada. Since my job, as Eugene Wigner's assistant, was to keep track of the multiplication constant, I quickly became good friends with George Volkoff and George Placzek, who were responsible for the theoretical work at Montreal. I recall how impressed we at Chicago were with the elegance of mathematical techniques that the Montreal group had developed to solve the transport equation. By contrast, our Chicago group used simple recipes - "Fist-Formulas" as George Placzek called them - to estimate the chain-reacting properties of natural uranium "piles" moderated by graphite, heavy water, and light water. I actually spent a few weeks in Montreal about this time helping Placzek, Volkoff and their team use these formulas in the design of the lattice for the experimental heavy water reactors that were forerunners of the wonderfully successful CANDU reactors. But at that time the Montreal project was focused on plutonium, not on power.

The Chicago project by this time had pretty well completed its design of the Hanford graphite-moderated reactors, and so much of our attention was turned to future applications of nuclear fission. We organized a New Piles Committee which met weekly for three months during the spring of 1944. Here the senior luminaries, such as Fermi, Wigner, Szilard, and Franck, together with a few younger assistants like myself, discussed various ideas for reactors: for power, for submarines, for production of plutonium, even for inducing endothermic chemical reactions. Our imaginations ranged widely as we considered various moderators, coolants, and configurations. Inventing a new reactor was an everyday occurrence, simply because no one else had thought about these matters. At that time we were under the impression that uranium would always be scarce. Our thinking was therefore directed mostly to breeders: unless breeders were successful, nuclear energy would not survive very long. Of course, this all happened at least ten years before Bennett Lewis thundered in his famous paper. Breeders are not necessary, that there are ways of skinning the cat of uranium scarcity short of full-fledged breeders. This question, in 1989, still remains moot!

Although at least 20 reactor concepts have received serious consideration since 1944, only five types have become commercial power plants: light water (in both pressurized and boiling versions); heavy water (CANDU); graphite-moderated, steam-cooled (RBMK) like Chernobyl; gas-cooled graphite; and liquid-metal cooled fastbreeders. The world's total nuclear electrical capacity in operation and under construction is now some 427 GW(e), or about 10% of the world's total installed electrical capacity. The 1650 terrawatt-hours of electrical energy produced last year by these reactors is more than 16% of the world's electricity, and about 6% of the world's primary energy. This widespread use of fission as a source for 16% of the world's electrical energy must be regarded as an extraordinary achievement.

Of the world's 500-odd commercial reactors, about 85% are moderated and cooled by light water. As one who was involved in the original decision to power NAUTILUS with a light water reactor, I have never outgrown my astonishment that LWR became the dominant reactor. After all, light water was chosen originally for the submarine because such reactors are compact, and at least in principle, relatively simple. They were not chosen because light water recommended itself as the best choice for generating electricity cheaply. To achieve compactness, we had to use highly enriched uranium, and at the time enriched uranium was very rare and expensive. Moreover, to retain the simplicity of its core design, the original NAUTILUS prototype simply burned U-235, without generating any new fissile material. Given our impression at the time that enriched uranium was very scarce, we could not visualize a light water reactor ever producing electricity at a competitive cost (see Table 1).

Two developments changed our perception. The first, I think largely attributable to Karl Cohen, was that gaseous diffusion, when fully rationalized, would produce enriched uranium at costs sufficiently low to allow its use in LWRs. Cohen occupied an all-but-unique position in those early days since he was probably the only person at the time who possessed an intimate knowledge of both gaseous diffusion and nuclear reactors. He had participated in the development of the original theory of the diffusion cascade at Columbia in 1942 and, when Harold Urey was banished to Chicago by General Groves in 1943, Cohen also came to Chicago to work with Wigner on the design of heavy water reactors. Cohen therefore was the first to command a detailed understanding of both reactors and diffusion plants. As matters turned out, slightly enriched uranium was eventually produced at a price that could be afforded in civilian reactors.

The second change was in the outlook for uranium ore. At the New Piles Committee, we spoke of 20,000 tons of uranium worldwide; by the late 1940's we realized that uranium was much more plentiful - as it turns out, at least 1000 times more plentiful - than we had estimated. Thus the incentive for breeding was very much diminished - a situation that persists to this day.

Was pressurized water for commercial power chosen because it was obviously the best choice? Not as I remember the matter. It was chosen for Shippingport after President Elsenhower had vetoed the Navy's proposal to build a nuclear aircraft carrier powered by a larger version of the NAUTILUS power plant. A demonstration of a power plant that would operate as part of an electrical utility was being urged by the Atomic Energy Commission. The only reactor that was on hand was the one designed for the canceled aircraft carrier - what was more natural than to rescue Rickover's carrier reactor by putting it on land, and operating it as part of the Duquesne Company's grid?

Given that LWR was chosen largely as a matter of expediency, was this a bad choice? At the time, Calder Hall, a Magnox, gas-cooled power reactor was operating; the CANDU was in final design; the Soviets had operated a small version of their RBMK; and two small prototypes aimed at breeding, Zinn's EBR-I, and Oak Ridge's Aqueous Homogeneous Reactor, had operated. The Atomic Energy Commission hardly had a serious alternative to choose from, given its desire to demonstrate a peaceful use of nuclear power. To have diverted effort from the main light water line would have involved risks - and the arguments favouring the alternatives were not really compelling. After all, they too were expedi- ent, not '"best" choices: Calder Hall was an outgrowth of the Windscale plutonium plants, just as the Russian Obninsk was a variant of their plutonium producers, and CANDU, based on natural uranium, was an outgrowth of Canada's NRX experimental reactor.

The two primary aims of nuclear power - inexhaustible energy, i.e. breeding - and economically competitive electricity - have both been demonstrated. The breeding ratio in France's sodium-cooled PHENIX has been shown to be 1.13, and Admiral Rickover's thorium-based U-233 seed-blanket light water breeder has been shown to have a breeding ratio of around 1.01. These demonstrations of actual breeding have passed rather unnoticed. I regard them as extremely important since we now can say with certainty that nuclear fission, based on breeders that burn very low grade ores, represents an all but inexhaustible source of energy. And, as I have already described, the goal of economically competitive nuclear electricity has been demonstrated in many places. What has not yet been demonstrated is electricity in a large scale breeder that is cost-competitive today. The largest breeder, the 1200 MWe Super-Phenix, is too expensive, and the same can probably be said of the light-water breeder. On the other hand, if successors to Super-Phenix could be built more cheaply - a very likely prospect - or if the life of a breeder could be extended to say 100 or 150 years (instead of the planned 30-40 years), the cost of the electricity over the entire lifetime could become competitive.

Super-Phenix has placed a cap on the cost of electricity for many future generations, if not for millenia, and this cap is surely less than twice current costs of electricity. Whether fusion or solar electricity will match this, and will eventually displace the breeder, remains to be seen. I therefore regard nuclear energy today as having all but achieved the primary goal we set for it at the New Piles Committee some 45 years ago - an inexhaustible energy source at a cost that ought not to escalate even as high grade ore is exhausted.

The Need for a Second Nuclear Era
As one of the surviving participants of the New Piles Committee, I should feel very good: our main aim, expressed 45 years ago - power breeding has been achieved! And competitive electricity from non-breeding light water reactors, a goal that we hardly considered important at the time, is a reality in many places. But, despite these extraordinary successes, nuclear energy hangs in the balance, and is all but dead in many countries. What went wrong, and how can we set it right?

Let me return to the New Piles Committee. Fermi even then seemed to sense that nuclear energy might encounter public opposition. I quote from Ohiinger's report of the April 26, 1944 meeting of the New Piles Committee, at which Fermi outlined his ideas for a fast breeder that fed its fissile plutonium to small satellite power plants: "There may be nontechnical objections to this arrangement, for example, the shipment of Pu-239 to smaller consuming plants offers the serious hazard of its falling into the wrong hands." And I can remember as if it were yesterday, though I cannot document it, Fermi's pronouncement at one of these meetings, that for the first time mankind would be confronted with enormous amounts of radioactivity; we must not assume that this will be accepted easily by society, he warned.

Fermi's warning is proving to be closer to the mark than the optimistic predictions of early enthusiasts such as myself. One explanation for this turn of events is that the dawning of the Nuclear Age has coincided with the dawning of what I call the "Age of Anxiety". Although life expectancy in most of the developed world has increased enormously since the turn of the century, we are much more anxious about survival - as individuals, as a society, and even as an inhabitable earth - than ever before. We worry about low levels of chemical insult, about the possibility of nuclear obliteration, about irreversible pollution of the planet. All of this is greatly exacerbated by television, and by new trends toward participatory democracy. Some of our worries are justified: nuclear war, possibly the greenhouse effect, possibility of a catastrophic accident like Chernobyl or Bhopal. Some are without scientific justification - like exaggerated claims that radiation (or many other toxins) at levels much lower than background pose any hazard to health. But in the Age of Anxiety the public does not distinguish between those worries that are real and those that are unjustified.

In the nuclear case, we must distinguish among the public's primary concerns. First, there is proliferation of nuclear weapons. I would judge this to be more of a worry for the professional arms control expert than for the public at large. I would classify this worry as justified, but as not evoking great public concern.

The second worry is radioactive waste disposal. We must remember that James Conant, President Roosevelt's personal monitor of the Manhattan Project, predicted in 1953 that nuclear power would not be worth the candle because waste disposal would prove to be intractable. The public is by and large convinced that Conant was right: wastes are the public's main concern about nuclear energy. Yet I would insist that wastes fall largely into the category of unjustified concerns. The reason is that, once the spent fuel has been removed from the reactor, and has been dispersed in a cooling pond or in a disposal cask, there is no longer enough energy being generated to cause widespread dispersal of large amounts of radioactivity. No analysis of a high level depository, including the accompanying transport system, has identified a credible accident that disperses really large amounts of radioactivity over really large amounts of land. An accident that imposes non-stochastic doses to a few people in the immediate vicinity of a depository - yes, though with small probability - or an accident that imposes very low doses on large numbers of people, these are also possible. But an accident, such as Chernobyl, that imposed large doses on large groups of people - no. And if, as I believe, the weight of evidence supports the view that doses of X-rays at less than background are hardly harmful, and may even be beneficial, I would insist that the public's concern over wastes is much exaggerated - it is a manifestation of our fearfulness in this Age of Anxiety.

Not so with the possibility of a reactor accident. As Chemobyl showed, and as Rasmussen had estimated in 1975, an uncontained meltdown of a large nuclear reactor could be a catastrophe of immense proportion. Unlike waste disposal, it can impose large doses on large numbers of people as well as contaminating large tracts of land. The nuclear community has tried to deal with this reality by invoking probabilistic arguments - the a priori probability is very small, say less than 10-6 per reactor year, or even lower for accidents as large as Chernobyl. But I don't think the public accepts such probabilistic arguments, even when the numbers show the absolute dangers are comparable to the dangers accepted in other, more familiar technologies, such as hydro-electric dams. What the public seems to want is a return to the situation when an engineer would say "This is safe - period", not "This is relatively safe", or the "Probability of an accident that causes unacceptable consequences is a small number, like l0-6 per reactor-year".

Can we develop nuclear reactors whose safety is deterministic, not probabilistic, and which, if developed, would meet the public's yearning for assurance of safety, not simply assurance of the probability of safety? This is the task that has engaged many nuclear engineers in a search, now ten years old, for an inherently safe reactor. Now, in some sense, a device that produces 200 megawatts of afterheat and is immune from meltdown under every circumstance, conceivable and inconceivable, is a contradiction in terms. But a device whose safety depends on the working of immutable laws of nature, with a minimum of interventions either mechanical, electrical, or human - a device in short whose safety is so transparent that the skeptical elite, as well as the informed public, will regard it as safe - this I regard as eminently possible and worthwhile. There are now at least two ideas for reactors that are largely inherently safe - the SECURE-P (also known as PIUS) of the ASEA-ABB company of Switzerland and Sweden; and the Modular HTGRs of General Atomic Company in the US and of the KWU Company in Germany. Argonne is developing a metallic-fueled small breeder that embodies almost as many passively safe features as do these two. The advanced developments sponsored by Westinghouse and General Electric aim at producing relatively small reactors that are incremental improvements over existing LWRs, yet provide substantially more passive safety than do existing reactors. And here in Canada, SLOWPOKE and a power version of SLOWPOKE, provide important elements of passive safety.

I cannot say where the quest for inherently safe, or at least transparently and passively safe, reactors will end. Will it lead to a second nuclear era, risen from the ashes of Chernobyl and Three Mile Island, in which the public accepts reactors as safe - not because quantitative arguments predict a very low probability of an accident with unacceptable consequences - but because the safety of these reactors is understandable and plausible? Or will the second nuclear era dwindle in a chaos of recrimination and protest because the public simply cannot be convinced, even by reactors that embody passive and inherent safety?

What the public's reaction will be surely depends on the alternatives to, and upon the incentives for, nuclear power in the next 50 years. As for incentives, greenhouse may be the key. I have estimated that to make a dent on greenhouse, the world might have to build approximately 5000 very large reactors, producing about 300 quads of primary energy. Such a nuclearized world is impossible if the core melt probability is as high as 10-4 or even 10-5 per reactor-year since this leads to a core melt every two to 20 years. And as Chernobyl, even TMI-2, showed a single core melt of the magnitude of Chernobyl puts an end to nuclear power in many countries. The public may regard greenhouse as worse than Chernobyl, and may not demand inherent safety as the price of a second nuclear era. I would not count on this though. I believe the nuclear community, descendants in a way of Fermi and the New Piles Committee, have no choice but to pursue these ideas for passively safe reactors.

For, in a way, there are alternatives to nuclear. There was no heavenly-ordained requirement that the age of fossil fuel be replaced by the age of fission, nor a cosmically-ordained anthropic principle that required nu to exceed 1 so that a chain reaction is possible, or exceed 2 so that breeding is possible. Had fission not been available we would, willynilly, be conserving energy at a much faster rate than now, we would be pushing fusion even harder - and as a last resort, we would turn to solar energy. A solar world in which primary energy is three times as expensive as it is today is hardly an impossible world, especially since in such a world, energy would be much more strongly conserved than it is now. We would not be relegated to a Malthusian poverty if the only reactor upon which man depended for energy was located 150 million kilometers away from the earth.

But Hahn and Strassmann's discovery of 50 years ago, and God's providence in adjusting the nuclear constants so as to make a power breeder practical, have given us another option. We nuclear engineers of the first nuclear era have had success, yes, with our 500 commercial reactors, and our practical breeders. But the job is only half-finished. The generation that follows us must resolve the profound technical and social questions that are convulsing nuclear energy. The challenge is clear, even the technical paths to meet the challenge are clear. All of us old-timers wish that we shall be here to see how these challenges are met, but even if we are not, we wish you well in fashioning an acceptable Second Nuclear Era!

1 comment:

Anonymous said...

The problem with solar isn't the amount of energy available to us, it's capturing it. We make those solar pannels using fossil fuels, and they ware out and need to be replaced. This is why the only real success we've had at capturing solar energy is hydro, because the water the sun evaporates concentrates in a river basin, making its capture vastly more efficient. Fission is the only proven alternative to fossil fuels available to us. Uranium lies in wait for the coming great switch, which only fusion may one day end.


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