BY DR. JOSE PALU-AY DACUDAO
WATER acts as the primary coolant. Water under high pressure is pumped to the reactor core. In the reactor core, uranium is fissioned, thus releasing heat. The heat transfers to the water. The water coolant is pumped to a steam generator. The heat energy from the high pressure primary water coolant is transferred to a secondary coolant, namely also water but under lower pressure. Steam is produced by the secondary water coolant. The steam drives turbines. The turbines spin an electric generator. Electricity is generated.
There should be a water source and outlet. Not a problem in the Philippines where there are lots of rivers and seas.
Pressurized water reactor (PWR) and the boiling water reactor (BWR) both use ordinary water (there are reactors that use deuterium enriched ‘heavy water’ such as the Canadian CANDU) as both coolant and neutron moderator.
US and French companies (and also the Russians) have had the most experience in building and maintaining PWRs.
There are other coolants. Canadian CANDU reactors use deuterium enriched heavy water. Its main advantage is that it can use unenriched uranium. (Natural uranium metal is made up of two isotopes, U-238, and the rarer U-235. It is U-235 that fissions in nuclear reactors. Thus light water-cooled reactors use uranium fuel enriched in U-235.) However, you have to have a process of getting deuterium.
Ordinary water mostly consists of an oxidized single proton, also called protium. It is chemically protium oxide. But heavy water is enriched in deuterium oxide. Deuterium is a hydrogen isotope with one proton and one neutron. (Thus it is heavier than regular water’s protium oxide.) When deuterium absorbs a neutron in the reactor core, it transmutes into tritium, a radioactive hydrogen isotope with one proton and two neutrons. It has a half-life of about 12.5 years. You have to have another system of isolating radioactive tritium, storing it until it decays into stable helium-3, or selling it. (Tritium has its uses for the military. It’s actually the hydrogen isotope used in fusion bombs.)
The most promising coolants in 4th gen nuclear reactors are lead, molten salt, and helium. More on that in later articles.
The third most important classifications system of nuclear plant reactors is by neutron moderator. Uranium fuel, specifically U-235 nuclide, fissions more easily if fast moving neutrons are slowed down. The term used is ‘moderated’. As previously mentioned, pressurized water reactor (PWR) and boiling water reactor (BWR) both use ordinary water as neutron moderator.
There are other neutron moderators. Graphite (elemental carbon) is the next most commonly used. The first nuclear reactor during WW2, which the US used to develop the atomic fission bomb, used graphite. China’s 4th gen pebble bed reactor uses graphite as moderator and helium as coolant.
The fourth most important classifications system of nuclear plant reactors is by related on its neutron moderation. Those that moderate neutrons from the fissioning uranium are thermal-spectrum or slow reactors. Today’s commercial reactors are mainly of this type, either moderated by water or graphite as mentioned above. Fast reactors, on the other hand, use unmoderated, also called fast, neutrons. There is no neutron moderator. Its advantage is that it can to breed fissile plutonium 239 and possibly heavier transuranic actinides from the common uranium isotope, the fertile U-238. Thus fast reactors are also called fast breeder reactors (FBR). They can breed more fuel than they use.
Furthermore, fast reactors have the potential to fission the transuranic actinides. Fast reactors can use these as fuel, and thus they cease to be waste. The most problematic nuclear waste are these actinides because they are so long-lived, with half lives in the hundreds of thousands or millions of years, and thus present a permanent radioactive hazard for humans. As of the present, only Russia’s sodium cooled fast nuclear reactors can fission these waste radioactive actinides.
More on this topic in future articles. (For comments and suggestions please email to mabuhibisaya2017@gmail.com)/PN
