Neutron moderator, nuclear reactor coolant and control rods

Yo peeps (notice my ‘Mat Salleh’ wannabe?). How y’all doing? (from Memphis apparently). Alright, let’s cut to the chase. For this third entry, we will talk about neutron moderator, nuclear reactor coolant and control rods.

Before we go any further, let’s take a ride on some nuclear lesson shall we? First off, what exactly is a nuclear reactor?

Nuclear Reactor:

·         A device to initiate and control a continuous nuclear chain reaction−splitting of the nucleus of an atom into smaller parts often producing free neutrons and photons (fission) of heavy isotopes like ²³⁵U−that releases several million times more energy per reaction.

·         It converts the thermal energy released from nuclear fission to boil water, producing pressurized steam which will then drive a steam turbine that generates electricity (just like conventional fossil fuels power stations).

·         It has undergone many revolution and currently, the Generation III is in operation and development like AP-1000(a 2-loop pressurized water reactor (PWR)),EPR(a  4-loop PWR), ATMEA-1(a 3-loop PWR)and many more.

Well then, enough of this confusion and let’s move on to the next puzzlement of the day, neutron moderator. Whenever nuclear fission occurs, neutrons that are normally bounded into an atomic nucleus are freed with energies of several MeV (mega electron volt). 

The initial high kinetic energy of the newly freed neutron is reduced by a process known as moderation where a material known as moderator transfers the neutron kinetic energy into it (remember, energy is always conserved!) Since along with the reduction of energy comes a reduction in speed, it is otherwise known as neutron slowing down.

In simpler words, a medium that reduces the speed of fast neutrons thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 is called a neutron moderator.

Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Two down, two more to go.

Where was I? Oh yeah, nuclear reactor coolant. As the name itself implies, a nuclear reactor coolant cools down a nuclear reactor by absorbing the heat from the core and transferring it to electrical generators and the surrounding environment. How does it work you ask? Good. I love questions (notice me pretending to be all enthusiastic and committed for a minute there?) It’s fairly simple really:

• The coolant (water, gas, liquid metal, molten salt) circulates past the reactor core and absorbs heat generated there
• The heat absorbed is then carried away to generate steam that moves a turbine

Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for example the boiling water reactor.

Frequently a chain of two coolant loops is used because the primary coolant loop takes on short-term radioactivity from the reactor (relax, sounds bad doesn’t it? But believe me the safety system now is really reliable. Honest!)

Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase change to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure. Current reactors stay under the critical point at around 374 °C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point.

Fast reactors have a high power density and do not need neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead and other metals have also been proposed and occasionally used.

Molten salts share with metals the advantage of low vapor pressure even at high temperatures, and are less chemically reactive than sodium. Salts containing light elements like FLiBe can also provide moderation. In the Molten-Salt Reactor Experiment it even served as a solvent carrying the nuclear fuel.

Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity, necessitating rapid circulation. Carbon dioxide has also been used. Gases of course need to be under pressure for sufficient density at high temperature.

Last but by no means the least, control rod. Now, we know when a neutron is absorbed normally fissions will occur. Control rods however, though absorbs neutrons is made of chemical elements that prevents them from fissioning. Due to this unique property, they are used in nuclear reactors to control the rate of fission of uranium and plutonium.

Then again, one control rod is made of different element than another so it is only logical that they differ in terms of its capture cross sections for neutrons of varying energies. Therefore, the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control.

• Light water reactors (BWR, PWR) and heavy water reactors (HWR) operate with "thermal" neutrons
• breeder reactors operate with "fast" neutrons

How does it work one might ask? Well you see:

·         Control rods are usually combined into control rod assemblies — typically 20 rods for a commercial Pressurized Water Reactor (PWR) assembly — and inserted into guide tubes within a fuel element.

·         A control rod is removed from or inserted into the central core of a nuclear reactor in order to control the neutron flux — increase or decrease the number of neutrons which will split further uranium atoms.

·         The control rods are partially removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor

·         This in turn affects the thermal power of the reactor, the amount of steam produced, and hence the electricity generated.

Finally, we’ve come to an end of this excruciating, torturous entry. You think you got it bad? Try writing them. All I can say is, this is only the third entry (how many more left? Huh? Pardon? 17 or more? Really? Really really?) yet I’m already spent (two shifts this week!) Still, what knowledge isn’t tiring right? Rome after all wasn’t built in a day. So does a nuclear power plant it seems. Till we meet again (and again, and again, and again...echo continues...), Assalamualaikum w.b.t and buh-bye.

neutron moderator

nuclear reactor coolant 

control rod

class notes