## Decay heat power for U233 fission products

Cyril R
Posts: 5037
Joined: Jul 14, 2008 3:12 pm

### Decay heat power for U233 fission products

ORNL 4541 has a graph with U233 fission product decay heat by time.

http://www.energyfromthorium.com/pdf/ORNL-4541.pdf

Fig. 6.4. Total afterheat production, p. 91 in document, p. 109 in Acrobat. Also compare table 3.11 on p. 57 in Acrobat.

Now this is something very different from the usual decay heat curves we're used to.

Code: Select all

``````T (seconds) PMWt P%
1           86   3.8%
10          75   3.3%
100         55   2.4%
1000        33   1.5%
10000       17   0.8%
100000      10   0.4%
1000000      5   0.2%
``````
There could be an error here. Perhaps the t=1 second is actually the t=100 seconds data point? This is what table 3.11 suggests, but figure 6.4 clearly states 10e0 seconds.
Last edited by Cyril R on Nov 23, 2012 8:21 am, edited 1 time in total.

Cyril R
Posts: 5037
Joined: Jul 14, 2008 3:12 pm

### Re: Decay heat power for U233 fission products

Ok, so the figure 6.4 apparently excludes the processing system, except at shutdown. Similar to table 3.11. That's 1.5 MW of FPs and 5 MW of Pa at shutdown.

So to get the figure for say a DMSR, with some FPs and Pa in the salt, add 6 MW medium term, later 5.

Lars
Posts: 3060
Joined: Jul 28, 2008 10:44 pm

### Re: Decay heat power for U233 fission products

Figure 6.4 is for the decay heat in the drain tank. The paragraph that refers to it (pg 90) talks about the time (7 minutes) it takes to drain from the core to the drain tank and the maximum power being 50MWatts. From the common formula the expected total power would be 58MW at 100seconds and 37mW at 1000 seconds. A peaks of 50MW seems reasonable - if I use the common formula and assume a linear transfer rate from t=0 to t=420 seconds the peak in the drain tank is 43.8MW.

I don't understand why the figure shows a significantly lower initial heat load than LWRs. The major difference I think would be that we have 233Pa decay and they have 239U decay.

Cyril R
Posts: 5037
Joined: Jul 14, 2008 3:12 pm

### Re: Decay heat power for U233 fission products

The drain tank, of course that makes sense, ORNL planned on having the brunt of offgas decay in the drain tank.

The lower initial heat load is indeed a mystery. Pa decay is only 5 MWt. U239 decay has to be lower than that. It doesn't explain the difference. I included Pa and the reprocessing system heat load (about 6.5 MW total) in the 86 MWt figure. So that's all we have, 3.8% of fullpower at 1 second after shutdown. In LWRs we expect around 6.5% at 1 second after shutdown.

Not that this matters much, everything below 10 seconds can just be ignored as the temp rise from that period is so small.

Lars
Posts: 3060
Joined: Jul 28, 2008 10:44 pm

### Re: Decay heat power for U233 fission products

I'm thinking that they used 10 seconds after shutdown as "at shutdown". It takes a few seconds to shutdown the fission power so a 1 second after shutdown is not so meaningful either. But like you said, in the end it doesn't matter since the time is so short that only a modest amount of thermal energy gets deposited during this time.