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US makes part for British nuclear warhead

ImageThe MoD has confirmed for the first time that a vital part of British Trident nuclear warheads is purchased off-the-shelf from an American nuclear bomb factory.  The Neutron Generator supplies neutrons to start the fission stage of the bomb.  A report of Sandia Laboratory's work in 2008 showed that last year the Laboratoriy supplied 14 Neutron Generator's to the UK.  Reported by Rob Edwards in Sunday Herald 29 November 2009.



Official statements

In 2004 the following reply was given to a written question:  

“Angus Robertson: To ask the Secretary of State for Defence in which countries the (a) arming, fusing and firing system components, (b) neutron generators and (c) gas reservoirs of Trident nuclear warheads are manufactured; and if he will make a statement.  

“Mr. Ingram: I am withholding the information requested under Exemption 1 (defence, security and international relations) of the Code of Practice on Access to Government Information.”[1]

The White Paper on the Future of the UK Strategic Deterrent, December 2006, included the following statement:“The UK produced a new nuclear warhead to coincide with the introduction into service of the Trident system.  This warhead was designed and manufactured in the UK by AWE, although it was decided that it would be more cost effective to procure certain non-nuclear components of the warhead from the United States”.[2] 

When told that Sandia had revealed that they were supplying Neutron Generators to the UK, an MoD spokesperson told Rob Edwards of the Sunday Herald (29 Nov 2009)-

"These non-nuclear components include the neutron initiator". 

2008 Neutron Generator Production

The Performance Evaluation for Sandia National Laboratory for 2008 says:

“The Responsive Neutron Generator (NG) Center completed all FY 2008 Directive Schedule requirements to a 100 percent on-time delivery. Sandia made the following neutron generator shipments supporting Directive Schedules: W76 – 224 units in 21 shipments, W78 – 148 units in 19 shipments, UK – 14 units in three shipments”.[3] 

Neutron Generator Function

The Neutron Generator supplies a boost of neutrons to the primary of a nuclear warhead to initiate an efficient fission reaction.  The units are designed and manufactured by Sandia National Laboratory, who have explained –“The proper function of a nuclear warhead depends on the presence of neutrons during primary implosion when the plutonium is supercritical.  Neutron Generators are located close to the warhead primary to produce a sufficient quantity of neutrons at that critical time”.

The Neutron Generator does not operate at the precise moment when the electrical signal is sent to the primary detonators, but a small fraction of a second later.  It must be timed so that the neutrons are available at the start of the fission process, after the high explosive has detonated and as the plutonium pit implodes.  The time delay is called “stand off”.  A computer model of Neutron Generator Stand-Off for the W76 Trident warhead was created in 1997.  This was an early target of the Accelerated Computing Initiative.  There was particular concern about potential problems with NG stand-off when the W76 was detonated on impact, by a contact fuze, rather than in the air.

The precise functioning of the Neutron Generator will have an effect on the initial fission yield, which is then magnified by the boosting process and by fusion.  Any inefficiency in the operation of the Neutron Generator will have a significant impact on the final thermonuclear yield.

Neutron Generator Manufacture

Neutron Generators for US and British warheads are manufactured at the Responsive Neutron Generator Product Deployment Center in Sandia National Laboratory, Albuquerque, New Mexico.   The first stage of the process is Neutron Tube Target Loading (NTTL).  NTTL “is an operation that involves the transferring (ie loading) of radioactive tritium gas onto metal target disks under an inert nitrogen atmosphere using a glovebox operation”.[4]  This is done by placing a reactive metal target disk into a high vacuum loading apparatus.  The chamber is then filled with specific quantities of tritium and deuterium which are absorbed by the disk, forming a metal hydride.  This is then known as a “loaded disk”.  The process must be very clean and is done inside a glovebox which has a nitrogen atmosphere.  During each loading run less than 0.1 gram of tritium is used.  The maximum amount of tritium used is around 2 grams per year.The loaded target disk is put into a Neutron Tube. 

The neutron tube is combined with a timer and a power unit to make the Neutron Generator.  The power unit uses an electro-explosive device to generate a high voltage.  In 2007 Neutron Tube sub-assembly took 43 days and Neutron Generator sub-assembly took 84 days.[5] 

The Responsive Neutron Generator facility at Sandia has an annual budget of $80 million and a staff of 217 who work in a 132,000 square feet complex.[6] 

Neutron Generators for the W76 and UK Trident warheads

The original Neutron Generator in the W76 was the MC2989.  In 1996 Sandia began a programme to recertify these units.  This involved replacing the timer and inspecting the neutron tube.  In 1999 Sandia was hired “to do the recertification work on the neutron generator in a similar British weapon”.[7] 

This recertification was only a temporary measure.  Sandia had already begun to design a new Neutron Generator for W76 in the early 1990s.  The new model, MC4380, was part of Alt 317 of the W76 warhead and entered service in 2002.  It is made up from the MC4277 neutron tube, MC4378 timer and MC4368 power supply.  Sandia revealed that the MC4380 had been delivered to the US Navy and to the UK in Summer 2002.[8] 

Limited Life of Neutron Generators

Neutron Generators are, in US parlance, Limited Life Components.  They are not units that can remain in service for several decades.  The tritium which is at their heart has a half-life of 12 years, so the Neutron Generator becomes progressively less effective over time.  As Sandia explain -

“Because it uses short-lived tritium, the neutron generator is one of the components in a nuclear weapon that must be replaced regularly.”[9]

The first UK Trident warheads were delivered to Coulport in November 1992.  The overhaul of the original Neutron Generators began 7 years after they entered service and their replacement with the MC4380 started 3 years later.

Neutron Generator manufacturing capability and stockpile levels

Because they need to be regularly replaced, the supply of Neutron Generators is one key factor which determines how many warheads are operationally available, as distinct from those which are stockpiled but cannot be quickly deployed.  A second factor is the availability of tritium reservoirs.

The Neutron Generator Facility at Sandia Laboratory was designed in 1994 to be able to manufacture each year 500 Neutron Generators for warheads, plus 100 for testing.  In 1997 DOE decided that this would not be sufficient to enable the US to restore reserve weapons to an operational status in the event of a defect with a deployed design.  So the manufacturing capability was increased from 600 to 1550 per year by 2008..[10]

[1] Hansard 8 July 2004, Column 829W
[2] The Future of the United Kingdom’s Nuclear Deterrent, Cm 6994, December 2006, page 30
[3] NNSA Fiscal Year 2008 Performance Evaluation report of Sandia Corporation, pages 138f
[4]  NTTL was carried out at Pinellas Florida until 1993 then at Los Alamos until 2005.
[7] Sandia Lab News 15 January 1999.
[8] Sandia Lab News February 2003
[9] Sandia Lab News 28 January 2000
[10] Testimony from Paul Robinson, Sandia, to Senate Armed Services Committee, April 2001.