BY DR. JOSE PALU-AY DACUDAO
WHAT ARE the types nuclear plants of the world? There are various classifications.
Below is a review mostly taken from the previous article, plus comments on how they can be used in the Philippine setting (as an earthquake zone).
By generation, there are first, second, third, fourth generations of nuclear plants. The newest, safest, most efficient are the fourth gens.
At present, only Russia and China are building them for commercial purposes. Russia is concentrating on lead-cooled and sodium-cooled nuclear plants. Russia has functioning commercial sodium-cooled reactors that can even consume nuclear wastes.
China at present has a functioning pebble-bed reactor providing commercial electricity to a power grid, and is also building a molten salt reactor prototype.
The main disadvantage to fourth gen nuclear plants is that they are relatively untested. However, their performances (few though they are) in Russian and China bespeak well of them thus far.
The main advantage for the Philippines setting (especially in the issue of resistance to earthquake damage) is that fourth generation nuclear reactors do not need a water coolant. In water-cooled reactors, failure of the water-cooling system will lead to a meltdown, as the fuel and waste radioactive material continue emitting heat. Eventually the encasing structures melt.
What are the present-day working fourth generation nuclear reactors?
One is the pebble bed reactor (PBR). China has two them. One is an experimental prototype. The other is commercial, and has just recently been connected to the local electric grid.
PBRs were first built by Germany (China has obtained patents for them) in the 1980s. They were discontinued after they ran into technical problems, including a leak. The fuel is uranium oxide encased in a pebble composed of multilayered graphite and silicon carbide. The moderator is graphite and the coolant helium.
Helium is quite safe as it is inert chemically and does not absorb neutrons thus transmuting into radioactive nuclides. However, theoretically the helium coolant might carry off radioactive graphite dust in case of a major breach.
Second is the Molten Salt Reactor (MSR). China has an experimental prototype. Again the coolant isn’t explosive water. Instead MSR uses molten salts of fluorine. The fuel is uranium fluoride, but theoretically can be thorium fluoride or even fluorides of waste actinides -plutonium fluoride, neptunium fluoride, and so on. MSRs can be designed as to become breeder reactors.
Note that the most long lived and dangerous radioactive wastes are actinides. MSRs, if designed as a breeder reactor, can consume actinides as fuel. You turn dangerous nuclear waste into fuel. In case of leaks, the molten salt fuel and coolant just solidifies under room temperature into salt crystals. Thus, quite safe.
The main disadvantage of the MSR is that fluoride salts are quite corrosive. Physical damage to the piping system may ensue. Research is still being done in order to solve this problem.
Russia is concentrating on metal-cooled reactors. These are also fast breeder reactors, capable of burning long-lived actinides. As mentioned above, you turn dangerous nuclear waste into fuel.
At present Russia has two commercial sodium-cooled reactors.
The main disadvantage is that sodium elemental metal combusts explosively when exposed to oxygen or water. (Thus the molten sodium is placed under an atmosphere of inert argon gas.) This has already happened to a Japanese experimental sodium-cooled reactor.
The most promising Russian design seems to be the lead-cooled reactors. Russia (then part of the Soviet Union) actually had a lot of experience in building nuclear powered submarine driven by lead-bismuth eutectic nuclear reactors during the cold war. The problem of incorporating bismuth is that it transmutes into highly radioactive polonium when exposed to neutrons.
Thus the Russians have done away with the bismuth and instead are building purely lead-cooled reactors. Lead is highly resistant to transmutation. (To be continued)/PN
