UK GAS-Prompt prices rise on cool weather, nuclear outages

* System undersupplied by 12.9 mcm/day

* Four nuclear reactors expected offline on Thursday

LONDON, May 6 (Reuters) - Prompt natural gas prices in Britain rose on Wednesday morning as cool weather and expected outages at the country's nuclear plants lifted demand from gas-fired power generators.

Prices for within day delivery were 45.25 pence per therm at 0837 GMT, up 25 pence since their last settlement. Prices for delivery on Thursday were up 0.25 pence at 45.10 pence per therm.

With supply flows at about 196.8 million cubic metres (mcm) per day on Wednesday and demand expected to be about 209.7 mcm, the system was 12.9 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.

Wednesday's demand was around 13 percent above the seasonal norm of 186 mcm, the data showed.

Expectations of lower power output from the country's nuclear generators and cooler weather forecasts led to higher demand from gas-fired power generators while supply was also dented by an ongoing outage at Norway's largest gas field Troll, traders said.

Three of Britain's 16 nuclear reactors are currently offline. EDF Energy's 550 megawatt (MW) Dungeness B-21 nuclear unit is also expected to go offline on Thursday, taking the total capacity of offline nuclear plants to 2.3 gigawatts (GW).

Britain's Met Office forecast temperatures would average around 12 degrees on Wednesday compared with average temperatures for May of around 14 degrees.

Meanwhile an outage at Norway's Troll extended into Wednesday, reducing capacity by 5.5 mcm a day, operators Gassco said.

Further along the curve, gas for delivery in June was 0.33 pence higher at 42.63 pence per therm and the Q1 16 contract was up 0.45 pence at 43.55 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was 0.25 euros higher at 20.45 euros per megawatt-hour (MWh).

The benchmark European Union carbon price hit a two month high of 7.75 euros on Monday morning on ICE Futures Europe, before slipping back to 7.63 euros, a 4 cent increase on Tuesday's close.

Carbon prices were up after the European Union late on Tuesday agreed to start reforms of the bloc's carbon market in 2019, earlier than the 2021 start date originally proposed by the European Commission. (Reporting by Susanna Twidale; Editing by Mark Potter)