Reactivity II

WARNING!!! ... Today topic is slightly nuclear science tuned. Therefore it might be a bit complex, but we are ready to be received comments and questions. =)

Recalling back, reactivity is define as fractional change in neutron population per generation. But today we are more into reactivity coefficient.

In general, reactivity coefficient is governed by 4 other important coefficients which are:

1) Moderator temperature coefficient

2) Fuel temperature coefficient

3) Pressure temperature coefficient

4) Void temperature coefficient

Moderator temperature coefficient, are found to be more effective for light moderators rather than heavy moderators. This is because, light moderator can remove more energy than heavy moderators. Do note that moderator can be in 2 conditions which is under moderated or over moderated. Under moderated condition will help the nuclear system to have self-regulating effect. This means that the reactivity and power produced by the nuclear power plant is under control automatically due to the nature of the system. This will be a relief to all nuclear engineers. On the other hand, over moderated is very dangerous. This is because, over moderated condition will caused the reactor to have higher reactivity, and this will lead to more power being produced. With the high amount of power being produced, the temperature of the core will significantly increase and causes the core in the nuclear reactor to be melted down. Hence, for every nuclear reactor, under moderated condition is always the best choice. But, how do we know if the system is under moderated or over regulated? This can be done by calculating the moderator-to-fuel-ratio, which is a ratio of moderator and fuel.

More explanation regarding moderator will be given in future, I promise. This is because, we need to fulfill certain criteria for our blog on a daily basis and i am rushing out of time.

Fuel temperature coefficient is rather hard to be explained without the graphs, but in short, it can be defined as the change in reactivity per degree change in fuel temperature. This means that there is an increased tendency of the fuel to absorb neutron as the core temperature begins to increase. This is because, an increase in the core temperature will causes the power level and the control chain reaction to be increased. Hence, it will contribute to higher reactivity.

Pressure temperature coefficient is usually negligible in reactors. This is because, most nuclear reactors uses sub-cooled liquid which means that the liquid density is nearly constant and does not change significantly within the nuclear power plant operating pressure.

Void temperature coefficient, is the change in reactivity per percent change in void volume. Hence, it means that void temperature coefficient is the result of the formation of the steam voids in the moderate. An example is, in theory air bubbles formation in most pipe (closed system) calculation is neglected if and only if the value is small enough. However in reality, air bubbles really does exists and it does contribute to a very minor extend such that its existence can be neglected.

That's all for now ....

P/S Sorry for no cool pictures were being uploaded, but it will ... tomorrow on the fun facts label!!