Claims
- 1. A method for producing an isotopically pure, ultralow-mass fissionable deposit which can be accurately calibrated and used as a nuclear reactor dosimeter, comprising the steps of:
- (a) holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed by the radioactive decay of the parent;
- (b) chemically separating the daughter from the parent;
- (c) electroplating the daughter on a substrate;
- (d) accurately determining the amount of daughter present by measuring its decay rate; and
- (e) holding the electroplated daughter until the daughter decays to the ultralow-mass fissionable deposit whose mass shall then be known since the amount of the daughter was previously determined.
- 2. A fissionable deposit prepared by the method of claim 1, wherein the fissionable deposit is effectively isotopically pure.
- 3. A method of using an electroplated fissionable deposit as a nuclear reactor dosimeter, comprising the steps of:
- (a) holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed by the radioactive decay of the parent;
- (b) chemically separating the daughter from the parent;
- (c) electroplating the daughter on a substrate;
- (d) accurately determining the amount of daughter present by measuring its decay rate;
- (e) holding the electroplated daughter until the daughter decays to the ultralow-mass fissionable deposit whose mass shall then be known since the amount of the daughter was previously determined;
- (f) combining the fissionable deposit with a solid state track recorder;
- (g) exposing the fissionable deposit to a neutron fluence; and
- (h) determining the value of the neutron fluence based on the number of fissions of the fissionable deposit.
- 4. A fissionable deposit prepared by the method of claim 3, wherein the fissionable deposit is effectively isotopically pure.
- 5. The method as recited in claim 3 wherein step (a) comprises the substep of:
- choosing the parent from the group consisting of .sup.243 Am and .sup.241 Pu.
- 6. The method as recited in claim 3, wherein the daughter is .sup.239 Np.
- 7. The method as recited in claim 3, wherein the daughter is .sup.237 U.
- 8. The method as recited in claim 3, wherein the fissionable deposit is .sup.239 Pu.
- 9. The method as recited in claim 3, wherein the fissionable deposit is .sup.237 Np.
BACKGROUND OF THE INVENTION
The invention which is the subject of this application was created under a contract with the U.S. Department of Energy.
This invention relates to preparation of fissionable deposits and, more particularly, to an electroplating method for producing ultralow-mass fissionable deposits for nuclear reactor dosimetry.
Ultralow-mass fissionable deposits have proved useful as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields. These fission rate measurements are used to derive information for neutron dosimetry purposes.
More particularly, a solid state track recorder placed adjacent a thin fissionable deposit records tracks from the recoiling fission fragments which result from the fissions in the deposit. The number of these tracks observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit. Thus, the number of fission fragment tracks per square centimeter, i.e., the track density, in the solid state track recorder can be used to calculate the fission rate per unit area in the fissionable deposit.
For typical high neutron fluence applications, such as reactor core dosimetry or reactor component dosimetry, it has been found that a limitation is placed on the use of solid state track recorders due to the maximum track density that can be used, usually about 10.sup.6 tracks/cm.sup.2, without excessive track overlap. In order to avoid excessively high track densities, low-mass fissionable deposits have been used to reduce the number of fissions that will occur at a given neutron fluence. For example, in dosimetry applications for liqht water reactor pressure vessel surveillance, .sup.239 Pu deposits with masses as low as 10.sup.-13 gram are required to produce a usable track density in the solid state track recorder. Similarly low masses of other isotopes such as .sup.235 U, .sup.238 U and .sup.237 Np are required for dosimetry in light water reactor pressure vessel surveillance.
It has been found that the technical problems associated with the manufacture of such low-mass deposits can be overcome by using isotopic spiking/electroplating techniques to characterize the masses of these fissionable deposits. For example, low-mass deposits can be produced by an electroplating technique using .sup.237 U (7 day half-life) as an isotopic spike for .sup.235 U and .sup.238 U, .sup.239 Np (2.4 day half-life) as a spike for .sup.237 Np, and .sup.236 Pu (2.85y half-life) as a spike for .sup.239 Pu. Typically, the shorter half-life isotopic spike is used as a chemical tracer to overcome the fact that the radioactivity of the respective fissionable deposit renders the principal isotope undetectable when present in such low masses as can.be employed according to the present invention.
However, it has also been found that the amount of the isotopic spike that can be added to a fissionable deposit is limited by the nuclear properties of the particular isotopic spike chosen. For example, .sup.237 U decays to .sup.237 Np and .sup.239 Np decays to .sup.239 Pu, each of which is also fissionable. In both cases, the amount of the isotope to which the spike eventually decays must be kept small enough (by limiting the amount of spike added) to keep the fission rate of the isotope to which the spike decays small relative to the decay rate of the isotope of interest in the deposit.
In particular regard to .sup.239 Pu deposits, the .sup.236 Pu isotopic spike itself is fissionable and must therefore be used in limited amounts. In addition, for .sup.239 Pu deposits spiked with .sup.236 Pu, several experimental problems arise. For example, in the case of a solid state track recorder using .sup.239 Pu to measure the fission rate at the mid-plane location of the reactor cavity in the annular gap of an operating commercial power nuclear reactor during a typical operating cycle, a .sup.239 Pu fissionable deposit with a mass of about 10.sup.-10 gram is required to produce an optimum number of fission tracks. Namely, due to the previously explained spiking limitations, the maximum allowable .sup.236 Pu/.sup.239 Pu spike ratio for such a mass of the deposit results in a count rate that is only about 1 disintegration per minute (dpm).
In order to desirably characterize the decay rate of this deposit to better than 2% for mass calibration purposes, a relatively long counting time of about four days is required. Also, due to the low sample count rate, counters with very low background count rates (e.g., 0.1 dpm) must be used. However, the decay properties of the isotopic spike make maintenance of the low backgrounds difficult. For example, .sup.236 Pu decays as follows: ##STR1## Thus, many radioactive decay products accumulate from the decay of .sup.236 Pu, which must be periodically removed from the counters by cleaning to maintain low counter backgrounds.
On the other hand, when electroplating is employed to produce .sup.237 Np deposits, it has been found that a .sup.239 Np tracer can be obtained by milking a .sup.243 Am source, and this tracer material must be chemically equilibrated with the .sup.237 Np. The chemical equilibration, which requires a series of chemical oxidations and reductions using HBr and HNO.sub.3, is necessary to ensure that the .sup.237 Np and .sup.239 Np are in the same equilibrium distribution of oxidation states and will therefore electroplate at the same rate, thusly quaranteeing that the .sup.239 Np/.sup.237 Np ratio is a constant. After characterization of the .sup.237 Np deposits via their .sup.239 Np beta activity, a measurement of the ratio of beta activity to .sup.237 Np alpha activity must be made to establish the mass scale for the deposits.
In light of the above, a simpler and more effective method is needed for producing ultralowmass fissionable deposits for nuclear reactor dosimetry.
Accordingly, it is an object of the present invention to provide an electroplating method for producinq ultralow-mass fissionable deposits capable of easier deposit characterization.
It is another object of the present invention to provide a electroplating method for producing ultralow-mass fissionable deposits which are isotopically pure.
It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits wherein detector background is no longer a problem.
It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits wherein chemical equilibrium of tracers or spikes is unnecessary.
It is another object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits capable of producing extremely low masses.
Finally, it is an object of the present invention to provide an electroplating method for producing ultralow-mass fissionable deposits capable of significant time savings in comparison with other methods.
To achieve the foregoing and other objects of the present invention, and in accordance with the purposes of the invention, there is provided a method for producing ultralow-mass fissionable deposits for reactor dosimetry, including the steps of holding a radioactive parent until the radioactive parent reaches secular equilibrium with a daughter formed thereby, chemically separating the daughter from the parent, electroplating the daughter on a suitable substrate, and holding the electroplated daughter until the daughter decays to the fissionable deposit.
US Referenced Citations (11)
Non-Patent Literature Citations (2)
| Entry |
| T. C. Liu and C. S. Su, "The Application of Fission Track Detectors as Reactor Neutron Temperature Monitor", Int. J. Appl. Radiation and Isotopes, vol. 22, p. 227 (1971). |
| Handbook of Chemistry and Physics, Weast, Ed., CRC Press, 56th Ed., Cleveland, OH, 1975, p. B-330. |
Patent Grant
4808271
