What would happen if the fuel rods in a nuclear reactor were completely depleted?

Question

My understanding is that in commercial nuclear reactor operations, fuel rods are not used up to the point where they're fully depleted and unable to support fission, but are replaced while they still contain an appreciable amount of fissionable isotopes, to ensure that the reactor stays in a stable operating regime at all times.

However, would it be possible, for any reason, to run a nuclear reactor without replenishing the fuel as long as any useful energy at all could be produced, that is, actually "running the fuel rods dry" (or, more formally, until criticality is irrecoverably lost and decay heat is the only remaining output left, at which point the reactor is essentially just a fancy spent fuel pool)?

Could such an end state be safely achieved, with power just gradually fading away while the control rods extend out more and more to compensate for the declining reactivity? Or would unstable and potentially dangerous operation ensue, as in Chernobyl where operator response to a poisoned core started a catastrophic chain of events?

The question is motivated by the realization that one of the safety-enhancing measures proposed for the RBMK reactor design after the Chernobyl disaster was to raise the enrichment grade of the fuel, apparently to reduce the type's susceptibility for core poisoning and the resulting power fluctuations (at least that's how I understood it). I'm interested if the converse is also generally true, that is, whether it would be hazardous to let the fuel deplete too much in modern PWR/BWR reactors that don't share the design flaws of the RBMK.

Answer 1

As the fissionable portion of the fuel tablets is expended, the reactivity of the fuel decreases and the control rods or the shim rods are withdrawn a bit to keep the power output of the reactor constant.

There will come a time where the rods are at their travel limits and the power output falls below its lower operating spec. beyond this point, the reactivity of the fuel continues an exponential decay, putting out progressively less and less power until the water stops boiling and the turbines come to a halt.

Beyond that point, the fuel rods continue generating heat, which must be carried away by the cooling system for a very long time as both the fission fuel and the fission products with long half-lives continue to decay.

In reactor designs common in the USA, they do not cross over any unstable operating regimes as they use up their fuel, but note that it may become necessary at any time for the reactor to be pushed up to its maximum rated power level if electrical demand is unusually high or if other generators on the grid go off-line for any reason. This means that the point where the reactor needs to be re-fueled will occur within a safety margin of when it can't reach full rated power anymore.

Note that from a reprocessing standpoint, there may be other criteria that must be taken into account to determine the optimum refueling time for any given reactor design and I invite the experts here to weigh in on those issues.

Answer 2

The issue is that there is a certain amount of enrichment (reactivity) that you need in the core to maintain criticality. For CANDU reactors with heavy water moderator, the reactors can be critical with natural uranium. For light water reactors used in the USA, there is some minimum amount to remain critical.

For the sake of argument, let's assume that you need an average of 3% enrichment to maintain criticality (the actual number depends on the core geometry and the loading pattern). You will start the cycle with some higher amount, say 4% average enrichment. As the core depletes, the enrichment will slowly decrease until the core can no longer maintain criticality. At this point, some of the fuel will need to be discharged and replaced with fresh fuel. This discharged fuel still has more U235 than natural uranium, but it isn't enough to keep an LWR critical.

Once the reactor can no longer maintain criticality, it is basically a "fancy spent fuel pool" until new fresh fuel is inserted. There is still decay heat being generated, but this is not the same thing as fission power.

There is no immediate safety issue with running the core too long. If you kept running the past where the core can be critical at 100% power, you will find that the power will slowly decrease (roughly something like 20% decrease over a month) until it eventually became zero power.

With that said, there are "burnup limits" on the fuel rods that limit the amount of power that can safely be extracted from the fuel. As the fuel rods deplete in the core, the cladding experiences radiation damage and there is a higher probability of failure. There are administrative limits on the fuel rods to only allow a certain amount of depletion per rod. In practice, the core is designed so that the burnup limits correspond to the natural end of the cycle.

As a thought experiment, there have been proposals in the past where you could take spent LWR fuel (which has higher enrichment than natural uranium), repackage the fuel into new rods, and then place the fuel into CANDU reactors. This would effectively use more of the fissile material in the fuel.