8th IAEA Technical Meeting on Fusion Power Plant Safety Vienna, July 2006 Clearance considerations for Slightly-Irradiated Components of Fusion Power Plants L. El-Guebaly 2, R. Pampin 3, M. Zucchetti 1 1 EURATOM/ENEA ENEA Fusion Association, Politecnico di Torino, Italy 2 University of Wisconsin-Madison, Madison, WI, U.S. 3 EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, U.K.
Overview In order to minimize the quantity of active materials that require long-term storage, arising during operation and after fusion power plant decommissioning, maximum use should be made of both recycling within the nuclear industry and clearance For clearance,, revised limits have been recently issued at the international level and in the US and Europe. Here, the implications ions for fusion materials of these new levels are considered, with particular attention to slightly-irradiated irradiated components Public acceptability of clearance remains an unsolved problem
Recent Clearance Regulations Clearance is the removal of radioactive materials or radioactive objects within authorized practices from any further regulatory control by the regulatory body Recently, both the U.S. Nuclear Regulatory Commission (NRC( NRC) ) and the International Atomic Energy Agency (IAEA( IAEA) ) have issued revised clearance levels, taking into account previous guidelines and studies.
2004 IAEA guidelines Concentration limits for clearance are issued in the 2004 IAEA guidelines for 257 nuclides The IAEA 2004 recommendations appear to be more stringent for some fusion-relevant radionuclides than the previous IAEA 1996 guidelines, upon which former evaluations for fusion materials were based. For instance, they call for lower clearance limits (i.e., more stringent) for 14 C, T, and 60 Co (factors of 300, 30 and 3, respectively).
US - NRC 2003 Proposed Clearance Limits Based on a detailed technical study, the NUREG-1640 document by the U.S. NRC contains estimates of the total effective dose equivalent (from which the clearance index can be derived) for 1151 15 radionuclides. The NRC has not yet issued an official policy on the unconditional release of specific materials. Herein, the proposed annual doses reported in the document will be referred to as the proposed U.S. limits
Comparison IAEA/NRC Even though the NRC and IAEA both recommended an individual dose standard of 10 µsv/y for cleared solids, we observed a notable difference between the clearance limits. Concerning some fusion-relevant nuclides, additional effort is needed to reduce the differences between the standards and understand the technical reasons for the major disagreements.
Ratios of the U.S. limits to those of the IAEA for steel.
Clearance of slightly-irradiated irradiated components Some selected examples of the effect of different clearance standards will be shown in the following. Case 1 (ARIES, US): a comparison of application of most recent IAEA and US clearance standards Case 2 Case 2 (PPCS, Europe): a comparison of application of IAEA old and new clearance indices
Differences in clearance standards: ex. #1 (ARIES) Due to ARIES compactness, most internal components are not eligible for clearance; however the external building surrounding the torus along the cryostat and magnet structure can be cleared, representing 80% of the total volume
Differences in clearance standards: ex. #1 (ARIES) Comparison of U.S. and IAEA clearance indices for steel of innermost segment of the confinement building.
Differences in clearance standards: ex. #1 (ARIES) Comparison of U.S. and IAEA clearance indices for concrete of innermost segment of the confinement building.
Differences in clearance standards: ex. #2# (PPCS) Clearance index of material from a part of the toroidal field coil of PPCS Plant Model A.
Differences in clearance standards: ex. #2# (PPCS) Clearance index of material from part of the vacuum vessel of PPCS Plant Model C
Clearance of slightly-irradiated irradiated components Case 3 (PPCS, Europe) Sometimes an entire component does not achieve clearance but individual constituents and/or radial/poloidal parts of it do so and viceversa Case 4 (PPCS, Europe) Sometimes impurities affect the clearance index and make the difference between clearance and disposal
Components vs. constituents clearance: ex. #3 (PPCS) clearance index 1.E+05 1.E+04 1.E+03 1 year 10 years 50 years 100 years 1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 0 45 90 135 180 225 270 315 360 poloidal angle (from bottom, anticlockwise) Poloidal variation of the TFC CI in PPCS-AB at different times after shutdown.
Components vs. constituents clearance: ex. #3 (PPCS) 30000 25000 IAEA 1996 IAEA 2004 IAEA 2004 with segregation 27% 24% 20000 17% 15000 10000 5000 0 TFC OB TFC IB VV OB VV IB total
Effect of impurities on clearance index: ex. #4 (PPCS) In particular, a 30-fold decrease of the Ni-63 level appears to become a particular concern in the case of conventional 316 grade steel used for the VV and TFC walls, casing and radial plates; its effect is crucial in the case of model AB outboard VV In the case of the 316 steel for VV and TFC applications the use of ferritic, low-activation variants has been recommended. Traces of impurities at the level of detection limits have certain influence in some cases, providing the difference between clearance and disposal: in 316 steel, even prior to irradiation, naturally occurring radioisotopes of Rb and Re impurities provide a background CI of 0.22.
Effect of impurities on clearance index: ex. #4 (PPCS) e index clearanc 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 TFC 316ss front TFC 316ss mid TFC 316ss back TFC cable TFC incoloy TFC epoxy average 1.E-02 0.1 1 10 100 1000 time (y)
Public acceptability of clearance There is no commercial market for the free release of slightly contaminated materials anywhere in the world. Nuclear industry favors clearance, while consumer and environmental groups do not Metals and concrete industries do not want slightly radioactive materials in their products Further action is necessary, to convince industrial and environmental groups on the safe feasibility of clearance
CONCLUSIONS - Specific Concerning materials clearance: Re-evaluations of the clearance indices, based on recently issued limits, show that the amount of clearable material could be lower than previously estimated or may require longer cooling period. Differences between standards show that further studies are needed to understand disagreements. An internationally agreed upon and complete set of fusionspecific clearance limits should be developed.
CONCLUSIONS - General One of the main goals for fusion is the minimization of radioactive materials that need permanent disposal. A strategy maximising the use of materials recycling (within the nuclear industry) and clearance could result in a clear advantage for fusion power, in view of its ultimate safety and public acceptance. Recent power plant studies have shown excellent potential results of the recycling and clearance strategy. Further development studies in both fields (recycling and clearance) are strongly recommendable.