Safeguarding the Nuclear Fuel Cycle

by William H. Hannum

Abstract and Summary

The purpose of this essay is to compare the safeguards challenges presented by two nuclear recycle approaches, relative to the current U. S. approach of a once-through fuel cycle.  If these nuclear fuel cycles are evaluated solely on the basis of the safeguards needed, one finds the following:

PUREX recycle offers no safeguarding advantage over the once-through fuel cycle.  Beyond that, this approach presents a significant concern over handling of separated plutonium in the power plant environment.  Since chemically pure Pu is inherent in the PUREX process, safeguards inspections must be highly intrusive.

Adding recycling fast reactors with pyroprocessing (“PYRO”) to an existing fleet of LWRs absorbs all of the plutonium produced by LWRs.  There will be no inventories of plutonium other than what is in active use. PYRO is a new class of facility requiring safeguards, but batch-process inventory controls, coupled with a simple mechanical layout, will make the inspectors’ job more straightforward than for a PUREX facility.  The facility for recovering usable material from used LWR fuel may require safeguards similar in approach to those in PUREX facilities, but no separated plutonium will be involved.  If plutonium were to be diverted from a PYRO facility or from the LWR recovery facility, it would be useless (for weapons use) without further processing in an otherwise unneeded PUREX type of facility. 

Realistically, a full transition to recycling fast reactors is a process that will take decades.  However, if all the LWRs were retired and replaced with recycling fast reactors, in addition to the above advantages, there would be no further need for uranium enrichment.

Discussion

Civilian nuclear power evolved from the technology of nuclear weapons, using the enrichment and reprocessing facilities developed for the military.  Such facilities, and the materials they produce, are the most sensitive parts of the nuclear fuel cycle.  To prevent diversion of such materials, which could lead to the uncontrolled spread of nuclear weapons, safeguards are needed.

Also, the nuclear reactors themselves require a degree of safeguarding.  In principle, any nuclear power plant can be used to produce plutonium for weapons.  Adapting a single-purpose commercial plant to produce high-quality plutonium would be very inefficient and has never been done, but such plants also require a degree of oversight.

In the early days of nuclear power, it was thought that uranium was a very rare material, and the U-235 could soon be exhausted.  Thus, the initial emphasis was on maximizing the amount of plutonium that could be recovered from the used fuel.  The plutonium was to be used as a replacement for the depleting stocks of U-235.  We now know that uranium is relatively plentiful, and the main  incentives to recycle nuclear fuel arises from concerns over nuclear waste disposal and huge increase in efficiency of resource utilization.

Recycling

Because of the perceived urgency of generating plutonium early in the nuclear age, the well developed technology and facilities for recovering plutonium for weapons use, PUREX, was selected for the commercial sector.  The downside of this approach is that it would present formidable safeguards challenges if widely deployed, since it would suggest an open market in separated plutonium among all states using nuclear power.  Responding to this concern, in the early 1970s the United States unilaterally terminated all plans for this type of reprocessing, in the vain hope that the rest of the world would follow suit.

Early on, a number of alternative recycle technologies had been explored, with the goal of removing fission products from the used fuel and recycling the remainder, rather than selectively extracting plutonium for recycle, as PUREX does.  Work on this approach resumed in the mid 1970s, and by the late 1980s, a successful high-temperature electrochemical process, “PYRO” for short, had been developed. An initial, pilot-scale demonstration was completed by the mid 1990s. Then, out of an exaggerated concern that an extra proliferation risk might accompany the new technology, Congress aborted the program.

PUREX vs PYRO

In the once-through fuel cycle, uranium is mined and refined, and then is enriched to increase the ratio of “fissile” U-235 to “fertile” U-238 for use in light-water cooled reactors (LWRs).  In the power plant, most of the U-235 is consumed, and some of the U-238 is converted into plutonium.  The used fuel, with the plutonium it contains, is set aside for disposal.

The PUREX (weapons-program) technology is currently in use in France.  Plutonium is recovered from used LWR fuel and recycled in other LWRs.  The residual uranium, fission products, and higher actinides (Np, Am, Cm, etc.) are left for later disposal.  In practice, the plutonium can be recycled only once, or perhaps twice.

The PYRO technology is effective only with fast reactors (FRs).  In this approach, fission products are extracted from the used fuel, and the remainder is recycled into the same (or another) fast reactor. Fast reactors can also be fueled with material recovered from used LWR fuel by an analogous process (removing fission products, and recycling the rest). 

Four Scenarios

Consider four simplistic scenarios, highly idealized to emphasize the differing implications of possible approaches to recycling.

Case 1: 200 large light-water reactors (LWRs) operating on a once-through fuel cycle.  This is consistent with some green-energy scenarios—a doubling of the current U. S. nuclear power-plant capacity.

Case 2: 200 LWRs, with maximum feasible PUREX recycle.

Case 3: 200 LWRs operating once through, supplemented with an equal amount of power from fast reactors (FRs) with PYRO recycle.

Case 4: 400 FRs with PYRO recycle, all LRWs having been phased out.  This is not a realistic near-term scenario, but is included to illustrate potential long-term opportunities.

Assumptions

•For case 2, PUREX facilities to separate the Pu necessary for recycle as MOX fuel are assumed to be available.  A fleet of 160 LWRs fueled with U-235 produces enough plutonium to fuel about 40 reactors with recycled Pu.
•For case 3, FRs are initially fueled with excess weapons material.  Additional fast reactors are fueled with material recovered from used LWR fuel.
•For case 4, the FRs are used to produce plutonium for additional FRs until the complete fleet is operating. Thereafter, they will be self-sustaining, neither requiring additional plutonium, nor producing an excess
•All excess weapons plutonium is used as feedstock for nuclear power.

Comparisons

Facilities needed
Case 1:

•Uranium enrichment facilities that produce the equivalent of 100 T weapons-grade HEU per year.  The material actually produced is not weapons-usable, but the enrichment facilities, if repro¬grammed, could produce that much weapons material.
•Facilities to manage and dispose of the initial backlog of used LWR fuel plus an annually additional accumulation of used fuel containing 40 T of generally low-grade plutonium.

Case 2:

•Uranium enrichment facilities; demand reduced by 20%;
•PUREX facility producing ~32 T per year of separated Pu.  With selected feed-stock, some weapons-usable material could be recovered;
•Facilities to manage and dispose of the wastes from the PUREX facility, containing long lived nuclear wastes more toxic than those in the original used LWR fuel; and an annual accumulation of used MOX fuel elements containing 10+ T of generally low-grade plutonium.

Case 3:

•Enrichment situation unchanged from Case 1, but power production doubled;
•A facility for recovery of recyclable materials from used LWR fuel;  No chemically pure plutonium is involved.
•PYRO facilities for recycling FR used fuel.   No chemically pure plutonium is involved;
•Facilities to dispose of the residue from FR recycling, containing essentially no long lived nuclear wastes.

Case 4:

•No enrichment facilities, unless part of a deliberate proliferation effort;
•PYRO facilities for recycling FR used fuel.  No chemically pure plutonium is involved;
•Facilities to dispose of the residue from FR recycling, containing essentially no long lived nuclear wastes.

Inventories
Case 1: Excess weapons plutonium depleted slowly.  Initial backlog of used LWR fuel (as of 2010, containing well over 700 T plutonium), accumulating further “waste” each year containing an additional 40 T plutonium.

Case 2: Excess weapons plutonium depleted slowly.  Plutonium recovered from used LWR fuel recycled as MOX.  Total plutonium production little changed, but less than a third as much is going to a repository.

Case 3: Excess weapons plutonium depleted rapidly.  The plutonium in used LWR fuel is directed to fast-reactor use.

Case 4: Excess weapons plutonium depleted rapidly.  No significant plutonium content in reactor waste.  All existing plutonium in a reactor, or in a heavily shielded, highly radioactive recycling facility.