REACTOR PHYSICS AND CRITICALITY SAFETY
Nuclear reactor physics, criticality safety, activity inventory, determination of source strength and fluence monitoring
Structure of fuel assemblies
In accordance with the requirements to be taken from rules and regulations for the national and international nuclear industry, TÜV NORD Nuclear carries out assessments of the following:
- The neutron physical structure of fuel assemblies and reactor pressure vessels,
- the radiation exposure of the reactor pressure vessel and its components,
- criticality safety during the handling of nuclear fuel in fuel assembly production plants and nuclear power plants, during the transport of nuclear fuel, and in storage at centralised and decentralised storage sites.
All of the above activities are based on the TÜV NORD Nuclear experience gained in the assessment of highly complex problems in neutron physical core design and criticality safety – partly using advanced computer codes to perform independent calculations – as well as during inspections carried out within the framework of regulatory and surveillance procedures for nuclear power plants.
With regard to the assessments and inspections performed for issues related to storage containers, interim storage facilities, waste storage facilities and compact storage facilities for fuel assemblies, TÜV NORD Nuclear reviews activity inventories, neutron and gamma source strengths as well as the decay heat of nuclear fuel and structural materials.
Neutron physical fuel assembly and reactor core design
The documentation on the neutron physical fuel assembly and the reactor core design – submitted by the licensee for instance for the approval of a reactor core reload – is assessed by the TÜV NORD Nuclear experts. For this task, they credit their experience gained in more than forty years in the operation surveillance of nuclear reactor cores and the application of advanced calculation systems for assessing the neutron physical design of fuel assemblies and reactor cores.
For the assessment, TÜV NORD Nuclear utilizes internationally proven codes that they have expanded through as developments. Specifically, the computer codes CASMO/SIMULATE/TRANSURANUS and the SCALE code package are routinely used. For three-dimensional calculations of reactor dynamics, the S3K code in combination with the plant dynamics code ATHLET is applied.
Criticality safety
TÜV NORD Nuclear verifies compliance of the manufacture process of nuclear fuel with the applicable safety requirements and criteria regarding criticality safety; equally, the transport, storage and handling of fuel assemblies in nuclear power plants as well as in the interim dry storage facilities of nuclear fuel are an issue of TÜV NORD Nuclear in the field of criticality safety.
For the evaluation of criticality safety analyses, the TÜV NORD Nuclear experts use modern computational systems (SCALE, MCNP). In this task, they credit again their long-term comprehensive experience from verifying the compliance with criticality safety requirements in fuel fabrication and nuclear power plants.
Storage of spent fuel assemblies and radioactive waste
Based on their independent calculations (SCALE), TÜV NORD Nuclear experts evaluate the nuclear characteristics (criticality safety, activity inventories and source strengths) of transport containers and of the storage facilities which are used to store spent fuel assemblies from nuclear power plants and radioactive waste from other nuclear facilities.
Activity inventories
TÜV NORD Nuclear determines the activation of core as well as structural components close to the core and assesses the generation of activation products.
Radiation monitoring programmes for reactor pressure vessels and for control rods are routinely reviewed by TÜV NORD Nuclear. To accomplish this, its experts use computer codes for reactor core design and the components of the SCALE program for neutron transport calculations, which – as mentioned above – has been expanded by TÜV NORD Nuclear through its own developments.
