Radiant energy – Invisible radiant energy responsive electric signalling – With radiant energy source
Reexamination Certificate
1999-06-11
2003-02-11
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With radiant energy source
Reexamination Certificate
active
06518579
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates in general to radiation detection techniques, and in particular to a new and useful non-destructive in-situ method and apparatus for determining the depth in a medium such as concrete, of radionuclides, between the radioactive source and a detector.
As a result of the Department of Energy's (DOE's) shift away from nuclear weapons production and the closing of nuclear power industry facilities, there has been a rapid increase in the number of sites and facilities which contain radioactive contaminants. A large part DOE and nuclear industry activity has been dedicated to the disposition of these facilities through various environmental restoration, and decontamination and decommissioning (D&D) projects. The resources spent on radiological characterization, monitoring and risk assessment contribute significantly to the anticipated total D&D budget of $265 billion dollars over the next 75 years.
Concrete is probably the most important medium or material for radiation shielding and operating structure in nuclear facilities. It has been estimated that there are approximately 73 km
3
of radioactively contaminated concrete material within 25 DOE facilities. Additionally the DOE estimates that over 550,000 metric tons of radiological contaminated metal will require characterization and disposition. In many cases, the contaminants are found not only on the surface, but also at depths below the surface of a medium.
Current practice in characterizing deeply contaminated concrete in DOE and nuclear industry facilities employ destructive approaches. In most cases, a surface survey is performed first, and then bore samples are taken manually. The samples are then sent to an off-site lab to analyze the depth profile. It usually takes days to weeks to obtain a result, whose accuracy is only as good as the bore sampling process. For a large contamination area, the task of making representative measurements can be too time-consuming to be practical. Further, because airborne radioactivity is generated when core bore samples are taken manually, occupational exposure is inevitable. The potential for internal exposure to radioactivity requires the use of protective equipment, which in turn slows down the worker's progress. The current practice in characterizing concrete contamination is inefficient and costly. The radiation protection philosophy of as-low-as-reasonably-achievable (ALARA) can be better accomplished through non-destructive means.
Gamma spectroscopy has been widely used for isotope identification and quantification by measuring the energies of the gamma rays, which can easily penetrate thick samples. When calibrated against a reference radioactive source, the radioactivity of the sample can be determined remotely. Despite these capabilities, gamma spectroscopy has not been demonstrated to be satisfactory for determining a contamination depth profile in-situ. The major difficulty is that while gamma spectroscopy utilizes the photopeaks from the uncollided gamma rays to identify radioisotopes, there has been no sufficient unfolding algorithm to determine the depth to which the gamma rays have penetrated. There is also a lack of tools that can quickly estimate the contamination activity levels and assess the doses or risk caused by the contamination. This information is crucial in deciding cleanup action and demonstrating compliance with regulations on releasing a facility.
Some of the more common contaminants have been reported recently. Table 1 lists the radiological information for some of these contaminants that emit gamma rays. The gamma energy lines are said to be the “finger prints” of these radiological contaminants. For example, it is possible to measure uranium isotopes by detecting the gamma energies, such as the 143 keV (10% yield) and 185 keV (50% yield) for
235
U. Highly efficient detectors are often used for isotopes with lower gamma energies and emission yields. The advantage of gamma ray spectroscopy is that gamma rays are much more penetrating than beta and alpha radiation, thus making non-destructive in-situ measurements possible. Table 1 provides the more important gamma lines and gamma yields for common isotopes that may be found in the DOE and nuclear industry. Additionally,
38
Cl is listed in Table 1 because the chlorine in salt, NaCl, may be neutron activated and subsequently emit two prominent gamma rays at 1642 keV (32.8% yield) and 2167 keV (44.0% yield).
TABLE 1
Gamma Energy
Radionuclide
b
(MeV)
% Yield
60
Co
1.17323
99.86
(NTH
59
Co)
1.33251
99.98
(NFA
60
Ni)
(NFA
63
Cu)
59
Fe
1.09922
56.50
(NTH
58
Fe)
1.29156
43.20
(NFA
52
Ni)
(NFA
59
Co)
38
Cl
1.6424
32.80
(NTH
37
Cl)
2.1675
44.00
(NFA
41
K)
(NFA
38
Ar)
137
Cs
0.66162
84.62
(NTH
136
Xe)
0.03219
3.70
(NFA
136
Ba)
(NFI 6.210)
234
U
0.01360
10.40
(NTH
233
U)
0.05310
0.12
(NFA
235
U)
0.12100
0.04
(NAT
238
U)
235
U
0.14376
10.50
(NAT
235
U)
0.16335
4.70
0.18572
54.00
0.20531
4.70
238
U
0.01300
8.70
(NAT
238
U)
0.04800
0.08
235
Pu
0.04910
2.34
(CHA
235
U)
0.09708
19.50
(CHA
233
U)
0.10107
34.80
0.11400
13.60
0.11750
4.50
239
Pu
0.01360
4.40
(NTH
238
U)
0.05162
0.04
240
Pu
0.01360
11.00
(NTH
239
Pu)
0.04524
0.05
241
Am
0.01390
28.00
(NTH
240
Pu)
0.02636
2.50
0.05954
36.30
231
Th
0.01330
92.00
(NAT
235
U)
0.02664
18.70
0.08421
8.00
0.08995
1.25
234
Th
0.01330
9.80
(NAT
238
U)
0.06329
3.90
0.09238
2.57
0.09289
3.00
239
Np
0.01430
56.00
(NTH
238
U)
0.09950
15.00
0.10370
24.00
0.10613
22.70
0.11770
8.40
0.12070
3.20
0.22819
10.70
0.27760
14.10
235
Np
0.09466
22.00
(CHA
235
U)
0.09844
38.00
(CHA
233
U)
0.11100
15.00
0.11450
5.30
237
Np
0.02938
9.80
(NFA
238
U)
0.08649
13.10
0.09229
1.82
0.09507
2.96
0.10800
1.02
Items in parenthesis indicate the nuclear reactions producing the radioisotopes based on the bombarding particle and the target nuclei as follows: NTH by thermal neutron isotopes; NFA by fast neutron; CHA by charged particles (alpha, proton, deuterons, etc.); NAT by natural occurring isotopes; NFI by fission with cumulative fission yield in percent for thermal neutron fission of
235
U.
The technique of the present invention, as will be explained later in this disclosure, can then be used to determine the depth of salt embedded in pavement or concrete shielding pads. Salt contamination of rebars leads to cracks, potholes, etc. in concrete and pavement.
Many researchers have developed models that use in-situ gamma spectroscopy to estimate the depth of contamination in media. Most of the research has been in the detection of
137
Cs contaminant distributions in soil as a result of post-Chernobyl environmental characterizations.
Russ et al. (1996) developed a method using in-situ gamma spectroscopy on transite panels at the DOE Fernald site that required measurements using an uncollimated high purity germanium (HPGe) detector on both sides of the medium. Although the method showed potential for predicting the contaminant depth distribution throughout the thin transite panel, the method requires access to both sides of a medium. Insufficient information was also provide to determine the technique used for any unfolding algorithm that was used. In conjunction with knowledge of the gamma-ray linear attenuation coefficient for the material, the method used the ratio of the photopeak areas at several energies to infer the “most-probable” contamination distribution. Because the method used uncollimated HPGe detectors the in-situ gamma spectroscopy results were of the entire transite panels and local depth profiles across the surface area could not be obtained.
Korun et al. (1991) developed a method to determine depth distribution of
137
Cs concentrations in soils based on the energy dependence of attenuation of gamma rays in soil. The method assumed a decreasing exponential distribution in the radionuclide concentration. The decreasing expone
Naessens Edward P.
Xu X. George
Hannaher Constantine
Lee Shun
Notaro & Michalos P.C.
Rensselaer Polytechnic Institute
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