NuclearReactor - a simplified Online Simulation if a nuclear reactor for your personal computer

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NuclearReactor is a so-called Monte-Carlo simulation of a strongly simplified nuclear reactor. With this programme you can try to control a chain reaction. Different experiments are possible, which illustrate the basic physical principles. You just have to download and to start the executable.

The new edition of NuclearReactor offers a couple of new features, among others the simulation of burnup, which means that you can study the effect of the expiring fuel during the reactor operation. In the end, the chain reaction stops by itself, because the quantity of fissile material decreases and the fission products start to absorb neutrons. The effect is of course again modelled in a qualitative way - the duration of a reactor campaign is only several minuts - a real reactor runs over a year or more...

To learn more about all new features, click here!

The present version 1.2 offers again the demonstration of the working principle of the reflector. in combination with the burnup option there are many nice new experiments possible. A reflector is basically nothing else than "moderator" substance put around the "reacotr core". The two terms "moderator" and "reactor core" are explained in the text below. For the description of the reflector simulation, click here! Beside this new feature, the reactor simulator is operated in the same way as before in version 1.0. Here is the description of the basic functions of the programme:

The programme calculates the life history of each neutron individually. This is a so-called Monte-Carlo simulation. The stochastic processes that determine the fate of a neutron are calculated by the help of a random number generator. The PC plays dice with the neutrons. The calculation efforts are huge, that's why only up to 100000 neutrons can be simulated in the same time, otherwise the calculation would become boringly slow. In a real reactor there are very much more neutrons.... That's why you should have a powerful PC - clock rates of 1, better 2 GHz are advised.

On the left side you find the reactor core, that's where the nuclear fuel is and where the fission takes place. It consists of fuel elements containing Uranium-235 (green), of the water as moderator (yellow) and the control rods (blue), which are made from a strongly neutron-absorbing material.

When the reactor is operating, a lot of neutrons fly around there. There are two kinds of neutrons: fast ones (red dots), which are liberated during the fission of a uranium nucleus, and thermal ones, which are the result of a collision of a fast neutron with a hydrogen nucleus of a water molecule. The latter are slowed down, because during the collision they transfer all their kinetic energy to the hydrogen. This is how the moderator works. Only thermal neutrons can cause a new fission when they hit another uranium nucleus. While fast neutrons literally run over the playground, thermal ones are quite slowly migrating. They are not faster than the thermal movement of the water molecules, that's why their name.

A thermal neutron can - with a certain probability - induce a fission, as long as it is located in green the fuel region. In the result, three new neutrons are born, which are fast in the beginning. With a little luck they will be thermalized in the moderator (= neutron brake) region (yellow), before they fly off the core. At this place the circle of life of the neutrons closes and the chain reaction becomes possible.

After staring the programme you'll not find many neutrons in the core. There are just very few of them generated by spontaneous fission or coming from cosmic radiation. They are soon absorbed by the safety and control rods and a chain reaction does not develop.

To start-up the reactor, you need to introduce the neutron source. In principle, the reactor can also be launched without it, since a few neutrons always are found. But using a source we'll immediately get a measurable neutron density in the core - a good condition for controlling the reactor, similar to switching the light on at the car by night. Press IN below the Neutron Source button. Now fast neutrons shot out-of the source in all possible direction. They're moderated and can already cause fissions from time to time. As a consequence, the neutron density increases. But the reactor is still subcritical. You can find that out by extracting the source again. After this, the neutron density rapidly decreases, that means there is no self-sustaining chain reaction.

Now take out the safety rods after re-inserting the neutron source. Since less neutrons are absorbed, their number further increases. Due to the multiplication of neutrons, the reactor works as a neutron amplifier. But it is still subcritical - after extracting the source, the number of neutrons decreases.

Now you can try to make the reactor supercritical. Pull control rods out, let's say until the half of the core height. Now the number of neutrons continues to rise even if the source is out. This rise occurs exponentially - take care! Don't forget to switch into the next measuring range of the neutron detector. If its reading exceeds 90 % of the measuring range, the reactor is scramed (scram = safety control rod axe man, a term introduced during the first chain reaction experiment of Fermi. One of the men of the team had to stand-by with an axe to cut the rope of the safety rod in case of emergency. Today the word "scram" is used for an emergency shut-down). All control and safety rods are immediately injected into the reactor core, to stop the chain reaction.

Be careful, don't change to fast to the higher measuring range neither. There is a minimal reading of 3 %, if you violate this, scram is triggered, too. This is a penalty for an inattentive reactor operator. There are still a couple of other interlocks. Control rods can be pulled only when safety rods are already out. Those can be extracted only if the neutron detector is in the lowest measuring range.

After a while, the supercritical reactor produces more and more fissions per unit time, causing the power meter to come close to the 100 % margin. Attention! At 120 % nominal power scram in initiated! Stabilize the power, for example at 100 % - for this lower the control rods. Seek for the point where the reactor becomes critical. that means where accurately one of the three neutrons generated during a fission act initiates one new fission in the next generation. The other two neutrons either leave the reactor or are absorbed in the control rods or even by the moderator. If you want to lower the power..... But I guess you already understood, how it works. Scram can be initiated also manually by pressing the corresponding button, with other words, nothing bad can happen! Go a couple of times up and down with the power and you'll soon get the right feeling for the machine. You can once try to start-up the reactor without neutron source or perhaps once extract all control rods completely. That's all very educating.....

That is, for the time being, what you can do with NuclearReactor 1.0. The next issue will demonstrate the function of the so-called neutron reflector. Later comes the burn-out etc. If you have fun or comments, let me know: mailto:hmprasser@yahoo.com. More information and other links about nuclear technology you can find under http://www.infokreis-kernenergie.org/ und natürlich bei http://www.ktg.org/.