Question 1: What is the difference between “decommissioning” and “D&D”?
Answer: D&D refers specifically to Decontamination and Dismantling — the physical operations of cleaning and taking apart the facility. Decommissioning is a broader term that encompasses the entire process from termination of the useful life of the facility, through D&D operations, delicensing actions, and management of radioactive waste, through to the final release of the site for unrestricted or partially restricted use.
Question 2: List four reasons why a nuclear facility might need to be decommissioned.
Answer: (Any four of the following): (1) End of operating life — the licence period has expired and is not extended; (2) Uneconomical operation — rising costs or falling revenues; (3) Technical obsolescence — the facility has become outdated; (4) Safety considerations — the regulator requires expensive safety improvements; (5) Change in government policy — the facility’s operation is contrary to national policy; (6) Accident situation — the facility has experienced a major incident.
Question 3: Describe the three stages of decommissioning and give the approximate timescale for each.
Answer: Stage 1 (Initial Decommissioning/POCO): Begins at shutdown; involves defuelling the reactor, removing process materials, transferring spent fuel; timescale varies but typically a few years. Stage 2 (Partial Dismantling): Involves sealing the reactor, decontaminating and dismantling everything outside the biological shield; the reactor is left under care and maintenance for approximately 50 years to allow radioactive decay. Stage 3 (Unconditional Release/Demolition): After the care and maintenance period, the biological shield, RPV and all remaining radioactive structures are dismantled and all radioactive material is removed from the site; the site is released for greenfield or brownfield use.
Question 4: When was the NDA established and under which legislation? What are its main responsibilities?
Answer: The NDA was established in April 2005 under the Energy Act 2004. Its main responsibilities are: decommissioning and cleaning up nuclear facilities; ensuring safe management of all waste products; implementing policy on long-term nuclear waste management; developing UK-wide LLW strategy and plans; and scrutinising the decommissioning plans of EDF Energy for their AGR fleet.
Question 5: What are the dose limits and constraints applicable to members of the public from decommissioning operations?
Answer: The dose limit for an individual member of the public is 1 mSv per year. The source-related dose constraint is 0.3 mSv per year (from a single source). The site-related dose constraint is 0.5 mSv per year (from all operations on a nuclear site). The risk target for underground waste disposal is 10^-6 per year, and the maximum dose from any waste disposal site must not exceed 0.1 mSv per year.
Question 6: Explain the difference between LLW and ILW in the UK waste classification system.
Answer: LLW (Low Level Waste) has a radioactive content not exceeding 4 GBq/te of alpha activity or 12 GBq/te of beta/gamma activity. It is suitable for disposal in near-surface engineered facilities such as the LLWR near Drigg. ILW (Intermediate Level Waste) exceeds these activity levels but does not generate enough heat for this to be taken into account in storage or disposal facility design. ILW requires disposal in a deep geological repository.
Question 7: A decommissioning worker is exposed to a source with a dose rate of 100 microSv/h at 1 m. They work at a distance of 2 m for 3 hours. Calculate the dose received.
Answer:
Using the inverse square law: D-dot(2m) = 100 x (1/2)^2 = 100 x 0.25 = 25 microSv/h
Total dose = 25 x 3 = 75 microSv = 0.075 mSv
This is well below the occupational dose limit of 20 mSv/yr but the ALARP principle still applies.
Question 8: Name three chemical and three mechanical decontamination techniques, and one emerging technology.
Answer: Chemical techniques: (1) Nitric acid (for stainless steel/Inconel); (2) LOMI (multiphase treatment for reactor circuits); (3) Foam decontamination (for porous and non-porous surfaces). Mechanical techniques: (1) High pressure water jetting (for metal and concrete); (2) Grinding (for floors and walls); (3) Scarifying/scabbling (for concrete surfaces). Emerging technology: Laser ablation — a dry process using concentrated light energy to remove surface contamination, with minimal secondary waste and capability for remote application using optical fibres.
Question 9: What is vitrification and why is it used for HLW?
Answer: Vitrification is the process of mixing high-level liquid waste (containing mostly fission products after uranium and plutonium have been separated out during reprocessing) with borosilicate glass at approximately 1100 degrees C. The resulting glass-like solid is contained in stainless steel canisters for storage. It is used for HLW because the vitrified product is highly stable, resistant to radiation damage, has very low leachability, and can be stored safely for long periods while awaiting geological disposal.
Question 10: Compare the disposal approaches of the UK, Sweden and the USA for high-level waste.
Answer: The UK is pursuing a Geological Disposal Facility (GDF) through a community consent model, where willing communities volunteer to host the facility; the geology has not yet been determined and the programme is at an early stage. Sweden has selected the Forsmark site and uses the KBS-3 method, placing spent fuel in copper canisters surrounded by bentonite clay in deposition holes at 500 m depth in granite bedrock; construction was approved in 2022. The USA designated Yucca Mountain in Nevada as its HLW repository site, but the programme was politically blocked and defunded in 2010; there is currently no operating civilian HLW disposal facility, and spent fuel remains in interim storage at reactor sites. The USA does operate WIPP in New Mexico for military transuranic waste, located in a salt formation at 655 m depth.