A core shroud is installed inside the pressure vessel in every reactor. This shroud is cylindrical in shape and is made of thick steel plate; it is not under pressure, and it does not perform a barrier function in order to contain radioactivity. ENSI assesses the integrity of core shrouds.
The core shroud is open at the bottom and the top, and it is installed permanently inside the reactor pressure vessel. It is welded together from several superposed steel plate cylinders, and is one of the reactor internals. As – unlike the reactor pressure vessel – it does not retain or conduct pressure, it walls are relatively thin (from about 3 to 5 centimetres).
The hydrodynamic conditions in pressurised water reactors differ from those in boiling water reactors. For this reason, the core shroud in pressurised water reactors performs a slightly different function. In this article, we discuss the core shroud in boiling water reactors such as those at Mühleberg and Leibstadt.
- Reactor Pressure Vessel Head
- Outlet for Steam
- Inlet for Coolant
- Core Shroud
- Fuel Elements
- Control Rods
The core shroud separates the flow directions of reactor cooling water and water at different temperatures.
It is not under pressure, and it does not perform a barrier function in order to contain radioactivity. The core shroud is open at the bottom and the top.
The hydrodynamic conditions in pressurised water reactors differ from those in boiling water reactors. For this reason, the core shroud in pressurised water reactors performs a slightly different function. The core shroud we are talking about here is installed in boiling water reactors such as those at Mühleberg and Leibstadt.
Essentially, the core shroud performs these three functions:
- Controlling the direction of flow
Inside the reactor pressure vessel, the core shroud separates the flow directions of reactor cooling water and water at different temperatures. Colder water flows from top to bottom in the annular gap between the core shroud and the wall of the reactor pressure vessel. In the inner zone of the core shroud, where the fuel assemblies are inserted, warmer water flows in the opposite direction – i.e. from bottom to top. Due to the heat from the fuel assemblies, these flow conditions establish themselves by way of natural circulation. The speed of the flow passing by the fuel assemblies can be increased with the help of the pumps in the reactor recirculation system. The reactor power (core thermal power, CTP) is directly related to the flow speed of the water. The power of a boiling water reactor is regulated by the recirculation pumps. When water flows through the fuel assemblies in a boiling water reactor, it is heated and partially converted into steam. This steam is then led away from the upper section of the reactor pressure vessel via the primary circuit pipes to the turbines.
- Stabilising the fuel assemblies
The “lower core plate” is located at the bottom of the core shroud, and the “upper core plate” is located at the top. The purpose of these two plates is to ensure the lateral position of the fuel assemblies so that the control rods can be retracted safely and reliably at all times.
Il mantello del nocciolo deve essere di spessore tale da consentire di immettere acqua nel nocciolo fino a due terzi della sua altezza anche in caso di rottura della condotta più grande del circuito primario. Si tratta di un requisito necessario per garantire il sufficiente raffreddamento del nocciolo del reattore in tale eventualità.
- Maintaining the water level
The core shroud must be impermeable enough to allow flooding of the core up to two thirds of its height, even in case of a break in the largest pipe in the primary circuit. This requirement must be met so as to ensure that the reactor core can be cooled adequately in such a case.
Periodic review of the core shroud
Based on experience from many years of operating boiling water reactors all over the world, it is known that cracks can form in the weld seams of core shrouds. Shrouds therefore undergo thorough examination. The formation and propagation of the cracks occur mainly on account of stress corrosion cracking, as it is known, which takes place under certain chemical and physical conditions. As the core shroud is a reactor internal that does not conduct or retain pressure, operation can continue even with a core shroud in which cracks have been found. However, it is a requirement that such cracks do not reach proportions that would jeopardise mechanical stability or maintenance of the water level in case of an incident involving major loss of coolant. For this reason, regular technical tests must be performed on the material of the core shroud, and safety assessments must be carried out.
Core shrouds can be reinforced, repaired or replaced, although the latter option involves substantial exposure to radiation for the staff. To provide additional reinforcement and stabilisation for the core shroud, especially if cracks are present in the weld seams, so-called anchor bolts are fitted; this has been done, for example, in boiling water reactors in Spain, Japan, the US, and also at Mühleberg in Switzerland.
Specific monitoring programmes are in place for the purposes of detecting the appearance of cracks at an early stage, tracking their growth and ordering actions as necessary. The weld seams are inspected at regular intervals during the planned maintenance outages, with the help of non-destructive testing methods. ENSI assesses the integrity of the core shrouds in Switzerland’s two boiling water reactors by means of weld seam inspections, crack growth measurements and fracture-mechanical evaluations.