Preamble

Sizewell is a small village, located within the civil parish of Leiston, on the Suffolk North Sea coast, in eastern England. It is the site where nuclear power plants are built in the UK. The power plants in this site are designated Sizewell A, Sizewell B and Sizewell C.

Sizewell A is  the first, built in the 60s. It used Magnox reactor and has ceased operation since 2006 and is presently being decommissioned. 

Sizewell B is the second nuclear power station built in the site – commissioned in 1995. It is currently in operation using Pressurised Water Reactor (PWR) to generating about 1.2 GW of power to provide electricity to about 2.5 million homes in the UK. The reactor operates at a temperature of about 325 °C and a pressure of 155 bar.

Sizewell C is a planned third station to be built on the site and it is expected to use two European Pressurised Reactors (EPRs) and to generate energy enough to power about 6 million homes.

This article focuses on the fabrication of the pressuriser for the Sizewell B nuclear plant – the only Pressurised Water Reactor (PWR) in the UK, currently operated by EDF Energy (see Fig. 1). The station is reported to provide 3% of the UK’s electricity, making an enormous contribution to the UK’s net zero ambitions and energy security.

Basic operation of Sizewell B plant

The basic operation of the plant is that a slow-moving neutron is absorbed in fissile nucleus – for example, absorbed in Uranium-235, to produce excited Uranium-236 which splits to produce fission fragments, energy and fast neutrons as illustrated Fig. 2.

Fast moving neutrons have very low probability of causing fission reaction. Hence, the fast-moving neutrons must be slowed down to sustain critical chain reaction.

At the Sizewell B nuclear power plant, light water is used both to cool the reactor core and to moderate neutrons, thereby controlling the nuclear reaction. The water surrounding the fuel rods slows down the fast neutrons produced during fission, increasing the likelihood of further fission events. This moderation allows slow neutrons to more effectively interact with Uranium-235, sustaining the chain reaction and enabling continuous energy generation.

Fig. 3 shows a typical Pressurised Water Reactor (PWR) nuclear plant. The energy released in the fuel rods heats up the light water. The water is pressurised to increase the water boiling point – hence called Pressurized Water Reactor (PWR).  The super-heated water from the reactor is circulated through steam generator and the steam produced is used to drive the turbine to generate electricity.

The Pressuriser

The pressuriser is a large cylindrical tank installed within the PWR coolant system. A typical example is shown in Fig. 4. Its main function is to control the pressure of the primary system’s light water coolant. Additional functions include monitoring water level and providing pressure relief.

The pressuriser is a safety-critical component, as it maintains the primary coolant in a liquid state, thereby reducing the risk of reactor core overheating. It has heaters mounted in inserts that penetrate through the bottom head, as shown in Fig. 4. These heaters and inserts are supported by two perforated plates

Welding Case Study – The Sizewell B pressuriser

The Sizewell B pressuriser was reported to weigh approximately 100 tonnes, with dimensions of 16 m in length and 2.5 m in diameter. It was fabricated from three 100 mm-thick forged strakes (ring forgings), along with 75 mm-thick top and bottom hemispherical heads. It has 78 electric heaters in tubes that keep the water inside at a slightly higher temperature than the rest of the primary circuit.

Design Standard and Material Selection

It was designed and fabricated in compliance with ASME ASME Boiler and Pressure Vessel Code (BPVC) Section III and the National Nuclear Corporation Specification IM 20713-Q Part F.  The pressuriser is a Class 1 component under the ASME BPVC Section III, Division 1, Subsection NB.

Fig. 5 shows the Sizewell B reactor pressuriser and the dished heads before machining and welding. The forged top and bottom heads were machined to the required dimensions. The material used in its construction was a quenched and tempered ASTM A508 grade 3 (MnNiMo) low alloy steel.

Forged materials offer good mechanical properties, and their eliminates longitudinal seams that would be required in cylinders manufactured from rolled plate, as well as welded-in nozzles in the heads – thereby reducing the number of welds and the extent of in-service inspection.

Welding Operation – Circumferential Seam Welding of Sizewell B Pressuriser

The welding of the vessel was carried out primarily along circumferential seams, using a single-sided J-preparation joint. Side 1 was welded using a manual metal arc (MMA) seal pass, followed by fill and cap passes using single-wire submerged arc welding (SAW). On the Side 2, the previous weld was gouged – using arc-air gouging, ground, and inspected using magnetic particle inspection (MPI) before backfilling.

The circumferential welds were ultrasonic tested prior to cladding. Additionally, the surface finish of both the welds and the cladding was specified to be 250 CLA (6.3 μm Ra) or better, with a flatness tolerance of 1.5 mm over a 50 mm span.

During welding, a preheat temperature of 150 °C was applied using radiant gas heaters, with temperature monitored and controlled by an infrared thermometer positioned just ahead of the welding head, along with a series of sliding thermocouples. Upon completion of welding, the preheat was increased to 250 °C and maintained for 4 hours to facilitate hydrogen release.

The Welding Sequence

Fig. 6 is a typical cut-away drawing of the pressuriser showing the strakes and the dish heads. The bottom head was welded to strake 3, then strake 3 to strake 2.

The internal cut-back was welded with the SAW process using a column and beam. The top head was joined to strake 1, with the closure weld made between strakes 1 and 2. The closure seam internal cut-back was welded using the MMA process – a challenging task due to the confined space within the enclosed vessel with a 150 °C preheat. Table 1 lists the welding parameters.

Non- Destructive examination (NDE) and Post-Weld Heat Treatment (PWHT)

Magnetic particle inspection (MPI), radiographic examination (RE), and ultrasonic testing (UT) were performed both before and after an intermediate post-weld heat treatment (PWHT) at 593 °C to 621 °C, as well as following the final heat treatment. The intermediate PWHT was reported to be necessary, as certain defects may become more readily detectable – i.e., sensitised, after heat treatment. The bottom-head/strake 3/strake 2 assembly and the top-head/strake 1 were furnace PWHT, while the closure seam underwent local PWHT.

The UT was carried out with 0° compression and 45°, 60° and 70° shear wave probes at 2 MHz and 4 MHz frequencies. Scanning was performed from both sides of the weld and from the inner and outer surfaces.

Miscellaneous Welds – Welding of the support skirt, perforated plates, and lifting trunnion pad

The support skirt  (see Fig. 6) was made from A516 Grade 70 steel and welded to a pintle, which was integrally forged into the bottom head. The welding procedure used was essentially the same as that applied to the main seam welds. This weld also underwent the same NDE as the main seams.

The two trunnion pads, shown in Fig. 6, were used to lift the pressuriser into a vertical position. These pads were welded onto opposing sides of strake 2 at a preheat of 150 °C using an ESAB submerged arc circle welder. The finished machined trunnion pad was partially lagged prior to an intermediate local PWHT. The perforated plates that support the heaters were manually welded using TIG welding.

The material selection and cladding of the pressuriser will be discussed in part 2 of this article.

Welding case study adapted from:

Mathers, G. (2025) ‘Fabrication of the Sizewell B Pressuriser’, Welding and Joining Matters, Issue 11, TWI, pp. 39 –41.

Contributor: VCI