Approach To Critical - 1/M and Penultimate Method

[dss91]

Can someone explain to me the basic process involved in an approach to critical with a nuclear reactor. I understand that you basically start with the rods all the way in and bring them out, while taking counts - waiting for them to stabilize. I understand with the penultimate method, you change the control rod position and take count rates, and then take the ratio of the count rates and extrapolate to the x-axis with each measurement in order to determine the critical point, however I'm confused as to how the 1/M method works to find the critical position.

Thanks,
Derek

 

[jim hardy]

...however I'm confused as to how the 1/M method works to find the critical position.

I likely don't understand the question:

It's a beautifully simple graphical method.

Plot rod position (or dilution) on Ordinate
1/m on Abscissa

Since multiplication is approaching infinity as you approach criticality,
1/m is approaching zero - but you don't multi-decade log graph paper to see it.

Extrapolate your slope to intersect ordinate and you have an estimate. Closer you get, the better your estimate.

After you do one it's second nature.


Probably i answered the wrong question, though.

 

[raining Pete]

Sure, I'd be happy to explain the basic process for approaching criticality with a nuclear reactor.

First, let's define what criticality means in this context. Criticality refers to the state of a nuclear reactor where the rate of fission reactions is balanced by the rate of neutron absorption, resulting in a self-sustaining chain reaction. In simpler terms, it is the point at which the reactor is producing enough energy to sustain itself.

Now, onto the process. The first step is to start with all control rods fully inserted into the reactor core. These control rods are made of materials that can absorb neutrons, and by inserting them, we are essentially limiting the number of neutrons available for fission reactions.

Next, we slowly start to withdraw the control rods, allowing more neutrons to enter the core and increase the rate of fission reactions. As we do this, we take measurements of the neutron count rate at different control rod positions. This is where the 1/M method comes in.

The 1/M method involves taking the ratio of the count rates at different control rod positions and extrapolating them to the point where the count rate would be zero (M = 0). This point is the critical position, where the reactor is producing just enough neutrons to sustain the chain reaction.

The penultimate method, as you mentioned, is a similar approach where we change the control rod position and take count rates, but instead of extrapolating to the x-axis, we plot the data on a graph and find the point where the count rate levels off. This point also represents the critical position.

Once we have determined the critical position, we can adjust the control rods to maintain the reactor at this point, ensuring a stable and self-sustaining chain reaction.

I hope this helps clarify the process for you. Let me know if you have any further questions.