Types of Nuclear Reactors

October 5, 2016 October 5, 2016

As of now, all commercial reactors in the world are nuclear fission reactors. Such reactors have a turbine and generator to turn and produce electricity. Most reactor types turn water into steam and use a steam turbine while others heat up a gas and use a gas turbine.

Both forms of water viz. light water (H₂O) and heavy water (D₂O) are used in nuclear reactors. The nuclear reactors that use the regular water in a purified form are called Light Water Reactors. On the other hand, some reactors such as CANDU (Canada Deuterium Uranium), employ natural Uranium, which is not enriched, and use “heavy water”. They are called Heavy Water reactors.

Light Water Reactors

The light water reactors are of two types viz. Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR). At present, the PWR are most popular kind of nuclear reactors. Key difference between a BWR and PWR is that

In a BWR, the reactor core heats water which turns to steam and then drives a steam turbine. The reactors at Fukushima Daiichi were among the first reactors of such kind.
In a PWR, the reactor core heats water, which does not boil ^{(because it is pressurised and increased pressure increases the boiling point).} Thus, no steam is produced in PWR because of high pressure. This water can reach higher temperatures and this hot water then exchanges heat with a secondary low pressure water system, which turns to steam and drives the turbine.

Heavy Water Reactors

The heavy water reactors use Deuterium oxide (D2O) as its coolant as well as moderator. These reactors use “un-enriched” natural Uranium for production of energy. Natural Uranium, as we all know is a mixture of many isotopes. It is primarily U-238 and a much smaller amount of U-235. The U-238 can be made subject fission only by fast energy neutrons. Moreover, the fast energy neutrons are quickly absorbed by U-238 and that is why, it is not able to sustain a nuclear reaction. Thus, no amount of U-238 can be made to a self sustaining chain reaction i.e. “critical” in nuclear energy production. This also implies that despite being fissionable, U-238 is not considered a Fissile Material.

The U-235 can sustain chain reaction and that is needed for common nuclear reactors. However, since U-235 is very low in abundance, the natural Uranium is enriched to increase the relative amount of U-235 in the mixture.

The scientists had devised a trick to use the natural un-enriched Uranium. What they did is to use moderators to slow down the neutrons to such a level that:

It increases probability of fission in U-235 within natural mixture
It increases probability of sustained chain reaction in the mixture as a whole.

The neutrons moderators, which absorb some of the neutrons’ kinetic energy play important role here. Water is an excellent moderator. The Hydrogen atoms in the water molecules are very close in mass to a single neutron. When a neutron and a hydrogen atom collide, there is an efficient momentum transfer, akin to the collision of two billiard balls. Apart from being good moderator, water is also a good absorber of Neutrons. This implies that use of normal water will though increase the probability of sustained chain reaction, yet, it will bring down the number of neutrons thus posing a hurdle in sustaining the chain reaction.

Examples of Moderators
Normal Water, Graphite, Heavy water, light metals such as Lithium or Beryllium, Salts of these metals and certain organic compound such as biphenyl and terphenyl are examples of moderators used in nuclear energy production.

On the other hand, heavy water does not absorb the neutrons as readily as light water. This is because, while heavy water collides with neutrons and moderates them similar to the light water but it does not absorb neutrons because it already has the extra neutron that light water would normally tend to absorb. This is the basic premise on which heavy water reactors work. The visible advantage is that heavy water reactors can be operated without the expensive uranium enrichment facilities. The drawback is the cost of heavy water.

Gas-cooled Reactors

The problem with the light water, heavy water, pressurised water and boiling water reactors is that they don’t have great thermal efficiency because they cannot work at very high temperatures. This problem is overcome in the reactors that use gas as a coolant and to drive a gas turbine. Such reactors are called High Temperature Gas-Cooled Reactors (HTGRs). Gas such as helium or carbon dioxide is passed through the reactor rapidly to cool it. HTGRs can operate at very high temperatures, leading to great thermal efficiency (around 50%). These reactors are not only useful for power production but also for other heat processes such in oil refineries, water desalination plants, hydrogen fuel cell production etc. But the drawback of these reactors is that they need highly efficient backup cooling systems because gas is a poor coolant. So, huge amounts of coolant are required for relatively small amounts of power. Therefore, these reactors must be very large to produce power at the rate of other reactors.

Fast Reactors

The above mentioned reactors (LWR, HWR, PHWR, and HTGR) are known as thermal reactors, which slow the high-energy (fast) neutrons down to low-energy (slow) by using moderators. However, in a fast reactor, this process is avoided. The fast reactors use fast neutrons. This means they don’t use neutron moderator. To sustain a chain reaction by fast neutrons, the fission material needs to be highly enriched. Since, Uranium enrichment is highly costly affair, the production of energy from Fast readers is so far uneconomical. To achieve criticality, they need higher amount of Uranium fuel also. The advantages they offer are that they reduce total radio toxicity of nuclear waste, and dramatically reduce the waste’s lifetime.

The fast breeders usually use liquid sodium metal as the coolant, at or near atmospheric pressure, thereby obviating the need for pressure vessels. Because the boiling point of sodium is quite high, fast reactors can operate at a considerably higher temperature than LWRs.

Breeder Reactors

In the above description, we have read that despite being fissionable, U-238 cannot be used in nuclear reactors on its own because it’s not a fissile product i.e. it cannot sustain a chain reaction. To be a useful fuel for nuclear fission chain reactions, the material:

Should be able to sustain a chain reaction
Should have a high probability of fission when bombarded with slow / fast neutrons
Should release two or more neutrons on average per collision so that it can compensate for non-fissions, and absorptions in the moderator
Should have reasonably long half-life
Be available in suitable quantities

U-235 fits in all of the above criteria except the last one. It is available in low amount in Natural Uranium, which needs to be enriched to increase ratio of U-235. It is only U235 that can be split using a slow neutron beam, producing enormous amounts of heat, to boil water, generate steam and run a turbine like in any other power station.

Breeder Reactor: Definition
A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile fuel from fertile material like uranium-238 or thorium-232. Breeders were at first considered attractive because of their superior fuel economy compared to light water reactors. Interest in breeders declined after the 1960s as more uranium reserves were found and new methods of uranium enrichment reduced fuel costs. In more recent decades, breeder reactors are again of research interest as a means of controlling nuclear waste and closing the nuclear fuel cycle.

U-238 is not fissile; but some of U-238 converts itself into Plutonium (Pu-239), if exposed to fast neutrons. This new element, Pu-239, can be easily burnt to produce power or to make nuclear weapons.

If we mix 25-30 per cent of Plutonium with U-238 and expose it to fast neutrons in a reactor, the Plutonium will burn and give us about 20 times more power than the natural uranium reactors now in operation. Meanwhile, some of the U-238 in the fuel would absorb some fast neutrons and get converted again into Plutonium. Since roughly 1.1 kg of plutonium comes out of the spent fuel due to this conversion, for every 1 kg that was initially put in the fuel rod, such reactors are called breeder reactors. Since fast neutrons are used to trigger the chain reaction, such reactors are called fast breeder reactors. Additionally, if we cover the reactor core with a blanket of either U-238 or Thorium, then this blanket captures some of the fast neutrons coming out of the core, which would have escaped and been wasted. On reprocessing this irradiated blanket, we could recover either Pu-239 or U-233, which is akin to U-235 is a fissile material.

Examples of Fissile Isotopes: U-233, U-235, Pu-239, Pu-241
Examples of Fertile Isotopes: U-238, Pu-240, Th-232

The above information can be summarized in the following:

U-235 is found in natural Uranium and is fissile
U-238 is found in natural Uranium and is NOT fissile. But, since U-238 can be converted into a fissile material by neutron absorption and subsequent nuclei conversions. Thus, it is called “Fertile Material”.
Plutonium-239 is a fissile material and is bred from U-238 by neutron collision (or capture).
When Plutonium-239 captures a neutron during nuclear reaction, some fraction of it would release energy but some fraction would convert itself in Plutonium-240, which is NOT fissile. So, for good health of a nuclear reactor (and also Nuclear Bomb), fuel needs to be as low in Pu-240 as possible. Again Pu-240 is a fertile material.
When Plutonium-240 captures a neutron, it converts into Pu-241 and it is fissile. Again, Pu-239 and Pu-241 are called weapon grade plutonium.
When Thorium-232 is used and captures a Neutron, it becomes Uranium-233 and that is also fissile. Thorium-232 is neither fissile nor fissionable but a fertile material.

Thorium Reactors / Thorium Fuel Cycle

Thorium-232, as we discussed above, is a fertile material. In natural conditions, trace amounts of Th-231are also found which is a fertile material but is so less than it is not even discussed among fissile materials. The first advantage of Thorium-232 is that it is four times more abundant in nature than Uranium, and is widely distributed throughout the Earth’s crust. Thorium from monazite sand can be converted into fissile uranium and used to feed nuclear reactors. Apart from that, it can be used multiple times to generate electricity, thus creating an endless cycle of fuel availability. This is called Thorium Fuel Cycle.