# What is Critical mass: Definition and 17 Discussions

In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fission cross-section), density, shape, enrichment, purity, temperature, and surroundings. The concept is important in nuclear weapon design.

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1. ### MCNP: Critical Mass of UO2 (10% Enrichment)

I wrote a program to determine the critical mass of uranium oxide with an enrichment of 10%. I got a keff approximately equal to 1 with the selected volume and density (attached a file). Is it possible to somehow run the program without writing the initial density and volume into the conditions...
2. ### B Do Black holes reach a critical mass?

Are there any theories on Blackholes becoming so massive that the forces generated by the matter within them exceeds the forces compressing them and the tipping point leads to a Big Bang?
3. ### Critical mass for nuclear fusion

Hello, can you please confirm this statement "Nuclear fusion does not require a minimum mass to occur (critical mass), which is instead a characteristic limit of fission. So you can make small fusion bombs as much as you want."? Thanks!
4. ### How Does Magic Merv Affect Plutonium Nitrate Waste Decommissioning?

Homework Statement In terms of criticality "magic merv" stands for the nine parameters which determine the level of criticality in a system: mass, absorption, geometry, interaction, concentration, moderation, enrichment, reflection and volume. The topic is decomissioning plutonium nitrate...
5. ### How to calculate critical mass?

Homework Statement How (or if) I can calculate critical mass with the information I'm given? I have to write a criticality safety evaluation for an assignment. The assignment is about decomissioning a dissolver which was used to recover plutonium. On the contingencies section I need to apply...
6. ### I Is there a way to lower critical mass?

Judging by https://en.wikipedia.org/wiki/Critical_mass , californium-252 has the lowest critical mass among known isotopes and isomeres. But it is still very high. Is there a way to reduce it significantly (down to microgramms) or, perhaps, a semi-stable isotope/isomere, which would have such a...
7. ### I Does the Universe's Expansion lead to Critical Mass < 1?

We understand that for the Universe to be flat, the critical mass of everything that has mass must total an omega of 1.0. At first, with our lack of knowledge regarding dark energy, we thought that the universe that we detected (ordinary matter and dark matter) did not even add up to a third of...
8. ### [mechanics] radius of a sphere with critical mass

Homework Statement Here is the question: "In the fall of 2002, a group of scientists at Los Alamos National Laboratory determined that the critical mass of neptunium-237 is about 60 kg. The critical mass of a fissionable material is the minimum amount that must be brought together to start a...
9. ### Neutronium/neutron-degenerate matter: critical mass for stability?

I've read (in wikipedia, at least) that neutron-degenerate matter is unstable in small quantities, and would decay into protons, electrons and antineutrinos. Is there any generally accepted estimate for the critical mass at which a ball of neutronium would be stable? (Trying to calculate...
10. ### Minimum critical mass with natural uranium

I wanted to know what is the minimum critical mass of natural uranium (99.28% U-238) when moderated by beryllium and light water, when in a optimumally designed lattice. As can be seen in this article (google Plutonium Production Using Natural Uranium) criticality is attained at about 10 tons of...
11. ### How Does Neutron Population Remain Stable in a Critical Mass Scenario?

The term critical refers to an equilibrium fission reaction (steady-state or continuous chain reaction); this is where there is no increase or decrease in power, temperature, or neutron population.(Ref: http://en.wikipedia.org/wiki/Critical_mass) 1. How can be the neutron population stable...
12. ### Critical mass of 235U and 239 Pu

Homework Statement Which of the two nuclides 235U and 239Pu has a smaller critical mass and why Homework Equations The Attempt at a Solution i'm thinking 239Pu since it produces more neutrons on average per fission. Would that be correct? Are there any other reasons I can add?
13. ### Understanding Critical Mass and its Role in Nuclear Fission

I can't quite understand critical mass. They say in a fission bomb they have to separate two blocks of fissile Uranium of subcritical mass so they won't undergo spontaneous fission. What exactly is critical mass? Is it just the amount of Uranium held in one place at one time and if so why...
14. ### Critical mass formula: The Los Alamos Primer and Rudolf Peierls paper from 1939

Dear Forum, this is my forst time here, so forgive me if I don't conform to any etiquette or rules. When I did nuclear physics in undergraduate school many years ago, I couldn't help noticing that little or no information was ever given on he issue of neutron multiplication and...
15. ### What Is the Peierls Critical Mass Formula?

Hi, I'm doing a project for my 3rd year maths degree about the development of the nuclear bomb and the mathematicians that contributed. I have so far wrote about early experiments that led to the discovery of radioactivity, the michelson-morley experiment and Einsteins Special relativity. I...
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### The density of Mass relative to a critical mass

I am currently writing a thesis on the matter of differentiating the mechanics that directly associate the quantum exchange that produces a critical mass to form and what influences the density of such masses and or substance. Actually how a vacuum in space:time influences this. There must be...
17. ### Estimating Critical Mass for Expansion

now, I'm a little confused on an important fact: how close are we now in our measurment to the critical mass for the expansion? the stuff I've been studying is from the 80's or something so I'm wondering what our current estimate is.