MCNP Summary Table: Explanation and Use

[DEvens]

MCNP (version 6 at least, possibly other versions) produces a summary table. An example is attached. The example shows the neutron portion of the summary table in a combined neutron/photon case. There is an SDEF card that produces photons, and the weights on that SDEF card give the relative strengths of various fuel bundles in a full reactor model. That's why the weights are so large.

The information in this table looks like it could be very useful. But in searching through the various documentation supplied with MCNP I can't find any explanation of what I'm seeing. At least, not in a form that points at the summary table. For example, the summary reports neutron creation from weight cutoff, reporting weight creation but no tracks. On the other side it reports neutron loss to weight cutoff with both weight and tracks. I don't understand what this is telling me.

I'd be glad to get an explanation. Or even a pointer to places in the docs I should be reading.

 

[DEvens]

I'm going to put an answer here so this drops out of the unanswered thread list. I don't think I will get an answer. No answer on the MCNP mailing list either. Oh well.

 

[rpp]

This table provides the "neutron balance" for the system. The left hand is all of the events that create a neutron and the right side is the events where the neutrons are lost.

The main source of neutrons is prompt fission, with smaller contributions from delayed fission, (n,xn), and photonuclear (must be a CANDU with D20).

The neutron losses are fission, capture, and loss to (n,xn). There is also some small amount of particles that escape the system (boundary?).

Note that the total source equals the total loss, so everything looks good. It is a good table to confirm that the events you think are happening are happening. I don't know what application you are running, but it seems a little strange that there is no neutron source. However, from your description, you say that you have a photon source, and I'm guessing the photonuclear source is starting the neutron reactions. You must be somewhat close to being critical.

Is there something particular that you are looking for, or a particular question?

 

[DEvens]

This table provides the "neutron balance" for the system. The left hand is all of the events that create a neutron and the right side is the events where the neutrons are lost.

The main source of neutrons is prompt fission, with smaller contributions from delayed fission, (n,xn), and photonuclear (must be a CANDU with D20).

The neutron losses are fission, capture, and loss to (n,xn). There is also some small amount of particles that escape the system (boundary?).

Note that the total source equals the total loss, so everything looks good. It is a good table to confirm that the events you think are happening are happening. I don't know what application you are running, but it seems a little strange that there is no neutron source. However, from your description, you say that you have a photon source, and I'm guessing the photonuclear source is starting the neutron reactions. You must be somewhat close to being critical.

Is there something particular that you are looking for, or a particular question?

Yes it's a CANDU. It's in the guaranteed shutdown state so KEFF=0.95 or there about. The question I was tasked with was how much activity is there from decay photons kicking neutrons out of deuterium. This table looks like it should tell me that. But I am not confident I'm reading it correctly.

The calculation consisted of getting the source term of decay photons using ORIGEN, then putting an SDEF into the reactor model and then running (if I recall) 4 billion photons. There's another whole summary table for photons.

Specifically in my question here I was looking to understand what the weight cuttoff row is telling me. What does it mean that weight cutoff produces weight but not tracks on the neutron creation side? And what does it mean on the other side that it loses both tracks and weight? How should I interpret that with regard to things like trying to estimate the net activity in the reactor? It's concerning because the weight affected by weight cutoff is the same order of magnitude as the prompt fission term. So my answer could in principle be drastically wrong.

But generally, I'm looking for a place in the docs that tells me how to read this table, what to be aware of to avoid getting tripped up, etc. I searched the docs but could not find anything that helped.

 

[Ariful]

@rpp Could you please clarify the distinction between "prompt fission" and "delayed fission" in terms of neutron creation? Specifically, are these terms referring to the number of neutrons created by prompt fission and delayed fission reactions?

 

[Astronuc]

@rpp Could you please clarify the distinction between "prompt fission" and "delayed fission" in terms of neutron creation? Specifically, are these terms referring to the number of neutrons created by prompt fission and delayed fission reactions?

Prompt fission neutrons are those that are released at the instant of fission (of a fissile nuclide). Usually, 2 or 3 neutrons are released 'promptly'.

Delayed neutrons are those neutrons that are released from certain neutron-rich fission products some time after fission, e.g., Br-87 decay to Kr-87 with a half-life (of Br-87) of approximately 54.5 s.