Is there any international mandate by UN which forces nations to accept refugees (For e.g. EU fines countries for not accepting refugees) and if not , can nations simply deny all refugees entering?
The key convention is the 1951 refugee convention.
The key points to this instrument is that refugees should not be returned to a country where they are under threat. Technically a person only becomes a refugee when they cross a border. "Under threat" means "having a well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a social group, or political opinion". Refugee status does not apply to those who are escaping economic hardship, natural disaster, or who have committed war crimes or serious non-political crimes.
Now this does not directly require signatories to allow refugees to enter their country. However if they have entered (legally or illegally) they should not then be removed to a country where they will be under threat. Nor can they face criminal charges relating to their illegal entry.
These are international norms, but there no requirement for a particular nation to be a signatory to this convention. While membership of some international organisations would depend on countries following international norms, if a country chose it could withdraw from these conventions.
There was a further convention in 1967 and UN resolution 2198 also applies to refugees
I think it's important to differentiate between a nation and a state. Assuming that you mean to say state and not nation there is nothing preventing them from denying the entry of refugees. There are no authorities above the state that can really compel a state to bend to it's well. Other states can pressure them and even threaten or carry out war but there is no enforcement mechanism that can compel states to behave in ways which they choose not too. So yes a state can outright refuse entry to refugees or anyone else they choose to deny entry to.