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Nuclear Power

Nuclear power is the safest, cleanest, most carbon free, and the energy source most capable of supplying the needed power for the world to transition from oil and gas to sustainable carbon-free energy within a reasonable time frame. The fuel source is essentially inexhaustible within the time frame relevant to humans.

Nuclear Power Plants

Summary

Global warming is the result of human activity releasing excess greenhouse gases. Nuclear power plants are capable of delivering prodigious amounts of greenhouse gas free electrical power or heat once they have been installed. Since their introduction nuclear power plants have provided a large proportion of the electrical power that does not contribute to global warming, out-producing the sum of wind, water, and solar power. Given the clear need to make the transition from fossil fuels to a carbon free or at least a carbon-neutral energy source, why is nuclear so slow on the implementation? The technology is rapidly advancing with emphasis on the large infrastructure installations and a newer interest in innovative small modular reactors. The modern installations are essentially meltdown proof with passive systems that do not depend on electricity, or types of fuels that cannot meltdown (such as thorium).

Economics of Nuclear Power Plants

Believe it or not, in the long run, large infrastructure nuclear plants produce inexpensive, carbon free energy 24 hours a day 365 days a year for many decades. Once the initial capital cost has been repaid, these plants can return a high profit, and deliver low-cost energy to consumers. There is no doubt that the initial cost of a large nuclear plant is expensive. But compared to solar or wind required to deliver energy 24 hours a day 365 days a year, the initial investment is either similar or less for nuclear. Assuming a 2000MW capacity installation for nuclear, the initial cost for capital is estimated at between $2700/KW in several Asian countries to about $5,530/KW for the US. Thus a 2000MW installation is about $5.4 billion. If we add financing and delays as a contingency the ballpark Asian cost is about $8.3 billion. A similar installation in the US might cost twice that much - approximately $17 billion. The difference is that the Asian countries have been doing this on a factory-built modular basis for some years in recent times, whereas the US has not built a large nuclear reactor very recently. By comparison a solar or wind installation delivering power consistently on demand would cost well over $25 billion to create the extra arrays and battery backups.

So what is the answer? Why is nuclear slow to be implemented? The answer is complicated but essentially boils down to public opinion and fear of radioactive contamination. The most widely cited reasons for the fear is from waste disposal and meltdowns. Waste disposal is seen as a huge issue whereas in fact nuclear waste can be and is currently handled quite effectively. Meltdowns have occurred and the industry has learned from those experiences. Modern reactors are for the most part either exceptionally well protected against meltdowns or are designed so meltdowns cannot occur.

Nuclear Waste

Summary

The amount of nuclear fuel waste is relatively small and can be handled using dry cask or inert ceramic form. Both of these are able to keep the radioactive material safely in a container that can be housed underground until the time comes for the fuel to be re-used as economic factors or technology gets ready to use it. Some of the newer forms of nuclear fuel are in the form of balls coated with very high heat-resistant metals that are resistant enough that very high temperatures (twice as high as meltdowns) would not release the fuels by melting the metal coating.

High Level Waste

Spent nuclear fuels are usually in the form of rods or small balls (pebble bed reactors). At each refueling stage, approximately one-third of the fuel in a nuclear reactor with fuel rods is removed and replaced with fresh fuel. In a pebble bed reactor the individual balls are replaced as they are spent and replaced with a new ball, so the process is more continuous. The fuel rods that have been removed are called "spent" fuel. Because the fuel is still quite hot (in terms of both temperature and radiation), the spent fuel is placed in deep pools of water right on the reactor site. The pools are deeper than swimming pools - usually about 10 to 15 m deep. These pools are strongly built to withstand damage. The water as well as the concrete and steel walls prevent the radiation from escaping and also cool the spent fuels.

Once the spent fuel has cooled sufficiently it is removed from the pools (usually after about 5 -10 years) and encased in dry casks. The dry casks vary in manufacturing design but essentially encase the fuel in layers of steel concrete and lead. The casks are also designed so that they can be handled using standard forklift trucks. The people who manage the casks do not need to wear protective clothing because the exterior has a nearly negligible radiation count. The exterior skin of the casks is slightly warm from the heat of the fuel inside the casks. The current disposition of most dry casks is in warehouse storage or just below grade. The ultimate intended storage is in deep geological repositories. The characteristics of a good deep geological repository is fairly short list: The area is not in a geologically active zone. The formation should be solid rock or of material that does not allow leaching or movement of water. The formation should be extensive enough that a large chamber can be created about 600 to 900m below the surface. The communities should be the active initiators of the request to house the nuclear waste, and a determination of the geological suitability should follow the invitation from the communities. The development of a large site will be a major infrastructure project and so the site should have reasonably good access suitable for large trucks. The community needs to be a part of the planning and be fully aware of the implication of a large multi-century project. The majority of the workers should be drawn from the community following suitable training. Because high level waste is small in volume a single large repository could hold all of the nuclear spent fuel from any one nation.

This will be the preferred storage system for the foreseeable future and the production of dry casks has a relatively high demand in the industry.

Low Level and Medium Level Waste

Low-level radioactive waste (LLW) is the most volumetrically significant waste stream through commercial activities such as nuclear power plant operations and medical treatments. Most nuclear pwoer plants and commercial operations dispose of LLW at their own sites. In the United States LLW is not necessarily defined by low levels of radioactivity. Instead it is defined by exclusion - LLW is not high-level radioactive waste, spent nuclear fuel, or byproduct material. LLW is physically and chemically diverse, ranging from lightly contaminated soils and building materials to highly irradiated nuclear reactor components (but not fuel). This means that LLW is broken into waste "streams" for specific types of disposal:

• Greater-Than-Class C [GTCC] waste and transuranic [TRU] waste
• Incident waste
• Sealed sources
• Very Low-level and Very Low-Activity Waste
• Depleted uranium (DU)

Most countries have a defined threshold for degrees of containment security based on radiation levels and the common themes stream approach. In the United States the regulations do not recognize a low-end threshold for radioactivity, so their regulations are much stricter and actually vary from state to state.

Meltdown Prevention

Introduction

From the first use of nuclear power to generate electricity, industry recognized the potential hazard of both nuclear criticality and release of radioactive materials from generating electricity with nuclear power. Strict safety measures have been implemented in the design and operation of nuclear power plants to minimise the likelihood of accidents, and if an accident does happen, to avoid major human consequences. 

There have been three major reactor accidents in the history of civil nuclear power. Three Mile Island was contained without harm to anyone. Chernobyl was caused by human error and resulted in an intense fire with insufficient containment. Significant radiation escaped from this accident. To date just over 50 deaths have been confirmed as a direct result of the Chernobyl accident but the World Health Organization suggests that may reach up to 4,000 premature deaths from cancer and other radiation-related causes. Fukushima, the most recent accident with triply redundant meltdown prevention mechanisms in the combination of a tsunami and earthquake breached the containment and allowed some release of radioactivity. 

The meltdown itself is the result of excess heat melting the steel structures holding the core together. If this fails then the molten fuel and metal fall into the outer containment device - usually a very strong steel and concrete building. A chemical reaction with the fuel and metal at high temperature can release high temperature hydrogen and in combination with the cooling water can cause a steam explosion. If the containment is not sufficient (such as in the Chernobyl case) radioactivity can escape.

These are the only major accidents to have occurred in over 16,000 cumulative reactor-years of commercial nuclear power operation in 33 countries. 

The evidence over six decades shows that nuclear power is a safe means of generating electricity. The risk of accidents in nuclear power plants is low and declining rapidly as the old reactors are decommissioned and new modern reactors replace them. The consequences of an accident or terrorist attack are minimal compared with other commonly accepted risks. Radiological effects on people of any radioactive releases can be avoided with careful planning.

Pressure and Boiling Water Reactors

Both these reactors require a constant supply of cooling water to maintain the stability of the reactor core. If the cooling water system relies on electrically driven mechanisms to keep the cooling water moving onto the core, there is a weak spot in the design and is the usual reason why a failure of the electrical system allows a meltdown of some degree to occur. While some of the systems still rely on electricity to release cooling water into the core in the event of an accident, most systems now use passive mechanisms that come into play even in the event that all the redundant electrical supply systems fails.

Molten Salt Reactors

Molten salt reactors combine molten fuel and molten salt. The fuel is chemically bonded to a salt coolant. Because the reactor temperatures (600°-950°C) never approach the boiling temperature of salt (about 1400°C), it remains as a liquid. If the system does somehow overheats a frozen plug melts and fuel drains harmlessly into passive cooling tanks, where further nuclear reaction is impossible, and no electricity or water is required. Unlike the light water reactors, the fuel can later be re-heated and pumped back into the reactor.

In the worst case of a terrorist attack damaging the reactor vessel for example, the fuel would simply spill out with no potential explosion. Radioactive leak is very unlikely because the molten salt and fuel mix is too dense to blow away and because there is no explosion it just sits there. Even if there is water added by the terrorist, the salt and fuel mixture doesn’t dissolve well in water, doesn’t interact with water.

Pebble Bed Reactors

As fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms, but the pebble-bed reactor is designed so that this effect is very strong. Also, it is automatic and does not depend on any kind of machinery or moving parts. As the rate of fission increases, temperature increases and the rate of fission decreases. This creates passive cooling.

Because the pebble-bed reactor is designed for higher temperatures, the reactor will passively reduce to a safe power level in an accident scenario. This makes the pebble-bed design (as well as most other very-high-temperature reactors) different from conventional light water reactors which require active safety controls.

The reactor is actively cooled by an inert, fireproof gas, not water, so will not have a steam explosion as has happened with light-water reactors. The coolant starts and ends as a gas. The moderator is solid carbon and does not act as a coolant.

A pebble-bed reactor can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. Instead the reactor just sits in an idle state. The reactor vessel continues to radiates heat, but nothing else happens. A safety test with the German AVR reactor demonstrated that everything was intact and the fuel balls were completely undamaged.