# Approach To Critical - 1/M and Penultimate Method

1. Feb 20, 2013

### 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

2. Feb 21, 2013

### jim hardy

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.