1. Yucca Mountain, even if completed to specifications, can only hold a certain tonnage of nuclear waste (63,000 metric tons according to the Nuclear Waste Policy Act of 1982). The present stockpiles of nuclear waste, plus the expected rate of waste production (approximately 2,000 metric tons/year), mean that sometime in the mid-2010s we will exceed the storage capacity of Yucca Mountain. This necessitates the search for a second long-term geological repository, which means we'd have to start the whole dreary 30-year, $9 billion+ search over again.
2. Yucca Mountain is a bad place to store waste in the longest terms (planners envision the waste being stored there for up to 1,000,000 years in the future). It is riddled with cracks, more full of water than the desert around it would suggest, and is difficult to access with trucks and heavy equipment. This is a big deal, because...
3. Establishing one national repository means that the waste from 104+ separate sites around the U.S. would have to be relocated there, a task that would require hundreds of separate trips by truck (with the waste encased in specially made containers). Furthermore, since our road system is designed to connect population centers, it's likely that most (if not all) shipments would run through one or more major metropolitan areas. The casks do not leak 'casual' radiation in harmful amounts, but if an accident were to happen, it could result in massive radioactive contamination of an American city.
4. Yucca Mountain was conceived as a "fire-and-forget" facility, which we (read: humanity) can dump the waste into and then forget about forever. However, the expectation is that we can build a facility which will remain secure for one million years in the future. For comparison, one of the oldest confirmed man-made structures on Earth, the Great Pyramid at Giza, is a mere 4,571 years old. Yucca Mountain would have to not only survive, but remain absolutely sealed and not release radiation, for 218 times longer than that. It would have to outlive the lifespan (to date) of the country that created it by 4,255 times. I respectfully submit that if humanity cannot forecast tomorrow's weather with certainty, how can we hope to predict local conditions (no volcanic activity, not much erosion, etc.) a million years in the future?
5. How exactly do you tell people, a mere 10,000 years in the future, to stay away from a given place? How can you communicate with them, knowing that the language future denizens of what is now Nevada will speak will be massively different from modern languages?
In short, Yucca Mountain is not a viable place to store nuclear waste, either in the short (next 30 years) or long (10,000 years) or longest (500,000 years) terms.
So... What Do We Do Instead?
Finding another repository site is out of the question. Yucca is the best site that the U.S. has for a geological repository, in terms of its isolated location, its (predicted) geological stability and its political defenses (Senator Harry Reid being one). If Yucca is inadequate after thirty years of study, it's likely that any other site would eventually be found inadequate as well.
We need a two-part solution.
However inadequately, Yucca would at least be able to fix two real problems by consolidating the spent nuclear fuel. The Yucca plan reduces the risk of both a terrorist attack and an accidental spill or leakage by consolidating the waste in one location. I don't disagree with this, but with the dangers of moving the waste by transport all the way to Nevada, I suggest a different solution. Set up regional waste collection centers at points throughout the U.S. where there's a high concentration of nuclear plants. The waste from Georgian plants can be moved to a center in the Southeast, the waste from Wisconsin's plants goes to a Midwest center, and so on. By doing this, we can reduce (though not eradicate) the risks associated with transporting the waste.
The thing about regional centers is that, unlike Yucca, they would not have to store the waste for a ludicrous target of one million years. Seventy years would more than suffice. The centers can simply be extremely large, well-guarded, well-sealed-off warehouses; they don't have to be mountains. Above all, they would allow us to eventually retake and reuse the waste, instead of throwing it into a mountain forever.
The second part solves both the issue of nuclear waste and the U.S.'s dependence on freshly mined uranium. Waste reprocessing plants can refine the nuclear waste, remove the material (lots of U-238, some smaller amounts of U-235 and Pu-239) that can be reused, and sell it back to commercial plants. Furthermore, the introduction of fast breeder reactors could allow the U.S. to adopt a closed nuclear fuel cycle, where our plants run on plutonium (which works just as well) and there is no need for new fuel, as it is created by the reactors.
If I were Energy Secretary David Chu, that is what I would recommend. Create regional waste collection centers, repeal the parts of the Nuclear Waste Policy Act that require the repository at Yucca Mountain to be opened, start construction of reprocessing plants (or incentives for their commercial construction) and start research into and development of fast breeder reactors, with the intention of creating a closed nuclear fuel cycle within 50 years or so.
The downside is the initial cost of closing Yucca, of building reprocessing plants and breeder reactors, is considerable. However, this will pay for itself in savings on new uranium once the closed fuel cycle is adapted, and in the removal of nuclear waste from the vicinities of dozens of American cities. Currently, the closing of Yucca has been blocked by lawsuits from Washington and several other states; however, they are motivated by a desire to prevent any eventual repository from being placed in their state (a site in Washington was a top contender after Yucca). I am positive that a commitment to regional centers and reprocessing plants would end their desire to block the closing of Yucca.
Yucca Mountain should be closed permanently, regional collection centers should be constructed and waste transported to them as soon as possible, and reprocessing plants and breeder reactors constructed as soon as funding can be found for them. If successful, this program will minimize the risks of transporting nuclear waste, and eventually eliminate our current stockpiles of waste entirely. It will also protect domestic nuclear power against future scarcity of uranium (current reserves are predicted to last about 100 years, and Fukushima notwithstanding, there is still something of a nuclear boom taking place worldwide), in the event of the adoption of a closed fuel cycle. In addition, it will eventually lower costs for the ratepayers who consume electricity generated by the breeder reactors, and allow the Nuclear Regulatory Commission to focus its energies on strengthening our nuclear fleet instead of dealing with the waste. The only drawback is the initial cost, and the only obstacle is political will.