1. BASIC PROFESSIONAL TRAINING COURSE Module XVIII Decommissioning Case Studies
Version 1.0, May 2015
This material was prepared by the IAEA and co-funded by the European Union.  1

2. Introduction
Each student should solve study cases for himself/herself.
Lecturer should decide if each student works on all cases or each student is assigned a certain number of cases of each topic.
Each study case should be presented on board, with the entire procedure and proper interpretation.
To facilitate discussion on particular cases description of a small nuclear research reactor is given in the introductory part.

3. Introduction (Cont.): TRIGA Mk II research reactor

4. Introduction (Cont.): TRIGA Mk II research reactor

5. Introduction (Cont.): TRIGA Mk II research reactor
Reactor core (STANDARD-12 Fuel Elements)
Fuel-moderator material: 11.8 wt% uranium in U-ZrH
1 wt% hydrogen
Uranium enrichment: 19.9% 235U
Fuel element dimensions: cm in diameter, cm in length
Cladding: mm steel (SS-304)
Mass of U in FE g
Mass of 235U in FE g
Active core volume: cm diameter, 38.1 cm height
Core loading: Approx. 59 fuel elements, approx. 3.2 kg of 35U
Core support: Upper and lower support grids, Al, thickness 1.91 cm
Number of FE in the core: 60

6. Introduction (Cont.): TRIGA Mk II research reactor
Reflector
Material: graphite with aluminium cladding
Radial dimensions: cm (inner dia.), cm (outer dia.)
Height: 53,8 cm

7. Introduction (Cont.): TRIGA Mk II research reactor
Construction
Reactor housing: heavy and standard concrete
6.55 m height
Reactor tank: 1.98 m in diameter
6.25 m in depth
5 mm thickness
Biological shielding
Radial: cm of graphite (reflector);
45.7 cm of water and at least
206 cm of heavy concrete
Vertical: above the core 4.90 m of water and
underneath the core 61.0 cm water,
91 cm standard concrete.

8. Introduction (Cont.): TRIGA Mk II research reactor
Irradiation devices
Four beam holes (columns) 15.2 cm in diameter.
One central irradiation tube (in the middle of the core).
Forty irradiation positions in rotary specimen rack;
Rack is made of aluminium and stainless steel parts (approx. 3 kg of SS).
One pneumatic transfer system (positioned near core edge).
The graphite thermal column with cross section 1.22x1.22 m and length m.
Bulk-shielding experimental tank (empty) with surface area 2.44x2.74 m and depth 3.66 m; connected to the reactor by means of a graphite neutron radiography collimator 0.61x0.61 m in cross section and 1.3 m long.

9. Introduction (Cont.): TRIGA Mk II research reactor
Control system
Three boron carbide control rods with electric motor and rack and pinion drive;
One boron carbide pulse rod with compressed air drive;
Total rod worth about 4.8% δk/k.

10. Introduction (Cont.): TRIGA Mk II research reactor
Characteristics in continuous operation
Thermal power output: 250 kW,
Fuel element cooling: natural convection of the tank water,
Tank water cooling: forced water circulation through heat exchanger,
Thermal flux: 1 ∙ 1013 cm-2s-1 in the central irradiation tube
1.7 ∙ 1012 cm-2s-1 in the irradiation tubes,
Prompt temperature coefficient: ∙ 10-4 δk/k /°C,
Mean prompt neutron lifetime: ∙ 10-5 s.

11. Introduction (Cont.): TRIGA Mk II research reactor
Characteristics in transient operation
Peak power: 250 MW
Prompt pulse energy yield: 10 MWs
Prompt pulse lifetime: 40 ms
Total energy yield: 16 MWs
Minimal period: 10 ms
Maximum reactivity insertion: 1.6%Δk/k = 2$
Maximum repetition frequency: 12/h
Number of fissions during a pulse: 3 ∙ 1017
Maximum fuel temperature: during the pulse 240 °C
9 seconds after the pulse 360 °C

12. Introduction (Cont.): TRIGA Mk II research reactor
Cooling system
During operation water from pool is circulated through heat exchanger and ion exchanger;
Two circulation pumps (primary and spare primary) are installed.
Heat exchanger: 0.3 m diameter and 5.4 m length, total mass is 2000 kg.
Heat exchanger consists of 64 stainless steel tubes with 6.2 m2 exchange surface.
Flow of primary water through heat exchanger is 23.6 m3 per hour.
Flow of secondary (cooling) water is 67.5 m3 per hour.
Secondary water is released into the river.
Ion exchanger unit with filter is installed in parallel with heat exchanger (cleaning bypass).
Flow of primary water through ion exchanger/filter: 2.4 m3 per hour.
Cooling system (pumps, heat exchanger and ion exchanger with filter) are located in the basement of reactor building.

13. Introduction (Cont.): TRIGA Mk II research reactor
Spent fuel pool
Located in the basement of reactor building.
It is a pit with dimensions 2.7 m ∙ 2.7 m and 4 m deep.
Walls are covered with 6 mm aluminium.
Pool is empty (all used fuel elements were returned).

14. Introduction (Cont.): TRIGA Mk II research reactor
History of operation
Reactor has operated for 50 years with average power 30kW (1050 FPH per year).
Reactor was mainly used for training and education, neutron activation analysis, isotope production, and neutron radiography.
There is no spent fuel on the site, since more than 200 used fuel elements were shipped back to United States in 1999.
During the first decade of operation one of the original fuel elements (with aluminium cladding) was leaking.
Fuel element was transferred to spent fuel pool and later returned to manufacturer.
Later on leak from cooling system was discovered.
System was repaired and decontamination performed.
Annual production of radioactive waste:
Spent ion exchange resins:  40 L per year,
Contaminated or activated material:  300 L per year.

15. Introduction (Cont.): TRIGA Mk II research reactor
Data from radiological survey
Reactor hall (during reactor operation):
Entrance: 0.24 µSv/h,
Beam ports: 0.5 µSv/h, 5.3 µSv/h, 22 µSv/h, 35.6 µSv/h,
No α or β/γ contamination measured.
Reactor bridge (during reactor operation):
Fence: 30 µSv/h to 60 µSv/h,
Over the pool: 560 µSv/h (max.),
Reactor basement (reactor is not operating):
Spent fuel pool: 3 µSv/h,
Old ion exchange resins and filters: max. 100 µSv/h (30 cm from barrels),
Ion exchanger/filter unit: 50 µSv/h (30 cm from unit),
Heat exchanger: 5 µSv/h (contact measurement),
Removable contamination was found behind heat exchanger (60Co, 137Cs and 154Eu were detected).

16. Case 1
When preparing the decommissioning plan the facility description is one of decommissioning plan topics.
What should be included in facility description?
During the research reactor design and build phase material selection is very important.
Why is material selection important?
What important characteristics should materials have?
What is happening with reactor materials during its operation?

17. Case 2
Selecting the decommissioning strategy is the another important topic.
Describe the possible decommissioning strategies.
Which issues must be considered during the selection process?

18. Case 3
During the design phase it is important to think about reducing the amount of waste generated during the decommissioning.
How can be amounts of radioactive waste reduced?
Discuss what must be considered in managing the waste from decommissioning.
During decommissioning cross-contamination and secondary waste generation must be avoided or at least minimized.
How to minimise cross-contamination and secondary waste generation during decommissioning?

19. Case 3 cont.
Around reactor core there is the biological shield. neutron flux detectors are placed inside biological shield.
Why is neutron flux in biological shield relevant for decommissioning?
There are different options for facility components after the removal of regulatory control. For some, there is an option of disposal in normal landfill sites. The other option is recycling or reuse of some materials outside the nuclear industry (steel, concrete).
What are the possible risks?
What considerations should be made to achieve a safe reuse of steel or concrete from research reactor?

20. Case 4
Safety assessment forms an integral part of the decommissioning plan. It includes, among other things, a systematic evaluation of nature, magnitude and likelihood of hazards and their radiological consequences.
Determine what kind of hazard are following examples:
External exposure from direct radiation,
Chemicals used for decontamination purposes,
Exposure due to ingestion,
High pressure,
Asbestos,
Dropped loads.
What are the internally arising, externally arising and human induced initiating events?

21. Case 4 cont.
Decommissioning is conducted using the defence in depth principle for safety.
Explain what is meant with defence in depth principle regarding the decommissioning.
Protection of safety barriers must be provided to protect workers, the public and the environment in all operating conditions and in a case of accident.
What accidents could occur during the decommissioning of research reactor?

22. Case 4 cont.
The results of safety assessments serves to demonstrate compliance with regulatory requirements or, in other words, that there are adequate safety measures in place.
What are these safety measures?
What needs to be done if the results of safety assessment does not demonstrate the compliance with safety requirements?
When safety assessment is finished it is reviewed by the regulatory body.
What are the objectives of the regulatory review of the safety assessment?
What are the results of the regulatory review of the safety assessment?

23. Case 5
In previous cases an overview of important topics of decommissioning plan was made.
What are the remaining important topics of decommissioning plan?

24. Case 6
During the design and construction phase of the research reactor initial decommissioning plan should be prepared. When the reactor is in operational phase there are still some considerations regarding the decommissioning.
What are these considerations? Give some examples.
A valve leakage was recorded several times at the research reactor and some areas were contaminated consequently.
What possible effects can the mitigating actions have on decommissioning plan?
What should be done, if decision is made to modify the section of pipeline with the associated valve?

25. Case 7
Important factor to reduce eventual decommission costs during the operational phase of research reactor is maintaining contamination control.
How can contamination control be achieved?
How can operator and maintenance training contribute to contamination control?
How can adequate and frequent monitoring contribute to contamination control?

26. Case 8
When conducting the decommissioning actions there are some critical tasks that have to be taken into account.
Why is initial characterization of the installation needed?
When (and how) is fuel removed from reactor?
Prior to dismantling materials, structures and equipment at nuclear installation should be decontaminated.
What is meant by the term decontamination?
What are the objectives of decontamination?
Why is effectiveness of decontamination strategy evaluated in advance? How?

27. Case 9
Compare advantages and disadvantages of different dismantling techniques.
What must be considered when selecting the proper dismantling technique?
What are the considerations that must be considered when analysing the most effective and safe dismantling method?
Which technique should be used for dismantling aluminium tank and which for concrete?

28. Case 10
For TRIGA Mark II estimate the volume (mass) of materials generated in decommissioning process.
Use the data from the introductory part;
You should focus only to main construction materials (standard concrete, heavy concrete, aluminium, steel, graphite);
You should also state which materials are activated, and which materials are contaminated;
Take into account that:
Up to 0.8 m of heavy concrete around the core is potentially activated (the most important activation products are 60Co, 113Ba and 152Eu); this applies also to concrete 10 cm around experimental columns;
Aluminium parts are made of an alloy, with the following most important activation products: 46Sc, 54Mn, 55Fe, 60Co, 63Ni, and 65Zn (prevailing radionuclides are underlined);
All graphite is activated (3H, 14C, 60Co, 152Eu, 154Eu);
Cooling system and some construction parts of rotary specimen rack are made of stainless steel.
The views expressed in this document do not necessarily reflect the views of the European Commission.