Renewal by annealing

THE REACTOR IS AN IRREPLACEABLE part of a nuclear power plant. The resilience of the power plant depends on the reactor’s life cycle because, over time, neutron irradiation from the reactor core causes the metal of the reactor vessel to become brittle.

Balakovo became the first nuclear power plant in the world where the annealing of reactor vessel metal was successfully carried out on a 1000MW VVER-1000 reactor. The procedure not only increases the reactor lifetime, adding 15 to 30 years, but also improves the overall safety of the unit.

Annealing is recognised as the most efficient way to almost entirely restore the physical and mechanical properties of the reactor vessel metal, significantly increasing the lifetime of a power unit. It has been successfully used at VVER-440 medium-power reactors (440MW), which made it possible to increase their lifetime from 30 to 60 years. Annealing has been carried out on VVER-440 reactors outside Russia: at Rovno (Ukraine), Metsamor (Armenia) and Kozloduy (Bulgaria).

The annealing carried out at Balakovo is unique because large capacity nuclear plants have technical characteristics that differ from smaller reactors. Compared with VVER-440 reactors, VVER-1000s reactor pressure vessels are much larger in diameter and have much thicker walls. In addition, elements of the vessel fastening system are in a reactor shaft very close to the heating area. Due to these differences engineers had to come up with a new annealing technology instead of using the one developed for VVER-440s.

Preparations for the operation began long before the programme began. Between 2009 and 2013, specialists from the Kurchatov Institute National Research Center and the KM Prometey Central Research Institute developed a specific temperature-time annealing mode. OKB Gidropress calculated the effects of applying the technology at a nuclear plant. NPF TermIKS in cooperation with NPO TSNIITMASH carried out the design and launched the manufacture of the annealing equipment.

Between 2015 and 2017, the equipment successfully passed comprehensive tests on a full-scale model of a reactor at Atommash in Volgodonsk. All the specialists who later participated in the operation at Balakovo, also underwent training at the Atommash site.

Work at Balakovo

The annealing process at Balakovo – from installation and testing of equipment, loading the heating device into the reactor vessel, to the heat exposure itself – was conducted under supervision of the Kurchatov Institute.

The equipment used for the annealing process weighed 32.4t and had a power of 840kW. The heat treatment, itself was carried out in several stages and lasted 220 hours.

“All the equipment necessary for the recovery annealing procedure, was delivered to the station on time. These included a heating device, assembly slipway, external thermal insulation for cutting off heat losses from the reactor vessel and power-supply control panels. We also ensured the required number of specialists were present on site. This allowed us to carry out the operation in an automated mode in strict accordance with the technology developed by the design organisations,” said Yuri Ryzhkov deputy chief engineer for safety and reliability at Balakovo.

After fuel and other in-vessel elements and equipment were removed, the heating device, consisting of 18 independent heating zones, was placed inside the reactor. During the annealing process, the reactor vessel was slowly heated to a temperature of 565 °C and it was kept in this state for 100 hours. This thermal effect meant that the welded joints and base metal in the reactor vessel core almost entirely regained their mechanical properties.

After the metal was gradually cooled down to a temperature of 70 °C, the heating device was dismounted. Standard procedures of visual and ultrasonic inspection of reactor vessel metal were carried out, confirming the success of the thermal operation.

All annealing operations were carried out with the participation of specialists from Rosenergoatom, OKB Gidropress and NPO TSNIITMASH, as well as specialists from the Balakovo plant, the Balakovo branch of JSC Atomenergoremont and NPF TermIKS.

On 22 December, upon completion of scheduled maintenance, Balakovo 1 was reconnected to the Russian grid. Heating and startup of the power unit were carried out in strict accordance with Russian nuclear plant operating regulations.

Export potential

“Annealing is new and, currently, the only technology in the world that enables the extension of the service life of a reactor,” said Rosatom Director General Alexey Likhachov. “This technology will be available for export.”

The technology could be developed for implementation in water-moderated power reactors of any design and capacity, Rosatom said.

Rosatom added that the extended operational service period of a reactor achieved using annealing could reduce the levelised cost of energy to as little as $29/MWt-hour less than the marginal cost of coal-fired generation. 

Material prepared by Olga Petrenko with input from Denis Zhurko, head of the Kurchatov Insitute