The subject was modifications made to reactor types that had major accidents, and specifically the RBMK-1000.
Last I checked, the VVER-440 is not an RBMK-1000, did not explode at Chernobyl, and has not had a major accident.
"The two VVER-440 units in Loviisa, Finland have containment buildings that fulfil Western safety standards.
A typical design feature of nuclear reactors is layered safety barriers preventing escape of radioactive material. VVER reactors have three layers:
1. Fuel rods: the hermetic zirconium alloy (Zircaloy) cladding around the uranium oxide sintered ceramic fuel pellets provides a barrier resistant to heat and high pressure.
2. Reactor pressure vessel wall: a massive steel shell encases the whole fuel assembly and primary coolant hermetically.
3. Reactor building: a concrete containment building that encases the whole first circuit is strong enough to resist the pressure surge a breach in the first circuit would cause.
Compared to the RBMK reactors – the type involved in the Chernobyl disaster – the VVER uses an inherently safer design because the coolant is also the moderator, and by nature of its design has a negative void coefficient like all PWRs. It does not have the graphite-moderated RBMK's risk of increased reactivity and large power transients in the event of a loss of coolant accident. The RBMK reactors were also constructed without containment structures on grounds of cost due to their size; the VVER core is considerably smaller"
It also appears that the VVER-440 does have a containment building, and older variants that had problems with said containment building were forced to shut down.
"One of the earliest versions of the VVER-type, the VVER-440, manifested certain problems with its containment building design. As the V-230 and older models were from the outset not built to resist a design-critical large pipe break, the manufacturer added, with the newer V-213 model, a so called Bubble condenser tower that – with its additional volume and a number of water layers – aims to suppress the forces of rapidly escaping steam without the onset of a containment-leak. As a consequence, all member-countries[citation needed] with plants of the VVER-440 V-230 type, as well as older types, were forced by the politicians of the European Union to shut them down permanently. Because of this, the Bohunice Nuclear Power Plant had to close two reactors and the Kozloduy Nuclear Power Plant had to close four. Whereas in the case of the Greifswald Nuclear Power Plant, the German regulatory body had already made the same decision in the wake of the fall of the Berlin Wall."
