Strontium-90 and yttrium-90 decay in ORIGEN

[David Holler]

Hi all,

I'm analyzing the GE 10x10 BWR library (ge10x10-8) in ORIGEN and I don't quite understand the results for Yttrium-90 (y-90) that I'm seeing.

After 3 cycles, ORIGEN returns a Strontium-90 (sr-90) activity of 7.069E+14 Bq and a y-90 activity of 7.415E+14 Bq. I understand sr-90 to be a fission product and this is reflected in the ORIGEN decay results. After 15 years of decay, the sr-90 activity should be 4.93E+14 Bq and this is indeed what ORIGEN shows.

After 15 years of decay, ORIGEN returns a y-90 activity of 4.93E+14 Bq. If y-90 is produced from sr-90 alone, then after 15 years of decay, the y-90 activity should be 1.25E+11 Bq.

Would there be another nuclide decaying into y-90 at a rate high enough to keep it this close to its initial value?

The ORIGAMI input is attached. This is an example and does not contain exact values of a real fuel assembly, but it produces the numbers I mentioned.

[Willem van Rooijen]

Consider a chain radioactive isotopes as follows:

A -> B -> C -> ....

a) set up the differential equation for the amount of isotope B.

b) solve the differential equation to find the time-dependent amount B(t), with A0 the initial amount of A and B0 the initial amount of B.

c) determine the time-dependent activity of B, and study the special case where the half life of A is much longer than the half life of B.

d) (bonus) determine the analytical solution for the time-dependent amount of each isotope in the chain in the special case that the sample initially contains only A. Study the special case where one of the isotopes in the chain has a much longer half life than all the other isotopes in the chain. Scandalous self-promotion: buy my book, "Exercises in Nuclear Reactor Analysis" where you will find the solution of this exercise. 

https://www.amazon.com/Exercises-Nuclear-Reactor-Analysis-Frederik/dp/460000955X/ref=sr_1_1?crid=33ZSN569X67OO&dib=eyJ2IjoiMSJ9.drPidmK6iGqHPwmeH2f7dJv6HGg-dpPoHwjW8DaPstBE4a0i5f2XXSz_k6UETq3EPd_qdY1JOGOU8y_gkm9iPuoZwtgosdsQLezS9Aj5yiAv9XgzQy2Y2BVRsL4G1sYaRGRniAeec_GTCgxwfqJ-v2d5BGIvsXvzqhoUPKh6PRwcgw_0VpxtSgJ7CDauY5Kb9T0DcDPlPA55yMmxy4y7SE3cV0sLWJmaGHGSMwFcw-o.phdKvGU_5Bh96KxdGbUm2uwdjJxTh1FgHSUpxgOYgPw&dib_tag=se&keywords=exercises+in+nuclear+reactor+analysis&qid=1779886937&sprefix=exercises+in+nuclear+reactor+analysi%2Caps%2C308&sr=8-1

Cheers,

Shinichi

2026年5月27日水曜日 9:35:45 UTC+9 David Holler:

[David Holler]

Maybe I need to rephrase my question. Which isotopes does ORIGEN decay into Yttrium-90 besides Strontium-90?

[Pavlo Ivanusa]

The activities will be equal in secular equilibrium where lambda_2 >> lambda_1. If you're using the equation N_2 = N_1 * lambda_1/lambda_2 to find the equilibrium values, it only works when you're counting atoms, but A_2 = A_1. ORIGEN does not really have any other branching to worry about for the Sr-90 to Y-90 decay path. 

[David Holler]

Thank you Pavlo. I had an incorrect term in my equation and now I get secular equilibrium.