Maentwrog Dam

Maentwrog New Dam, Llyn Trawsfynydd

Date	8 December 2012
Location	Afon Prysor / Llyn Trawsfynydd	SH 67455 37711; 52.92112°N, 3.97334°W
Information	The artificial lake Llyn Trawsfynydd, which later also provided a supply of cooling water for Trawsfynydd nuclear power station, was originally created as part of the Maentwrog hydroelectric power scheme, constructed from 1925 to 1928. Four dams were built to contain the reservoir, the principal one being the Maentwrog Dam on the Afon Prysor at the northwest corner of the lake. The original structure was at the time Britain’s largest arch dam. However, it suffered from long-term leakage problems, caused by, amongst other things, vertical cracks in the arch near the abutments and leaching of porous areas. Work began in 1988 on a replacement dam at the end of the Ceunant Llennyrch gorge 75m downstream from the original. The new S-shaped gravity dam was completed in 1992. It is 219m long, 39m high, and consists of 54,500 cubic metres of concrete. The heat produced by the chemical reaction when cement cures can prose serious problems in massive concrete structures with regards to thermal cracking. During the construction of the new dam, pre cooling was employed to mitigate this problem: in the hot summer of 1989, the temperature of a 600 cubic metre pour was reduced by 6°C by injecting 3000 litres of -200°C liquid nitrogen into the mixed concrete. Maentwrog Power Station and Dam are currently owned by the Nuclear Decommissioning Authority and are operated on its behalf by Magnox. Maentwrog Hydroelectric Power Station (Magnox)

Trawsfynydd Power Station

Trawsfynydd Power Station with the Moelwyns behind. Located in the Snowdonia National Park, the power station was designed by architect Basil Spence to look like ‘a castle in the landscape’.

Date	29 September 2012
Location	Trawsfynydd	SH 69197 38031; 52.92370°N, 3.94713°W
Information	The 390 MW twin-Magnox-reactor nuclear power station at Trawsfynydd generated electricity between 1965 and 1991. Construction of the plant, designed by architect Basil Spence to look like ‘a castle in the landscape’, started in 1959 and lasted six years. The station was officially opened in 1968. Trawsfynydd was the UK’s only civil nuclear power station to be built inland, taking advantage of Llyn Trawsfynydd as its source of cooling water. Following its shutdown in 1991, defuelling of the reactors started in 1993. The decommissioning phase commenced in 1995 and is set to continue until 2016, when an ‘Interim Care and Maintenance’ stage will be entered. A purpose-built storage facility for Intermediate Level Waste was constructed on site between 2005 and 2008. In 2020 work will start to reduce the height of the monolithic concrete reactor towers from 170 ft to 100 ft and to encase them for conversion into safestores. On completion of this in 2026, the site will then again be put into ‘Care and Maintenance’ until ‘Final Site Clearance’ — scheduled for 2076 – 2085 — at the end of which all radioactive waste will have been transferred to a national repository, all structures will have been demolished, and the land finally returned for public use. Trawsfynydd (Magnox Ltd)

Trawsfynydd Footbridge

Footbridge over Llyn Trawsfynydd

Date	29 September 2012
Location	Trawsfynydd	SH 70336 35247; 52.89896°N, 3.92909°W
Information	Llyn Trawsfynydd is an artificial lake just south of the Vale of Ffestiniog. It was constructed between 1924 and 1928 as the water supply for Maentwrog hydro-electric power station and it also later served as the supply of cooling water for Trawsfynydd nuclear power station, which was in operation from 1965 until 1991. When the reservoir was created, around twenty properties were flooded and public rights of way across the land were lost. To compensate for the latter, a public footbridge was built across the southern end of the lake. The steel bridge comprises 20 lattice-girder spans supported by trestle towers, is a quarter of a mile long, and links Trawsfynydd village with the site of a former local chapel. Trawsfynydd (Wikipedia)

Moelwyn Mawr and Moelwyn Bach

Llyn Stwlan, the upper reservoir for Ffestiniog Power Station, from Craigysgafn (B). The concrete dam and intake towers can be seen on the right, with the lower reservoir at Tanygrisiau below.

Date	26 June 2011
Location	Moelwynion	(A) SH 65821 44862; 52.98423°N, 4.00016°W (B) SH 65991 44332; 52.97951°N, 3.99740°W (C) SH 66028 43754; 52.97433°N, 3.99661°W (D) SH 63574 43762; 52.97378°N, 4.03314°W
Information	The Moelwynion group of mountains in Snowdonia are to be found roughly in an area with Porthmadog and Capel Curig at its south-west and north-east ends, respectively. The highest peak in the range is Moel Siabod (872 m) and the name of the group comes from Moelwyn Mawr (Big White Hill), 770 m, and Moelwyn Bach (Little White Hill), 710 m. A rocky ridge, Craigysgafn, separates these two mountains and overlooks Llyn Stwlan, the top reservoir of Ffestiniog Power Station at Tanygrisiau below. This 360 MW pumped-storage hyroelectric power station began service in 1963 and was the first major facility of its kind in the UK. Ffestiniog Power Station (First Hydro)