A spokesman for the GMB Union says Sellafield "will not agree a way forward" after failing to carry out an agreement made in January last year to increase monitors' salaries due to the responsibility and accountability of the role.
"The industrial action taken today by the monitors at Sellafield was initiated by the company’s failure to carry out the agreement made in January 2014, when an independent panel decided that the monitors' role, because of the responsibility and accountability, warranted an upgrade to Band 4.
"It is 18 months now since that decision was made and the company will still not agree a way forward.
"We have informed the company that we are willing to sit down at any time to try and resolve this situation, but it seems that they are reluctant to do this.
The quote from the Director of Stakeholder Relations this morning ‘that the company could not always guarantee safety’ is something that causes the GMB great concerns. We provided 17 monitors to make sure that we were in a position to respond to any emergency and guarantee the safety of all personnel on the site."
Four-hundred monitors, who carry out radiation checks at the nuclear reprocessing plant in Cumbria, are taking part in today's industrial action.
Sellafield's management say contingency plans are in place and that the strike has caused "low key" disruption.
Stakeholder Relations Director at Sellafield Ltd, Rory O'Neill, says the company believes its workers are "fairly recognised for what they do."
The GMB Union says more than 97% of members at the plant voted in favour of industrial action over the current pay deal which, Sellafield says, is "appropriate."
More than a thousand construction workers at Sellafield have voted for strike action.
Unite union says the move follows a 10 month dispute over health and safety and a disagreement over the employment of a full-time shop steward.
Sellafield says the workers are contractors, employed by other companies, and their actions are not affecting safety or site operations.
Rail workers in Carlisle participated in a Union "day of action" between 7am and 9am this morning.
They are joining staff across the north in a campaign to have a publicly owned system in the region.
The RMT Union claims jobs, services, and safety are at risk if franchises are awarded to companies outside the UK.
Members of the Fire Brigades Union in Cumbria are returning to work this morning after a 24 hour strike.
They walked out at 7am yesterday morning as part of a long-running national dispute over pensions.
The Government says firefighters have "one of the best pensions in the public sector" but the striking workers say their conditions are being downgraded.
1,300 contractors have walked-out at Sellafield for the second time this week.
They originally downed tools on Monday after a dispute over scaffolding on a project to replace old ventilation.
While the issues surrounding that have been resolved, the workers are now unhappy with contractor Doosan Babcock and several health and safety Issues.They have said they plan to remain on strike until Monday.
Doosan Babcock hasn't commented.
Contractors that walked-out at Sellafield on Tuesday have returned to work today.
Talks to settle the dispute are also set to start as part of the agreement to workers ending the "unofficial action".
The mass walk-out was caused by a disagreement between two companies over who owns a piece of scaffolding at the site.
The mass walk-out at Sellafield has reportedly been caused by a dispute between two companies over who owns a piece of scaffolding at the site.
The union UNITE is claiming its members may not be insured to touch the equipment.
The main contractor Doosan Babcock has said it's in talks with unions to try and resolve the matter.
"Doosan Babcock can confirm that it is in consultation with its employee union representatives to fully understand and resolve the unofficial industrial strike action undertaken at Sellafield.
"While those talks are underway, we are unable to comment further."
Around 1,300 contractors based at Sellafield have walked out over a dispute between a main contractor and a sub-contractor.
Those striking work for a number of sub-contracting firms that operate under the umbrella of the National Agreement for Engineering and Construction Industry and were reportedly working on building projects.
Sellafield Ltd has responded to the "unofficial action", stating that the dispute does not directly involve the company.
"Sellafield Ltd is not directly involved in the dispute, which does not involve any of our employees, but we have taken steps to ensure that safety and security at the site are unaffected by this unofficial action, and we will be closely monitoring further developments.
"This unofficial action is as a result of a dispute between a main contractor and a sub-contractor"
Around 1,300 contractors based at Sellafield have walked out.
Workers downed tools at midday and are not expected to return to the site until Wednesday.
The industrial action is the result of a dispute between a main contracting firm and a sub-contracting firm on the nuclear plant where more than 10,000 workers are based.
Staff employed on the Settle and Carlisle Railway are set to take two more days of strike action as part of an ongoing dispute over compulsory redundancies.
Those striking provide at-seat catering on Northern Rail services between Carlisle and Settle. They will walk out for 24 hours on both the 14 and 21 of February
The Partnership that runs the service has previously said it cannot agree to its employees demands as they would undermine the financial security of the not for profit organisation.