* New climate target, 2021-27 budget dominate EU leaders' summit
* Differences over whether nuclear can be part of energy mix
* Wrangling over climate ambition to feed into EU budget standoff (Adds Austria, other detail)
By Jonas Ekblom and John Chalmers
BRUSSELS, Dec 12 (Reuters) - Poland, Hungary and the Czech Republic set out demands on Thursday that could block agreement on the European Union's new push for climate neutrality by 2050, saying the poor should not have to bear the cost.
Tense talks are expected at a summit in Brussels of 27 EU leaders, at which the majority will offer generous financial incentives to the eastern member states to win their support for the goal of net-zero greenhouse gas emissions by mid-century.
"We cannot allow Brussels bureaucrats to have poor people and poor countries to pay the costs of the fight against climate change," said Hungarian Prime Minister Viktor Orban. "We must receive clear financial guarantees and we will negotiate the terms of that."
The summit is the first since Germany's Ursula von der Leyen took office as the new head of the EU's executive commission, and a failure to agree on the Green Deal, her biggest initiative, would be a setback for the new leadership.
With floods, fires and droughts wrecking lives around the world, Greenpeace climate activists scaled the glass-fronted Europa building where the leaders were to meet, unfurling banners reading "Climate Emergency", firing off red flares and blaring fire alarm sirens.
Some activists were detained by police and the protest ended well before the summit started.
The eastern countries want more money to fund a transition to lower emissions, including a role for nuclear power, which emits no carbon but which Germany and others aim to shut.
"It is important to have certainty that nobody will stop us in the construction of nuclear power units," Czech Prime Minister Andrej Babis told reporters. "We have to have electricity for people, for firms, and heating."
While the bloc's draft summit decision includes some of the easterners' demands, Austria and Luxembourg led opposition to specifically naming nuclear as eligible for EU green funds.
"Austria's position is clear. No nuclear energy. We support climate neutrality in 2050, but without nuclear energy, which is not safe and not sustainable," said Chancellor Brigitte Bierlein.
THORNY BUDGET DEBATE
The EU's new chief executive proposed a Green Deal this week to mobilise 100 billion euros worth of investment to help economies move away from fossil fuels.
But the three eastern states were not on board, demanding that any decision spells out in more detail the scale and scope of financing available, as well as including nuclear energy.
The Czech Republic generates about half of its electricity from coal and wants to phase most of it out over the next 20 years, replacing part of it with new nuclear power plants.
Hungary relies on nuclear power for about a third of its energy needs, with coal making up less than 15% of its energy mix. It wants to abolish coal by 2030 and replace it with a mix of nuclear, gas, renewables and energy imports.
Poland produces some 80% of its power from coal and discussions about introducing nuclear energy have not yet been settled, partly due to high costs.
French President Emmanuel Macron said those countries may have to have nuclear in their future energy mix.
He said it was clear some countries opposed this, but was confident they could reach an agreement: "We just need to find the right mechanisms, be trustworthy, allow predictability, but we can do it."
One EU diplomat estimated chances for an agreement at the summit - which must be unanimous - at 50/50, while another added that some countries have reservations about the strain that a 100 billion euros price tag might put on their economies.
The climate discussion feeds into another tricky debate, over the next long-term budget, where no agreement is expected after a latest proposal to cap joint spending at 1.087 trillion euros for 2021-27 was rejected by both the frugal camp and those seeking a heavier outlay.
"Some people want to pay less, some people want to get more, others to do new things," said Luxembourg Prime Minister Xavier Bettel. "I wasn't the best at mathematics but this, I think, is not going to add up." (Additional reporting by Michel Rose, Christian Levaux, Marton Dunai, Jason Hovet, Jonas Ekblom, Robin Emmott, Philip Blenkinsop, John Chalmers, Gabriela Baczynska, Andreas Rinke, Marcin Goclowski, Francesco Guarascio and Jan Lopatka, Writing by John Chalmers and Gabriela Baczynska, Editing by Angus MacSwan)
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