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Fri Feb 21, 2020, 03:15 AM

Material balance evaluation of pyroprocessing for minor actinide transmutation nitride fuel.

The paper I will discuss very briefly is this one: Material balance evaluation of pyroprocessing for minor actinide transmutation nitride fuel. (Sato et al., Journal of Nuclear Science and Technology. 2020, VOL. 57, NO. 3, 224235.)

The actinide nitrides are very attractive nuclear fuels because of their high thermal conductivity and the ease by which they are reprocessed. During his quest to learn how to fix nitrogen, Fritz Haber explored a uranium catalyst for the purpose. Uranium nitride when exposed to water rapidly decomposes to give aqueous ammonia and uranium oxides. (Haber was able ultimately to utilize hydrogen and ammonia to accomplish the task, which was then industrialized by the German chemical engineer Carl Bosch. The invention allowed Germany to sustain itself for four years in the first world war without access to Chilean salt peter.)

The property makes it very convenient to reprocess nuclear fuels.

The paper above is about an ADS system, an accelerator driven system, which uses neutron spallation by a beam of high energy protons to fission actinides in a subcritical state. Although my son worked this summer at a neutron spallation facility, I'm not a fan of ADS reactors for various reasons, but what is interesting about this paper is the pyroprocessing of used nuclear fuels for separation of valuable elements therein.

From the paper's introduction:

The Japan Atomic Energy Agency has been pursuing research and development regarding the partitioning and transmutation of long-lived minor actinides (MAs). Partitioning and transmutation technology will help to reduce the potential toxicity and volume of high-level radioactive waste [1]. One approach is the double-strata fuel cycle, which comprises a commercial reactor fuel cycle and a dedicated fuel cycle for transmutingMAs using an accelerator-driven system (ADS). MAs partitioned from high-level liquid waste generated in the commercial reactor fuel cycle would be fed into the transmutation fuel cycle. The mass flow of actinides in the transmutation fuel cycle would be approximately two orders of magnitude smaller than in the commercial reactor fuel cycle [2]. It has been demonstrated that a mononitride solid solution of MAs and Pu diluted with ZrN is a prime candidate for MA transmutation fuel because of the superior thermal and neutronic properties of this compound [3]. The role of Pu in this fuel is to mitigate the burn-up reactivity swing. Mutual solubility among actinide mononitrides ensures flexibility in the fuel composition. Nitrogen enriched in 15N is planned to be used in the nitride fuel to prevent the formation of long-lived 14C.

Nitrogen-14, 14N, when bombarded with high energy neutrons, undergoes a nuclear reaction in which a proton is ejected from the nucleus, giving carbon-14, 14C. In general, most writers think that this is a bad thing, since carbon-14 is radioactive. However, I disagree. Carbon 14 has a very low neutron capture cross section compared to the two other carbon isotopes, the non-radioactive carbon-12 and the other non-radioactive (but rarer) isotope carbon-13. 14-C is a slightly less efficient moderator than natural carbon, meaning that regions of fast neutrons and epithermal neutrons are easier to maintain in breeder blankets where actinide 14-C carbides are used. I think we would be better off with more 14C, not less, but that's just my opinion.

As I've noted in this space, all of the transuranium isotopes have a critical mass in a fast neutron spectrum, and therefore it is not actually necessary to treat them with neutron spallation, as in an ADS system, but no matter. The processing is what is interesting, not the mechanism of fission.

Here's the flow chart for the process described:

There are several references to "waste" here. In my opinion, they are all unnecessary. Nothing that is useful is waste, and given the nature of our intractable chemical pollution problem, radiation can serve to remediate some very severe cases; in fact in some cases, it may be the only tool for remediation. I'd stay away from that zeolite thing, myself. Under these circumstances, this particular molten salt has a number of things that don't recommend it, one being the presence of chloride and the other being the problem of tritium being generated from lithium. (On the other hand, if people ever get fusion reactors to work, tritium will be a valuable fuel.)

To my way of thinking, this is not an ideal pyroprocessing approach, but it's nice one at which to look to stimulate thought.

A table of fuel composition on loading and on discharge:

Recovery of metals in the liquid cadmium cathode:

Decay heat:

The isotopic mix of the plutonium is actually quite wonderful. This is an example of proliferation-proof plutonium, because of it's heat load.

Nice stuff.

Esoteric, but this sort of thing represents the only path out of climate change in my opinion.

There are many, many, many other possible similar processing schemes. This is just one.

Enjoy your Friday.

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