At 9:50 on the evening of 17 March, Sizewell B operators noticed a higher-than-normal humidity reading inside the 1200 MWe PWR’s containment building. Although the reading suggested a leak rate which was only slightly above safety limits, operators began a controlled shut down in any case, by increasing borate concentrations in the primary loop – a controlled shutdown is preferred because it puts less stress on the plant. Seven hours later, at 4:50 am the next morning, the reactor had shut down, and would remain so for the next six months, until 29 September, at a cost of between 4 – 6 TWh of electricity production.

Standard procedures for responding to the alarm code dictated that staff check for a leak in the primary circuit, where 20 tons of water per second circulates through the reactor. It enters the reactor at a cool leg temperature of 290°, and leaves the reactor with a hot leg temperature of 325°. Together, the four steam generators produce 2 tons of steam per second. Records indicated that the circuit was leaking at a rate of below 0.5 L/min of unidentified loss before the humidity alarm. Although the current value was not much greater, between 4-4.5 L/min, the disparity was significant enough to lead to a decision to shut the unit down.

When the alarm sounded there was no indication where the leak was; it might have been coming from gland leakage from a valve, for example, which would require repacking. So maintenance workers went into containment to find the leak which is where they encountered the increased moisture levels.

In the Westinghouse SNUPPS PWR (Standardised Nuclear Power Unit Power Plant System Pressurised Reactor), one 16m-high pressuriser holds a steam bubble that pressurises the four-loop primary circuit to 155 MPa (2204 PSI) – the pressure is the same wherever you are in the circuit.

The top of the pressuriser vessel contains a water spray nozzle; the pressuriser is usually half full of water. At the bottom, 78 heating elements have been installed to heat the water to create the bubble. Each one of the elements is a 3m-long, 22mm-wide metal tube, inside which run heating elements that generate heat through electrical resistance. The wires connect to the power supply immediately outside the pressuriser vessel.

It transpired out that the seal on one of the heating elements failed and water infiltrated the element. Inside was magnesium oxide insulation, which swells to more than twice its volume when wet. The force of expansion caused a 14mm-long but narrow split in the circuit enclosure. Primary water from the pressuriser seeped down through the element, and on to the containment floor.

The exact design of the pressuriser heaters was to meet the particular specifications of the Central Electricity Generating Board, the commissioning utility when the station was being designed and built in the 1980s and 1990s. When the electricity industry was privatised, British Energy, (which was acquired by EDF in 2008), took over the station.

Sizewell B staff have been planning to replace the pressuriser heating elements in the next 5-10 years, explained Sizewell B station director Jim Crawford. Only 38 are required to operate at any one time to generate sufficient heat to form an adequate steam bubble. Now, because of the leak, element replacement had become a critical path project.

Inside the pressuriser, the vertical heating elements are supported at two points by a couple of 25mm (one-inch thick) horizontal stainless steel plates. The elements are gripped in closely-machined circular cut-outs. To remove individual elements without damaging the entire pressuriser, engineers would need to find a way to separate them from the upper and lower support plates.

Most of the elements could be removed from below, but some had to be removed from above within the pressuriser. While two repair plans were developed in tandem by suppliers Rolls Royce and Areva, the fuel was offloaded from the reactor for safety. Doosan built a full-scale replica of the pressuriser at its Gateshead facility so the repair plan could be tried and tested.

In one method, a small tethered robot with a mechanical cutter would be lowered into the pressuriser from the only access point, a 400mm (16-inch)-wide hatch at the top. The robot would swim around, controlled remotely, and cut off the tubes above and below the support plates. It was designed to fit through the gaps between the side of the support plate and the interior wall of the pressuriser.

In the other method, an electrical discharge cutter mounted in a jig assembly with a hollow tube termination would again be lowered through the access hatch and down over the top of an element to remove it. Electrical current at its bottom cutting edge would burn a hole through the support plate around the tube. The work would be carried out under water to reduce radiation dose exposure.

Ultimately EDF Energy chose this second approach, an Areva design carried out by Areva and Doosan, because it was simpler and more straightforward, Crawford explained.

To operate the equipment, a worker had to enter the pressuriser for a work period limited to 40 minutes at a time to keep the radiological dose to an acceptable level. Workers wore a two-layer protective suit and radio-monitored back-mounted personal dosimeter. The highest individual dose was 3.8 man mSv, twice the annual geological background radiation in parts of the UK.

To guard against the risk of the problem recurring, extra sensors have been installed. Strain gauges at the bottom of remaining elements will monitor any signs of expansion. A new camera system has been installed to watch over the bottom of the elements. All of these systems are intended to be backups; before the element cracks it will lose electrical conductivity, which is already being monitored. “We are building up layers of defence,” Crawford said.

When asked what lessons had been learned from the incident, Crawford said: “Our entire culture is based on learning and seeking continuous improvement in everything we do. We are a learning industry. We will take away any lessons learnt from this experience and share them with colleagues across the industry.“

The station sent out an incident report to industry organisation the Institute of Nuclear Power Operations a few weeks after staff identified the problem. A World Association of Nuclear Operators committee was due to produce a ‘significant event report’ in October to provide industry-wide learning about the incident, which was rated 0, ‘no consequence’, on the International Nuclear and Radiological Event Scale.