In the middle of May 1986 a Governmental Commission decided to make a long-term covering of the Unit 4. The design of a protective sarcophagus ‘Shelter’ was started on 20 May 1986. Numerous research and development institutions, design organizations, scientific subsidiaries of ministries and their offices were involved in the design process. The design idea was to constrict a cover consisting of structural elements that would be up to 50 meters long and use the intact walls and structures as supports. Among the 18 options proposed, the one selected was the most risky.
Developing a design without preliminary research of a support had no precedent in construction practice. The risk was offset only by an expected reduction in the construction term and the fastest cessation of radioactivity releases into the environment. Alexey Bytskiy, who was appointed as chief engineer, personally collected all the required information flying over the damaged reactor in a helicopter in a special cabin hanging under a hoisting crane. For the most part, the design were developed using only photos, which were taken by pilots every day. The design process itself was protracted for five and a half months – the whole period while the sarcophagus was being built. Lots of corrections were made on the spot. The construction process was divided into three stages. The first stage lasted from May 20th to July 15th, 1986. The main objective was to produce the structure and elements of the Shelter necessary to protect working site from major sources of radioactivity, perform decontamination, and make preparations for the building works. The second stage, from July 16th to September 15th, 1986, accomplished the major part of construction activities with the third stage, which involved the completion of building and assembly works, lasting from September 16th to November 30, 1986.
For separating the destroyed Unit 4 from Unit 3 the partition and wall were constructed. The reinforced concrete walls were constructed along the Unit 4. The Northern Buttress Wall was made of concrete up to 12 meters in height in the form of cascades. The cascades were filled with worn and damaged metal structures and also with containers with high-level waste. The undamaged western wall was shielded from the outside by a wall with counterforce up to 50 meters high. The number of workers involved was about 90 thousand. During 1986-87, when the radiation exposure was the highest, the total number of people working in the Chernobyl region was about 200 thousand.
On November 30th, 1986, the State Admission Commission took control of Chernobyl NPP Unit 4, which was brought to a safe storage state for maintenance.