Effect of continuous refueling on decay heat

[marlh]

Estimate the decay heat rate in a 300 MWth reactor in which 3.2% mU-enriched
U02 assemblies are being fed into the core. The burned-up fuel stays in the core for 3 years before being replaced. Consider two cases:

1. The core is replaced in two batches every 18 months.

2. The fuel replacement is so frequent that refueling can be considered a continuous process. (Note: The PHWR reactors and some of the water-cooled graphite-moderated reactors in the Soviet Union are effectively continuously refueled.)

Compare the two situations at 1minute , 1hour, 1day, 1month, and 1year.

I don't understand about cases 2. how many time the fuel replacement is frequent? Are you help me?

 

[Astronuc]

Is that 300 MWth (~100 MWe), or 3000 MWt (~1000 MWe)? One can then determine a core average LHGR, or core average kW/kgU (specific power), from which one then determines kWh/kgU or kJ/kgU.

So the batch load is 1/2 core, so at the end of the second or third cycle, one half the fuel in the core has been resident for 3 years, and the other half has been resident for 1.5 years.

The continuous refueling is difficult to understand, but CANDUs can do online refueling in which sets of 4 or 8 assemblies are pushed through the fuel channel while the equivalent number is collected on the other side. They can be refueled from either side. Normally, some fraction of fuel is replaced on a schedule, so batch sizes are quite small.

I'm not sure how to calculate the frequency of refueling for the essentially continuous process because for a truly continuous process, the fuel would be irradiated only as is traverses the core, so one would have to determine a feed/traverse rate, which would determine the burnup at exit. One would have to determine the kWh/kgU across the core.

 

[Hologram0110]

The continuous refueling is difficult to understand, but CANDUs can do online refueling in which sets of 4 or 8 assemblies are pushed through the fuel channel while the equivalent number is collected on the other side. They can be refueled from either side. Normally, some fraction of fuel is replaced on a schedule, so batch sizes are quite small.

I'm not sure how to calculate the frequency of refueling for the essentially continuous process because for a truly continuous process, the fuel would be irradiated only as is traverses the core, so one would have to determine a feed/traverse rate, which would determine the burnup at exit. One would have to determine the kWh/kgU across the core.

I recently took a graduate level course in fuel management at a Canadian university (RMC) so there was a lot of focus on the fueling cycle of CANDU reactors. A part of the course we developed a continuous fueling model for CANDU reactors for approximate calculations.

The calculation is essentially as you described. For the continuous model, all fuel has the same exit burn up. This is equivalent to calculating the time integral of the flux as it passes through the core. This is simplified because, as you mentioned, each plane perpendicular to the center axis of the reactor has the same burn-up. This is even simpler, if you assume that the fuel channels are fueled in opposite directions. This is required anyways to effectively control the local reactivity of the core. The average fuel burn up of a group of near by channels is then always the same throughout the reactor. Then approximating this as a homogeneous reactor you can calculate the flux distributions and then the feed rate of each of the channels required to maintain a perfectly critical reactor. Note that this is a highly simplified model, but it is used to understand some aspects of CANDU fuel management.

For the continuous process think of it like a pasta press where each noodle can come out at a slightly different speed. The center of the core has a higher flux and therefore burns the fuel faster, so you want to push the fuel through faster. The paragraph above is describing how fast you would need to push each noodle (aka fuel bundle) to get even burnup.

I think the question wants you to calculate the approximate power coming for the decay of fission products of the fuel. The first case you would have fresh and 18 month old fuel to start and you would have to calculate the decay power 1m later, 1 hour... 1 year later. You'll find that the decay power in this case should change as a function of time. The continuous case, I believe should have a constant decay heat because the average core burnup is always the same.

 

[Astronuc]

Compare the two situations at 1minute , 1hour, 1day, 1month, and 1year.

I don't understand about cases 2. how many time the fuel replacement is frequent? Are you help me?

I wonder if the 1 minute, 1 hour, 1 day, 1 month, 1 year applies to the residence time for which one determines the maximum decay power, or this after the decay power.

As for continous, does this assume the same discharge burnup? If in both cases, i.e., in either case, all the fuel in the core is replaced after 2 cycles (3 years), then it's a matter of determining the core average burnup, at any given time, or at EOC/EOL.

 

[marlh]

This is Problem 3-5: Effect of continuous refueling on decay heat (section VIII) into pages 72 - book: Nuclear Systems I - Thermal Hydraulic FundamentalsAnswers in book:

Case 1 / Case 2
1 minute: P = 81.9 MW / P = 81.0 MW
1 hour: P = 33.2 MW / P = 32.2 MW
1 day: P = 1 5 .0 MW / P = 14. 1 MW
1 month: P = 4.97 MW / P = 4.26 MW
1 year: P = l.28 MW / P = 0.963 MW

I can calculate cases 1 but I can't calculate cases 2 because time replace indefinite. I think it has 4 cores, the replacement will turn to each core for 9 months. Expect people to help. Thank a lot!

 

[marlh]

Thank for help friends. I had solved this problem, because I can't type formula in forum so I can't post solution. If you need it, you could send me a PM.