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Fission of U-235

Posted: Wed Apr 24, 2019 5:58 am
by fredrikmalmros
Dear forum users and developers

I have encountered some FISPACT issues that I’m not able to find any answers to. I am studying the dose rate contribution from specific nuclides arising from fission of U-235.

According to my FISPACT output, all parameters in terms of i.e. nuclide specific activity and heat seem ok, but I get very high dose rate from nuclides with low specific activity. I tried to change different parameters in my FISPACT code to see how the code responded to the change. I changed radiation time, total flux and the amount of U-235 in the source. Here I found something really strange. The change in flux and radiation time responded by change in activity and corresponding dose rate accordingly. But, if flux and radiation time are constants and I only change the amount of U-235 in the source, I get a change in activity but not in dose rate. This may generate nuclides with low specific activity but unreasonable high dose rate contribution. I should also mention that I keep the density constant. Maybe a constant density and larger source (volume) can explain the source self-absorption of gamma radiation, but if I compare activity and dose rate, the results are not realistic.

It seems like dose rate is independent of U-235 mass in the source. This is confusing.
Can someone please explain how to interpret the output results regarding nuclide specific activity vs. nuclide specific dose rate?

Thanks in advance!
/Fredrik

Re: Fission of U-235

Posted: Wed Apr 24, 2019 10:20 am
by Thomas Stainer
Dear Fredrik,

Thank you for raising this issue.

Before I start to assume anything or proceed any further, I would first like to be able reproduce your issue. Are you able to send (you should be able to attach files here) your input file, files files and fluxes file? If I have those three things I can attempt to reproduce your issue and investigate further.

Kind regards,
Thomas

Re: Fission of U-235

Posted: Wed Apr 24, 2019 1:23 pm
by fredrikmalmros
Dear Thomas

I have attached the requested files from my FISPACT run.

I also thought that there was something wrong with my FISPACT input code or files file or fluxes file, so I also used the "fission_U235_1E5s" example in the getting_started library on the FISPACT disc to see if I could reproduce this strange behavior. And yes, I got the same strange output from the getting_started files when I changed the source amount in the input file.

Kind regards/
Fredrik

Re: Fission of U-235

Posted: Wed Apr 24, 2019 3:08 pm
by Thomas Stainer
Dear Fredrik,

Thank you for providing those files. I think I understand the issue now.

If I take the input file you provided and run it, I get the following activities and dose rates for U235:

First inventory step (irradiation)
Total elapsed time: 157788000.0 s
Activity (Bq): 70.2179089893019
Dose rate (Sv/hr): 3.856573780302555e-05

Second inventory step (cooling)
Total elapsed time: 255616560.0 s
Activity (Bq): 70.21791009879098
Dose rate (Sv/hr): 3.855269014930463e-05

Does this match with your results?
Note I used the JSON output (if using F-II version 4) to give higher precision.

Following this, I did as you suggested and increased the initial inventory to 1e25 (your input is 1e19 - so factor 1e6 higher) and then got the following:

First inventory step (irradiation)
Total elapsed time: 157788000.0 s
Activity (Bq): 70219460.97306284
Dose rate (Sv/hr): 3.8566363942111626e-05

Second inventory step (cooling)
Total elapsed time: 255616560.0 s
Activity (Bq): 70219462.0825535
Dose rate (Sv/hr): 3.855331927051491e-05

Indeed, dose rates are almost unchanged (small differences). Here the activity is now a factor 1e6 larger, as we would expect, but the dose rate does not change by this magnitude.

Is this the issue you are reporting?

If so, whilst this make look odd, it actually makes sense because the specific activity (Bq/kg) is used to calculate the dose rate, not the activity (Bq). Hence, the specific activity is per unit mass.

The actual formula used in the code to compute this can be found here: http://culhamem.co.uk/fispact/sd2.pdf on pages 36-37.

I hope this offers a suitable explanation, that is, if I have understood your issue. Otherwise, if I have missed something please let me know.

Kind regards,
Thomas

Re: Fission of U-235

Posted: Wed Apr 24, 2019 4:55 pm
by fredrikmalmros
Dear Thomas

Thank you for prompt reply.

Then it would make much more sense.

I tried to get this information from the FISPACT manual but I couldn´t find any information regarding this issue. I found though similar info if using the control file keyword “DOSE” in input file, that FISPACT output will deliver dose rate per 1 gram of source material. But no information of the default dose rate output.

Thank you!

Kind Regards /
Fredrik

Re: Fission of U-235

Posted: Thu Apr 25, 2019 7:54 am
by Thomas Stainer
Dear Fredrik,

I have added this information to the DOSE keyword page, and I hope it is now more transparent.
https://fispact.ukaea.uk/wiki/Keyword:DOSE

Kind regards,
Thomas