The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

ecause of their effects on the neutron energy), such as water, slow down the fast neutrons
resulting from fission. Thus, fast reactor designs that use non-moderating coolants such as
liquid metallic sodium are able to obtain fast spectra and achieve high conversion ratios (up
to 1.3) for uranium-based fuel. In contrast, current LWRs have thermal spectra (low average
neutron energies) which result in conversion ratios between 0.5 and 0.6.
Water-cooled reactors can produce conversion ratios near unity if an epithermal (between
thermal and fast) spectrum is achieved. This can be done by reducing the moderator-to-fuel
ratio and/or using heavy water (D 2 O) as a coolant since it is a less efficient moderator.
Neutrons do not lose as much energy per collision when scattering off of heavy water (D 2 O)
compared to light water (H 2 O) since the deuterium nuclei in heavy water molecules are
twice as massive as the hydrogen nuclei in light water. Therefore, the neutron spectrum
is harder (faster) when H 2 O is replaced with D 2 O in a water-cooled reactor with a low
moderator-to-fuel ratio. This is one way to obtain a higher conversion ratio. However, the
use of heavy water comes with some disadvantages: (1) heavy water costs several hundred
dollars per kilogram, (2) the reactor system must be designed to minimize losses of the
expensive coolant and avoid contamination with H 2 O, and (3) neutron capture by deuterium
produces tritium, a radioactive isotope of hydrogen that must be efficiently collected
through recovery systems.
Because most of the world’s reactors are pressurized water reactors (PWRs), a type of LWR,
most of the early work on sustainable LWRs was associated with PWRs. A high-conversion
PWR using the plutonium-uranium fuel cycle was first suggested by Edlund [3] in 1976.
The concept was to harden the spectrum by reducing the water-to-fuel volume by redesigning
the core of a Babcock & Wilcox PWR into one with a hexagonal pin lattice and hexagonal
assemblies. This resulted in a conversion ratio of about 0.9 while maintaining sufficient
cooling and a small negative void reactivity coefficient.
The concept of retrofitting an existing PWR using a hexagonal lattice for a high-conversion
ratio was proposed by Broeders in 1985 for a Kraftwerk Union 1300 MWe PWR [4]. This
design also introduced heterogeneous seed and blanket reactor core designs that had two
major beneficial impacts; (1) it increased the conversion ratio to 0.96 and (2) it resulted in a
large negative void coefficient [better nuclear safety upon overpower occurance]. When the
water temperature increases, the resultant decrease in its density yields a harder spectrum
resulting in more fast neutrons leaking from the fissile regions into the fertile regions resulting
in more neutron absorption and lower power levels. Seed and blanket designs became
a fixture of all future sustainable LWRs. Seed and blanket concepts imply the seed operates
at a high power output and the blanket operates at a low power output—characteristics
that tend to reduce thermal margins and increase pumping power requirements. The safety
constraint is cooling the higher powered seed under loss of coolant accident conditions.
Ronen [5] proposed a 1000MWe PWR with a conversion ratio of 0.9, which featured axially
heterogeneous rods with alternating layers of fissile plutonium (MOX) fuel and fertile
(natural UO 2 ) regions. This was followed by the high-gain PWR proposed by Radkowsky
[6], which features two seed-blanket cores: a prebreeder and breeder. The concept used
rapid fuel reprocessing to minimize the loss of 241 Pu through beta decay (14.4 year half life).
241
Pu has the highest η (number of neutrons emitted per capture of a neutron) of all uranium
and plutonium isotopes and can greatly increase the conversion ratio. The prebreeder
core uses a soft spectrum to produce the 241 Pu through sequential thermal neutron capture
appendix B: advanced Technologies 193