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

2. Basic Professional Training Course; Module XVIII Decommissioning 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. 2

3. Basic Professional Training Course; Module XVIII Decommissioning Introduction (Cont.): TRIGA Mk II research reactor 3

4. Basic Professional Training Course; Module XVIII Decommissioning Introduction (Cont.): TRIGA Mk II research reactor 4

5. Basic Professional Training Course; Module XVIII Decommissioning 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% 235 U − Fuel element dimensions: 3.73 cm in diameter, 72.06 cm in length − Cladding:0.51 mm steel (SS-304) − Mass of U in FE276.55 g − Mass of 235 U in FE55.04 g − Active core volume:44.2 cm diameter, 38.1 cm height − Core loading:Approx. 59 fuel elements, approx. 3.2 kg of 35 U − Core support:Upper and lower support grids, Al, thickness 1.91 cm − Number of FE in the core:60 5

6. Basic Professional Training Course; Module XVIII Decommissioning Introduction (Cont.): TRIGA Mk II research reactor Reflector − Material:graphite with aluminium cladding − Radial dimensions:45.5 cm (inner dia.), 106.2 cm (outer dia.) − Height:53,8 cm 6

7. Basic Professional Training Course; Module XVIII Decommissioning 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: 30.5 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. 7

8. Basic Professional Training Course; Module XVIII Decommissioning 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 1.68 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. 8

9. Basic Professional Training Course; Module XVIII Decommissioning 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. 9

10. Basic Professional Training Course; Module XVIII Decommissioning 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 ∙ 10 13 cm -2 s -1 in the central irradiation tube 1.7 ∙ 10 12 cm -2 s -1 in the irradiation tubes, − Prompt temperature coefficient: -1.2 ∙ 10 -4 δk/k /°C, − Mean prompt neutron lifetime: 6.0 ∙ 10 -5 s. 10

11. Basic Professional Training Course; Module XVIII Decommissioning 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 ∙ 10 17 − Maximum fuel temperature:during the pulse 240 °C 9 seconds after the pulse 360 °C 11

12. Basic Professional Training Course; Module XVIII Decommissioning 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 m 2 exchange surface. − Flow of primary water through heat exchanger is 23.6 m 3 per hour. − Flow of secondary (cooling) water is 67.5 m 3 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 m 3 per hour. − Cooling system (pumps, heat exchanger and ion exchanger with filter) are located in the basement of reactor building. 12

13. Basic Professional Training Course; Module XVIII Decommissioning 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). 13

14. Basic Professional Training Course; Module XVIII Decommissioning 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. 14

15. Basic Professional Training Course; Module XVIII Decommissioning 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.), − No α or β/γ contamination measured. − 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 ( 60 Co, 137 Cs and 154 Eu were detected). 15

16. Basic Professional Training Course; Module XVIII Decommissioning 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? 16

17. Basic Professional Training Course; Module XVIII Decommissioning 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? 17

18. Basic Professional Training Course; Module XVIII Decommissioning 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? 18

19. Basic Professional Training Course; Module XVIII Decommissioning Case3 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? 19

20. Basic Professional Training Course; Module XVIII Decommissioning 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? 20

21. Basic Professional Training Course; Module XVIII Decommissioning 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? 21

22. Basic Professional Training Course; Module XVIII Decommissioning 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? 22

23. Basic Professional Training Course; Module XVIII Decommissioning Case 5 In previous cases an overview of important topics of decommissioning plan was made. What are the remaining important topics of decommissioning plan? 23

24. Basic Professional Training Course; Module XVIII Decommissioning 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? 24

25. Basic Professional Training Course; Module XVIII Decommissioning 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? 25

26. Basic Professional Training Course; Module XVIII Decommissioning 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? 26

27. Basic Professional Training Course; Module XVIII Decommissioning 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? 27

28. Basic Professional Training Course; Module XVIII Decommissioning 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 60 Co, 113 Ba and 152 Eu); this applies also to concrete 10 cm around experimental columns; − Aluminium parts are made of an alloy, with the following most important activation products: 46 Sc, 54 Mn, 55 Fe, 60 Co, 63 Ni, and 65 Zn (prevailing radionuclides are underlined); − All graphite is activated ( 3 H, 14 C, 60 Co, 152 Eu, 154 Eu); − Cooling system and some construction parts of rotary specimen rack are made of stainless steel. 28 The views expressed in this document do not necessarily reflect the views of the European Commission.