| En Name: | Udachny |
| Ru Name: | Удачный |
| Loc Name1: | Удачнай |
| Loc Lang1: | Yakut |
| Coordinates: | 66.4°N 131°W |
| Map Label Position: | top |
| Image Coa: | Udachny coat of arms (2009).png |
| Federal Subject: | Sakha Republic |
| Adm District Jur: | Mirninsky District |
| Adm Selsoviet Jur: | Udachny |
| Adm Selsoviet Type: | Town |
| Adm Ctr Of: | Town of Udachny |
| Inhabloc Cat: | Town |
| Inhabloc Type: | Town under district jurisdiction |
| Mun District Jur: | Mirninsky Municipal District |
| Urban Settlement Jur: | Udachny Urban Settlement |
| Mun Admctr Of: | Udachny Urban Settlement |
| Leader Title: | Head |
| Leader Name: | Artur Prikhodko |
| Pop 2010Census: | 12613 |
| Established Date: | 1968 |
| Current Cat Date: | 1987 |
| Postal Codes: | 678188 |
| Dialing Codes: | 41136 |
| Website: | http://мо-город-удачный.рф |
Udachny (Russian: Удачный|p=ʊˈdatɕnɨj, lit. successful or lucky; sah|Удачнай, Udaçnay) is a town in Mirninsky District of the Sakha Republic, Russia, located on the Markha River, 508km (316miles) from Mirny, the administrative center of the district. As of the 2010 Census, its population was 12,613.
The Udachnaya pipe diamond deposit was discovered in 1955. Due to its isolated location, it was not exploited until the 1960s. In conjunction with the beginnings of diamond production, the urban-type settlement of Udachny was founded in 1968; town status was granted to it in 1987.[1]
As part of a plan to create the basin for a tailings dam for the nearby diamond mine, a 1.7 kiloton atomic bomb was detonated 98m (322feet) underground near Udachny on October 2, 1974. Original plans had called for eight similar explosions to be conducted; however, due to radioactive fallout being far greater than expected, the project was halted after the first blast. The shaft in which the explosion was held was not plugged until eighteen years later, with an estimated 7to thick concrete sarcophagus.[2]
As an inhabited locality, Udachny is classified as a town under district jurisdiction. Within the framework of administrative divisions, it is, together with one rural locality (the selo of Polyarny), incorporated within Mirninsky District as the Town of Udachny. As a municipal division, the Town of Udachny is incorporated within Mirninsky Municipal District as Udachny Urban Settlement.[3]
Diamond mining remains the principal economic activity in the town. Udachny is the second most important diamond production site after Mirny for the state-owned ALROSA corporation. It is served by Polyarny Airport 12 km outside of town.
Udachny has an extreme subarctic climate (Köppen climate classification Dfd). Winters are extremely cold, with average temperatures from to in January, while summers are mild, with average temperatures from to in July. Precipitation is quite low, but is significantly higher in summer than at other times of the year.