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After the Three Mile Island (TMI) accident and the more recent Fukushima disaster, nuclear energy as a viable clean energy alternative is questioned. Germany has planed to phase out nuclear power plants starting from 2008 while others, especially developing countries, are still seeing nuclear energy as a modern and cheap alternative comparing to wind power or solar power. Brazil still focus its renewable energy plan on nuclear power, and China, with recent controversies regarding Fukushima disaster, has quite a few nuclear construction projects under way. Although Chernobyl, TMI and Fukushima disasters highlight weaknesses in safety measures of some nuclear power plants, safety can be improved. The first traffic light prototype killed a policeman, well-designed twin tower can be stroke down and an earthquake can shake a nuclear power plant down. These accidents, though worrying people, but hardly can be a showstopper for nuclear power in general. However, the problem with nuclear waste was underplayed for many years may well be the killer switch for nuclear power installation. This survey intended to summarize recent developments of nuclear reactors, and hopefully, would also provide an insight into the possible future of nuclear energy development regarding some popular concerns.

Background

The nuclear waste issue was downplayed for a long time. During the first commercial nuclear power plant installation, manufacturer assured that they will collect and reprocess the nuclear waste so that can be used again. Although the technology hadn't been developed yet then, they were expecting a time frame within 10 years that these wastes can be properly handled (accelerated radioactive decay or reprocessing). Soon after the first installation, to encourage the nuclear power usage, the Congress has passed a law to ensure that nuclear waste will be properly handled by The Federal government. But after more than 30 years, there is still no viable way to reprocess these highly radioactive wastes. The Federal government was bounded by the law to help nuclear power plant to handle the nuclear waste they've collected on-site for the last 30 years.

The Bush administration after years’ consideration came up with a proposal that using Yucca Mountain as a permanent burying site for nuclear waste, which in theory would last for 10,000 years. Although disposal is always an option, many details that involved make the proposal much less desirable. The criticisms mainly focus on the transportation safety and the option of sealing off the site forever after disposal without monitoring in place. Because of these concerns, The Obama administration has suspended the operation indefinitely.

If only newer nuclear plants can produce much fewer nuclear wastes, or we can expect nearly to none nuclear waste produced, the disposal solution may be viable again. After all, the nuclear wastes that we are sealing off are in finite amount, and we can be free of worrying after the burying.

In-Operation New Types

In this section, I will investigate several nuclear reactor types that based on new technology and are operational currently.

Pressurised heavy water reactor (PHWR) is one of the most used commercial nuclear reactor types to date. PHWR is categorized by its cooling system design and coolant within. A PHWR uses the same mechanic of a pressurised water reactor (PWR). A pressurised loop used to transport heat out of the core chamber. Since coolant in the loop was directly contacted with nuclear fuel in the chamber, it will be contaminated. Then, the coolant will heat up the stream generator that ultimately will generate electronic power. During the whole process, the coolant (heavy water) is kept inside the close loop to avoid any radioactive contamination to stream generator. The pressurised loop also keeps the boiling point of coolant high enough so that the system can be operated in higher temperature, thus, more energy-efficient. The "trick" of using heavy water is because heavy water won't absorb nearly as much neutrons as light water (normal water), thus, natural uranium can be fed into the reactor without enrich process.

These early designs such as PHWR were only focused on the economy aspect (e.g. how to make a cheap, commercial-viable reactor without any consideration of environmental impact). As a result, even tough a PHWR can deliver energy with higher efficiency and cheap natural uranium deposit; it also can produce more plutonium and tritium than its light water counterpart.

Since PHWR is still the main type of commercial nuclear reactor to date [1], it shows that the primary goal of nuclear production still focus on safety (pressurised coolant-based reactor is considered to be safest in operation) and efficiency (both the efficiency in energy production and economic efficiency, e.g. the price of fuel). It is clear that the by-product processing was not a central concern in past because "it can be solved with technology advancement".

In-Construction New Types

In this section, I will investigate several nuclear reactor types that merit some new technology development and currently are in construction.

Fast breeder reactor (FBR) is the kind of reactor that can "refuel" itself in theory. Many FBR prototypes were built for research purpose. Some ere built and operated but only shutdown. [2] In recent years, FBR picked up some traction and some are currently under construction. U.S., China, Japan, and India have several projects on FBR building or prototyping.

A FBR is mainly different from ordinary nuclear reactor because it uses fast neutrons for its reaction. Fast neutrons can produce far more neutrons in fissile reaction that in return can increase the concentration of Pu/U ration that is needed to sustain the chain reaction. Thus, a much higher breed ratio (the ratio of fissile atoms created per fissile event) can be obtained with FBR. The high hope on FBR is that with initial fuel, later, it can be fed on natural uranium or even depleted uranium. The hope of exploiting all power in fissile reaction with FBR was diminished when the price of uranium mine dropped and the process to enrich uranium became commercial-viable. It becomes interesting again in recent years because it turns out that the FBR process will produce much less plutonium and minor actinides (both are main nuclear waste component). Due to the heated debate around nuclear waste, researchers hope that based on FBR, they can construct a reactor that produces much less long half-life nuclear waste.

Proposed New Types

Some nuclear power reactor types are showing promising future and were considered in Generation IV International Forum [3], but for the time being, are still on paper or only research projects.

Integral Fast Reactor (IFR) in principal is much like FBR we discussed before. Due to the ability of "burning" long last nuclear waste, FBR is considered to be integrated an on-site electrowinning fuel-reprocessing unit, thus, resulted IFR. The electrowinning fuel-reprocessing unit could potentially recycle all the transuranics and uranium through electroplating, leaving only short half-life materials in the waste. To address concerns on safety, IFR also has a passive safety measurement deployed in reaction chamber. The IFR design promises to have high-efficiency (99.5% in theory) with minimal "safe" (half-life less than 20 years) by-product.

Pebble Bed Reactor (PBR) unlike most deployed nuclear power reactors, PBR uses gas cooling system that in principle enables it to operate under very high temperature. The high temperature enables the reactor to be more efficient at energy production (higher thermal energy to mechanic energy transfer ratio). Also, the fact that it uses a gas-cooling system to avoid many complexities that introduced by traditional water-cooling system (double loop design etc.). PBR system promises to have a small, compact and much efficient reactor that can even be deployed in home or on vehicles.

Conclusion

The survey of nuclear reactor types shows that a nuclear reactor with our desired property can be achieved (low nuclear waste production and high-efficiency). Unfortunately, the first few years’ research (1960s ~ 1980s) was incentivized by economic efficiency and complete left environmental consideration out of the equation. Nuclear power is our most productive new energy to date, if only we could incentivize research to the right direction, the problems around nuclear waste and safety can be solved within given time.

[1] International Atomic Energy Agency, Nuclear Power Reactors in the World (Reference Data), 2006

[2] Superphenix in France, SNR-300 in Germany, 1 in Enrico Fermi Nuclear Generating Station U.S.

[3] GIF, http://www.gen-4.org/

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