Under JCPOA Iran was supposed to change the design of the Arak IR-40 research reactor to make it harder to produce (weapons grade) plutonium.
Iran announced in 2016 that they filled the core of the old Arak reactor with concrete.
Iran has removed the core of its Arak heavy-water nuclear reactor and filled it with cement, according to the country's Fars news agency.
Also that 2016 report was apparently confirmed by some Western sources:
U.S. State Department spokesman John Kirby confirmed the concrete has been poured into the reactor's central vessel. Secretary of State John Kerry said on Wednesday the core of the reactor had been removed and was to be filled with concrete and destroyed.
This month however, Iran threatened to restart work on the old heavy-water design at Arak within 60 days.
Also, in 2017 Iran apparently contradicted their previous (2016) claims regarding the status of Arak's old reactor:
In an August 22, 2017 interview on IRINN TV, Ali Akbar Salehi, head of the Atomic Energy Organization Of Iran (AEOI) and a member of Iran's nuclear negotiating team, reiterated the threats made by President Rohani on August 3, 2017 regarding Iran's ability to quickly revive its nuclear program at the point it was left prior to the JCPOA. He said that Iran will be able to start enriching uranium to 20% in the Fordo facility within only five days, and that it can also reactivate the old reactor in Arak, because cement was poured not into its core but only into a few external pipelines that are easily replaced. [...]
"We [actually] poured cement only into some of the reactor's pipelines, [pipes] several centimeters in diameter and two to three meters long. [We poured it] not into the reactor itself but [only] into the external pipes. If we are instructed to restore the former reactor and advance the former program that is unsuitable to [the present time] and is 40 or 50 years out of date, we will remove the front and back parts of these pipes and put in new pipes, which will take only several months. They made so much noise [about this], and we said nothing [at the time], out of humility, or if we said something nobody listened to us. That's why we have decided to present photos of the reactor [to the public].
Are there some published Western expert opinions how quickly can Iran bring back the old heavy-water reactor design at Arak to a level of readiness equal to the one before its JCPOA (partial) decommission? E.g. do they need a new heavy water reactor core? How long would that take to build? Or can they dig out the concrete of the old one (without significantly damaging it)? And how long would that alternative take? Presumably Arak was the subject of some Western inspections under JCPOA.