Decision to create Semipalatinsk Nuclear Test Site was made by Central Committee of the Communist Party and by USSR Council of Ministers on August 21 1947. So, the Decree of the Central Committee and USSR Council of Ministers has established Mining Seismic Station (site 905) re-named in 1948 into Training Site of the Ministry of Defense and, later, into State Central Science-and-Research Test Site No.2 (GOS-CNIIP 2). First units of the test site (military unit 52605) started their re-deployment to the Site on June 1 1948. Works on preparation to the tests were completed at the Site in July 1949.
The first Soviet nuclear bomb «RDS-1»
A distinguished political and scientific event of the global scale was the successful test of the first Soviet nuclear bomb performed at 7:00 a.m. 29th of August 1949; 12.08.1953 the first fusion device was tested and on 29.11.1955 – the first hydrogen bomb. One should also note a range of other important such as a test with release of a bomb from bomber aircraft on 18th of October 1951; the first commercial well-type explosion in order to create artificial lake at junction of the rivers Shagan and Ashisu on 15th of January 1965. Total 456 nuclear tests with 616 nuclear explosions were performed at the test site from 1949 to 1989. Majority of the tests were performed in the beginning of 1960th when tests in the atmosphere were shifted to underground ones. Nuclear weapon tests at Semipalatinsk Test Site can be divided into 2 stages: 1 – nuclear explosions in the atmosphere in 1949 – 1962; 2 – underground tests in boreholes and tunnels in 1961 – 1989. At SNTS besides nuclear tests were performed 175 explosions with chemical explosives. 
The following types of tests were conducted at the Site:
- tests of nuclear charges and ammunition in low atmosphere;
- ground and atmospheric nuclear explosions with the purpose to study damaging factors of nuclear explosion; tests of armament and ammunition, shelters, buildings, constructions and biological objects subjected to nuclear explosion;
- underground nuclear explosions in tunnels performed to test nuclear charges and ammunition, large-scale tests of linings and cans for underground constructions of various design, their technological equipment and other objects;
- underground nuclear explosions in wells to test nuclear charges and ammunition, large-scale tests of objects for Strategic Missile Forces (silo launchers including those equipped with missiles, unified command posts), fragments of State management system, nuclear arsenals and their technological equipment;
- peaceful underground nuclear explosions. There were developed technologies to use underground nuclear explosions for creation of artificial water reservoirs, channels, seismic probing of the Earth crust for minerals, creation of underground reservoirs, extinguishing of flames of burning gas and oil fountains, etc. At SNTS were performed 7 underground nuclear tests for commercial purposes. Explosion in the well 1004 performed to create an artificial reservoir, a so-called “Atomic Lake”, has shown that the harm from commercial nuclear technologies based on underground nuclear explosions may exceed dramatically the economical profit from the explosion.
High-altitude and near-surface (atmospheric) nuclear explosions were performed at the testing ground Opytnoye Pole (“The Experimental Field”). Tests in tunnels were performed at the testing ground of the mountain range Degelen; well tests were performed at the grounds Balapan and Sary-Uzen. Reactions with incomplete chain reaction were performed at the testing ground Aktan-Berli; technologies of commercial explosions were performed at the testing ground Telkem. In the final years of tests at the Site, there were performed works aimed at creation of an additional testing ground for well explosions – Novaya ground, but no nuclear explosion has been performed there. 
Main impact on the environment in terms of radioactive contamination was made by ground and atmospheric nuclear explosions; lower impact – excavational explosions, i.e. explosions in low-depth wells with excavation of ground. Tests in boreholes and tunnels (except those with accidents), as a rule, caused no contamination at the day surface.

Kurchatov Igor Vasilievich (1903-1960)
Khariton Yurii Borisovich (1904-1996)
Sakharov Andrey Dmitrievich (1921-1989)

Reactor tests. Starting from 1958 the USSR Council of Ministers issued a series of decrees about organization of research aimed at creation of nuclear missile engines (NME) and nuclear power installations (NPI). To test NMEs and NPIs and their units at the site was commissioned in 1961 a pulse graphite reactor IGR with thermal neutrons. Other tests required construction in 1962-1970 a reactor facility “Baikal-1”. Tests of fuel assemblies for nuclear missile engines and nuclear power installations in the reactor IVG-1 as well as tests of IRGIT reactors (ground-based prototypes of NME reactor) were performed at the stand facility of “Baikal-1” in 1974-1988.   
Tests of gas-cooled FA and reactors IRGIT at the stand facilities “Baikal-1” were performed in open cycle with release of coolant to atmosphere. Release of decay products at these tests was close in its composition to radioactive releases of an NPP at its normal operation and exposition does rates at the Site boundary comprised just 15÷20 μR/h, i.e. was very close to the background values for this region. Total 28 “hot” starts of the reactor IVG-1 with release of activity to atmosphere were performed at reactor power ranging from minimal 4.9 to max 230 MW. Total 178 gas-cooled FAs were tested in 4 active cores.
Reactor IGR was used to test fuel elements and fuel assemblies for reactors YaRD and YaEDU and for gas-dynamic installations with various heat carriers (hydrogen, nitrogen, helium, and air). Environmental safety of the tests at the stand facility IGR was assured by a pressurized system for release of gaseous heat carrier. Spent heat carrier was aged in tanks of the closed release system long enough to decrease its activity down to acceptable levels.
Therefore, in 1970-1980th the second largest source of radioactive contamination at SNTS was reactor tests with releases of radioactivity to atmosphere. 

Demilitarization of SNTS. During 1994-98 were performed large-scale works on demilitarization of SNTS and liquidation of its infrastructure. On May 31 1995, the last nuclear device was destroyed at the testing ground Degelen. It was exploded in the bore-well 108-K using conventional chemical explosives with no nuclear reaction involved. Destruction of the device has been confirmed employing distance control methods. Radiation control performed during the following five days revealed no exceeding over natural radiation background both in the bore-well and at its mouth.
In 1996 – 2000 within the “Agreement between the US Ministry of Defense and Kazakhstan Ministry of Science and New Technologies about liquidation of nuclear infrastructure” were performed works on sealing of all 181 tunnels in the mountain range Degelen; sealing was made using concrete plugs, boring of blast-holes to explode the tunnels from inside and outside as well as explosions with laid on charges. Liquidation of the tunnels was followed by remediation works at near-mouth sites to improve the radiological situation at Degelen. At the testing ground Balapan were destroyed 13 ready-to-use wells and 12 SILOs for intercontinental missiles. Conventional chemical charges were used to destroy these SILOs.