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Fraction of Fission Products That Becomes Stable Fission Gas Is

Fission gases are released from nuclear fuel between 1300-1900 K via diffusion and above 1900 K via gas bubbles. This release, along with accumulated fission products, causes fuel swelling. Fast reactor cores have higher neutron flux, harder spectra, and higher temperatures and burnups, requiring fuels with high melting point, radiation resistance, and atom density. Potential fuel candidates include metallic U-Pu-Zr alloy and ceramic (U,Pu)O2, C, and N, with metals having highest atom density and carbides/nitrides being intermediate. Boron, europium, and tantalum are considered as absorber materials to control and shut down the reactor, with boron carbide commonly used but exhibiting swelling issues

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41 views12 pages

Fraction of Fission Products That Becomes Stable Fission Gas Is

Fission gases are released from nuclear fuel between 1300-1900 K via diffusion and above 1900 K via gas bubbles. This release, along with accumulated fission products, causes fuel swelling. Fast reactor cores have higher neutron flux, harder spectra, and higher temperatures and burnups, requiring fuels with high melting point, radiation resistance, and atom density. Potential fuel candidates include metallic U-Pu-Zr alloy and ceramic (U,Pu)O2, C, and N, with metals having highest atom density and carbides/nitrides being intermediate. Boron, europium, and tantalum are considered as absorber materials to control and shut down the reactor, with boron carbide commonly used but exhibiting swelling issues

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FISSION GAS RELEASE & SWELLING

• Fission gas release


• fraction of fission products that becomes
stable fission gas is 0.27
• At temp.< 1300 K, gas mobility is very low
– hence no gas escape
• Between 1300 to 1900 K, gas release
through diffusion
• > 1900 K, large thermal gradients drive
gas bubbles and hence more release

• Fuel swelling
• occurs due to accumulation of low dense
fission products
• fission gases are insoluble in fuel matrix
• the unreleased fission gases cause
swelling
FAST REACTOR CORE

• Core environments

‑ higher neutron flux


‑ harder neutron spectrum
‑ higher temperature
‑ higher fuel burnup

• FUEL REQUIREMENTS
* high burn-ups
* high specific power
* high temp. gradient
• DESIRABLE FEATURES OF AN IDEAL FUEL:

‑ high K and high m.p


‑ high radiation damage resistance
‑ high fuel atom density
‑ good compatibility with cladding & coolant

‑ ‑ve prompt Doppler coefficient

‑ Avoidance of phase change below m.p.

‑ Easiness for fabrication

‑ High neutron yield


Condidates for Fuel
• Metallic U‑Pu‑Zr alloy (Zr=10%)
‑ m.p. 1155oC
• Ceramic (U,Pu) O2
(U,Pu) C
(U,Pu) N

Ceramic Metallic
Low Heavy Atom Density High Atom Density
Low K High K
High m.p Low m.p
Low Thermal Expansion High Thermal Expansion
• Carbides & Nitrides ‑ intermediate between Oxide &
Metals but closer to metal.
* SWELLING
* FABRICATION COST
* REPROCESSING
* DOPPLER coeff.
* THERMAL EXPANSION
• In TOPA ‑ oxide perform better
• In ULOFA ‑ carbide, nitride & metal serve better due to
high K
• metal best
✔ no core melt down, reactor shuts down by itself
• BURNUP
• Oxides ‑ high burnup (~200 000 MWd/t)
‑ doubling time : 25 to 30 years
• Carbide & Nitride
‑ good burnup
‑ doubling time : 15 ‑ 20 years
• Metal ‑ tried in USA
‑ doubling time : 10 ‑ 15 years
• In Indian Context
* Doubling time is more important than economics
* More FBRs before local resources are exhausted
* Metal is right choice
• PFBR ‑ Oxide - to establish technology of
manufacturing & reliable operation of plant
• For long term program, metal being considered.
• ABSORBER
‑ to control & to shut down the reactor
• Preferable features:
‑ High absorbtion cross section
‑ good compatibility with cladding & coolant
‑ Long effective life time (2 to 3 years)
‑ low swelling under irradiation
‑ high thermal conductivity
‑ high m.p &
‑ low cost
• Boron, Europium & tantalum ‑ have been given most
considerations
• Boron
✔ 19.6% B10 in Natural Boron
✔ B4C is used as absorber.
✔ In SPX1, B4C with 93% enriched B is used.
(n, α)
10 1
• 5
B + on ‑ ‑‑‑> 3
Li7 + 2He4
(n, α)
10
• 5
B + on ‑‑‑‑> 2 2He4 + 1T3
1

(This process is only at very high energy level; i.e., E > 1 MeV)

Since both 3Li7 & 2He4 atoms are larger than B10, B4C matrix
swells approximately linearly with neutron exposure.

• Problems
‑ Swelling
‑ Buildup of Helium in B4C rod
‑ larger plenum must be provided or rod has to be vented
leading to a gas release problem
THANK YOU
• Europium
✔ In natural 63Eu, 47.8 % Eu151 & 52.2 % Eu152 are present.
✔ Averaged fast neutron cross section of Eu is twice that of
B 4C

Eu2O3 Euro Oxide


✔ absence of swelling & gas release problem due to Eu (n,
γ) reaction.
✔ daughter nuclei of Eu (n, γ) are also good neutron
absorbers and hence life of absorber rod can be
extended.
✔ because of self-shielding effect, effective worth of Eu2O3
is only equivalent to that of Natural B4C.
✔ low K
• EuB6 Euro Boride
✔ reactivity worth is about 10% more than Eu2O3; i.e.,
equivalent to that of 25 % B10 enriched B4C.
✔ better K than Eu2O3.
✔ 2He4 gas release problem is much worse than for B4C.
Hence venting out is essential.
✔ Being a rare earth element, supply of Europium is quite
limited. Both Eu2O3 & EuB6 are costly.
• Tantalum
• In natural 73Ta, 99.9 % Ta181 with trace amount of
Ta180 is present. σa is about 1/3 of that of B10.
• 73Ta181 (n, γ) 73Ta182 β- 74W182 + 1.2 MeV
115 days
• No gas is released
• Daughter product - Ta182 is also a good absorber.
• Being a metal, good K.
• Since it is not a rare element, it is not too
expensive.
• However, it is soluble in sodium and it has decay
heat removal problem due to 115 days half-life β-
decay of Ta182.

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