This invention relates to the determination of the plutonium content
in plutonium contaminated material containing fluorine, particularly fluorine in
the form of plutonium tetrafluoride.
Fluorine is used in the process for manufacturing plutonium metal
so that many waste items made from this material may contain fluorine or fluorides.
When disposing of these waste items they are deposited in large drums. It is important
that the plutonium content of these drums is accurately measured so that the drums
can be filled to a maximum amount without exceeding the fissile limit. If the
capacity of the drums is efficiently utilised a reduction in overall disposal
costs can be achieved.
It has been found when using a total neutron counting technique for
such measurements that the quantity of plutonium present can be overestimated due
to the emission of interfering (alpha, n) reaction neutrons. The (alpha, n) neutron
emission is particularly severe when the plutonium is in the form of plutonium
tetrafluoride.
The assessment of plutonium content using total neutron counting
requires an accurate knowledge of the specific neutron emission rate. For plutonium
contained in waste material the specific neutron rate is dependent on the isotopic
composition, the chemical composition and the matrix composition. The isotopic
composition defines the spontaneous fission neutron rate and also the alpha emission
rate which then influences the (alpha, n) reaction rates due to the chemical and
matrix compositions.
The chemical composition affects the (alpha, n) neutron rate. For
example, alpha interactions with fluorine produce many more neutrons than alpha
reactions with oxygen. The presence of light elements, for example oxygen and
fluorine, in the matrix also affects the (alpha, n) neutron rate, although to a
lesser extent than their presence in the chemical composition.
The use of a conventional neutron coincidence counting technique
for plutonium assaying is not considered suitable where large quantities of plutonium
tetrafluoride are present. This is because the coincidence circuitry of the measuring
equipment is unable to cope with very high random pulse rates and measurement
precision tends to be very poor.
It is an object of this invention to provide a method of determining
the plutonium content of plutonium contaminated material which overcomes the problem
of overestimation caused by the presence of fluorine.
According to the present invention there is provided a method of
determining the plutonium content of a material which also contains fluorine, wherein
said method includes the step of determining the mass ratio of plutonium in a
compound comprising plutonium and fluorine to the total plutonium content of the
material using a high resolution gamma ray spectrometry technique.
In a preferred embodiment the said compound is plutonium tetrafluoride.
Preferably the method includes obtaining a count rate in a pair of
gamma ray photopeaks, one of said photopeaks having an energy level of 414 keV
resulting from the decay of plutonium-239, and the other of said photopeaks having
an energy level resulting from the interaction of plutonium alpha particles with
fluorine.
The other of said gamma ray photopeaks may have an energy level of
583 keV resulting from the decay of an excited sodium-22 nucleus following an (alpha,
n) interaction with fluorine.
Preferably the photopeak having an energy level of 583 keV is corrected
for thorium decay series contribution using a thorium decay series gamma ray at
a 238 keV photopeak energy level.
Alternatively, the other of said gamma ray photopeaks may have an
energy level of 891 keV or 1274 keV, which are unaffected by thorium decay series
gamma emissions.
The invention will now be described with reference to the accompanying
Figure 1, which is a diagrammatic representation of a typical high resolution gamma
spectrometry system for carrying out the invention.
A preferred method of carrying out a high resolution gamma ray technique
for determining the plutonium (as fluoride) to total plutonium mass ratio is based
on the following:
An (alpha,n) reaction may leave the remaining nucleus in an excited
energy state. The nucleus loses this excess energy via the emission of radiation
as gamma rays. Thus, (alpha,n) reactions may be identified by the detection of
characteristic gamma rays.
In a material containing plutonium tetrafluoride (PuF&sub4;), alpha
particles of plutonium isotopes interact with fluorine atoms to produce gamma rays.
The most intense gamma rays seen in the high resolution gamma spectrum are those
at 583 keV and 1274 keV.
The 583 keV gamma ray results from the (alpha, n) reaction:
19F + 4He (α) → 22Na*
(* indicates that the nucleus is formed in an excited state of energy)
+ 1n 22Na** → 22Na
+ gammas
The 1274 keV gamma ray arises from two sources:
i) as a prompt gamma ray from the (alpha, p) reaction:
19F + 4He (α) → 22Ne* + 1p
22Ne*
→ 22Ne + gammas
ii) as a delayed gamma ray from the de-excitation of Ne-22 following the decay
of Na-22.
Plutonium-239 alpha decays spontaneously, resulting in the emission
of gamma rays such as that having an energy level of 414 keV.
A relationship exists between the emission rate of the 414 keV gamma
rays, the emission rate of gamma rays resulting from the interaction of alpha particles
with fluorine, and the plutonium tetrafluoride to total plutonium mass ratio.
A count rate in the 414 keV photopeak (P) can be derived from:
P = E&sub4;&sub1;&sub4;.M.B&sub4;&sub1;&sub4;
where:
E&sub4;&sub1;&sub4;
= detector efficiency at 414 keV
M
= mass of plutonium
B&sub4;&sub1;&sub4;
= 414 keV gammas per second per gram of plutonium (determined from the plutonium
isotopic composition).
The plutonium and americium-241 composition can be determined from
a gamma spectrum obtained by high resolution gamma spectrometry during analysis
or, alternatively, it can be entered separately if the isotopic composition is
already known.
Note. Americium-241 is the daughter product resulting from the decay
of plutonium-241 and is often present in plutonium containing material having 'grown
in' with time.
A count rate in the 583 keV photopeak (F) can be derived from:
F = E&sub5;&sub8;&sub3;.M(PuF&sub4;).G.A
where:
   E&sub5;&sub8;&sub3; = detector efficiency at 583 keV
   M(PuF&sub4;) = mass of plutonium tetrafluoride
   G = 583 keV gammas per alpha (this is a constant for plutonium
tetrafluoride and can be determined experimentally).
   A = alphas per second per gram of plutonium tetrafluoride
(determined from the plutonium and americium-241 isotopic composition).
The mass ratio (R) of plutonium tetrafluoride to total plutonium
is derived from:
R = M(PuF&sub4;)/M
and so:
M(PuF&sub4;) = M.R
Incorporating this in equation (ii), F can be rewritten as:
F = E&sub5;&sub8;&sub3;.M.R.G.A
From equations (i) and (iii), the ratio (F/P) of net counts in the
583 keV and 414 keV photopeaks is given by:
and so:
R = (F/P).B&sub4;&sub1;&sub4;/Er.G.A
where Er = the relative detection efficiency at 583 keV
relative to that for 414 keV.
The relative efficiency is, among other things, affected by the intrinsic
efficiency of the detector at 414 keV and 583 keV photopeaks. For most germanium
detectors, as used in the gamma ray spectrometry technique of the present invention,
the intrinsic efficiency is a defined characteristic and can be determined using
known radionuclide sources.
The foregoing formula (iv) is valid when the 583 keV photopeak comprises
purely PuF&sub4; (alpha, n) reaction gamma rays. Certain applications arise where
the 583 keV photopeak contains a contribution from thorium-228 decay series gamma
emissions. To compensate for this situation, the contribution at 583 keV from PuF&sub4;
reaction gamma rays may be determined using other PuF&sub4; reaction gamma rays
at energy levels of 891 keV and 1274 keV, which are unaffected by thorium decay
series emissions, or by removing the thorium contribution from the 583 keV photopeak
using the thorium decay series gamma ray at an energy level of 238 keV.
In general, the technique for determining the contribution of a radionuclide
in one photopeak from the net photopeak count of another photopeak is as follows.
The net photopeak count Ca, at energy Ea, is used to determine
the net photopeak count Cb from the same source at energy Eb.
It is therefore necessary to multiply Ca
by the intrinsic efficiency of
the detector at energy Eb
relative to that at energy Ea. This
is then multiplied by the intensity of the gamma rays at energy Eb relative
to that for gamma rays at energy Ea. The intensity for a particular
gamma ray emission of a particular radionuclide is defined as the number of gamma
rays of that energy emitted per disintegration of that radionuclide.
This relationship is expressed as follows:
Cb = Ca.(EFFb/EFFa) (Ib/Ia)
where:
   EFFa and EFFb are the intrinsic detection
efficiencies at energy Ea and energy Eb, respectively, and
Ia and Ib are the intensities of gamma ray emissions of energy
Ea and energy Eb, respectively.
Using the general procedure described above a technique for removing
the thorium decay series component from the 583 keV photopeak has been derived,
as follows:
The contribution to the 583 keV photopeak from PuF&sub4; reaction
gamma rays, C583f, is determined according to the relationship:
C583f = C583tot - C583th
where:
   C583tot is the total photopeak count at 583 keV,
and C583th is the thorium-228 decay series contribution.
Therefore, using formula (v) above:
C583th = C238th.[EFF&sub5;&sub8;&sub3;/EFF&sub2;&sub3;&sub8;].[I583(Th-228)/I238(Th-228)]
where:
   C238th is the thorium-228 decay series contribution
to the 238 keV photopeak,
   EFF&sub2;&sub3;&sub8; and EFF&sub5;&sub8;&sub3; are the detection
efficiencies at 238 keV and 583 keV, respectively,
   I&sub2;&sub3;&sub8;(Th-228) is the intensity of
the 238 keV gamma emission from thorium-228 decay series, and
   I583(Th-228) is the intensity of the 583 keV gamma
emission from thorium-228 decay series.
Therefore, substituting detector efficiency values for a particular
germanium detector and available data for the gamma emission intensities, equation
(vii) can be expressed as follows:
C583th = k&sub1;.C238th
where:
k&sub1; is a constant appropriate for a particular germanium detector.
Assuming the 238 keV photopeak comprises only thorium series gamma
rays, then:
C238th = C238tot
where:
   C238tot is the total photopeak count at 238 keV.
Therefore:
C583th = k&sub1;. C238tot
Therefore, equation (vi), using the 583 keV photopeak with basic
thorium interference correction can be expressed as follows:
C583f = C583tot - k&sub1;. C238tot
Similarly, using the above technique for determining the contribution
of PuF&sub4; reaction gamma rays to the 583 keV photopeak using the 891 keV reaction
gamma ray emission of PuF&sub4;, the following equation is derived:
where:
   k&sub2; is a constant appropriate for a particular germanium
detector.
To determine the contribution of the PuF&sub4; reaction gamma rays
to the 583 keV photopeak using the 1274 keV reaction gamma ray emission of PuF&sub4;,
separate formulae have been derived for PuF&sub4; which has existed for over 10
years and for fresh PuF&sub4;. These formulae are of the same general form, but
have different values of constants since the intensity of the 1274 keV gamma emission
varies with time until equilibrium is reached after about 10 years.
For fresh PuF&sub4; samples, equation (v) may be written:
C583f = C1274.[EFF&sub5;&sub8;&sub3;/EFF&sub1;&sub2;&sub7;&sub4;].[I583(PuF4)/I1274(PuF4)F]
where:
I1274(PuF4)F is the intensity for the 1274 reaction gamma ray emission
for freshly prepared PuF&sub4;.
Therefore, substituting germanium detector efficiency values and
available data for gamma emission intensities, equation (viii) can be expressed
as follows:
C583f = k&sub3;. C&sub1;&sub2;&sub7;&sub4;
where:
k&sub3; is a constant appropriate for a particular germanium detector.
For PuF&sub4; which has existed for over 10 years, equation (v) may
be written:
C583f = C&sub1;&sub2;&sub7;&sub4;.[EFF&sub5;&sub8;&sub3;/EFF&sub1;&sub2;&sub7;&sub4;].[I583(PuF4)/I1274(PuF4)A]
where:
I1274(PuF4)A is the intensity for the 1274 keV reaction gamma ray emission
for PuF&sub4; of more than 10 years old.
Therefore, substituting detector efficiency values and available
data for gamma emission intensities, equation (ix) can be written:
C583f = k&sub4;.C&sub1;&sub2;&sub7;&sub4;
where:
k&sub4; is a constant appropriate for a particular germanium detector.
For some plutonium samples to be analysed the 238 keV photopeak may
contain a contribution from plutonium-239 which is significant compared to the
contribution from thorium-228 decay series gamma rays. There is also a minor plutonium-239
contribution to the 583 keV photopeak.
The contribution of PuF&sub4; reaction gamma rays to the 583 keV
photopeak from PuF&sub4; gamma spectrum, C583f, is as follows:
C583f = C&sub5;&sub8;&sub3; - C583th - C583(Pu-239)
where:
   C&sub5;&sub8;&sub3; is the net count for the 583 keV photopeak
   C583th is the thorium-228 decay series contribution
to the 583 keV photopeak, and
   C583(Pu-239) is the plutonium-239 contribution to
the 583 keV photopeak.
Each of the C583th and C583(Pu-239) terms can
be evaluated using the aforementioned general formula (v). Substituting the values
obtained in formula (x) the following formula is derived:
where:
C&sub5;&sub8;&sub3;, C&sub2;&sub3;&sub8; and C&sub4;&sub1;&sub4; are the net counts
for the 583 keV, 238 keV and 414 keV photopeaks, respectively.
EFF&sub5;&sub8;&sub3;/EFF&sub2;&sub3;&sub8; is the detection efficiency ratio for
gamma rays at 583 keV relative to that for 238 keV gamma rays.
EFF&sub5;&sub8;&sub3;/EFF&sub4;&sub1;&sub4; is the detection efficiency ratio for
gamma rays at 583 keV relative to that for 414 keV gamma rays.
I583(Th-228)/I238(Th-228) is the intensity ratio of 583 keV,
Th-228 decay series gamma rays relative to that for 238 keV, Th-228 decay series
gamma rays.
I238(Pu-239)/I414(Pu-239) is the intensity ratio of 238 keV,
Pu-239 decay series gamma rays relative to that for 414 keV, Pu-239 decay series
gamma rays.
I583(Th-228)/I238(Th-228) is the intensity ratio of 583 keV,
Th-228 decay series gamma rays relative to that for 238 keV, Th-228 decay series
gamma rays.
I583(Pu-239)/I414(Pu-239) is the intensity ratio of 583 keV,
Pu-239 decay series gamma rays relative to that for 414 keV, Pu-239 decay series
gamma rays.
Formula (xi) can be rewritten in the following way:
C583f = C&sub5;&sub8;&sub3; - k&sub5;.C&sub2;&sub3;&sub8; + k&sub6;.C&sub4;&sub1;&sub4;
where:
k&sub5; and k&sub6; are constants which are appropriate for the particular germanium
detected used.
In practice, the plutonium tetrafluoride measurements are carried
out using a high resolution gamma spectrometry system, a typical representation
of which is shown in Figure 1. A sample 1 containing plutonium tetrafluoride is
placed in front of and on the germanium crystal axis of a high purity germanium
detector 2. The item to be analysed may range from a small sample, as shown, to
a large container holding intermediate level radioactive waste containing plutonium.
To reduce interference from background radiation the detector 2 is equipped with
a lead collimator 3. A thin cadmium shield 4 is placed between the sample 1 and
the detector 2 to reduce the effects of high intensity, low energy gamma radiation.
A high voltage power supply unit 5 and a preamplifier power supply
6 are connected to the detector 2. Output signals from the detector 2 are transmitted
to a spectroscopic amplifier 7 from which the output signals are directed to an
analogue-to-digital converter 8. Digital signals from the converter 8 are processed
by a personal computer and multi-channel analyser system 9 which is equipped with
software suitable for analysing plutonium tetrafluoride.
In use, the system shown in Figure 1 is operated so that a gamma
spectrum is collected of the radiation emitted by the sample 1 by the multi-channel
analyser system 9. The relevant net photopeak counts are obtained from the gamma
spectrum by the analyser system 9 and used to determine the plutonium and americium
isotopic composition of the sample. From this information, the mass ratio of the
plutonium tetrafluoride to the total plutonium ccntent is subsequently obtained
using the formulae, as disclosed earlier in the specification, which are incorporated
in the computer software program. The analysis can be carried out by inputting
data into the program manually, or by setting up the system to carry out the analysis
automatically.
The technique can be incorporated as an integral part of a complete
measurement system as, for example, in a drum assay system, or it can be implemented
using portable equipment and employed to assay in a wide variety of situations.
Examples range from scrapings from glovebox walls taken during decommissioning
procedures through to in situ analysis of large items such as crates or drums
holding intermediate level radioactive waste containing plutonium. By obtaining
an accurate measurement of the plutonium content of the waste material and drums
enables the drums to be filled to a capacity up to the fissile limit without exceeding
it. This results in the drums being filled to their optimum capacity, so reducing
the cost of disposal.
The foregoing techniques for analysing the gamma spectrum to give
a measurement of the plutonium (as fluoride) to total plutonium mass ratio is quick
and easy to carry out. It can be used in combination with total neutron counting
in situations which cannot be dealt with satisfactorily by conventional techniques,
such as total neutron counting alone or neutron coincidence counting. In the presence
of plutonium tetrafluoride these conventional techniques result in large overestimates
of the amount of plutonium present, so incurring excessive storage costs and possibly
giving rise to perceived criticality safety problems when in actuality no such
criticality hazard exists.
Anspruch[en]
A method of determining the plutonium content of a material which also contains
fluorine, characterised in that the said method includes the step of determining
the mass ratio of plutonium in a compound comprising plutonium and fluorine to
the total plutonium content of the material using a high resolution gamma ray
spectrometry technique.
A method as claimed in Claim 1, characterised in that the said compound is
plutonium tetrafluoride.
A method as claimed in Claim 1 or Claim 2 characterised in that the method
includes obtaining a count rate in a pair of gamma ray photopeaks, one of said
photopeaks having an energy level of 414 keV resulting from the decay of plutonium-239,
and the other of said photopeaks having an energy level resulting from the interaction
of plutonium alpha particles with fluorine.
A method as claimed in Claim 3, characterised in that the other of said gamma
ray photopeaks has an energy level of 583 keV resulting from the decay of an excited
sodium-22 nucleus following an (alpha,n) interaction with fluorine.
A method as claimed in Claim 4, characterised in that the count rates obtained
are substituted in a formula for calculating the mass ratio (R) of plutonium tetrafluoride
to the total plutonium content of the material, said formula being
R =
(F/P). B&sub4;&sub1;&sub4;/Er.G.A
where
F =
count rate in the 583 keV photopeak
P =
count rate in the 414 keV photopeak
B&sub4;&sub1;&sub4; =
the 414 keV gamma per second per gram of plutonium (determined from the plutonium
isotopic composition)
Er =
the relative detection efficiency at 583 keV relative to that for 414 keV
G =
the 583 keV gammas per alpha (a constant for plutonium tetrafluoride, determined
experimentally)
A =
the alphas per second per gram of plutonium tetrafluoride (determined from
the plutonium and americium-241 isotopic composition)
A method as claimed in Claim 4, characterised in that the photopeak having
an energy level of 583 keV is corrected for thorium decay series contribution using
a thorium decay series gamma ray at a 238 keV photopeak energy level.
A method as claimed in Claim 3, characterised in that the other of said gamma
ray photopeaks has an energy level of 891 keV which is unaffected by thorium decay
series gamma emissions.
A method as claimed in Claim 3, characterised in that the other of said gamma
ray photopeaks has an energy level of 1274 keV which is unaffected by thorium decay
series gamma emissions.