Advanced designs in various development stages promise to remedy the drawbacks of conventional nuclear energy
By Jonathan Tennenbaum
This is part 4 of a series.
Nuclear energy, in its presently utilized form, has many, many problems. The main ones are worth recalling briefly, in order to understand the motivations behind the effort to develop new reactor designs.
(1) Nuclear fission reactors generate considerable amounts of radioactive materials from which humans must be shielded and whose release into the environment must be prevented under all circumstances. A complicating factor is the “messy” nature of the fission process itself: it produces dozens of different radioactive isotopes scattered around the periodic table – substances with widely differing chemical and physical properties. Dealing with this poses enormous technical challenges, necessitating multiple containment and shielding structures and elaborate measures for handling, transport and storage of radioactive material. All of this multiplies the cost of construction and operation of nuclear power plants.
(2) Energy-generation in fission reactions is based on fission chain-reactions, which have an inherent tendency toward exponential growth, requiring active and passive mechanisms to limit the power output and to prevent a runaway event. For physical reasons it is impossible for a fission reactor to explode like a bomb; but even a slow runaway reaction could have disastrous consequences, as may have occurred in the Chernobyl reactor.
(3) The radioactive material in the core of a fission reactor continues to generate large amounts of heat (decay heat) for days after the fission chain-reaction has been turned off. This heat must be removed somehow, by active and/or passive cooling systems, to prevent a meltdown of the reactor core, with potentially disastrous consequences.
For example: at the moment the Fukushima power station was hit by a 15-meter tsunami, in March 2011, the three operating reactors on the site had automatically shut down in response to the preceding earthquake. But the impact of the tsunami wave knocked out the external power supply and the plant’s own emergency power systems. A complicated series of events ensued in which an effective cooling of the reactor cores could not be maintained, and the cores of all three reactors subsequently melted down.
Hydrogen was generated by a reaction between zirconium in the broken fuel rods and steam from the boiling cooling water, leading to explosions in the upper reactor buildings. Significant amounts of radioactive elements were released to the environment. Fortunately, the direct health impact on the general population from elevated radiation exposure – including long-term life expectancies – appears to be very small. Much more serious were the effects of stress and hardship from the emergency evacuation.
(4) The fuel cycle of present-day nuclear power plants carries a risk of proliferation of nuclear weapons, connected with its dependence on large-scale isotope separation and with the generation of plutonium during reactor operation. The system of international controls (by the IAEA) was created to address this problem. However, some countries have not accepted the controls – North Korea, for example, and Iran before the nuclear agreement).
(5) The reserves of uranium that can be economically exploited by present-day light-water reactors, are limited in quantity and very unequally distributed among the world’s nations.
(6) Present-day light-water reactors require enriched uranium fuel. Suitable enrichment plants are expensive to build and very few nations have them.
(7) Generally speaking, nuclear-produced electricity today offers – at best – no decisive cost advantage over conventional fossil fuel plants, especially when fossil fuel prices are low. Historically, the decision to use nuclear power was primarily motivated by strategic considerations, especially energy security, rather than economic reasons alone.
(8) Due to their complexity and the elaborate licensing and permission requirements, building new nuclear power plants involves long lead times and long construction times.
(9) Present-day nuclear power plants employ very large reactor units, giving rise to extremely high unit capital costs, on the order of $5-to-10 billion for a plant with 1 GW electrical output. This makes financing difficult and puts nuclear energy out of the range of many potential users, including developing nations.
A glance at this list conveys the magnitude of the challenge faced by nuclear energy today.
New designs transformative
There are relatively good solutions to most of the problems listed above, when each is taken separately. For example, it is possible to build reactors for which a runaway reaction is ruled out by their physical properties, whatever the plant operators do and whatever damage may occur to the reactor structure.
It is possible to build reactors with sufficient built-in, passive cooling by physical mechanisms alone, so that a meltdown is ruled out even if all active cooling systems fail or are shut down.
There are realizable reactor designs which produce few or no long-lived radioactive isotopes and which can even “burn” radioactive waste to produce energy.
There are reactors that require no isotope separation to produce their fuel, and reactors that are virtually proliferation-proof.
There are nuclear plant designs – small modular reactors – requiring only a fraction of the capital investment of today’s plants. There are ways to greatly reduce electricity production costs by nuclear energy.
The big problem is how to combine these advantages in a single system.
At present there exist a multitude of advanced nuclear reactor designs, in various stages of development, which promise to remedy the drawbacks of conventional nuclear energy – at least to a substantial extent. I shall limit myself to four basic options which I consider the most promising: (1) molten-salt reactors, (2) traveling-wave reactors, (3) pebble-bed high-temperature reactors and (4) small modular reactors. At the risk of sounding somewhat like a salesman I shall not discuss the problematic aspects of these four options. I also shall not go into the countless variations and combinations currently under study.
Physics and engineering
Before trying to explain what is special about those four advanced reactor types, I should say a few words about the basic physics – hoping not to scare the reader too much.
The key player in nuclear fission is the neutron. This wonderful particle is one of the two building blocks of atomic nuclei, the other being the positively-charged proton. But when freed up to move around in space, neutrons become powerful agents of change. Whoever can produce large numbers of neutrons is king in the nuclear world. Being electrically neutral, neutrons can easily penetrate into the nuclei of atoms. This can result in two outcomes of interest. Which one of the two occurs depends on the type of nucleus and the speed of the incoming neutron.
One outcome is that the nucleus splits up into fragments – nuclear fission. With the right choice of nuclei we get not only energy out, but also more neutrons. If there are more nuclei of the appropriate type around, we can get a self-sustaining chain-reaction. This, of course, is the basis of energy generation in fission reactors – and it’s the basis of using fission reactions as a source of neutrons to do other things.
The second possible outcome from absorbing a neutron is that the nucleus becomes unstable, but without splitting into fragments; instead, it sooner or later undergoes “radioactive decay,” emitting a high-energy particle (alpha, beta or gamma particle). In the case of alpha- or beta-decay we get an isotope of a different element – transmutation.
The potential to transmute nuclei using neutrons opens up tremendous possibilities, which conventional nuclear reactors hardly make use of.
One is to “breed” more nuclear fuel. Presently nuclear power is nearly entirely based on a single fuel: the uranium isotope U-235, which makes up 0.7% of natural uranium. The remaining 99.3% consists of U-238, which for physical reasons does not support a self-sustaining chain reaction.
If we have enough neutrons, however, we can transmute U-238 into an excellent nuclear fuel, namely plutonium (more precisely Pu-239), thereby increasing the resource base for nuclear power by 140 times.
We can generate even more nuclear fuel by transmuting thorium into another isotope of uranium, U-233. U-233 is not only an excellent fuel, but its fission produces only relatively short-lived isotopes, thereby greatly reducing the problem of nuclear waste. Thorium is at least 3 times more abundant in nature than uranium.
Secondly, we can use neutrons to “burn up” nuclear waste. This occurs by transmuting radioactive isotopes into stable isotopes, or at least isotopes having short lifetimes. In the process we get additional energy, too. The often-repeated statement that so-called nuclear waste absolutely must be stored for thousands of years or more is not true. On the contrary, some reactors on the drawing boards will be able to use “nuclear waste” as a fuel. As a scientist once remarked: If we bury nuclear waste, then people will later mine it as a valuable resource.
The key requirements for doing these things are:
(1) To be able to produce a surplus of neutrons, over and above what are needed to sustain the fission reaction. This is sometimes referred to as “neutron economics.” In a reactor neutrons are constantly being generated. Either they are absorbed or they escape to the outside.
(2) To be able to “tune” the neutrons to the right energies for the desired uses. Nuclear reactions are governed by various kinds of resonance phenomena.
Playing with the choice of fuel mixture, coolant, core composition and the geometrical layout of the reactor, gives us great leeway to improve the neutron economics and to shape the spectrum of neutron energies to fit our requirements. In turns out, for example, that fission reactions often occur much more effectively when the neutrons are slowed down from their original energies – a task which can be achieved by making them interact with certain materials (the so-called moderators).
The reactor itself is only the heart of a nuclear power plant, of course. The rest of the power plant’s body – its coolant circuits, heat exchangers, pumps, turbines, control systems, containment building, etc. – is no less important for safety and, especially, for the cost. In modern plants the reactor itself makes up only about 20% of the cost. But new reactor types can open the way to greatly simplifying and reducing the costs of the rest of the plant and of the whole infrastructure required to fuel it and operate it.