NRC Science 101 – Quantities and Units of Measure

Suzanne Schroer
Reliability and Risk Analyst

In the last Science 101 blog post, we discussed measurements made in various units of measure, particularly meters, grams and liters. You might be wondering where these units of measure come from and how they relate to one another. If so, you’re in luck, as that is the topic for today’s post.

Quantities are characteristics or properties we are trying to measure, such as the length of an object. Units of measure are how we express measurements of quantities. For length, the unit we would use in science is meters. A unit is really only a particular amount of some quantity used as a reference point for measurements of that quantity. Put differently, units of measure are chosen and accepted by the people who use them.

Often, units of measure were agreed upon many years ago. One meter is as long as it is because that’s what scientists agreed to use as a base unit for length. A meter could have been some other length. For the sake of establishing well-defined and easily-accessible units of measure, the General Conference of Weights and Measures (a collection of scientists from multiple countries) created in 1960 the Sytème International d’Unités (otherwise known as the SI¬). This system relies upon the base units of measure listed in the following table.

Often times, however, the base unit of measure can be either too large or too small to be useful in describing a particular measurement. For example, while we could talk about the distances between cities in meters, we would be using very large (and, as such, cumbersome) numbers. For example, from Portland, Maine, to St. Louis, Missouri, is 2,060,000 m or 2,060 km. Similarly, if we are talking about the size of atoms, the basic building blocks of matter, speaking in terms of meters would be difficult as the diameter of a hydrogen atom is only 0.000000000120m. So, instead, for the sake of convenience, we often use prefixes to modify the size of the base unit. The following table lists a number of such prefixes using meter for the base.

The above table lays out conversion factors, ratios that can be used to convert one unit to another. For example, one such ratio, expressing the relationship between kilometers and meters, would be 1 km / 1000m. Using this ratio, we can convert 12348m into 12.348km (12348m x 1km / 1000m).

The NRC uses these same ideas when measuring radiation. These measurements will be discussed in an upcoming Science 101 post. As always, thank you for reading the NRC’s Science 101 blog series.

Keeping Knowledge “In the Family” at the NRC

Tom Boyce
Branch Chief, Regulatory Guide Development Branch

The NRC’s Office of Nuclear Regulatory Research (RES) produces regulatory guides to provide guidance that nuclear utilities can follow to meet NRC rules for the design and operation of their nuclear plants. The NRC staff also use regulatory guides to review applications from the utilities for plant permits and licenses. New guides are developed when needed – and existing ones are regularly reviewed and updated.

The Regulatory Guide Development Branch (part of RES) is very concerned with promoting “knowledge management” within the agency. We have an obligation to preserve key technical information especially as people leave the agency. Therefore, we have taken steps to make sure we capture, preserve, and transfer important technical knowledge.

NRC knowledge management especially is focused on two major ideas. First of all, it is focused on capturing and preserving agency knowledge (technical information) while the information is fresh and available. Second, and equally as important, knowledge management also means ensuring the collected information is transferred to the next generation of NRC staff.

To do this, we encourage NRC staff members to write their knowledge into regulatory guides as they identify technical and regulatory issues. This helps keep the knowledge in a permanent and long-lasting record. This also helps prevent technical information from becoming difficult to find over time. This is a very important responsibility, especially because of staff retirements or reassignments.

In addition, the NRC staff teams up its junior staff with senior staff to perform the reviews and updates of the regulatory guides. This approach effectively helps pass on regulatory information to the junior staff. It also ensures that this important guidance continues to be available to the NRC staff, its stakeholders, and the public.

Critical knowledge can’t be lost if the NRC is to continue to meet its important mission. We make sure it isn’t.