Most countries that generate nuclear power are in the process of develop­ing criteria, designs and sites for the permanent disposal of spent nuclear fuel, but they have yet to become licensed realities (Adamson et al, 2010). The most significant fuel related criteria for dry storage are compared in Table 5.1. Meanwhile the pools at the nuclear plant sites are filling up with spent fuel and the utilities are transferring the spent fuel from the pools to dry cask storage sites that are mostly located at the plant sites. Exceptions are the central, large intermediate pool facilities that serve all the plants in Sweden (CLAB facility) and all the plants in Finland (KPA-STORE). The lack of a licensed permanent fuel repository in any country has placed total reliance on intermediate storage. As a result the dry storage technology has become a major activity and business component of today’s back-end fuel strategies.

The key differences between dry storage and in-reactor performance of fuel are (Adamson et al., 2010): [4]

Dry storage criteria USA Germany Hungary S. Korea (CANDU only) Spain (follows USNRC) Switzerland Pellet clad °С, storage, drying, other 400° 570° 370° 370°, 100 hr 410° 410° None but maintain clad integrity 400° 570° None 570° Temperature cycling Max. no. of cycles and Max. ДТ (°С) per cycle 10/65° None None None 10/65° 10/65° BU, GWD/MT None 65 assembly avg. 49-50 None None None Clad hoop stress, MPa 90 (ifTclad > 400 °С) 120 @370° None None 90 Depends on fuel supplier*; typical values are 90 and 120 Clad strain limit None 1% circumferential None None 1% for HB 1% during storage Creep rupture limits None None None None Limited by °T and stress limits Cladding condition limits Oxide thickness for stress calculation Oxide thickness for stress calculation None None Limited by °T and stress limits None

Any assembly with defected rods

Yes in special container (ZIRAT11/IZNA6, Sect. 11.3.2)

0.95 normal 0.97 accidents

Yes, with actinides & fission products

Failed assembly identified by sipping	Clad penetration that emits fission products	NRC ISG1, rev. 1	None
Not approved	Not approved	Yes, case by case to meet safety criteria	Not approved
Keff+AKeff 0.95	0.95	0.95	<1
No; storage is based on enrichment of fresh fuel	“none”	Yes, with actinides, only for transportation license	Yes, acceptable for transportation license
Yes	“none”	Yes	Yes
No	“none”	No	No
PSA approach for	Coincident	No criticality	Airplane crash +
max. radioactive	failure of	in transport	kerosene fire,
exposure	“cylinder” and 600 CANDU assemblies	accident with flooding	earthquake for storage + 9m drop etc. for transport

{Continued)

	Dry storage criteria	USA	Germany	Hungary	S. Korea (CANDU only)	Spain (follows USNRC)	Switzerland
	Fuel failure	Yes	Yes	Yes	No failure during Yes, within off-site	Yes
	allowed in				cooling	dose limits
	worst case				blockage
	accident
	Acceptable	10CFR72&	10% failed rods	Retrieve bi I ity	—	% fuel failures	Retrievability
$	failure	source	1% fuel release	Criticality		1% — normal
о CL	conditions	term limit		Coolability		10%-off
CD						normal 100%
CL						— accident +
C_						maintain max
С/Г						cask internal
=з’						pressure
CQ	Modelling	ABACUS	MCNP	MCP4, SCALE 5.1,	—	MCNP, SCALE,	CSAS (AREVA)
3′	codes	LS DYNA		MONK 6B, RISK		ANSYS,	for normal
CD		FALCON		SPECTRUM,		FLUENT	conditions
				TRANSURANUS,		ABACUS, LS
О				ENIGMA + others		DYNA 3D
CD	ANT International, 2011

Table 5.2 Maximum BUs achieved vs Regulatory limits (excludes LUAs), A. Strasser in Adamson et al. (2010, ZIRAT 15 Annual Report) Country BU (GWD/MT) Batch Assembly Rod Pellet Regulatory limit USA 54 58 62 73 62.5 peak rod Belgium 50-55 55 UO2 assy, 50 MOX assy Czech Republic 51 56 61 60 peak rod Finland 45.6 46.5 53 45 assy France 47 51 UO2 42 52 assy MOX Germany 58 62 68 65 assy Hungary 50 62 Japan 50 55 62 55 UO2 assy, 45 MOX assy Korean 46 60 rod Republic Netherlands 51.5 58 64.5 60 rod Russia 45 56 60 68 Spain 50.4 57.4 61.7 69 Sweden 47 57.2 63.6 60 assy, 64 rod Switzerland 58 60 65 71 75 pellet Taiwan 60 rod (P), 54 assy (B) UK 44.3 46.5 50 55 pellet Ukraine 50 Source: A. N.T. International, 2011

• He gas pre-pressurization during fabrication

• fi ssion gases

• He from transmutation of B in burnable absorbers

• alpha decay of the Pu isotopes in the fuel during storage

and their pressure is further raised by the fuel decay heat. All of these pres­sure sources, except the pre-pressurization level, are burnup dependent.

The pressure outside the cladding in the cask is only slightly above atmo­spheric. Creep deformation of the cladding will occur at a relatively constant rate early in life, when the internal gas pressure, the cladding stresses and the cladding temperature are at their highest. As the decay heat decreases with time, the gas pressure and the cladding temperature both decrease. In addition, the internal free volume of the fuel rod increases as the cladding creeps outward, decreasing the gas pressure and cladding stresses further. All three of these factors eventually reduce the creep rate to a negligible value (Adamson et al, 2010).

Creep-rupture is the most likely cladding failure mode during dry storage and there is general consensus on this mechanism among the industry and the regulators (Adamson et al., 2010). The parameters that determine the potential for creep rupture are the cladding stress level, the cladding tem­perature and the rate of decay heat decrease, all parameters that are burnup dependent.

The cladding temperature, currently one of the primary USNRC licens­ing criteria, is determined by the decay heat generated in the fuel, the heat transfer capability of the cask and the surface temperature of the cask in its storage environment (Adamson et al., 2010). The decay heat is generated primarily by absorption of the alpha decay either directly or indirectly from the plutonium (Pu) isotopes. Increasing burnup will increase the level of Pu isotopes formed by transformation of the 238U, and increase the cladding temperature in dry storage conditions. In comparison, MOX fuel will have significantly higher temperatures under the same conditions.

Several other potential failure mechanisms were considered, but elim­inated as highly unlikely (Rashid, 2006). They are summarized below (Adamson et al, 2010).

Stress corrosion cracking (SCC) is not a credible failure mechanism in dry storage because:

• There is insufficient elemental iodine present to cause SCC.

• At the stress and strain rates in dry storage, initiation of intergranular cracking is nearly impossible; the 180-200 MPa stresses needed for SCC are well above those for high burnup fuel rods.

• Hydrides, including radial hydrides, will not affect iodine induced SCC.

• The occurrence of all the conditions that cause DHC is highly unlikely, but cannot be ruled out. The initial conclusions are based on the follow­ing evaluation:

• Analyses indicated that at a hoop stress of 250 MPa (well above dry storage conditions) in a cladding wall thickness reduced by 100 pm oxide with an 83 pm crack size, the stress intensity factor is below that needed to initiate the DHC process,

• Hydride re-orientation that might assist crack propagation is intended to be minimized or prevented by current regulations and industry practices, but cannot be ruled out.

• In addition, propagation of a crack assisted by radial hydrides may not occur for many of the hydride morphologies.

Also under accident conditions during storage or subsequent transporta­tion, the fuel must remain subcritical and should be recoverable by nor­mal methods (Adamson et al, 2010). The hypothetical accident conditions that these criteria have to meet, as defined by the USNRC, are specified in 10CFR71.73 (NRC, Rules and Regulations, Title 10 Code of Federal Regulation, Chapter 71). Of all the accident scenarios the most limiting scenario is a free drop of the cask for a distance of 9 m (30 ft) onto a flat, unyielding horizontal surface, striking the surface in a position that would cause the maximum fuel damage.

Radial hydrides in zirconium alloy cladding are undesirable because they reduce the critical stress intensity required to propagate a radial crack through the wall of the cladding during handling or transportation (Adamson et al, 2010). The objectives of the dry storage regulations are to limit the conditions that could result in hydride re-orientation.

A certain fraction of the hydrogen (H) picked up during the oxidation reaction is soluble in the zirconium matrix and the remainder forms zirco­nium hydrides (Adamson et al, 2010). The solubility of the H is a function of temperature, alloy composition and microstructure. Solubility is also a function of irradiation history, heating or cooling rates during service. The orientation of the hydrides formed during normal reactor operation are generally circumferential near the cooler cladding OD and remain so dur­ing wet storage of the spent fuel.

The hydrides can reorient in the radial direction if they are precipi­tated from solid solution by cooling the alloy from a higher temperature under a tensile or hoop stress (Adamson et al, 2010). The hydrides will align themselves in the direction perpendicular to the tensile stress. This can occur during reactor operation although it is generally unlikely. It could occur during dry storage if the internally pressurized cladding is at a high temperature, holds sufficient hydrogen in solution and is then cooled while under the hoop stress. The hydrides in solution will precipitate in the radial orientation (provided the hoop stresses are large enough), while the hydrides that did not dissolve will remain in their original circumferen­tial orientation. This is most likely to occur during rapid cool-down from high temperatures after cask drying or evacuation procedures rather than during storage when the rate of temperature and pressure reduction that control the stress levels are extremely slow.

In summary, the factors that affect hydride re-orientation in irradiated cladding are (Adamson et al, 2010): [5]

• Microstructure features such as grain size and shape, amount of CW, and perhaps others.

• Texture.

The radial hydrides can be present in a wide variety of sizes and distribu­tions as well as fractions of the total hydrides present and each type of struc­ture can have a different effect on mechanical properties. This emphasizes the importance of characterizing the structures when they are related to the mechanical properties measured.