China has built and put plants into opperation withen ~5 years.

The Haiyang 1 and 2 reactors started planning in 2005 and began commercial operation in 2018 and 2019. The Taishan 1 and 2 reactors were bid in 2006 and also didn't begin commercial operation until 2018 and 2019. This is much longer than 5 years, and timelines in other countries of nuclear projects tend to be much longer.

The cost of nuclear is high because of the relatively low amount of nuclear plants, as more are made the economics of scale come into play. It is high-cost upfront to build a plant, but cheaper than renewables once a plant is in operation.

The cost of nuclear has actually been going up, while the cost of renewables has been going down:

The third risk of nuclear power related to its ability to reduce global warming and air pollution is the high cost for a new nuclear reactor relative to most WWS technologies. In addition, the cost of running existing nuclear reactors has increased significantly, and the costs of new WWS technologies have dropped so much, that many existing reactors are shutting down early due to high costs. Others have requested large subsidies to stay open. In this section, nuclear costs are discussed briefly.

The levelized cost of energy (LCOE) for a new nuclear plant in 2018, based on calculations by Lazard (2018), is $151 (112 to 189)/MWh, where $100/MWh equals 10 ¢/kWh. This compares with $43 (29 to 56)/MWh for onshore wind and $41 (36 to 46)/MWh for utility-scale solar PV from the same source (Table 7.9). A good portion of the high cost of nuclear is related to its long planning-to-operation time, which in turn is partly due to construction delays.

This nuclear LCOE is an underestimate for several reasons. First, Lazard assumes a construction time for nuclear of 5.75 years. However, the Vogtle 3 and 4 reactors, though will take at least 8.5 to 9 years to finish construction. This additional delay alone results in an estimated LCOE for nuclear of about $172 (128 to 215)/MWh, or a cost 2.3 to 7.4 times that of an onshore wind farm (or utility PV farm).

Next, the LCOE does not include the cost of the major nuclear meltdowns in history. For example, the estimated cost to clean up the damage from three Fukushima Dai-ichi nuclear reactor core meltdowns in 2011 (Section 3.3.2.2) was $460 to $640 billion (Denyer, 2019). This is equivalent to a mean of about $1.2 billion, or 10 to 18.5 percent of the capital cost, of every nuclear reactor that exists worldwide.

In addition, the LCOE does not include the cost of storing nuclear waste for hundreds of thousands of years. In the U.S. alone, about $500 million is spent yearly to safeguard nuclear waste from about 100 civilian nuclear energy plants (Garthwaite, 2018). This amount will only increase as waste continues to accumulate. After the plants retire, the spending must continue for hundreds of thousands of years with no revenue stream from electricity sales to pay for the storage.

The spiraling cost of new nuclear plants in recent years has resulted in the cancelling of several nuclear reactors under construction (e.g., two reactors in South Carolina) and in requests for subsidies to keep construction projects alive (e.g., the two Vogtle reactors in Georgia). High costs have also reduced the number of new constructions to a crawl in liberalized markets of the world. However, in some countries, such as China, nuclear reactor growth continues due to large government subsidies, albeit with a 10- to 19- year time lag between planning and operation (Section 3.3.1.1) and escalating costs.

In sum, before accounting for meltdown damage and waste storage, a new nuclear power plant costs 2.3 to 7.4 times that of an onshore wind farm (or utility PV farm), take 5 to 17 years longer between planning and operation, and produces 9 to 37 times the emissions per unit electricity generated. Thus, a fixed amount of money spent on a new nuclear plant means much less power generation, a much longer wait for power, and much greater emission rate than the same money spent on WWS technologies.

Further more, there are even signs that China might start to (slowly) shift away from nuclear and towards renewables:

Officially China still sees nuclear power as a must-have. But unofficially, the technology is on a death watch. Experts, including some with links to the government, see China’s nuclear sector succumbing to the same problems affecting the West: the technology is too expensive, and the public doesn’t want it.

The 2011 meltdown at Japan’s Fukushima Daiichi plant shocked Chinese officials and made a strong impression on many Chinese citizens. A government survey in August 2017 found that only 40% of the public supported nuclear power development.

The bigger problem is financial. Reactors built with extra safety features and more robust cooling systems to avoid a Fukushima-like disaster are expensive, while the costs of wind and solar power continue to plummet: they are now 20% cheaper than electricity from new nuclear plants in China, according to Bloomberg New Energy Finance. Moreover, high construction costs make nuclear a risky investment.