US 4,483,816 A · Apparatus and method for quantitative assay of generic transuranic wastes from nuclear reactors

Abstract

A combination of passive and active neutron measurements which yields quantitative information about the isotopic composition of transuranic wastes from nuclear power or weapons material manufacture reactors is described. From the measurement of prompt and delayed neutron emission and the incidence of two coincidentally emitted neutrons from induced fission of fissile material in the sample, one can quantify .sup.233 U, .sup.235 U and .sup.239 Pu isotopes in waste samples. Passive coincidence counting, including neutron multiplicity measurement and determination of the overall passive neutron flux additionally enables the separate quantitative evaluation of spontaneous fission isotopes such as .sup.240 Pu, .sup.244 Cm and .sup.252 Cf, and the spontaneous alpha particle emitter .sup.241 Am. These seven isotopes are the most important constituents of wastes from nuclear power reactors and once the mass of each isotope present is determined by the apparatus and method of the instant invention, the overall alpha particle activity can be determined to better than 1 nCi/g from known radioactivity data. Therefore, in addition to the quantitative analysis of the waste sample useful for later reclamation purposes, the alpha particle activity can be determined to decide whether "permanent" low-level burial is appropriate for the waste sample.

A method for quantitative assay of a sample of generic transuranic waste containing a mixture of the fissile isotopes .sup.239 Pu, .sup.233 U, and .sup.235 U, and the non-fissile isotopes .sup.240 Pu, .sup.241 Am, .sup.244 Cm and .sup.252 Cf, using a first and second neutron detection apparatus, which comprises the steps of:

(a) calibrating the first neutron detection apparatus using a standard .sup.252 Cf neutron source;
(b) calibrating the first and second neutron detection apparatus using a standard .sup.239 Pu sample and a pulsed neutron source external to said .sup.239 Pu sample;
(c) repetitively quantitatively determining passive neutron emission from the sample for specific time intervals until a useful magnitude of signal has been accumulated using the first neutron detection apparatus, which further comprises the steps of:
(i) measuring passive neutron coincidence multiplicity yields, P.sub.2, P.sub.3, and P.sub.4, to quantitatively determine the amount of spontaneously fissioning isotopes present;
and (ii) measuring the noncoincidence neutron yield, P.sub.1, which in combination with said passive neutron coincidence yield measurements provides the amount of non-spontaneously fissioning .sup.241 Am present through the known production rate of neutrons from the reaction of emitted alpha particles with any oxygen isotopes present in the sample;
(d) periodically irradiating the sample with pulsed neutrons from a source external to said sample to induce nuclear fission in the fissile isotopes present in the sample;
(e) quantitatively determining the active neutron emission subsequent to said neutron irradiation step and resulting from said pulsed neutrons inducing fission with release of fission neutrons in the fissile isotopes present in the sample using the second neutron detection apparatus which is insensitive to said passive neutron emission repetitively and during alternate periods to said passive neutron emission determining step until a useful magnitude of signal has been accumulated, which further comprises the steps of:
(i) measuring the prompt neutron yield, Y.sub.p ;
(ii) measuring the delayed neutron yield, Y.sub.D ;
and (iii) measuring the coincidence prompt neutron yield, Y.sub.C ;
(f) solving the following set of four linear equations in four unknowns for the I's, where I.sub.252 is the actual number of microcuries (.mu.Ci) of .sup.252 Cf present in said sample and I.sub.241, I.sub.240 and I.sub.244 represent the actual numbers of millicuries (mCi) of .sup.241 Am, .sup.240 Pu and .sup.244 Cm present in said sample, respectively, and
P.sub.1 =C.sub.1 [I.sub.252 +0.0351 I.sub.244 +0.00283I.sub.240 +0.00154I.sub.241 ]
P.sub.2 =C.sub.2 [I.sub.252 +0.0236I.sub.244 +0.00066I.sub.240 ]
P.sub.3 =C.sub.3 [I.sub.252 +0.0148I.sub.244 +0.00032I.sub.240 ]
P.sub.4 =C.sub.4 [I.sub.252 +0.0076I.sub.244 +0.00015I.sub.240 ],
where
P.sub.1 is said measured background corrected noncoincidence single neutron rate of detection, and P.sub.2, P.sub.3 and P.sub.4 are said measured, background corrected multiply coincident neutron rates of detection for two, three, and four simultaneously formed neutrons, respectively, and C.sub.1, C.sub.2, C.sub.3 and C.sub.4 are the observed multiplicity responses for a 1.0 .mu.Ci .sup.252 Cf calibration source;
and
(g) solving the following set of three linear equations in three unknowns for the M's, where M.sub.239, M.sub.233 and M.sub.235 are the actual masses present of .sup.239 Pu, .sup.233 U and .sup.235 U, respectively, and C.sub.1, C.sub.2 and C.sub.3 are calibration constants representing the number of prompt neutrons per unit mass of .sup.239 Pu, the number of delayed neutrons per unit mass of .sup.239 Pu, and the number of coincidence prompt neutrons per unit mass of .sup.239 Pu, respectively;
Y.sub.p, Y.sub.D and Y.sub.c having been determined in said active neutron emission determining step:
Y.sub.P =C.sub.1 [M.sub.239 +0.67M.sub.235 +0.62M.sub.233 ]
Y.sub.D =C.sub.2 [M.sub.239 +2.02M.sub.235 +0.76M.sub.233 ]
Y.sub.C =C.sub.3 [M.sub.239 +0.70M.sub.235 +0.75M.sub.233 ].

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Apparatus and method for quantitative assay of generic transuranic wastes from nuclear reactors

Life of this patent

Apparatus and method for quantitative assay of generic transuranic wastes from nuclear reactors

Life of this patent

Abstract

Claims5