
#1
Jan2413, 01:24 PM

P: 53

So, In Lamarsh,
when he talks about the burnup , he's talking about the fission energy released per unit mass of fuel where as when he talks about consumption rate, he's talking about the amount of fissile matter consumed per unit energy produced? 



#2
Jan2513, 08:13 AM

Admin
P: 21,637

Burnup is often expressed as energy produced (power integrated over time) per unit mass of initial fuel. It is also expressed, by some, as FIMA, fissioned initial metal atoms. Roughly 1% FIMA or 0.01 fissioned atoms corresponds to ~950 GWd/tU, based on 200 MeV of recoverable energy per fission. The conversion varies based on what is assumed as recoverable energy. The energy per fission is less for U235 than for Pu239/241, so the relationship between FIMA and GWd/tU changes with cycle exposure, or fissile content. MOX (U,PuO_{2}) would have a higher energy per fission at BOL. 



#3
Jan2513, 10:42 AM

P: 53

In class we were taught that (CR) = (1+alpha)BR
Is it just me, or is there no uniformly agreed upon definition? 



#4
Jan2513, 04:48 PM

Admin
P: 21,637

Burnup Vs. Consumption RateI assume CR = conversion ratio, and BR is breeding ratio? Breeding means beyond that which is used in the reactor. LWR convert U238 to Pu239, but they consume some of the Pu239, and produce less fissile material than consumed, so they are not considered breeders. 



#5
Jan2613, 03:07 PM

P: 53

We're using "Introduction to Nuclear Engineering"
CR > consumption rate BR > Burnup Rate. 



#6
Jan2713, 07:59 AM

Admin
P: 21,637

Consumption rate as used in the text refers to consumption of U235 (or rather depletion = loss of U235). Then they use burnup rate for grams (of U235) fissioned per day. IMO, this muddies the water, since not all fission occur in U235. The burnup rate is based on fissions, while the consumption rate is based on fission + neutron absorption by U235. In addition to fissioning, the U235 may absorb a neutron and become U236, which has a low fission crosssection. In fact, U236 will absorb a neutron and become U237, which decays to Np237. In reality, about 8 to 10% of fissions occur in U238 as a result of fast fissions. Some U238 is converted to Pu239, 240 and 241, and at high burnups, ~40 GWd/tU, most thermal fissions occur in Pu239/241 in high enrichment (> 4%) fuel. See chapter 2 in http://www.nap.edu/catalog.php?record_id=9263  particularly page 18 and Fig. 25. Most of the time, burnup is calculated using the thermal energy, which does not distinguish among fast fissions and thermal fissions or the isotopes producing fission. Another factor to consider is that large nuclear reactor have fuel of different vintage, which are divided into batches. At the beginning of a cycle, there is fresh fuel (no exposure), which may be one quarter to almost one half of the core. There is an almost equal amount of fuel (onceburned) being reinserted for a second cycle. There is some smaller fraction of twiceburned fuel in for a third cycle, if the core loads between onethird and onehalf of the core. In plants using more than three batches, there may be some thriceburned assemblies in for a fourth cycle. The third and fourth cycle assemblies are loaded on the periphery of the core, and they function more as a reflector since the power density is generally less than onethird of core average power. High burnup fuel, with exposures (burnups) > 40 GWd/tU will have most fissions in Pu239/241, as well as some portions of fissions in U238. 


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