Was Brexit always going to include a withdrawal from the European Atomic Energy Community

Question

Today is officially Brexit. The Prime Minister, Theresa May, has submitted a letter to EU President Tusk to formally notify him of the action. Here is a snippet from that letter (emphasis mine):

Today, therefore, I am writing to give effect to the democratic decision of the people of the United Kingdom. I hereby notify the European Council in accordance with Article 50(2) of the Treaty on European Union of the United Kingdom's intention to withdraw from the European Union. In addition, in accordance with the same Article 50(2) as applied by Article 106a of the Treaty Establishing the European Atomic Energy Community, I hereby notify the European Council of the United Kingdom's intention to withdraw from the European Atomic Energy Community. References in this letter to the European Union should therefore be taken to include a reference to the European Atomic Energy Community.

This is the first I'm hearing of a withdrawal from the European Atomic Energy Community. Was this always part of the plan? Was it possible to leave the EU without leaving the EAEC? If this was a not a given based on last year's Brexit vote, is there a reason why leaving the EAEC was included?

Answer 1

Yes, the UK government has mentioned this before in the explanatory notes published with the EU Notification of Withdrawal Bill (Article 50 bill).

18 The power that is provided by clause 1(1) applies to withdrawal from the EU. This includes the European Atomic Energy Community (‘Euratom’), as the European Union (Amendment) Act 2008 sets out that the term “EU” includes (as the context permits or requires) Euratom (section 3(2)).

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Basically, Euratom was explicitly mentioned as the government had not stated its intention as seen in this article by Politco:

The British government had not previously stated its intentions to sever its ties to Euratom, but EU lawyers had warned that leaving the EU would automatically trigger a Euratom exit too. The agency is the lesser known of the three communities that led to the European Union’s creation.

However, it's worth noting that its creation precedes the EU and is technically not part of the EU though it has the same membership as the EU as seen in this article by The Guardian:

Signed in 1957 as the European Atomic Energy Community, it is a separate treaty from the EU with the purpose of creating a single market for nuclear knowledge and resources in the peaceful pursuit of science and nuclear energy.

Whilst currently its only full members are EU countries, it is in fact a legally separate organisation to the EU.

To sum up, some may think that Brexit automatically triggers an exit from Euratom since the European Union (Amendment) Act 2008 includes Euratom in its scope while others may think that it's not the case as Euratom is legally separate from the EU (though its only full members are EU countries).

So, the UK Gov basically makes their intention clear by explicitly mentioning it.

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As to why the UK would want to leave Euratom, it's likely because they want to exit the EU completely and control all their laws and policies, including their nuclear plants etc., which is in line with what the government has repeatedly conveyed.