Context: Recently, Radiation sensors in Stockholm have detected higher-than-usual but still harmless levels of isotopes produced by nuclear fission, probably from somewhere on or near the Baltic Sea. 
Background 

• The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) oversees a network of hundreds of monitoring stations that use seismic, hydroacoustic and other technology to check for a nuclear weapon test anywhere in the world.

Key Points

• Stations scanning the air for radionuclides, the radioactive particles that can be carried long distances by the wind, detected unusually high levels of three radionuclides: caesium-134, caesium-137 and ruthenium-103. 
• Seismic refers to earthquakes or other vibrations underneath the earth's surface, while the term hydroacoustic refers to an underwater sound.   
• The detection used a borderless map showing where the particles might have come from in the 72 hours before they were detected - a large area covering the tips of Denmark and Norway as well as southern Sweden, much of Finland, Baltic countries and part of western Russia including St Petersburg. 
• Nuclear fission refers to the atomic chain reaction that generates heat in nuclear reactors  used in nuclear power stations. 

HOW DO NUCLEAR REACTORS WORK? 

• A nuclear reactor creates energy by splitting atoms of uranium.
• The energy released from these atoms is then used to boil water. 
• This, in turn, drives a turbine.  
• A reactor core contains the uranium pellets and a 1000 megawatts (MWe) facility would have about 75 tonnes of enriched uranium.
• Uranium-235 is bombarded with neutrons to split the atom, which then creates different elements or another isotope of Uranium. 
• Either way, it releases energy. 
• These often also undergo radioactive decay and a chain reaction is triggered - contributing to the net energy output. 
• Steam is produced, condensed and then recycled so the only waste products are often the radioactive compounds created from the fission. 
• Control rods can be added or removed from the reactor core to either increase or decrease the rate of reaction. 
• These are made of stable elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without undergoing fission.  

About Comprehensive Nuclear-Test-Ban Treaty Organization

• CTBT was negotiated at the Conference on Disarmament in Geneva and adopted by the United Nations General Assembly in 1996.
• The Treaty intends to ban all nuclear explosions - everywhere, by everyone. It was opened for signature in 1996 and since then 182 countries have signed the Treaty, most recently Ghana has ratified the treaty in 2011.
• A comprehensive test ban has been defined as a “zero yield” test ban that would prohibit supercritical hydro-nuclear tests but not sub-critical hydrodynamic nuclear tests.
• Hydronuclear tests study nuclear materials under the conditions of explosive shock compression. Their yield ranges from negligible all the way up to a substantial fraction of full weapon.
• Subcritical (or cold) tests are types of tests involving nuclear materials and possibly high-explosives that purposely result in no yield.
• The Treaty will enter into force after all 44 States listed in Annex 2 to the Treaty will ratify it.
• These States had nuclear facilities at the time the Treaty was negotiated and adopted.
• As of August 2011, 36 of these States have ratified the Treaty. Eight States still need to do so: China, North Korea, Egypt, India, Iran, Israel, Pakistan and the United States.
• India, North Korea and Pakistan have not yet signed the Treaty.
• All three have also undertaken tests after 1996; India and Pakistan in May 1998 and North Korea six times between 2006 and 2017.
• The CTBT has therefore not entered into force and lacks legal authority.
• The organization was founded in 1996. It is headquartered in Vienna. It employs a staff of roughly 260 from the CTBT’s Member States.

About Baltic Sea

• The Baltic Sea is positioned in Northern Europe and bordered by Sweden (a part of the Scandinavian Peninsula), Finland, Russia, Estonia, Latvia, Lithuania, Poland, Germany, and Denmark and its numerous islands.
• The Baltic states also known as the Baltic countries, Baltic nations, or simply the Baltics, is a geopolitical term, typically used to group the three sovereign states in Northern Europe on the eastern coast of the Baltic Sea: Estonia, Latvia, and Lithuania.  
• All three countries are members of NATO, the eurozone, and the OECD, and are members of the European Union.

About Baltic States

• Baltic States lie in the north-eastern region of Europe containing the countries of Estonia, Latvia, and Lithuania, on the eastern shores of the Baltic Sea.
• Climate: The climate across Baltic States is cool and damp, with greater rainfall in the interior uplands than along the coast.
• Temperatures are moderate in comparison with other areas of the East European Plain, such as in neighbouring Russia.
• People Majority: Balts, Finnic, Baltic Finns, Roman Catholics.
• Culture – The Baltic States offer a mix of old European charm and soviet memories, as per Baltic Run.