Reprocessed residual fission products

For reprocessed uranium to be used again in a reactor, it must again be enriched to about 3.5%. Because new uranium is in plentiful supply, it has not been necessary to re-emich uranium recovered from reprocessing to meet the requirements for new fuel. If and when it becomes necessary to enrich reprocessed uranium, the potential occupational hazard associated with UFe conversion, enrichment, and fuel fabrication processes due to the small amounts of residual fission products in the uranium will have to be addressed. 

The sample also contained ppb levels of Pu and Np, and high Pu content was consistent with reactor burn-up of light-mass U isotopes. The residual fission products Cs, Cs, and Sb were also detected, providing supplemental evidence of reactor irradiation and reprocessing. The absence of other fission products gave some clues to the reprocessing chemistry. 

According to the generally accepted specifications, the maximum allowable content of residual fission products in the reprocessed uranium amounts to about 10% of the natural uranium activity as an upper limit for residual y-emitting fission products 1.1 10 MeV Bq/d kgU is usually specified. In most cases, the residual fission product activity in reprocessed uranium is below 1000 Bq/g. During the enrichment process of the reprocessed uranium the fission products behave in... 

Although the retention of selective fission products in fissile materials may not adversely affect the performance of fuel in a reactor, the intensity of the gamma radiation is such that the fissile material must be handled, transferred, and fabricated remotely. As a result, it is both technically difficult to divert the fissile material and fabricate a weapon, and nearly impossible to do so without detection. The levels of residual radioactivity in the product of some of the pyrochemical or dry processing methods is close to that found in spent unreprocessed fuel and hence the reprocessed product presents a risk to proliferation only trivially less than that of spent fuel. Pyrochemical and dry processing methods can be used that will... 

Early Work. The irradiated fuel, upon discharge from the reactor, comprises the residual unbumt fuel, its protective cladding of magnesium alloy, zirconium or stainless steels, and fission products. The fission process yields over 70 fission product elements, while some of the excess neutrons produced from the fission reaction are captured by the uranium isotopes to yield a range of hew elements—neptunium, plutonium, americium, and curium. Neutrons are captured also by the cladding materials and yield a further variety of radioactive isotopes. To utilize the residual uranium and plutonium in further reactor cycles, it is necessary to remove the fission products and transuranic elements and it is usual to separate the uranium and plutonium this is the reprocessing operation. 

Some of the MA nuclides (Np, Am, Cm) contained in residual waste from reprocessing have extremely long-term radio toxicity. Means of reducing the radio toxicity of the MA nuclides are presently under investigation. The MA nuclides could produce useful energy if converted into short-lived fission products by neutron bombardment. From this standpoint, a nuclear reactor provides the obvious means for transmutation of MA nuclides. Among the various nuclear reactors, a fast reactor is considered to have the greatest potential to transmute MA effectively, because of its hard neutron spectrum. 

HLW (high level waste). The radioactive liquid containing most of the fission products and actinides present in spent fuel — which forms the residue from the first solvent extraction cycle in reprocessing — and some of the associated waste streams this material following solidification spent fuel (if it is declared a waste) or any other waste with similar radiological characteristics. Typical characteristics of HLW are thermal powers of about 2 kW/m and long lived radionuclide concentrations exceeding the limits for short lived waste [2]. 

Chemistry used in the recovery of plutonium from irradiated fuel must provide a separation from all these elements, other fission and activation products, and the actinides (including a large amount of unburned uranium), and still provide a complete recovery of plutonium. The same issues apply to the recovery of uranium from spent thorium fuel. Most of the processes must be performed remotely due to the intense radiation field associated with the spent fuel. As in the enrichment of uranium, the batch size in the later steps of the reprocessing procedure, where the fissile product has become more concentrated, is limited by the constraints of criticality safety. There is a balance between maximizing the yield of the precious fissile product and minimizing the concentrations of contaminant species left in the final product These residual contaminants, which can be detected at very small concentrations using standard radiochemical techniques, provide a fingerprint of the industrial process used to recover the material. 

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