Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

Currently, there are 438 nuclear power reactors in operation in the world today. Two are currently being decommissioned, yet 71 are undergoing construction. Together, they generate almost 400,000 MWe of power. In 2014, nuclear reactors produced over 11% of the entire world's energy production. All that power coming from a radioactive source begs the important question: What would happen during a nuclear meltdown?

There are many insinuations attached to nuclear power. In history, there have been two catastrophic nuclear meltdowns that resulted in human casualty and untold environmental damage. However, since the events following Chernobyl and Fukushima, nuclear reactors around the world have undergone significant modifications to ensure events that have happened in history's past never occur again.

Perhaps the safest reactors in the world belong to no other than Canada, one of the world leaders in nuclear power generation and technologies.

The CANDU Reactor

The CANDU reactor earns its name from the land it was invented in - CANada. It also used deuterium oxide (heavy water) as a moderator, and uranium as a fuel source.

The reactors are unique in that they employ technologies most other reactors cannot MATCH.

The advanced power generator is the most efficient of all uranium-powered reactors. In comparison to other reactor types, the CANDU reactor uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced. The reactor also does not require enriched uranium, cutting out the necessity of an extra refinery step.

"CANDU reactors can be refueled while operating at full power, while most other designs must be shut down for refueling. Moreover, because natural uranium does not require enrichment, fuel costs for CANDU reactors are very low," explains the Canadian Nuclear Association.

Without the necessity of enriched uranium, CANDU reactors operate with comparatively less uranium, and therefore, less cost. Furthermore, the radioactive waste is significantly less dangerous.

How it works

Nuclear reactors are remarkably quite simple. With the CANDU reactor, it generates energy by harnessing the energy from a fission reaction. Fission occurs when an unstable atom splits, releasing radiation and heat.

The reactor's fuel source is comprised of naturally occurring Uranium. The unstable properties of Uranium cause the atom to split into more stable isotopes, resulting in the release of heat and radiation.

The radiation that results creates a chain reaction by splitting other nuclei, creating more heat and more radiation. Part of the decay process relies on the emission of neutron radiation. As neutrons are ejected at high speeds, they collide with other nuclei to initiate the decay of other atoms,  continuing the chain of fissile reactions.

All of the uranium is contained within specialized fuel rods.

The fuel rods heat up significantly and need to be cooled by water. Water flows over the rods to cool them down while simultaneously causing the water to heat up rapidly. The heat and pressure can then be harnessed by a steam turbine.

In CANDU reactors, heavy water is used to cool the rods. However, since the water passes over the rods, it is exposed to dangerous amounts of radiation.

To prevent radiation leaks, the heavy water flows through a heat exchanger which transfers most of its heat to a separate cooling system without mixing the coolant. The heat is transferred to an independent water flow that remains non-radioactive.

From there, the water boils to produce steam and pressure. A turbine can then harvest the energy and produce copious amounts of energy for even the smallest of reactors.

Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

[Image Source: CANDU via Canadian Nuclear Association]

Small reactors can provide power to millions of homes

Canada's smallest CANDU reactor, located in Pickering, Ontario, contains just four CANDU reactors. Despite the small size, the power plant provides enough energy to supply 2.5 million households with electricity.

CANDU reactors are incredibly safe and efficient to operate. However, within the reactor remains highly radioactive isotopes. If handled incorrectly, the outcome would be devastating.

To ensure the absolute safety of their plants, CANDU reactors make use of some of the most advanced and safe technologies that prevent the worst case scenario: a nuclear meltdown.

Preventing a Nuclear Meltdown

At the heart of a nuclear power plant is the nuclear reactor. Water pumps continually circulate coolant over the rods and through the reactor to ensure the temperatures are sustained at safe levels. The entire reaction process is contained within the calandria, a sort of highly reinforced shell that completely encircles the reactor.

Under normal operation, the reactor is controlled by increasing, decreasing or stopping the chain reaction happening inside the reactor.

Control rods within the reactor core can be raised and lower to adjust the fission rate of the Uranium. Control rods are comprised of elements including Boron, Silver, Indium, and Cadmium - all of which are sufficient at absorbing neutrons - an important feature in slowing neutrons down (the particles that initiate and increase the chain reaction).

During the reaction of Uranium, neutron radiation is released. When neutrons are ejected from Uranium during the fission process, they collide with other atoms and initiate more reactions. Since control rods are sufficient at absorbing neutrons, when introduced into the reactor core, they intercept rogue neutrons and substantially slow down the fission process.

Backup safety systems

However, should the control rods fail to slow the rate of reaction down to sustainable levels, a secondary safety system will detect the irregularity and will automatically inject a poison that will immediately stop the chain reaction.

The liquid poison control system introduces a solution of boron as boric anhydride, and gadolinium as gadolinium nitrate, dissolved in D2O (heavy water).

Similar to the control rods, the poison intercepts neutrons, preventing a chain reaction from cascading into a nuclear meltdown.

Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

Both the control rods and poison system act as first and secondary measures against nuclear chain reactions. The vault contains the calandria and also houses heavy water. [Image Source: Canadian Nuclear Safety Comission]

Both the control rods and the poison injection system are activated automatically and function without power. However, they can also be manually controlled. The systems are regularly tested and checked under strict regulation.

What happens during power failure

In the event of a power failure, both the control rods and injection systems will automatically activate, stopping the chain reaction.

However, the fuel rods still generate heat and require cooling. The heat produced, known as decay heat, represents a small fraction of the heat that is produced during normal operation.

The power plant has multiple sources of backup power including the power it generates itself to keep the pumps circulating water and keeping the reactor cool. The nuclear power plant requires just one reactor to power all of the water pumps to cool the fuel rods.

However, should every reactor be shut off with no availability to external power, emergency power generators are kept on-site to ensure the water pumps are continuously powered.

Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

Backup generator [Image Source: Canadian Nuclear Safety Comission]

At every nuclear power plant in Canada are at least two or three standby power generators, two or three emergency power generators, and emergency batteries.

In the extremely unlikely event of a total station blackout, nuclear power plants have even more backup systems to ensure the plant does not melt down.

Natural Circulation

At this point, with no access to external power, and with the failure of multiple safety systems, emergency safety procedures would begin to initiate.

Assuming there is no external power, internal power, and no means of power from backup generators, CANDU reactors will continue to naturally cool the reactors via natural circulation. The decay heat of the reactor core will constantly be fed a supply of water without pumps, as long as the water basin above the reactor is kept full.

A backup water supply will provide water to the steam generators to maintain coolant circulation. Over prolonged periods, water will need to be continually added to the basin to ensure constant circulation.

Emergency backup equipment

During this time, emergency mitigation equipment is brought in to ensure the reactor is constantly cooled. In response to the Fukushima disaster, all Canadian power plants now have mobile emergency equipment on standby. Mobile pumps and fire trucks can be used to cool the reactor.

Steam may be released from the steam generators to reduce the amount of heat and pressure build up. This steam comes from the secondary coolant system and is completely safe and is not radioactive.

Up until this point, no radiation has been released and the reactor has sustained no damage. According to the Canadian government, the power plant can still be brought back online after undergoing a series of checks.

Total System Failure: The Beginning of a Meltdown

Assuming all backup safety equipment fails and natural circulation is not maintained, the heavy water will begin to boil within the vault. Radioactive steam is produced, however, the reactor building will contain all of the radiation.

The heavy water will continue to boil until it completely evaporates. The heavy water contained within the calandria would also boil, causing damage to the fuel rods.

It is important to note that emergency systems can stop the damage to the reactor by adding water to the calandria.

However, if no emergency measures intervene, the water will continue to boil and the reactor will sustain significant damage. More radioactive steam is generated, causing the pressures inside the reactor building to rise.

Pressure reduction systems

To prevent damage to the reactor building, the pressure must be lowered.

In single reactor plants, emergency water is sprayed into the building. The water cools and condenses the steam, significantly reducing the pressure.

To control the internal pressure at a multi-unit reactor, pressure can be relieved by releasing steam into a massive vacuum chamber.

 Nuclear Meltdown: What Would Happen in the Worst Case Scenario?

Cutaway of a multi-unit nuclear CANDU reactor [Image Source: Canadian Nuclear Safety Comission]

Like the safety systems mentioned before, the vacuum building will still operate without power.

Water can also be injected into the vacuum chamber to further reduce steam. As one of the final safety stages, a backup liquid nitrogen supply will be injected to cool the reactor.

If emergency operations still fail to add water to the calandria, the heavy water will completely evaporate, causing the nuclear fuel to melt. The fuel would begin to heat up the water that remains in the vault which contains the reactor.

Production of Hydrogen

When the Uranium melts, it produces hydrogen. Further safety devices convert some of the Hydrogen into water, preventing the explosive gas from accumulating within the reactor building.

Up until this point, there have not been any radiation leaks into the environment. However, at this stage, emergency operations are put into effect so controlled venting can release some of the radioactive hydrogen gas and radioactive heavy water.

If sufficient emergency services are still not employed, the fuel will evaporate all of the water in the vault. The fuel will melt through the foundation onto a thick concrete slab. Evacuation procedures would have initiated to remove people around a large radius. Then, recovery operations would be put in effect to contain the site.

However, the probability of an event cascading into such a severe scenario is extremely unlikely. In modern nuclear reactors, many fail-safes ensure the utmost safety of the environment and the people around it.

Beyond the Dangers

Nuclear power offers a viable alternative to fossil fuel power generation. In the last few years, nuclear reactors have significantly reduced the carbon load on the planet. In history, there have been a few minor incidents two major incidents involving the release of radiation.

However, when employed properly, nuclear power generation is an efficient means of power generation. Currently, there are not enough renewable energy sources available to amass the incredible amount of energy nuclear power plants produce.

With global warming, the world cannot afford the addition of fossil fuel power plants to make up for nuclear. For the time being, nuclear power plants are necessary to provide the world with enough electricity.

That being said, substantially more research needs to be invested into finding viable renewable alternatives. Also, discoveries still need to be made to devise methods of how to safely deal with radioactive waste. Perhaps the solution may be abandoning fission technologies altogether in lieu of fusion power. However, at this point, governments are not investing enough money into alternative sources.

Until then, it is imperative to ensure nuclear power plants that continue to operate today are forced to abide by the strictest regulations governing the implementation and safety of their operation to prevent a nuclear meltdown. It is not a perfect solution, alas, it is a solution that works - for now.

Sources:  Canadian Nuclear Safety ComissionInternational Atomic Energy Agency, Canadian Nuclear Association

SEE ALSO: Turning Nuclear Weapons into Nuclear Fuel

Written by Maverick Baker

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