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By Charles Rhodes, P.Eng., Ph.D.

A good technical reference is: Nuclear Reactor Physics

Avoidance of production of long lived nuclear waste causes fundamental changes in nuclear reactor design principles.

It is shown on this web site that some of the most difficult nuclear wastes to safely dispose of are the long lived low atomic weight radioactive isotopes:
Be-10, C-14, Cl-36, Ca-41, Ni-59, Se-79 and Sn-126.

The only practical way to avoid long term disposal problems with these isotopes is to design new nuclear power reactors to minimize production of these isotopes. The isotopes Be-10, C-14, Cl-36, Ca-41 and Ni-59 arise due to neutron activation of the elements beryllium, carbon, chlorine, calcium and nickel. These elements should not exist in the reactor's neutron flux. There are significant implications of this constraint on fast neutron nuclear reactor design.

The isotopes Se-79 and Sn-126 arise as fission products. After 300 years in storage the elements selenium and tin should be selectively extracted from other decayed fission products and placed in isolated long term storage.

All fission reactor cores should be surrounded by a removeable blanket of U-238 sufficient to absorb all excess neutrons that escape form the fission reactor core. Over time this U-238 will partially convert into Pu-239 and other transuranium actinides that will be required to operate future fast neutron reactors.

The fission products include the inert gases. The problem with the inert gases is that there is no practical way to contain them for long periods of time. They can be liquified but for storage periods in excess of decades they are impossible to reliably contain and generally must be released to the atmosphere. In this respect Kr-85 is a problem because with a half life of 10.76 years it is difficult to store but the rapidity of its decay still makes it potentially hazardous in high concentrations. Kr-81 is a potential very long term problem because with a half life of 210,000 years it is impossible to contain and over time it will accumulate in the atmosphere. At this time there is no practical way of either preventing Kr-81 from forming or of isolating and storing Kr-81 until it naturally decays. If mankind is going to rely on fission energy mankind will have to adapt to the presence of Kr-81 in the atmosphere.

From a practical nuclear reactor design perspective inert gas fission products must remain contained sufficiently long to allow the other short half life inert gas isotopes to naturally decay. If the inert gases must be stored for 20 half lives, the required storage time for krypton is:
20 X 14.8 hours = 296 hours = 12.3 days
and xenon must be contained for:
20 X 12 days = 240 days

From the perspective of FNR operation the fuel tubes must be stored out of the neutron flux for at least 240 days before they are openned to allow all the contained radioactive inert gases except Kr-81 and Kr-85 to naturally decay. This requirement is generally not an issue since used fuel should be stored immersed in liquid sodium at the reactor perimeter for about 6 years before being recycled.

The element thorium is about four times more abundant in Earth's crust than is uranium. There have been numerous proposals to use thorium instead of uranium as a primary nuclear fuel. However, those proposals are not without serious complications, especially related to the byproducts Pa-231 and U-232.

In order to convert fertile Th-232 into fissionable U-233 thermal neutrons are used to convert Th-232 into U-233 according the reaction sequence:
n + Th-232 = Th-233,
Th-233 = Pa-233 + e, (half life = 22.1 minutes)
Pa-233 = U-233 + e (half life = 27.0 days)
U-233 = Th-229 + He-4 (half life = 162,000 years)

However, in a practical nuclear reactor some of the neutrons have higher than thermal energy and a significant fraction of the thorium is consumed via the alternate reaction path:
n + Th-232 = Th-231 + 2 n,
Th-231 = Pa-231 + e,
Pa-231 = Ac-227 + He-4, (half life = 32,500 years)
Ac-227 = Th-227 + e, (half life = 21.6 years
Th-227 = Ra-223 + He-4, (half life = 18.2 days)
Ra-223 = Rn-219 + He-4, (half life = 11.435 days)
Rn-219 = Po-215 + He-4, (half life = 4.00 seconds)
Po-215 = Pb-211 + He-4, (half life = 1.778 X 10^-3 seconds)
Pb-211 = Bi-211 + e, (half life = 36.1 minutes)
Bi-211 = Ti-207 + He-4, (half life = 2.16 minutes)
Ti-207 = Pb-207 + e, (half life = 4.79 minutes)
Pb-207 = (stable isotope)

Note that this alternate reaction path seems to superficially improve the availability of neutrons but it breeds the isotope Pa-231. The Pa-231 contains a lot of potential decay energy but it has a half life of 32,500 years. Hence it is a hugely dangerous nuclear waste. The Pa-231 also has a high neutron absorption cross section.

Part of the Pa-231 reabsorbs neutrons and follows the decay path:
n + Pa-231 = Pa-232
Pa-232 = U-232 + e, (half life = 1.31 days)
U-232 = Th-228 + He-4, (half life = 72 years)
Th-228 = Ra-224 + He-4, (half life = 1.91 years)
Ra-224 = Rn-220 + He-4, (Half life = 3.64 days)
Rn-220 = Po-216 + He-4 , (half life = 55.3 seconds)
Po-216 = Pb-212 + He-4, (half life = 0.145 s)
Pb-212 = Bi-212 + e, (half life = 10.64 hours)
Bi-212 = Po-212 + e, (half life = 60.60 minutes)
Po-212 = Pb-208 + He-4, (half life = 3.04 X 10^-7 seconds)
Pb-208 = (stable isotope)

Note that this decay path results in U-232 in the spent fuel. This decay path, after consuming neutrons, yields hard gamma rays but does not yield energy fast enough to be of interest for central nuclear power generation. If the available neutrons are preferentially used for breeding more fissionable U-233 fuel what is left is a mountain of extremely dangerous spent fuel containing Pa-231 mixed with U-232. Possibly Pa-231 and U-232 could be fissioned in a fast neutron reactor (FNR), but that technology is somewhere in the future. It appears that as long as uranium is readily available and relatively inexpensive the complications of Pa-231 and U-232 in spent reactor fuel can be avoided by use of uranium instead of thorium. Uranium supplies, if reasonably recycled in a fast neutron reactor, will last over 1000 years. However, some time in the future the use of thorium as a primary reactor fuel and the issue of disposal of spent reactor fuel containing Pa-231 and U-232 will have to be faced. Possibly by then fast neutron reactor (FNR) technology will be suffiently advanced to fission the Pa-231 and U-232.

1. Avoiding neutron activation of carbon means that B4C cannot be used for neutron absorption or reflection. Furthermore, fuel tube steel must have the lowest practical carbon content.

2. Avoiding neutron activation of chlorine means that no PVC plastics can be located anywhere in the neutron flux. This constraint may affect the choice of temperature sensing thermal couple wire insulation in the neutron flux.

3. Avoiding neutron activation of calcium means that concrete cannot be used as a neutron shield or neutron absorber. Hence a 2.8 m thickness of sodium surrounding the reactor fuel tubes is required for neutron absorption / shielding. This constraint has the further effect of making many small modular nuclear reactor designs non-viable due to their use of concrete for absorbing leakage neutrons. From an economics perspective the reactor must be large enough to justify the minimum sodium requirement.

4. Avoiding neutron activation of nickel means that stainless steel and other alloys that rely on nickel to achieve high strength at high temperature cannot be used in the neutron flux. Hence the stress, temperature and pressure rating of the fuel tubes and fuel bundles is severely restricted. Hence the reactor fuel bundles must operate at a relatively low pressure and the intermediate heat exchanger must be physically outside the neutron flux. This geometry requirement increases the minimum sodium requirement and the fuel tube plenum size. The maximum primary liquid sodium temperature should be restricted to < 488 degrees C with a nominal maximum operating temperature of 450 degrees C.

5. The reduced stress handling capability of low carbon and low nickel steel at high temperatures leads to structural design constraints in the fuel bundles.

6. Avoiding problems with inert gas fission products means keeping fuel tubes in storage outside the neutron flux for at least 8 months before openning them for material recycling.

7. Avoiding spent fuel problems relating to Pa-231 and U-232 means keeping thorium out of the reactor fuel.

The aforementioned design constraints lead to the Fast Neutron Reactor (FNR) design presented on this web site. These design constraints, which are based on minimizing production of long lived nuclear waste, rule out operation of electricity power generation reactors at higher primary sodium temperatures.

This web page last updated April 14, 2020

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