LAMBDA in the Delayed Neutron Parameters Output

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Sasha
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LAMBDA in the Delayed Neutron Parameters Output

Post by Sasha » Thu Sep 10, 2015 5:51 am

Dear Jaakko,

Excuse me, two questions regarding LAMBDA in the delayed neutron parameters part of the output (SERPENT 2.1.24):

1) Is my understanding correct that LAMBDA=Lambda_eff, which is the prompt neutron reproduction time, and It is equal to l_eff (the effective prompt neutron lifetime)/keff (the effective multiplication factor)?
2) Is LAMBDA value in the SERPENT output in seconds?

Thank you very much in advance!

With best regards,
Sasha.

jesse.johns
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Re: LAMBDA in the Delayed Neutron Parameters Output

Post by jesse.johns » Fri Sep 11, 2015 2:15 am

Hello Sasha,

It is my understanding that LAMBDA is the delayed neutron decay constant (units of 1/s), where the total delayed neutron decay constant is the first term and the six groups are the following terms. This corresponds directly with BETA_EFF.

The neutron lifetimes you are looking for will be:

*_LIFETIME for the prompt neutron lifetime
*_GEN_TIME for the prompt neutron generation time, which is the "l_eff (the effective prompt neutron lifetime)/keff (the effective multiplication factor)?" you are asking about.

The units for these are in seconds.

Which method you elect to use depends on your modeled system.

I hope this helps.

Best,

Jesse

Sasha
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Re: LAMBDA in the Delayed Neutron Parameters Output

Post by Sasha » Fri Sep 11, 2015 4:02 pm

Hello Jesse,

Looking at the order of magnitudes of LAMBDA, *_LIFETIME, *_GEN_TIME, I think that you're right.

Thank you very much for the clarification!

With best regards,
Sasha.

Diego
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Re: LAMBDA in the Delayed Neutron Parameters Output

Post by Diego » Fri May 20, 2016 11:28 pm

Hi,
I am experiencing a problem when trying to understand the contents for ..._LAMBDA array. Basically I would expect that the 1 group constant is the result of --> lambda = sum (lambda_i/Beta_i) * Beta , but when I perform such calculation, the results differ.
For example, for a given case run in jeff 3.2 using S2.1.24 I get:

ADJ_NAUCHI_BETA_EFF (idx, [1: 18]) = [ 7.58068E-03 0.00232 2.49465E-04 0.01285 1.14606E-03 0.00603 7.21075E-04 0.00757 1.49329E-03 0.00519 2.46769E-03 0.00404 7.01660E-04 0.00752 6.20301E-04 0.00815 1.81142E-04 0.01519 ];
ADJ_NAUCHI_LAMBDA (idx, [1: 18]) = [ 4.10452E-01 0.00376 1.24667E-02 6.9E-09 2.82917E-02 0.0E+00 4.25244E-02 7.3E-09 1.33042E-01 0.0E+00 2.92467E-01 6.8E-09 6.66488E-01 4.1E-09 1.63478E+00 0.0E+00 3.55460E+00 5.1E-09 ];

Then I can calculate --> sum (lambda_i/Beta_i) * Beta = 0.076866968 [1/s] but the first value in ADJ_NAUCHI_LAMBDA is 4.10452E-01

Is this 1 group constant divided by another parameter in ADJ_NAUCHI_LAMBDA?

Thanks,
Diego

Ville Valtavirta
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Re: LAMBDA in the Delayed Neutron Parameters Output

Post by Ville Valtavirta » Mon May 23, 2016 1:32 pm

Hi Diego,

shouldn't the one group decay constant just be a weighted average of the individual decay constants, where the weight of group i is Beta_i?

So lambda = sum (lambda_i*Beta_i) / Beta, which gives lambda = 0.41054 in your case. Reasonably close to the tallied one group lambda.

-Ville

Diego
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Re: LAMBDA in the Delayed Neutron Parameters Output

Post by Diego » Mon May 23, 2016 3:56 pm

Hi Ville,
Ussually we use the 1 gruop constant weighted by 1/Beta_i , but I suppose that both could be correct.
Thanks for the clarification,
Diego

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